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Surface & Coatings Technology

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A review of electrodeposited Ni-Co alloy and composite coatings:Microstructure, properties and applicationsAbdossalam Karimzadeha, Mahmood Aliofkhazraeia,⁎, Frank C. Walshb

a Department of Materials Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iranb Electrochemical Engineering Laboratory, National Centre for Advanced Tribology, Faculty of Engineering and the Environment, University of Southampton, SouthamptonSO17 1BJ, UK

A R T I C L E I N F O

Keywords:CobaltCorrosionElectrodepositionMicrostructureNickelTribology

A B S T R A C T

NieCo alloy electrodeposits have been widely employed in industry due to their good corrosion and wear re-sistance, high mechanical strength, moderate thermal conductivity and outstanding electrocatalytic and mag-netic properties. This review aims to provide an insight into the mechanism of electrodeposition and effect ofoperational parameters and deposit microstructure, together with the mechanical, electrochemical and tribo-logical characteristics of NieCo alloys and included particle, composite deposits. Potential applications of thecoatings have also been considered in applications as diverse as additive manufacturing, micro-tools, micro-sensors, electronic imaging and electrochemical energy conversion.

1. Introduction

Most failures in metallic components originate from the interactionof their surface with the environment. Problems can include wear,abrasion, fatigue, fretting, corrosion, oxidation, stress corrosioncracking (SCC). The practical use of engineering components is severelylimited by these failures. The surface of the component plays an es-sential role and can restrict its uses. Appropriate attention to surfaceengineering can help to transform the microstructure, chemical com-position and the resultant properties of materials. Coating is an effectivemethod of improving the service life of the components [1]. Electro-deposited NieCo alloys and nanocomposite coatings provide a goodexample of such coatings. NieCo alloys possess great practical im-portance due to their protective and decorative properties. Among theproduction methods of NieCo coatings, electrodeposition is commonlyused because of its moderate costs, flexibility (single layer or multilayerdeposition as intermittent or gradient), high efficiency and simple massproduction procedure with little need for high temperatures and highpressures, compared to other production processes, such as chemicalvapour deposition (CVD), sputtering and flame spraying [2–4].

While the electrodeposition of Co or NieCo alloys is more expensivethan Ni, due to the higher price of Co and its salts, significant benefitscan be achieved due to the improved engineering properties of the alloydeposit. The electrodeposition of Ni and Co single metals has beenconsidered in classical texts and reviews. The benefits of

electrodepositing nickel and cobalt single metals, their binary alloysand ternary composites in a nanostructured form was highlighted in aseries of three reviews [4A–4C] [5–7]. The often superior corrosion andtribological characteristics of CoeNi compared to Ni are accompaniedby higher costs; the specific cost of cobalt metal is often more thantwice that of nickel.

Many coating techniques are available, such as chemical vapourdeposition, CVD, high velocity oxygen fuel (HVOF) flame spraying,plasma spraying and laser cladding. The advantages of electroplatinginclude versatility, an acceptable rate of deposition and moderate coststogether with the ability to control the thickness and structure of thedeposits, even on complex shapes. In the case of NieCo alloy electro-deposits, A review [6] considered that there was a need for continueddevelopments in several areas:

1. A clarification of the role of tribofilms and wear debris on the tri-bological behavior of NieCo deposits.

2. Developing techniques, such as compositionally graded coatings andapplication of pulsed current have limitations including complexequipment and high cost.

3. The deposit microhardness needs to be increased further and directnanostructured alloy deposition needs to be achieved by optimizingbath additives and plating conditions.

4. Despite many studies on coarse-grained deposits, there have beenrelatively few investigations of nanocrystalline NieCo coatings.

https://doi.org/10.1016/j.surfcoat.2019.04.079Received 5 March 2019; Received in revised form 20 April 2019; Accepted 25 April 2019

⁎ Corresponding author.E-mail addresses: [email protected], [email protected] (M. Aliofkhazraei), [email protected] (F.C. Walsh).

Surface & Coatings Technology 372 (2019) 463–498

Available online 29 April 20190257-8972/ © 2019 Elsevier B.V. All rights reserved.

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These aspects are considered in this review, which highlights thecontinuing importance of NieCo electrodeposited coatings, includingbath composition, deposition mechanism, types of controlled currentdeposition, the effect of key operational parameters on microstructureand chemical composition as well as the physical, mechanical, mag-netic, corrosion, high temperature oxidation resistance and thermalstability of NieCo deposits. Selected highlights during the developmentof NieCo electrodeposits are shown in Fig. 1. Several aspects deserveattention:

1. The sulfate based Watts bath, introduced in 1916, remains the mostpopular electrolyte. This and other bath compositions are con-sidered in Section 4.

2. The benefits of modern baths, such as the use of ionic liquids [8] inavoiding hydrogen evolution as a secondary cathode reaction, arealso seen in Section 4.

3. The introduction of P as an alloying element in NieCo electro-deposits [9] is mentioned in Section 16.

4. NieCo composite deposits containing included particles are treatedin Section 17.

5. Special cases of NieCo electrodeposition have included ultra-sonically assisted jet plating [10].

6. Traditionally, NieCo deposits have offered better corrosion andwear resistance than Ni deposits and have seen increasing use inspecialized tribological engineering applications since the 1960s;they have been one of the possible replacements for high chromiumover the last twenty years.

7. Over the last decade, NieCo electrodeposits have found diverse usesin electronics and as electrocatalysts in electrochemical energyconversion.

The types of NieCo electrodeposits considered in this review are

Symbols

A Electrode area, cm2

b Burger's vector magnitude, dimensionlessC Concentration in the bath, g dm−3

Cb Bulk concentration in the bath, g dm−3

Cs Concentration at the metal surface, g dm−3

dave Average grain size of deposit, cmD Diffusion coefficient of ion, cm2 s−1

E Electrode potential, Vf Pulse frequency, s−1

F Faraday constant, A s mol−1

G Shear modulus of coatingH Hardness, PaHc Magnetic coercivity, OeI Current, AINi Current for nickel deposition, AICo Current for cobalt deposition, AIH2 Current for hydrogen evolution, Aj Current density, A dm−2

ja Anode current density, A dm−2

jc Cathode current density, A dm−2

jave Average current density, A dm−2

jp Peak current density, A dm−2

jcorr Corrosion current density, A dm−2

jL Limiting current density, A dm−2

K Coefficient of friction, dimensionlessKH Overall rate constant for a reaction involving two mole-

cules, dimensionlessM Molar mass of metal, g mol−1

Mc Magnetisation, TMfilm Mole fraction of Co in the alloy deposit, dimensionlessn Amount of material, molN Flux of material, mol cm−2 s−1

q Electrical charge, A st Time, ston On time, stoff Off time, staa Anodic reverse time, stc Cathodic forward time, sT Temperature, KTm Melting point, KTp Peak temperature, Kv Electrolyte velocity, cm s−1

z Electron stoichiometry, dimensionlessW Probability of nucleation, dimensionlessL Applied load, N

x Deposit thickness, cmv Poisson ratio, dimensionlessY Young's modulus, dimensionless

Greek

δ Nernst diffusion layer thickness, cmη Overpotential, Vθ Surface coverage, dimensionlessψ Current efficiency, dimensionlessϕ Duty cycle, dimensionlessρ Density of metal, g cm−3

σ Internal stress, Paν Kinematic viscosity of electrolyte, cm2 s−1

ω Angular speed of RDE, s−1

2θ Scattering angle in XRD, deg

Abbreviations

ASTM American Society for Testing MaterialsBD 2-butin-1,4-diolCOF Coefficient of frictionCVD Chemical vapour depositionDC Direct currentDES Deep eutectic solventsEBSD Electron backscatter diffractionECAM Electrochemical additive manufacturingEDS Energy dispersive X ray spectroscopyEIS Electrochemical impedance spectroscopyEPA Environmental Protection Agencyfcc Face centred cubic (crystal structure)FIB Forced ion beam (microscopy)GMR Giant magnetoresistancehcp Hexagonally close packed (crystal structure)HER Hydrogen evolution reactionHVOF high velocity oxygen fuel (spraying)LAED Local area electron diffractionMEMS Microelectromechanical systemsOER Oxygen evolution reactionPC Pulsed currentPCR Pulsed current reversalSAD Selected area diffractionSCC Stress corrosion crackingSDS Sodium dodecyl sulfateSEM Scanning electron microscopyTEM Transmission electron microscopyXRD X ray diffraction

A. Karimzadeh, et al. Surface & Coatings Technology 372 (2019) 463–498

464

indicated in Fig. 2.

1.1. Characteristics of Ni, Co, and NieCo alloys

Ni belongs to the transition group with a molar mass of 58.69 g moland density of 8.90 g cm−3 at 25 °C. It has a face-centered cubic (fcc)crystal structure in the solid state. It melts at 1453 °C and is resistant tocorrosion and oxidation at moderate to high temperatures. The elec-trical resistivity of Ni is negligible at low temperatures, having a valueas low as 68.44 nΩ m at 20 °C. It is chemically inactive and its corrosionresistance decreases in oxidizing conditions. Ni is extremely versatileand will readily form an alloy with most of the metals. Cobalt is posi-tioned between Fe and Ni in the periodic table. The molar mass anddensity of Co are close to that of Ni, at 58.93 g mol−1 and 8.85 g cm−3,respectively. As in the case of Ni, Co is a ferromagnetic metal. De-pending on the temperature, Co can take two different crystallinestructures; hexagonal closed packed (hcp; ε-Co) at T < 417 °C, and fcc;α-Co) at 417 °C < T < 1493 °C (melting point). Alloying Co with Niresults in lowered corrosion and wear rate [17]. The physical, me-chanical and magnetic properties of nickel and cobalt are shown inTable 1.

Ni and Co-based alloys have been studied as prominent engineeringcoatings due to their enhanced properties compared to pure Ni or Co.They possess higher strength, better wear and corrosion resistance,suitable electrocatalytic activity and special magnetic properties [18].An important aspect of the NieCo alloy system is that these two

Fig. 1. Selected highlights relevant to the development of NieCo electrodeposits [6,11–16].

Fig. 2. Types of NieCo electrodeposit.


465

elements can form solid solutions (α-Co, α-Ni) throughout the range ofconcentration, enabling alloys with any chemical composition to beproduced, in principle [19] as indicated in the binary NieCo phasediagram (Fig. 3).

Electrodeposition is a simple, controllable and cost effective way ofproducing NieCo alloy coatings. However, the chemical composition ofNieCo electrodeposited coatings is limited by anomalous codepositionof Ni and Co. Ni, Co and Fe belong to the iron-group metals. Four alloyssystem (three binary alloys and one ternary alloy) can be formed viacodeposition of these metals. Electrodeposition of these alloys is theeasiest among various alloy systems since these metals have closestandard potentials (E°Fe = −0.440 V, E°Co = −0.277 V, andE°Ni = −0.250 V vs. SHE) and the deposition of these metals takes placeat very negative overpotentials [21]. The codeposition of Ni and Co iseither anomalous or abnormal. Normal codeposition of metals includespreferential deposition of the more noble metal; in anomalous beha-viour, the bath ratio of Co2+/Ni2+, as discussed in Section 3.

1.2. Properties and applications of NieCo coatings

The incorporation of Co ions into such baths is readily achieved bythe use of soluble cobalt anodes or addition of cobalt salts to the bath.The resultant NieCo deposits have become prominent engineeringmaterials and coatings due to their attractive mechanical properties(high tensile strength and high hardness), good tribological propertiesand suitable heat resistance and anti-corrosion properties. Such prop-erties have allowed these coatings to develop as anti-wear resistant andcorrosion coatings [22–24]. It has also been reported that NieCo de-posits have good electrocatalytic properties, rendering them competi-tive catalysts for the cathodic hydrogen evolution reaction (HER) andanodic oxygen revolution reaction (OER) during electrolysis of water inalkaline solutions [25–27]. The electrocatalytic activity of the coating isgenerally increased at higher Co levels in the alloy [28].

One of the important properties of NieCo alloys is their unusualmagnetic properties. Much attention has been given to magneticproperties of these alloys for their potential applications in electronics

and computer industry, including discs, memory cards, drums andmicroelectromechanical systems (MEMS). The physical and mechanicalproperties of coatings largely depend on their microstructure and che-mical composition. It has been reported that a higher Co content canimprove the magnetic properties of NieCo coatings [2,29].

These alloys can also be used for high-temperature applications.Golodnitsky et al. [30] have investigated the impact of Co content onthe mechanical properties and high-temperature oxidation of thesecoatings. However, Chang et al. [31] have shown that the corrosion rateof NieCo coatings was accelerated in the presence of salt deposits suchas NiCl2 at 800 °C.

NieCo alloys have several specialized applications. In some cases,these coatings are used in the electronic industry such as NieCo coatedchips [32,33], guide blocks and microprobes [34–36], micro-connectors[37] and cantilever type micro-connectors [38]. Recently, NieCo filmshave been employed in the fabrication of micro-lenses. In fact, theNieCo electrodeposition process was used for the creation of metallicmold with micro-lens arrays, resulting in enhanced brightness andscattering [39].

The diverse application areas of NieCo electrodeposits are sum-marised in Fig. 4.

2. Environmental and health concerns

Electroplating is commonly used in industry for a variety of pur-poses, such as improvement of the properties of the material. However,electroplating process of Ni and Co produces hazardous wastewatercontaining various metallic and nonmetal ions, which are highly ef-fective on the environment and human health (both Ni and Co give riseto selective skin irritations as well as being toxic metals, etc.) [40].Many important issues hinder the application of Ni, Co, and NieCocoatings, including (1) health and environmental problems, and (2) theprice of raw materials, especially cobalt salts. However, these coatingshave lesser health problems than chromium coatings. According to the(EPA), Cr, Ni and their compounds are classified as extremely ha-zardous substances [41]. At present, replacement of hard chromiumwith low-cost materials is unrealistic and Ni and Co can be consideredas acceptable choices until they can be satisfactorily replaced with non-hazardous coatings. As mentioned above, they possess unique proper-ties that have attracted a great deal of industrial attention. NieCocoatings possess such great importance that the ASTM B994 standard isfully dedicated to these coatings. These coatings are deposited on themajority of the substrates without limitation, and owing to the negli-gible friction coefficient, they provide a dry lubricant on the surfaces

Table 1Selected properties of Ni and Co.

Properties Nickel Cobalt

Atomic number 28 27Molar mass (g.modm-3) 58.69 58.93Electron configuration [Ar] 3d8 4s2 or [Ar] 3d9

4s1[Ar] 3d7 4s2

Melting point (°C) 1455 1495Boiling point (°C) 4946 2927Density (g cm−3) 8.9 8.9Crystal structure fcc hcpElectrical resistivity (nΩ m) 69.3 62.4Magnetic ordering Ferromagnetic FerromagneticYoung's modulus (GPa) 200 209Vickers-Hardness (MPa) 638 1043Thermal conductivity (W/(m·K)) 90.9 100Thermal expansion (μm (m·K)−1) 13.4 13.0Applications

-Alloys

-Electrodeposition

-Batteries

-Fuel cells

-Magnetic applications

-Alloys for corrosionand wear resistance

-Batteries

-Catalysts

-Electrodeposition

-Pigments

Fig. 3. NieCo equilibrium phase diagram [20].


466

that are in contact with each other. Thus, there is no need for variousharmful lubricants. In addition, the easy fabrication of three-dimen-sional morphologies with high surface area provides high potential forusage of NieCo coatings as an electrocatalyst for energy conversion infuel cells and water electrolysers.

Efforts have been made to reduce the environmental impact ofNieCo electroplating baths. Replacement of some of the raw bathmaterials has been regarded as an effective way of reducing the risks ofthese materials. Nickel electrodeposition became more prominent afterthe optimization of a practical bath by Watts in 1916, which was fol-lowed by more than a century of innovation, e.g. [16]. Citrate bathshave been considered, in which boric acid is substituted by citric acid[42,43] although such baths show poorer stability than classical Watts'solutions. Environmental discharge regulations restrict the usage ofbaths containing boron ions and the wastewater must be cleaned by

removal of boron but this treatment is difficult [44]. Recovery of Co andNi from the industrial effluents is essential, since they pose grave risksof environmental pollution. According to effluent discharge regulations[45], the maximum permissible level of Ni and Co is 0.1 and0.05 mg dm−3, respectively. The industrial treatment methods such asprecipitation by lime-soda do not possess sufficient efficiency of re-moval [46]; additional treatment is required [47]. Recovery of Nithrough affordable electrochemical methods (without additional treat-ment) has been successfully carried out at industrial scales by two ro-tating cylinder electrode reactors with pH control [48]. Electrodialysishas been introduced as a recent and economic method to purify thewastewater generated in Ni electroplating [49] similar to other tech-niques such as electrolysis and capacitative deionization [50,51]. Thelatter method includes ion-exchange membranes through which theions selectively migrate due to an electrical driving force [52]. The

Fig. 4. Applications of NieCo coatings.


467

other methods of recovering or removing Ni from wastewaters arediscussed further in ref. [53]. It is very difficult to replace Ni, Co andNieCo coatings due to their attractive engineering properties. By op-timizing the methods used to treat wastewater and used baths fromNieCo electroplating, harmful effects on the environment and humanhealth can be mitigated by lowering Ni and Co levels in electroplatingeffluents and wastes.

3. Electrode reactions

The principles of NieCo electroplating are shown in Fig. 5. Nor-mally, an undivided cell is used with soluble Ni and Co anodes are usedin an acidic electrolyte although the Ni and Co concentrations can alsobe maintained by periodic addition of their soluble salts. In the case ofsoluble anodes (in the form of foil, plate or rounds) a microporouspolymer bag is often used to retain anode slime (e.g., fine metal oxide/hydroxide particles) to prevent bath contamination. The figure high-lights some of the important aspects involved in choice of electrolyteand control of operational conditions.

In the usual case of a soluble anode, metal ions are formed:

Ni 2e Ni2+ = + (1)

Co 2e Co2+ = + (2)

When an insoluble anode, such as platinised titanium is used,oxygen is evolved:

2H O 4e O 4H2 2+ = + + (3)

At the cathode workpiece, the main reaction is Ni and Co deposi-tion, as described by reactions (1) and (2). In some cases, these areaccompanied by deposition of phosphorus, boron or tungsten (seeSection 17).

Hydrogen evolution takes place as a secondary reaction at thecathode surface:

2H 2e H2+ =+ (4)

This reaction not only lowers current efficiency and decreases localpH but can result in deposit porosity due to hydrogen gas bubblessticking to the surface, if the electrolyte/electrode movement is limitedor a surfactant is not present in the bath.

4. The overall rate of deposition

The overall rate of an electrode process can be described byFaraday's laws of electrolysis, where the electrical charge, q flowingthrough a cell due to the movement of z electrons is:

q n z F. .= (5)

where the electrical charge, q in the time interval t2 - t1, is:

q Idtt

t

1

2

=(6)

If t2 = 0, the overall charge is involved; otherwise, the charge in thetime interval (t2-t1) is involved.

It is common to electroplate at constant current, when the expres-sion for the mean rate of deposit thickness is:

xt

j Mz F

=(7)

where j is the current density [A cm−2], i.e., the current, I per unitelectrode area, A:

j I A/= (8)

x is the deposit thickness [cm], ρ is the density of the deposit [g cm−3],z is the number of electrons in the electrode process [dimensionless], Fis the Faraday constant [96,485C mol−1], t is the time [s], ϕ (whichis < 1) is the current efficiency for metal deposition [dimensionless]and M is the molar mass of deposited metal [g mol−1]. In the case ofpure nickel deposition at a current efficiency of 95%, a current density

Fig. 5. An undivided cell for electrodeposition of a NieCodeposit with the options of alloys containing additional ele-ments and included-particle composite coatings. Importantaspects of electrolyte choice and control of operational con-ditions are indicated. Co2+ ions may be replenished in thebath by using a soluble anode or addition of soluble cobaltsalts. The elemental addition to the alloy deposit, can includeX = W, Fe, Cu, P, B, Mo and Zn.


468

of 2 A dm−2 (i.e., 20 mA cm−2) is expected to produce a uniform de-position rate of approx. 23 μm h−1.

The current efficiency (charge yield), the fraction of total currentused in the primary reaction, i.e., metal deposition, is defined as apercentage by:

I I% 100 ( )/INi Co= + (9)

where the total current has contributions from NieCo deposition andhydrogen evolution, i.e.

I I I INi Co H2= + + (10)

5. Electrolytes used in NieCo electrodeposited coatings

Various baths are used for electrodeposition of NieCo coatings.Three different electrolytes that are commonly employed in the pro-duction of NieCo alloys consist of sulfate (Watts type), sulfamate andchloride baths. In this section, the classification of baths is based on thesource of Ni and Co. Recently, ionic liquids have been introduced aspossible electrolytes. Other baths with special additives such as pyr-ophosphate, gluconate, and glycine baths have been developed whichare considered further in Section 7.4. Advantages and disadvantages ofthree common baths are listed in Table 2.

5.1. Chloride bath

The NieCo coatings obtained from chloride baths have been studiedin previous research [54–58]. This bath consists of nickel chloride andcobalt chloride. Sodium chloride and ammonium chloride are oftenutilised as supporting agents to enhance the cathodic current efficiency,and trisodium citrate is sometimes employed as a complexing agent forNi and Co ions [55,56]. Chloride baths have disadvantages such asexpensive make-up chemicals, corrosivity, a greater tendency to pitformation compared to the other baths [55]) and higher internal stressin the deposits [59]. However, the deposits from this bath can realize ahigher microhardness [54,59].

5.2. Watts-type (sulfate) bath

This bath is a modified Watts (sulfate-chloride) bath in which theNieCo alloy is electrodeposited from simple ions [60]. Nickel sulfateand cobalt sulfate are the primary sources of Ni and Co ions, respec-tively, chloride ions provide good solution conductivity, reducing cellvoltage requirements, and are vital to achieve satisfactory dissolution ofnickel anodes [61]. Abd elRehim et al. [60] have shown that sound,smooth and bright coatings are obtained from this bath and that theproperties are improved by the increment of Ni content in the bath.Watts-type bath exhibits a strong ability for passive film formation, sogood corrosion resistance is a characteristic of the deposits resultingfrom sulfate bath [55]. Established and developing sulfate baths [9A]have multiple advantages such as low cost, low risk of equipmentcorrosion, simplicity of use and maintenance, and also the fact that itsdeposits have lower internal stress and larger grain sizes compared tochloride bath [59]. The presence of boric acid as a buffering agent isnecessary in order to attain high current efficiencies [19].

5.3. Sulfamate bath

Usage of sulfamate baths in the electrodeposition of NieCo coatingshas been reported in numerous studies [62–64]. Nickel sulfamate andcobalt sulfamate act as the sources of the Ni and Co ions in the bath.These salts are extremely soluble, and using chloride instead of sulfa-mate helps dissolve the anodes more uniformly while preventing anodepolarisation [65]. The internal stress of deposits from a sulfamate bathcan be lower but this is very sensitive to impurities and the bath is moreexpensive [59]. Sulfamate baths have higher solubility, which enables Ta

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469

high current densities in the production units [65]. Excellent stabilityfor different ranges of pH from 2 to 4 and for temperatures by up to60 °C is another advantage of this bath [66–68]. There is a local pHincrement close to the surface of the electrode in alloy electrodepositionof iron group metals. Hydroxides may precipitate and partially blockthe cathode surface. The pH increase close to the cathode is also ob-served in the sulfamate bath [69]. Addition of boric acid buffers the pH[70]. NieCo alloys electrodeposited from sulfamate baths can exhibitgood mechanical properties at deposition rates of 7–20 μm min−1 (de-pending on operational parameters) [64]. In fact, the addition of Co toNi increases the yield and ultimate strength but fracture resistance maydecrease [62].

Goldbach et al. [71] have shown that the presence of cobalt insulfamate bath does not affect the deposition of Ni, however the pre-sence of Ni in the electrolyte prevents the deposition of Co. Tury et al.[72] have reported that the coatings deposited from the sulfamate andWatts type NieCo baths exhibit uniform chemical composition andmicrostructure throughout the entire of cross-section, but had lowermicrohardness compared to those produced from chloride electrolytes.The hydrogen evolution potential is low for the Watts bath, and is in-creased for chloride and sulfamate baths. The cobalt content of NieCocoatings largely depends on the utilised electroplating bath. Due to the

anomalous deposition of NieCo alloys, it can be expected that per-centage of Ni in the deposited layer is raised by increasing the currentdensity of deposition. For the aforementioned baths, the deposit Cocontent showed a near linear decrease with applied current. Accordingto this behaviour, Ni deposition likely to be activation-controlled [55].

5.4. Acetate bath

A survey of the literature reveals that there are very few studiesavailable for NieCo electrodeposition in acetate-based baths. Thesebaths are greatly attractive owing to their inherent low toxicity, lowbiodegradability [73] and eco-friendly properties [74]. Singh et al. [75]have used acetate-based baths for the electrochemical deposition ofNieCo and Ni-Co-Fe films. The sources of Ni and Co in the electrolytewere nickel acetate (Ni(CH3CO2)2·4H2O) and cobalt acetate (Co(CH3CO2)2·4H2O), respectively. The mechanism of electrodeposition(for iron-group metals such as Ni, Co, Fe) in an acetate bath is anom-alous codeposition similar to the other baths. The amount of Co acetatein the electrolyte is lower than Ni acetate [76,77]. Buffering capacity isanother advantage of this bath. To avoid passivation and improve thedissolution of Ni anodes, Cl− ions are added to the electrolyte [76,78].The cathode current efficiency of the bath can be increased to about99% by the use of additives [78].

5.5. Ionic liquid baths

Electrolytes based on ionic liquids can be suitable alternative elec-troplating baths (as they may provide green electrolytes) [79] althoughtheir cost and stability must be scrutinised. Recently, ionic liquids suchas deep eutectic solvents (DES), have attracted a great deal of attentionin electrodeposition of Ni, Zn, Cr, Cu and Co. Electrodeposition of nickelbased alloys from DES baths has been studied in previous research[80–83]. You et al. [80] have investigated the electrodeposition of pureNi and NieCo coatings from choline chloride/ethylene glycol DES (themost common DES [84]). The DES contained dissolved by Ni chlorideand Co chloride. Co content in the NieCo coatings was lower than theCo ions in the electrolyte. This hints at the absence of an anomalouscodeposition mechanism for non-aqueous baths, unlike the anomalouscodeposition for aqueous baths.

6. Mechanism of NieCo electrodeposition

NieCo codeposition behaviour is recognised as abnormal (anom-alous) codeposition according to the Brenner classification [21]. Thistype of codeposition is often observed in codeposition of iron groupelements (including Ni, Fe, or Co) [85,86], and also in codeposition ofone of the elements of the iron group with zinc [87] or cadmium [88].In fact, in the electrochemical deposition of iron group alloy systems,the composition of the coatings is clearly different from the compositionof components in the electrolyte since Co is deposited preferentially tothe more noble element (Ni) under many conditions.

The Co content in the coating is higher when the ion fraction ofnoble element is higher [21,56,89]. For instance, in a Watts type bathwith a cobalt sulfate to nickel sulfate ratio of 1:10 in the bath, thedeposit yield was 40 wt% cobalt [64]. The cyclic voltammograms of Ni,Co, and NieCo alloys are presented in Fig. 6. In Fig. 6a), the currentduring NieCo deposition is higher than that for pure Ni and Co de-position. Thus, both Ni and Co determine the charge reduction in thepositive scan. Fig. 6b) shows that the partial current of Co is improvedin comparison to Ni in electrolytes with lower Ni(II)/Co(II) ratios(curves 1 and 2), indicating that anomalous codeposition is preferred atlower Ni(II)/Co(II) ratios in the bath. By increasing the Ni(II) ionconcentration in the electrolyte, the reduction peaks shift towards morenegative potentials and give higher currents.

The results of cyclic voltammetry are in agreement with linearsweep voltammetry results (Fig. 7). Comparison of NieCo coatings

Fig. 6. Cyclic voltammetry for deposition of (a) Ni, Co and NieCo deposits and(b) NieCo coatings deposited at electrolytes with different Co(II)/Ni(II) ionratios [90].


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fabricated in solutions with different Ni(II)/Co(II) ratios reveals that byincreasing the ratio, scans shift towards the positive direction. Thisdemonstrates that the kinetics of Ni(II) reduction are faster at a higherNi(II) ion concentration in the bath. Scanning in the bath with lowerratio led to lesser polarisation. According to Fig. 7, the current densityafter the potential of about −0.5 V is measurable for all the deposits.The low overpotential of region I is related to H+ reduction. Region IIappears only for the bath containing higher amounts of NiSO4 in thepotential of −0.7, which is due to the starting of Ni(II) reduction. In thebath containing 0.1, 0.3 and 0.4 mol dm−3 NiSO4, the reduction of Ni(II) is inhibited due to the presence of Co(II) ions.

Recently, various models including experimental and mathematicalones, have been developed which try to explain this behaviour[85,86,91–98]. Among the iron group binary systems, NieFe is studiedthe most and owing to the mechanism of codeposition in this systemand NieCo exhibiting similar behaviour (anomalous codeposition),some explanation of the mechanism is based on NieFe system. An earlyinvestigation of iron group codeposition was conducted by Vagramyanand Fatueva [99]. They found retardation and facilitation deposition ofnoble and active metals in the alloy system. Dahms et al. [92] suggestedthat the metal hydroxide is absorbed on to the surface prior to the re-duction and reduction of Ni is prohibited as a result of surface hydro-xides. Measuring the surface pH indicated that it was insufficient toprecipitate metal hydroxide [100,101] although true surface pH mea-surements are difficult to realize. Many studies have proposed thatmonohydroxyl (MOH+) metal ions discharge at the electrode[96,102,103]. Such models suggest that hydrolysis of metal ions playsan important role in the electrodeposition of iron group metals. Hes-sami et al. [96] and Grande et al. [103] suggested a model based onone-dimensional diffusion, conversion, and hydrolysis of Fe ions, Niions, and H2O. According to this model, the cathode current densityconsists of a partial current density for reduction of H+ and H2O as wellas the reduction of NiOH+, FeOH+, Ni2+ and Fe2+ ions. Sasaki andet al. [94,95,104] later extended the formation of MOH+ (Grande andTalbot model [103]) to other iron group systems (NieCo and FeeCo).Equilibrium constants for the NieCo alloy compared less favorably withexperimental results. The models were then revised and surface hy-drogen adsorption blockage was added to competitive adsorption ofmonohydroxyl intermediates of metals [94,95]. The model proposed byMaltosz was based on the competition between the Fe(I)ads and Ni(I)ads

which were adsorbed on the surface. These monovalent intermediateswere formed during the reduction of the dissolved Fe and Ni [97]. In

fact, the model was proposed as a two-step reaction mechanism: re-duction and adsorption of metal cation on the surface as a monovalentintermediate ion (first step), and reduction of the intermediate to metal(second step). Baker and West [98,105] used EIS to examine this modelwith experimental data in agreement with the Matlosz model. Zechet al. [85,86,91] developed a model which measured partial currentdensities of metal co-reduction during electrodeposition compared toelectrodeposition of pure metal individually. Their investigations in-dicated that in the electrodeposition of NieFe and NieCo, deposition ofNi is inhibited in presence of Fe and Co ions, whereas the deposition isaccelerated by the presence of Ni ions active metal (Fe and Co). Ac-cording to these results, Arenas and Pritzker [106] suggested that 3reactions occur simultaneously during codeposition of Ni and Co.

Reduction of the noble and active species (Ni2+ and Co2+) can beshown as:

Ni(II) e Ni(I)ads+ (11)

Ni(I) e Niads0+ (12)

Co(II) e Co(I)ads+ (13)

Co(I) e Coads0+ (14)

Ni(II) Co(II) NiCo(III)ads+ (15)

NiCo(III) e Co Ni(II)ads0+ + (16)

Reactions (11) to (14) are mutually independent. (15) and (16) arereductions in which Ni(II) and Co(II) ions interact (as assumed by Po-dlaha and Landolt [107,108] for induced behaviour during MoeNicodeposition). These act as catalytic reactions for reduction of Co(II)ions and are secondary pathways for cobalt deposition, having an ac-celerating effect on codeposition of components. However, the NiCo(II)ads intermediate is capable of covering the surface, effectively in-hibiting the deposition of Ni. NiCo(II)ads and blocking other valid sitesfor Co(I)ads and Ni(I)ads [85,86]. Reduction of the intermediate CoNi(II)ads can then be considered:

CoNi(III) e Ni Co(II)ads0+ + (17)

All studies show that only the active metal is brought up in theanomalous codeposition of iron group metals and this reaction is notpossible [86]. During electrodeposition, reduction of H+ ions takesplace simultaneously with metal adsorption on the electrode surface.Increasing the pH at the surface during the electrodeposition process(due to the consumption of H+) and formation of hydroxide ions is the

Fig. 7. Steady state linear sweep voltammetry of Ni and Co deposition at 0.1 mV s−1 at a Cu rotating disk electrode at 1000 rpm in electrolytes of differentcompositions [19].


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other hydroxide mechanism [109]:

2H 2e H2++ (18)

2H O 2e H 2OH2 2+ + (19)

In these circumstances, the pH at the surface is sufficiently high forhydrolysis of metal ions with OH−. Precipitation of Co hydroxide (Co(OH)+) on the cathode hinders the deposition of Ni [109]. According tothe previous studies [19,98,110–114] based on the deposition of pureCo, Fe, and Ni metals, hydrogen evolution reaction consists of adsorbedspecies (such as Hads). This is a weakness in the model proposed by Zechet al. [86] which does not consider such adsorbed species:

H e Hads++ (20)

2H Hads 2 (21)

The active sites on the electrode surface can be obstructed by Hads,otherwise Co(I)ads, Ni(I)ads and NiCo(III)ads will occupy these sites.Regarding the competition between the adsorbed species, their inter-action is vital and therefore its effects need to be considered. Reaction(10) may be stopped at higher overpotentials [111,115], leading to[111]:

2H O e H OH2 ads+ + (22)

Studies have shown the impact of H2O reduction on the depositionof iron group metals [85,86,91]. It has been indicated[71,90,91,109,116–123] that the Co content in the electrodepositedNieCo alloy is lower at more negative overpotentials. This result is,however, contrary to the behaviour shown in FeeNi systems[93,103,124,125].

There is some disagreement regarding the mutual impact of bathconstituents. So far, all models have been based on the inhibiting effecton the codeposition of the active metal in anomalous codeposition.Indeed, the majority of studies indicate that the presence of Fe(II) or Co(II) inhibits the deposition of Ni [56,58,71,89,95,117], despite the factthat it was revealed that Co(II) did not have a strong effect on the Nideposition in some cases [71,95]. Moreover, the fact that the reductionof Co(II) is considered an improvement in the electrolyte containing Ni(II) is not yet fully elucidated. Some researchers [85,86,95,117,120]have shown that Co(II) reduction is accelerated in presence of Ni(II),while others have claimed that it is unaffected in this condition[96–98,103,125]. It has even been claimed that the reduction of Co(II)can be completely prevented [71]. Most recently, Vazquez et al. [126]studied the competitive adsorption of Ni(II) and observed that Co(II)and the desirable Co(II) adsorption energies were achieved instead of Ni(II). In the last study [127], however, anomalous codeposition is relatedto the hcp phase structure formed during deposition. Ni solubility islimited in the hcp structure and thus the deposition of Ni atoms in thehcp lattice structure is hindered. In contrary, normal deposition is re-lated to the deposition with fcc phase structure, resulting in full Nisolubility in this phase. According to the arguments above, it can beconcluded that the mechanism of anomalous codeposition is still chal-lenged, however, the schematic of this codeposition behaviour deducedfrom the models (the most common hypothesis, hydroxide suppression

[128,129], is illustrated in Fig. 8.In contrast to the anomalous nature of codeposition of NieCo alloy

deposition, attempts have been made to limit anomalous CoeNi de-position. In the laboratory, this may be achieved using techniques suchas cyclic voltammetry and pulse reversed electrodeposition in chlorideelectrolytes [57,130,131]. The inhibition of the anomalous codeposi-tion in cyclic voltammetry is ascribed to dissolution of the newlyformed alloys and co-dissolution of the adsorbed hydroxide layer in theanodic dissolution region [130]. Grill et al. [132,133] have shown, byelectrodeposition of CoeNi in a modified 3D-printed Hull cell andsulfate baths including trisodium citrate or glycine, that a transition tonormal codeposition is occurred. Fan and Piron [58] have revealed thatthe anomalous behaviour in NieCo system is different from the NieFesystem, and that electroplating in high current densities leads to atransition from anomalous to the normal codeposition. Hu et al. [127]have observed a deviation from anomalous behaviour of electro-deposited CoeNi nanowires in some conditions. Electrodeposition ofNieCo alloys in non-aqueous electrolytes, despite the aqueous elec-trolytes, displays no anomalous codeposition process [80].

7. Effect of electrodeposition parameters

In this section, the effect of various parameters such as the com-position of electrolyte, temperature, current density, agitation of theelectrolyte and additives are considered. The effect of current type(direct current (Section 6.1), pulsed current (Section 6.2) and pulsedcurrent reversal (Section 6.3) on the deposition of NieCo coatings isconsidered further in Section 7.

7.1. Electrolyte composition (metal ratio of the bath)

Nickel and cobalt form solid solutions in any concentration range,and therefore NieCo alloys with any composition can be electro-deposited [134]. In fact, this phenomenon enables deposition of NieCoalloys with a wider range of composition. However, electrodepositionof alloys with a high Ni content (Ni-rich alloys) is difficult due to theanomalous behaviour of this system [21] and the Co percentage in thealloys is usually higher than the electrolyte [6]. However, NieCo alloyscontaining low Co (Ni-rich alloys) have been electrodeposited from avery low Co2+/Ni2+ ratio in the bath [135,136]. Based on previousstudies [2,136–139], the relationship between Co percentage in thealloy deposit and Co2+/Ni2+ ratio in the bath is non-linear, as shown inFig. 9. However, the linear relationship was observed in some rare cases[140]. Tian et al. [2] employed cyclic voltammetry to study the elec-trodeposition behaviour of NieCo films. They observed that the de-position current is increased at a higher Co2+/Ni2+ ratio. In addition,the contributions of the partial current for Co, compared to Ni de-position, is much higher even at low Co ion content in the electrolyte(anomalous codeposition). The anomalous codeposition degree definedas Mfilm/cb (Mfilm is the mole fraction of Co in the alloy and cb is themolar cobalt ion concentration in the bath) is increased by decreasingCo2+ percentage in the electrolyte [90]. Studies have proven that theimpact of operational factors, including temperature, pH and current

Fig. 8. Schematic of anomalous NieCo codeposition: (a) hy-drolysis of water and OH– ions increase the pH near thecathode, (b) Bonding of OH− ions with Co and Ni ions formsCo(OH)+ and Ni(OH)+, respectively. (c) Adsorption of Co(OH)+ ions at the electrode surface is easier than Ni(OH)+

ions and further reduction to Co and obstructs active sites; Nideposition is limited and Co is deposited in preference to Ni,(d) 3D view of anomalous codeposition.


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density, on properties is influenced by the composition of the bath. AbdEl-Rehim et al. [60] have shown that the bath composition affects thecathodic current efficiency but it has a greater impact on cathodic po-larisation and NieCo alloy composition.

7.2. Electrolyte pH

The literature reveals that bath pH during the electrodeposition ofNieCo alloys is in the acidic range. pH is a critical parameter foranomalous codeposition of iron group metals because of two reasons[58,103,141]: (1) adsorption of metal hydroxides, mono-hydroxides orthe active metal (Co) on the surface which are caused by the local in-crease of pH, thus if pH value on the cathode surface is lower than theformation of the hydroxide, codeposition occurs; (2) hydrogen evolu-tion reaction occurs simultaneously which depends on pH. In addition,it affects the coating composition and morphology [131,142]. Fig. 10indicates the impact of pH on alloy composition of NieCo system. In thecase that Co content is increased by increasing the pH at low pH values,formation and adsorption of hydroxides are hindered and normal de-position is predominant. Therefore, a deposit with lower Co content isformed. This is in agreement with the results obtained by Orinakovaet al. [142]. There is no general consensus about the effects of ionsconcentration and pH in the electrolyte. According to some researchers[131,137,142,143], by increasing the pH, the amount of Co in the de-posit is increased while others [64,144] have demonstrated that it isdecreased instead. Vazquez-Arenas et al. [19] studied the effect of pH(bath containing 0.018 M CoSO4, 0.2 M NiSO4, and 0.5 M H3BO3) on thecurrent efficiency. They observed that current efficiency was raised byincreasing pH from 2 to 4. The current efficiencies at pH values of 2, 3and 4 were 82%, 72 and only 42%, respectively. The usual range of pHin electrodeposition of NieCo coatings is acidic in the range of 2–6. Insome cases, such as pyrophosphate baths, pH values up to 10 wereexamined [145].

7.3. Temperature

Correct control of the operating temperature in the electrodeposi-tion process is a vital factor for the suitable performance of the elec-trolyte [147]. Different and inconsistent reports have been providedregarding the impact of temperature on Co content and properties ofNieCo deposits in the literature. Przyluski et al. [148] have in-vestigated the effects of bath temperature (30–60 °C) on Co deposition.They observed that by increasing the temperature of a NieCo Wattstype bath, the cobalt content of CoeNi coatings is lowered from about

80 to 60 wt%. This is in agreement with the results obtained by Qiaoet al. [10]. However, Bai and Hu [131] have demonstrated that theimpact of raising temperature from 25 to 50 is negligible. Feninecheand Coddet [149] showed that by increasing the temperature from 10to 80 °C, both current efficiency and hardness are decreased. Coatingsformed at a lower temperatures (< 40 °C) tend to be brighter andsmoother than those deposited at higher temperatures (40–80 °C). Theresults of different studies related to the effect of temperature on Cocontent in the coating in different studies are summarised in Fig. 11.These differences mainly depend on both the composition and the de-position current density.

7.4. Current density

Fig. 12 demonstrates the impact of applied current density on thecomposition of the coatings in baths having different Ni2+/Co2+ ratios.Current density is a major control parameter in electrodeposition sinceit controls the rate of deposition, the electrode kinetics, deposit com-position, texture, surface morphology and coating properties. It is un-fortunate that electrode potential is not always monitored and relatedto current density by steady state polarisation curves. Burzynska andRudnik [151] observed that by decreasing the current density, theamount of Co was raised in accordance to the previously obtained re-sults [19,109,152]. However, Kamel [89] has reported the contra-dictory influence of current density on the alloy composition in glu-conate baths. Chung et al. [109] have also shown that the effect ofdirect current and pulse deposition techniques on the composition isinsignificant. A number of researchers [60,95,116,118] have studiedthe dependence of efficiency on current density for iron group alloysystems, and concluded that current efficiency is increased by raisingcurrent density (current density range was up to 6 A dm−2). Arenaset al. [19] demonstrated that the current efficiency was lowered in thecurrent densities values above 6 A dm−2. falling to 0% current effi-ciency at 8 A dm−2. On increasing the current density, metal ions arequickly consumed; the concentration and diffusion of the metal ions aredecreased with deposition time, so anomalous behaviour is hindered (inthis situation the effect of mass transport is enhanced [19]). As men-tioned, on decreasing the current density, Co content in the deposit isincreased, so Ni peaks in XRD patterns will be more intense [109]. Itwas demonstrated that increasing the current density in the range(0.5–5 A dm−2) resulted in more compact and smoother deposits [3].

Fig. 9. Co content in the deposit as a function of Co2+/Ni2+ ratio in the bath[2,29,136,138,139].

Fig. 10. the Co content in the NieCo coating as a function of bath pH. [29,146].


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7.5. Agitation of electrolyte or cathode rotation

In general, bath agitation and cathode rotation enhance the homo-geneity and thickness of the deposits [153]. Air agitation, impeller [61],magnetic stirrer [154], high speed jet [155,156] and peristaltic pump[151] are common methods of electrolyte agitation. In agreement withall studies on NieCo alloys electrodeposition [10,91,157–160], in-tensifying the agitation leads to higher cobalt percent in the NieCoalloys, since the thickness of diffusion layer (δ) is reduced by increasingthe metal ions contained within [10]. In the laboratory, electrolyteagitation may be achieved by a rotating cylinder electrode (RCE) inturbulent flow or a rotating disk electrode (RDE) in laminar flow, Forthe RDE, the Nernst diffusion layer thickness, δ is decreased, i.e., masstransport of metal ions towards the cathode surface is enhanced, byincreasing the angular speed of rotation according to the Levich equa-tion [147]:

D v1.61 1/3 1/6 1/2= (23)

D is the diffusion coefficient of metal ions; ν is kinematic viscosityand ω is the rotation speed of RDE. The limiting current density (jL) canbe increased using agitating the electrolyte according to [155]:

j zFD C CL

b i= (24)

where F is the Faraday constant, D is the diffusion coefficient of themetal ions, Cb and Cs are the concentration of metal ions in the balk and

at the deposit surface, respectively and δ is the Nernst diffusion layerthickness. Qiao et al. [10] observed that by increasing the agitation ofthe bath, NieCo grain size became finer. The relationship between theprobability of nucleation (W) and overpotential (η) can be described by:

W B bexp 2=(25)

where B and b are constants. Bath agitation decreases the diffusionlayer thickness, effectively increasing the maximum current densitythat can be used. According to Eq. (25), the deposit grain size is re-duced.

In electrolytes containing nanoparticles, bath agitation and elec-trode movement serve two purposes: (1) to suspend the particles in theelectrolyte and (2) to transfer them to the electrode surface. RDE, RCEand parallel plates in channel flow are suitable laboratory techniquesfor quantitative characterisation of these electrolytes. Bahadormaneshand Dolati [161] reported that by choosing a stirring rate between 120and 770 rpm, SiC particle content in Ni-Co-SiC nanocomposite coatingwith 480 rpm is the highest. This result is in agreement with NieConanocomposite coatings maintaining different nanoparticles [139,162].In fact, by increasing agitation, the particle content in the deposit isenhanced in general. Excessive stirring can cause a lower degree ofparticle in the coating due to the strong hydrodynamic forces in thebath. Excessive tangential force can remove the nanoparticles from thesurface before they become entrapped in the deposit.

7.6. Electrolyte additives

Additives can be added to the electrodeposition baths of NieCoalloys to alter the electro-crystallization process for various purposes,such as mirror-like surfaces and improvement of properties [78]. Theadded additives to NieCo electrolytes are listed in Table 3. Amongvarious additives, saccharin as a grain refiner and internal stress re-liever, boric acid as a buffering agent, sodium dodecyl sulfate (SDS) as asurfactant and ammonium/sodium chloride as a support for increasingthe conductivity are most the commonly used. The effects of additiveson the alloy content, grain size, microstructure and mechanical prop-erties of the coatings have been investigated. The influence of H3BO3 onanomalous codeposition of iron group deposits has been focused onFeeNi coatings [102,103,124]. However, Lupi and Pilone [163] havereported that increasing boric acid to 20 g dm−3 causes the currentefficiency to increase. Arenas et al. [19] showed that the boric acid hada negligible effect on the NieCo alloy composition, in contrast to theclaims of the past studies [93,103,124] on iron group codeposits.H3BO3 is a buffering agent and its main function is to control bath pH.H3BO3 is added to the bath to avoid precipitation of metal hydroxidesand oxides at the cathode surface [164,165]. The buffering effect ofboric acid is attributable to the hydrolysis:

3H BO B O (OH) H 2H O3 3 3 3 4 2+ ++ (26)

In acidic conditions, pH < 6, boric acid dissociates and remainsstable. At high pH levels (7.5–9.5), B3O3(OH)4

− is stabilised[106,110]. The effect of boric acid in the water reduction region isimportant:

H O e H OH2 ads–+ + (27)

SDS is an anionic surfactant which promotes the uniform distribu-tion of particles and prevents the agglomeration of ceramic particles[166–168]. Lari baghal et al. [169] have proven that adsorption of Ni2+

and Co2+ ions on SiC nanoparticles is enhanced by increasing the SDSconcentration from 0 to 0.75 g dm−3 in the NieCo bath. They reportedthat, at SDS concentrations higher than 0.5 g dm−3, the number ofnanoparticles was decreased. SDS content higher than 0.25 g.dm−3 hada negative impact on the ductility of NieCo deposits. 2-butin-1,4-diol(BD) is another additive that reacts with adsorbed hydrogen (Hads) andlowers the growth rate of the crystal. BD can promote the formation of

Fig. 11. Co content in the NieCo deposit as a function of bath temperature.[10,89,148,150].

Fig. 12. The effect of current density on the Co content of the deposit inelectrolytes containing different concentration of NiSO4 and CoSO4 [19].


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(1010)hcp fibre texture in Co-rich coatings. It has been demonstrated thatthe impact of saccharin at concentrations as low as 0.1 g dm−3 is in-significant. Absorption of saccharin is accomplished by formation ofcoordinate bonds through filling its non-sharing lone pair electronswith the 3d electrons of Ni and Co atoms. In higher amount of saccharin(0.5 g dm−3), it was adsorbed by close-packed planes of (0002)hcp/(111)fcc. 2 g dm−3 saccharin and 0.5 g dm−3 BD result in higher in-hibition effect greater XRD peaks of (0002)hcp/(111)fcc. With the ad-dition of saccharin and BD, the texture was changed from (200)fcc to(111)fcc [14]. Li et al. [135] studied the relationship between saccharincontent (3–5 g dm−3) and physical-mechanical properties of NieCodeposits. The results indicated that saccharin leads to inhibition of Codeposition (because of increasing the overpotential [10]) and dete-rioration of microhardness due to grain refinement. However, Bakhitand Akbari [162] have reported that hardness was changed slightly inthe presence of 4 g dm−3 saccharin. Saccharin can lead to some sulfurand carbon impurities in the coatings, causing embrittlement, lowerductility [170–172] and poor corrosion resistance [173]. Marikkannuet al. [78] demonstrated that the addition of various additives such assaccharin, dextrin, coumarin, formaldehyde, glycine, and crotonalde-hyde to acetate baths result in a lower corrosion rate of the deposits,together with a higher hardness and mirror-bright surface in somecases. Gomez et al. investigated the effect of dodecyl trimethylammo-nium chloride (DTAC) on CoeNi electrodeposition. The process wasmodified in the bath containing DTAC surfactant, and increased the Cocontent in the coating. Even the magnetic properties of the deposit wereaffected, due to a phase transformation from fcc to hcp structure.

Gluconate is another additive which is often added to electro-deposition bath of NieCo coatings. Such electrolytes are not only in-expensive, but also non-toxic and eco-friendly similar to the acetatebaths. This bath contains 0.02–0.12 mol dm−3 cobalt sulfate,0.8–0.18 mol dm−3 nickel sulfate, mol dm−3 sodium sulfate and0.20–0.5 mol dm−3 sodium gluconate. The pH of the electrolyte wasadjusted to 5 [89]. Boric acid can be added as a buffer to control the pH[174]. Sodium gluconate is used as a supporting electrolyte [175]; its

addition to the bath lowers cathode polarisation due to the enhancedstability of Ni2+-gluconate complex species [89].

A few studies have investigated the formation and properties ofNieCo alloys in glycine-containing electrolytes [78,176–178]. Glycineis a complexing agent which eases the electrodeposition of rare-earthelements from aqueous electrolytes. In addition, it possesses remarkablebuffering properties, so it stabilises the pH of the electrolyte [179]. Inanother study [178] it was indicated that the addition of glycine to thebath affects the alloy composition of the Ni-Co-Mo coatings, enhancingthe deposition rate of Ni and Mo, while having a negligible impact oncurrent efficiency at high current densities.

8. Current control during NieCo electrodeposition

Various methods have been employed for the production of NieCocoatings. Among them, electrodeposition is commonly used due to itslow cost, flexibility (coatings with different structure and patterns),high efficiency and simple mass production procedure with little ne-cessity for high temperatures and high pressures compared to otherproduction techniques. Three techniques have been commonly used forNieCo films: (1) direct current (DC), (2) pulsed current (PC) and (3)pulsed reversal current (PRC). Fig. 13 shows the current modes forelectroplating of NieCo deposits.

8.1. Direct current deposition

In DC electrodeposition procedure, a constant and continuous cur-rent is applied throughout the process. Only current density (or voltage)is effective in this method. DC electrodeposition has several advantagesincluding low cost and ease of operation. In many studies[57,64,114,131,135,191] on the morphology, microstructure, corro-sion behaviour, microhardness, internal stress, and magnetic propertiesof NieCo alloy depositions fabricated by the DC technique, PC, RPC andcyclic voltammetry have also been investigated. Some researchers havestudied the effect of different currents and their intensity on anomalous

Table 3Bath additives used in NieCo electrodeposition.

Additive Formula Performance Ref.

Boric acid H3BO3 Buffer [163]Ammonium sulfate (NH4)2SO4 Buffer [163]Sodium dodecyl sulfate (SDS) C12H25NaSO4 Surfactant [163]Thiosemicarbazide hydrochloride CH6ClN3S Surfactant [180]Thiourea CH4N2S Surfactant, brightener [180–182]Sodium saccharin C7H5NO3SNa Stress reducing, grain refinement, surface roughness reducing,

brightener[180,183,184]

Saccharin C7H5NO3S Stress reducing, grain refinement, surface roughness reducing,improving the brightness and quality of coatings

[135,162,185,186]

Dextrin (C6H10O5)n Surfactant [187]Gum Arabic – Surfactant [187]Gelatin – Surfactant [187]Sodium acetate C2H3NaO2 Internal stress reducer, complexant, stabling the structure and

properties[63,64]

Citric acid C6H8O7 Internal stress reducer, stabling the structure and properties [63]L-Ascorbic acid C6H8O6 Surface smoother [140]Triton X-100 C14H22O(C2H4O)n(n=9,10) Anti-pitting agent [188]1,4-Butyne diol C4H6O2 Brightener [78]Coumarin C9H6O2 Reducing the current efficiency [78,189]1,3-Naphthalene sulphonic acid C10H8O3S Stress reducer [78]Formaldehyde CH2O [78]Glycine C2H5NO2 Complexing agent [78]Crotonaldehyde C4H6O [78]Glycolic acid C2H4O3 Complexant [64]Oxalic acid C2H2O4 Complexant [64]2-Butin-1,4-diol (BD) C4H6O2 To control the internal stress; and grain size and texture [14]FC 95TM a blend of sodium salts of

uoroalkylsulfonateWetting agent [71]

Sodium gluconate C6H11O7Na Complexing agent [89]Dodecyltrimethylammonium chloride (DTAC) CH3(CH2)11N(CH3)3Cl Cationic surfactant [190]


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codeposition and cathodic current efficiency. Under DC deposition, thecodeposition behaviour of NieCo deposits in sulfate baths is alwaysanomalous. The deposition current efficiency in PC and RPC methodshave been higher than that of DC deposition [114].

8.2. Pulsed current deposition

Pulse or unipolar plating is a recent and attractive choice for im-proving the properties of electrodeposited coatings in comparison to DCelectrodeposition. In this process, the current/potential alternativesbetween two values. Thus, there are series of pulses having the sameamplitude, time period and polarity. During electrodeposition, metalions are reduced when they reach the cathode surface. There is a zeroconcentration of metallic ions at the cathode surface and the transportrate of ions to the cathode surface controls the deposition rate.Application of a current (greater than limiting current density) resultsin a more negative charge on the double layer [61]. In DC

electroplating, this layer is thick and inhibits the ions from reaching thesurface. Throughout the PC process, the off-time period allows the layerto discharge, so the ions can easily pass the layer and arrive at thecathode surface [192]. PC electrodeposition has various advantages incomparison to DC electrodeposition [192–194]: (1) in PC electro-deposition, the grain growth is inhibited and nucleation rate is boostedon the surface, resulting in a more compact structure; (2) this methodintroduces lower internal stresses compared to the DC plated coatings;(3) chemical composition, structure and properties can be controlled bymodifying the PC parameters; the need for additives in PC plating canbe approx. 50% lower than in DC plating and (4) incorporated ceramicnanoparticles content in PC are more numerous than the DC method.PC can be interpreted by many parameters including average currentdensity (Ia), peak current density (Ip), on time (ton), off time (toff), dutycycle (ϕ) and pulse frequency (f) as shown in Fig. 13b). IP, ton and toff areindependent variables [195]. The microstructure and properties onNieCo coatings are affected by changing PC parameters (IP, ϕ and f)which are defined in the next section:

8.2.1. Duty cycleThe duty cycle, which is in the range of 1–100%, is defined as the

ratio of on time to total time (t = ton + toff) [192]:

tt t

100on

on off=

+×

(28)

In a PC process, at very low duty cycles (like 5%), a high Ip is ne-cessary for deposition rate to be equal to DC plating. At very high dutycycles, PC is similar to DC [192]. Arenas et al. [114] have shown thatthe NieCo alloy composition strongly depends on the duty cycle(Fig. 14) but it does not affect anomalous behaviour. Indeed, the de-crease in duty cycle leads to a shorter on-time duration and a slowerdepletion rate of Co2+. At longer toff values, Co2+ is enriched in theboundary layer and consequently, the Co content in the deposit will behigher. A study of the Co-Ni-Fe pulse electrodeposition from a sulfatebath demonstrated that by decreasing duty cycle and pulse currentdensity, current efficiency is reduced [196]. By raising duty cycle, thebrightness and smoothness of NieCo deposits will be increased and thegrain size of coatings will be reduced [114], in accordance with theresults obtained by Yang and Cheng [197].

Duty cycle has a significant role in determination of the number ofceramic nanoparticles in composite coatings. In a study, it was reportedthat with decreasing duty cycle from 80% to 20%, the difference involume fraction of SiC nanoparticles in Ni-Co-SiC composite coatingswas 4 vol%. According to the early Guglielmi model (which focuses oncharge transfer, neglecting mass transfer) [198] the adsorption ofceramic nanoparticles takes place in two continuous steps: (1) weakFig. 13. Controlled current modes of electrodeposition: (a) direct current, (b)

pulsed current and (c) pulsed current reversal.

Fig. 14. The effect of pulse frequency and duty cycle on the composition of aNieCo deposit obtained from pulsed current. Baths 1 and 2 contain a Ni2+/Co2+ ion ratio of 200/100 and 400/18, respectively [114].


476

adsorption on the cathode surface along a high coverage of cations and(2) electrophoretic transfer and attraction of charged nanoparticles tothe surface and thus adsorption on the surface due to the Coulomb forcebetween the nanoparticles and metal cations which were adsorbed. Inshort off-time duration at a high duty cycle, there is no chance oftransferring nanoparticles to the surface. Thus, the amount of SiC na-noparticles in the NieCo coating is decreased.

8.2.2. Pulse frequencyFrequency is the rate of pulsing expressed as Hz (s−1) unit, and

defined as the inverse of the total cycle time (t = ton + toff) according tothe following equation [192]:

ft t

1on off

=+ (29)

At high frequencies, ton and toff are very short and the double layercannot completely charge or discharge during ton and toff durations,respectively. Alloys with a uniform composition are often electro-deposited at high frequencies [192]. Fig. 14 shows the impact of pulsefrequency on composition of the coating. As can be observed, by in-creasing the frequency, Co content of the coating is slightly changed.However, the composition is affected by the bath composition. Chungand Chang [109] have shown that changing pulse frequency in therange of 10–100 Hz had a negligible effect on the Co content. NieCocoating with smoother morphology and finer grain structure was ob-tained using increased frequency [109,114,199], which is due to theformation of more nuclei on the substrate [200].

Pulse frequency has a significant effect on the properties of NieCocoatings. It is reported that the hardness of NieCo coatings at10–100 Hz and a constant current density of 3 A dm−2 was between6.67 and 6.82 GPa, whereas under DC conditions, it was measured to be8.45 GPa [109]. This difference can be ascribed to the finer grain size,smoother surface or lower internal stress under higher frequencies. Theeffect of pulse frequency in NieCo nanocomposite coatings is strongerthan the alloy deposits. Yang and Cheng [197] have stated that by in-creasing frequency between 0.1 and 100 Hz, the microhardness GPa ofa Ni-Co-SiC nanocomposite coating was changed to between 510 and

670, which is attributable to increasing SiC nanoparticle contact withthe surface. High frequencies result in a higher overpotential but moreenergy is provided for adsorption of SiC nanoparticles.

8.2.3. Pulsed currentMetal deposition rate in PC is the same as in DC, which is the

average (mean) current density. The average (mean) current density(jave) is calculated from the peak current density (jp), according to[192]:

j j .ave p= (30)

This parameter can affect chemical composition, structure, mor-phology and properties of the coatings. Several studies [72,109,114]have revealed that NieCo coatings with a compact structure, smoothermorphology, and finer grain size were formed at lower pulse currentdensities. Li et al. [135] have shown that by increasing the peak currentdensity, the amount of Co in the coating was decreased. This is inagreement with the results obtained by other researchers [10,72,193].Ni deposition in deposits is controlled by activation. Increasing thecurrent density leads to higher overpotential, resulting in higher acti-vation. Under these conditions, the Ni content is enhanced in the de-posit.

8.3. Pulsed current reversal electrodeposition

PRC electrodeposition is a bipolar process. In this technique, thepolarity of a direct current (including anodic and cathodic currents) iscontinually reversed. PCR parameters include [201]: (1) cathodic cur-rent density (jC); (2) anodic current density (jAA); (3) anodic (reverse)time (tAA), and (4) cathodic (forward) time (tC). The average currentdensity (jA) and duty cycle (ϕ) are defined below [192]:

jj t j t

t tAC C AA AA

AA C=

+ (31)

tt t

C

AA C=

+ (32)

Not all of the current is used for deposition. During reverse

Fig. 15. Comparison of (a) residual stress and (b) hardness in DC and PCR methods of Ni-Co-Al2O3 electrodeposition [204].


477

polarisation, a small part of the coating is dissolved [201]. In general,the current efficiency in PRC electrodeposition is lower than that of PCelectrodeposition [202] and higher than DC plating [114]. According tothe formation mechanism of iron group metals, M(OH)ads

+ ions areproduced in pulse period and dissolved in reverse period. Thus, theanomalous codeposition of CoeNi can be improved by PRC, which is apowerful technique for easy control of composition [57]. However,depending on the bath type, the deposited layer can be passivated [55].Arenas et al. [114] observed that PCR is unable to efficiently reduce theanomalous behaviour of NieCo alloys in sulfate baths and differentfrequencies, duty cycles or current densities, whereas Bai and Hu[57,130,131] were successful in overcoming the anomalous behaviourof NieCo deposits in chloride baths. The increase in anodic currentdensity and frequency and the decrease in duty cycle lead to higher Cocontent in the alloy. Numerous studies [57,149] have reported similarresults. Examination of the microstructure has revealed that anodiccurrent during the electrodeposition promoted a crack-free NieCocoating [203] due to the diminishing hydrogen reduction and loweringthe internal stresses in the deposit (Fig. 15a) [204,205]. PRC results inthe enhancement of the ceramic nanoparticles content in Ni [206–208]and NieCo [204,205,209] nanocomposite coatings (in comparison withDC and PC). Chang et al. [205] have shown that the microhardness ofNi-Co-Al2O3 coatings prepared by PRC was higher than the DC plating(Fig. 15b). Higher hardness in PRC electrodeposition can be due to abetter distribution of Al2O3 nanoparticles in the coating.

8.4. Other controlled current techniques

In addition to the square shape of current, various other shapes suchas triangular can be used for electrodeposition of NieCo alloys. Cyclicvoltammetry (CV) is an electrochemical technique which is conductedthrough cycling the potential of a working electrode and measuring theobtained current. In this method, the working electrode is scannedlinearly by an applied voltage in a forward and reverse direction, and in

repetitive cycles. The current is the response of the applied potential,and the differences between scans yield information about the reactionmechanisms [210]. The potential range and number of cycles are im-portant parameters of CV [131]. This technique has been employed inthe previous research [56,57,114,130,131,211,212] for electrodeposi-tion of iron-group alloys such as NieCo and NieFe films. However,potential cycling is not a common method for electroplating of metalsand alloys. In fact, the only valid reason for employing this method is tosurvey the anomalous codeposition mechanisms of iron-group metals[130]. CV is commonly used as a laboratory technique in which thepotential of the electrode is swept at a relatively fast, rate between setpotential limits [57]. However, CV is not feasible on an industrial scaleinvolving large cathodes, since double layer capacitative charging be-comes excessive and it is difficult to design fast, large current po-tentiostats. Hu et al. [130] have shown that anomalous codeposition ofalloys such as CoeNi, FeeCo, ZneFe, FeeNi, and ZneNi was preventedusing CV electrodeposition method, which is in agreement with thepreviously obtained results [131,211,212]. They have suggested thatthe inhibition of anomalous codeposition by CV is because of the fol-lowing steps: (1) dissolution of the deposited alloys, (2) removal of theM(OH)ads

+ layer by anodic dissolution, (3) so the less noble metalsdissolve slowly, and (4) finally the anomalous behaviour is prevented.Bai et al. [131] have investigated the effect of bath pH, temperature,number of cycles and the potential range of CV on the composition ofNieCo coatings. The results indicate that the anomalous behaviour ofdeposits is increased at higher pH values. Morphologies of NieCocoatings were affected by CV potential range. Moreover, finer grainNieCo deposits were obtained by decreasing the temperature andnumber of CV cycles.

9. Microstructural characteristics

Numerous NieCo electrodeposition studies [56,63,119] have statedthat different properties, microstructure, and phase structure of these

Fig. 16. (a–f) XRD patterns of NieCo alloys containing 0–81 wt% Co [6,154], (g) TEM image of the selected area diffraction (SAD) patterns for Ni-49 wt% Co alloy[215] and (h) the percentage of relative texture coefficient for (111), (200) and (220) in NieCo alloys with different Co content [220].


478

alloys are a function of the Co content of the coating, which is con-trolled by process variables such as pH, temperature, electrolyte com-position and deposit current. According to the phase diagram for theNieCo binary system (Fig. 3). Ni and Co can form two different phases,consisting of a face-centered cubic (fcc) phase and a hexagonally close-packed (hcp) phase, depending on alloy chemical composition [213].Wang et al. [154] have investigated the phase structure and composi-tion of NieCo deposits by XRD. The results indicated that the pure Nicoating has fcc phase grown in the preferred orientation of (200). Byadding cobalt ions to the Ni electroplating bath, the solid solution ofNieCo is deposited. As can be seen in Fig. 16a)–e), for alloys consistingCo < 49 wt% (Ni-rich alloys), the alloy structure is completely fcc(XRD peaks location at 2θ = 44.5 deg., 51.8 deg., 76.4 deg., and93.1 deg., in agreement with previous studies [63,214]. TEM micro-graph of selected area diffraction (SAD) confirms the fcc single-phasestructure of Ni-49 wt% Co alloy [215] (Fig. 16g). Myung and Nobe[216] have reported that in Ni-rich alloys, by increasing the alloycontent from pure Ni to 50 wt% Co, the grain size remains constant andis equal to about 140 nm. By increasing the Co content, the growthorientation of (111) will be predominant. In addition, according toVegard's law [10], higher Co content in the alloy can increase the latticeconstant. Based on this law, the relationship between the lattice con-stant and Co content is linear [10,217]. Fig. 16g) shows that the relativetexture coefficient (RTC) is changed depending on the Co content. Inaddition, at 66 wt% Co, the hcp lattice begins to form in accordance to(100) peak at XRD pattern (Fig. 16h). At this point, the structure ofNieCo alloy is a mixture of both fcc (major phase) and hcp (minorphase), as suggested in other studies [139,218]. Based on nanos-tructured materials, a mixed structure of fcc + hcp can be formed[219]. In this structure, a cluster of hcp-Co includes an fcc-Ni (structure(a) in Fig. 16), or there is a hcp-Co in the fcc-Ni in Fig. 16b) [72]. Thesmallest grain size of the alloys belongs to the mixed phase region[216]. In Co-rich coatings (alloys containing > 80 wt% Co), a vigorous

texture of (002), (100) and (110) peaks is observed which belongs tohcp structure, as shown in Fig. 16f). By gradually increasing the Cocontent, the structure is transformed from fcc to hcp.

As mentioned in Section 4, the alloy composition of NieCo depositsis altered by operational parameters, which can also affect the structureand phase of the coatings. Park et al. [146] have studied the impact ofpH and additives such as saccharin on the XRD patterns. The resultsindicated that NieCo coatings obtained from a bath containing sac-charin exhibited the same peaks with lower intensity in comparison tothe coatings from baths without saccharin. The dominant phases at lowcurrent densities (5 and 10 A dm−2) are similar to Fig. 16b)–e). Byincreasing the current density to 20 and 25 A dm−2, the dominantphases are changed to fcc (220) and hcp (110). However, Chung andChang [109] have stated that the dominant phases remain unchangedby changing the current density. Bath pH has no influence on XRDpatterns [146]. Pulse parameters such as frequency do not change thepeaks [109]. Hu et al. [127] have studied the impact of depositionpotential. There is a prominent peak at 2θ = 41.2 deg. (related to the(100) plane of hcp) and a small peak at 2θ = 76.0 deg. (related to thefcc planes of NieCo) for NieCo alloys coated at −0.9, −1.3 and−1.6 V. However, the alloy deposited at −3.0 V showed a dominantpeak at 2θ = 76.0 deg.

According to the discussion above, the study of microstructure andtexture of the top surface using XRD is limited to the deposit surfacethat is in contact with electrodeposition bath. Thus, it is not useful forinvestigating the microstructure, phases or texture on the cross-sectionof the electrodeposited coating. Fig. 17 represents the microstructure ofthe coating/substrate interface, free surface and FIB image of the cross-section. It can be observed that the grains of coating/substrate interfacehave a smaller size than that of the free surface. FIB image (Fig. 17d)reveals a columnar grain structure. The majority of the grains are si-tuated along the direction of grain growth. However, several smallergrains form clusters as indicated in Fig. 17d). It can be concluded that

Fig. 17. The microstructure of a Nie80Co coating: (a) schematic cross-section, (b) coating/substrate interface, (c) top surface and (d) FIB image of cross-section[221].


479

the microstructure includes a bimodal distribution of grain size.Electron backscatter diffraction (EBSD) and forced ion beam (FIB)

microscopy are powerful techniques for determining the micro-structure, crystallographic properties, texture and phases in the cross-section and interface of NieCo electrodeposited films. Bastos and et al.[221] investigated the microstructure and texture of Ni-80 at.% Cocoating by FIB and EBSD. According to the XRD pattern, Ni-80 at.% Cocoating is a single phase alloy (hcp structure) [186]. However, EBSDresults revealed that this coating contains two phases (a mixture of hcpand fcc structures) consisting of about 6% fcc phase in the coating/substrate interface, and 3% at the top surface [221] (Fig. 18). This valuein cross section depends on the thickness of the coating. By increasingthe thickness, the amount of the fcc phase is reduced.

Grain boundaries have a significant effect on physical and me-chanical properties, and therefore the determination of their char-acteristics is of great importance. Researchers have used 3D EBSD todemonstrate that the formation of NieCo columnar grains is because oftriple junctions with low energy [222]. In some cases, the grains are notcolumnar [223]. According to cross-sectional studies, 1120< > is theprominent growth direction.

10. Mechanical characteristics

10.1. Microhardness

The hardness of coatings depends on various factors includingchemical composition, microstructure, the strength of coatings, etc. Allthese factors are influenced by the coating method and the involvedprocesses. Electroplating variables change the microhardness of NieCocoatings. By increasing the amount of Co by up to 49 wt%, micro-hardness is raised from 250 to about 480 Hv. However, a further in-crease in Co content leads to a drop in the microhardness down to 350Hv. This behaviour can be explained by the reverse Hall-Petch relation[224,225]. In fact, in Ni-rich alloys, Co results in finer grains, however,Co content increment in Co-rich alloys increases the grains size. Thegrain size can be determined from TEM micrographs and XRD data. Thedata from these methods reveal differences that can be explained byresidual stresses [10,225]. Coatings with a grain size above 15 nmfollowed the Hall-Petch relationship. Interactions of dislocationboundary and twin boundaries are effective for the strengthening ofalloys in this range of grain size. However, there is a breakdown in Hall-Petch behaviour with the grain sizes lower than 15 nm. Thus, maximum

Fig. 18. Phase distribution using EBSD maps: (a) coating/substrate interface, (b) top surface and (c) FIB image of cross-section [221].


480

hardness occurs at a certain critical grain size.The mechanism of an inverse Hall-Petch relationship in Ni and

NieCo deposits is the same due to identical hardness data in bothcoatings. The critical size of grains for breakdown in Hall-Petch beha-viour can be defined based on dislocation pile-up in the grain. At thegrain sizes smaller than the critical size, the dislocation pile-up in thegrain can be stopped. According to Nieh and Wadsworth analysis [226],the critical grain size (lc) can be defined by Eq. (21)

l Gbv H

3(1 ).c =

(33)

In which H is the microhardness of the material; v, G, b and v are thePoisson ratio, the shear modulus of the coating and Burger's vectormagnitude, respectively. According to this equation, at grain sizesgreater than the lc the Hall-Petch relation is stable, but the relationshipis reversed at the grain sizes smaller than lc. However, the analysis isbased on dislocation pile-up, which is inadequate to elucidate thereason for Hall-Petch breakdown. Indeed, the breakdown in this rela-tion is associated with deformation mechanisms such as sliding in grainboundary (twins or general high- and low-angle boundaries) and in-tergranular diffusion. This can be related to the structure of the coating.Fcc structures such as nano-structured Ni [225], Cu [227] and Al [228],possess low twin density, so the reverse Hall-Petch effect occurs bysliding and boundary diffusion. Nanostructured coatings such as Co[229] and NieCo [186] have a high density of twins resulting in theirhcp structure. The Hall-Petch breakdown in these coatings is probablydue to predominant migration of twin boundaries at very fine grains.This issue requires detailed and extensive further studies.

In addition to the above considerations, it is necessary to considervariables influencing the grain size and hardness of NieCo coatings.Presence of ceramic nanoparticles such as Al2O3 [205], SiC [197] andZnO [230] can improve the microhardness and other mechanicalproperties of NieCo coatings. The movement of grain boundaries anddislocations due to dispersion strengthening is limited in the presenceof nanoparticles which act as an obstacle against them, resulting inhardness improvement of the coating. In general, the hardness of na-nocomposite material is influenced by two important factors [231]: (1)the content of strengthening nanoparticles and (2) the hardness of themetal matrix. In a specific metal matrix, nanoparticles are the mostimportant factor influencing microhardness.

The amount and size of nanoparticles have two different hardeningmechanisms [197]: (1) the strengthening based on the dispersionhardening and (2) the strengthening based on the particle. Pulseparameters including duty cycle and frequency affect the microhard-ness in nanocomposite coatings. Yang et al. have reported that de-creasing the duty cycle from 80 to 20% resulted in enhancement of themicrohardness of Ni-Co-SiC coatings from 620 Hv to 670 Hv due tochanging the nanoparticles content from 6 to 10 vol%. Increase in pulsefrequency led to increases in microhardness as a result of grain re-finement.

10.2. Tensile properties

Ni coatings are known as high strength and tough materials in in-dustrial applications. NieCo alloys have higher mechanical properties(strength, toughness, and tribological behaviour) compared to the Nicoatings. Therefore, they are suitable candidates for replacing electro-deposited Ni [135,154]. The higher mechanical properties of NieCodeposits are due to the effect of cobalt in the refinement of micro-structure and reduced requirement of different organic structure re-finers [135]. Using organic refiners results in precipitation of C and N atgrains boundary which prevents the sliding of grain boundaries, re-sulting in lower toughness and strength [170]. Alloying with Co pre-vents the toughness and tensile strength from decreasing due to grainrefiners such as saccharin [135,149,154]. By changing the alloy com-position from pure Ni to 45 wt% Co, the yield strength and ultimate

tensile strength can be enhanced, probably due to finer grains and thestrengthening mechanism of Co solid solution in the deposits. Fur-thermore, the increase in Co content by up to 55 wt% leads to a drop inyield strength and ultimate tensile strength down to 650 and 1100 MPa,respectively. The reason for this could be the different density ofcoatings [138]. However, it is reported that NieCo coating with 49.2 wt% Co exhibits higher tensile strength in the range of 1920–2250 MPa(depending on the strain rate) [215]. The remarkable strength of thisalloy is attributed to the fine grain size and hardening by the solidsolution. The tensile properties of coatings can be affected by variablessuch as temperature [232] and strain rate [18,215,233]. Increasing theCo content by up to 25 wt% led to an increase in ductility from 8.5% to13.5% because of finer grains [138,234]. In alloys with higher Cocontent, the ductility was significantly decreased [138]. In fact, in Co-rich alloys, the porosity of the coatings is increased which leads toformation of voids and nucleation of cracks in the NieCo coatings[154,234]. The grain size of the deposits is an important factor in theductility of coatings. Ductility can be affected by the strain rate[215,233]. Low strain rates may increase the elongation of NieCocoatings resulting from grain boundary activities that collaborate in thisstrain rates. In Ni-8.6 wt% Co coating, by increasing the strain rate(1.04 × 10−5−1.04 s−1), a transition behaviour of ductile-brittle-ductile was observed [233].

10.3. Fracture surface and fracture toughness of NieCo coatings

According to the tensile strength results of NieCo deposits, the ex-istence of plasticity on the fracture surfaces can be predicted. As statedin the previous section, changes in Co content lead to different tensileproperties such as elongation, reduction in area, tensile strength, yieldstrength, and various other properties. Thus, the fracture surface of thecoatings can be influenced by these differences. The morphology of allcoatings included obvious dimples resulting in the necking of cross-section [18]. The presence of many grain boundaries in nanostructuredmaterials leads to sliding resistance and the extension of local shearplanes to more than one grain size. Consequently, it provides the con-dition for dimple morphology [235]. This type of fracture surface inNieCo coatings has been observed in the previous studies[18,169,215,233]. The size of dimples ranges from 400 nm to 1 μm,which is larger than several grains [18,215]. By increasing the Cocontent up to 25 wt%, size of the dimples is decreased. This is probablybecause of increasing the grain boundary, twining, and triple junction.The large voids on Ni-55%Co fracture surface can be attributed to theporosities due to electrodeposition process at the bath containing highCo ions. These porosities result in lower ductility in Co-rich NieCocoatings [138].

Addition of various nanoparticles, such as SiC, leads to a greaternumber of dimples with a smaller size than NieCo deposits due to theprone sites including the interface of nanoparticles and matrix.Addition of a surfactant such as SDS improves the ductility of NieConanocomposite coatings [169]. However, tensile testing parameterssuch as strain rate can affect fracture mode. Gu et al. [233] have re-ported that the fracture surface of Ni-8.6 wt% Co deposits deformed atstrain rates within the low and high ranges (1.04 × 10−5 and 1.04 s−1)were ductile, including deep dimples and rough surfaces. At strain ratesof 2.08 × 10−3 and 1.04 × 10−2 s−1, however, the morphology offracture surfaces revealed brittle and ductile modes with a mixedstructure containing shallow dimples and cup-and-cone morphologies.

The fracture toughness of NieCo coatings, as an indication of re-sistance to fracture for a material containing a crack, is rarely in-vestigated. Anthony et al. [62] determined this property for the coatingwith Co content, according to ASTM E399-72. Raising the Co contentresults in a decrease in fracture toughness. There is a reverse relation-ship between strength and toughness. The difference between tough-ness of Nie43Co was due to the different dimension of specimens andas-electrodeposited and heat treated specimens.


481

10.4. Superplasticity

Superplasticity is defined as a state in which any given material isable to bear a large degree of deformation before its breaking point,usually larger than 200% during the tensile deformation [236]. Thischaracteristic is often determined at elevated temperatures(T > 0.5Tm), in which Tm is the melting point. Superplastic behaviourof NieCo coatings is similar to conventional materials resulting fromincreasing stress with raising rate of strain and reducing the tempera-ture [171,232]. The relationship between elongation and strain rate atdifferent deformation temperatures for NieCo and Ni-Co-Si3N4 wasinvestigated [171,232]. It was shown that the process temperature wasan important factor for elongation of coatings. There was no super-plasticity of NieCo coatings at temperatures of 723 and 823 K. Super-plastic behaviour for NieCo coating (279% elongation) was only ob-served at 773 K and 5 × 10−3 s−1 strain rate, in comparison to 692%elongation in Ni-Co-Si3N4 at the 723 K temperature and the same strainrate. The difference in superplasticity of NieCo and Ni-Co-Si3N4 coat-ings is because of thermal stability and normal grain growth of thenanocomposite coating. In fact, Si3N4 nanoparticles prevent the growthof NieCo grains. The high percent of elongation could be related toboth dislocation slip and grain boundary sliding. Change in grain size isan important factor for superplastic behaviour of materials. At theelevated temperature of 773 K, Co atoms are unable to inhibit the graingrowth of Ni. Comparison of grain size in Ni [237], Ni-SiC [238],NieCo [171] and Ni-Co-Si3N4 [232] reveals that Ni-Co-Si3N4 coatingpossesses the lowest grain growth rate. The elongation of Ni is lowerthan that of NieCo and Ni-Co-Si3N4 coatings. Therefore, it can beconcluded that the solid solution of cobalt in the Ni matrix will exhibitlimited superplastic behaviour.

10.5. Internal stresses

Internal stresses in NieCo coatings are affected by operationalconditions. These stresses can be measured by XRD according to thedisplacement of the peaks [205], deflection of strip equipped with ananalyzer of deposit stress [146,239,240] and various other methods[241]. According to the latter method, the internal stress (σ) can becalculated by Stoney's equation:

Yz x x tx L

( )3s s d

d

2

2= +(34)

where Y, z and L are the Young's modulus, measured displacement ofthe electrodeposited strip, and the length of strip electrodeposited, re-spectively. xs and xd are the thickness of electrodeposited strip and thetotal thickness of the coating.

Fig. 19 shows the effect of pH, current density and saccharin on theresidual stresses of NixCoy coatings electrodeposited in a chlorideelectrolyte. The coatings exhibited minimum internal stress (220 MPaand 20 MPa with and without saccharin, respectively) at the pH valueof 4. In the low current density range (1–10 mA cm−2), the internalstress was measured at 220 MPa in a saccharin-free bath, whereas it hada value of 360 MPa at 15 and 20 mA cm−2. In general, the residualstresses are decreased with the addition of saccharin to the bath. Sac-charin, as a source of sulfur, helps transform tensile stresses to com-pressive. Studying the deposit stress in Ni-Co-Fe coatings reveals that byincreasing the temperature from 25 to 50 °C in the electrolytes con-taining saccharin, the internal stress is reduced from 61 to 32 MPa[240].

The effect of Co content on deposit stress indicates that by in-creasing the Co content in the deposit, the internal stress mode waschanged from compressive for Ni-rich alloys (Co < 49 wt%) to tensilefor deposits with Co > 49 wt% in Fig. 20a). These results were ob-tained in a sulfate bath containing saccharin in accordance with Parket al. [146]. It is shown in Fig. 20a) that, by controlling the composition

and structure and deposition of the functionally graded NieCo coatings,the internal stress drops to 4–6 MPa. Internal stresses in NieCo com-posite coatings are essential. By increasing alumina nanoparticlescontent from 0 to 5 wt%, the internal stress is increased from 230 to300 MPa in the additive-free bath and DC condition, respectively, asshown in Fig. 19b). PRC electrodeposition leads to a reduction in re-sidual stress resulting from the uniformity of coating and suppression ofhydrogen adsorption.

11. Magnetic properties

Magnetic properties of a material include magnetic anisotropy,magnetic flux density, saturation magnetisation, perpendicular mag-netic anisotropy, perpendicular coercivity, permeability, magnetic re-manence and squareness which may be affected by the Co content ofNieCo deposits, thickness, surface roughness, additives, and otherelectrodeposition parameters [242–247]. Fig. 21a) and b) demonstratethe impact of bath composition and pH on the saturation magnetisation(Ms) and coercivity (Hc), respectively. According to Fig. 21a), by raisingthe Co2+/Ni2+ in the bath (increasing the Co content in the coating),Hc and Ms are increased to 125.2 kA m−1 and 175.1 kA m−1, respec-tively. This result is reported in previous studies as well [140,248]. Infact, the magnetic dipolar moment of Ni is lower than that of Co,therefore the Co/Ni ratio in the NieCo deposit influences on Ms. Thehigher value of Hc at pH 2.0 can be related to the higher concentrationof defects in the NieCo coatings [137]. Magnetic properties can bealtered by heat treatment at different temperatures. By increasing theannealing temperature from 25 to 450 °C, Ms will be in range of144.8–342.6 kA m−1, but the value of Hc will be lowered from 15.3 kAm−1 reduced to 7.3 kA m−1 [2] as a result of the new phase of NieCo,finer grain size and separation of the hydrogen adsorbed during theelectrodeposition process [2,249]. However, the preferential texture ofcoating probably affects Hc [250]. It has been shown [176] that

Fig. 19. The effect of (a) pH and (b) current density on deposit stress of NieCocoatings: (○) without saccharin and (∆) containing 0.01 M saccharin [146].


482

addition of glycine to the electrolyte leads to the emergence ofmorphologies such as acicula at high amounts of Co with harder mag-netic deposits, yet NieCo deposits containing high amounts of Ni pos-sess subtler magnetic behaviour with round shaped grains.

Magnetoresistance (MR) involves changes in electrical resistance ofa material in the condition where a magnetic field is applied. MR stu-dies have proven that NieCo coatings containing 0–80 wt% Co ex-hibited magnetoresistance (AMR) properties. Increasing the magneticfield resulted in higher longitudinal magnetoresistance (LMR) andlower transverse magnetoresistance (TMR) properties. By increasing theCo content in the NieCo coatings, LMR and TMR were altered by3.5–11% and 3–8%, respectively [248].

Giant magnetoresistance (GMR) has a negligible magnetic impacton alternative ferromagnetic and non-magnetic conductive multilayers.Electrodeposition of NieCo (Cu)/Cu multilayers on gold substrates hasdemonstrated that GMR is effected by Ni and Co content in the ferro-magnetic layer, thickness and number of layers. The multilayer whichwas electrodeposited in citrate bathes with lower pH values did notexhibit GMR [251]. The role of the interface of the layer in GMR issignificant. There is a linear relationship between a number of inter-faces and GMR [252]. Toth et al. [253] have shown that the impact ofsurface roughness was important in GMR of Ni-Co/Cu electrodepositedmultilayers.

12. Corrosion resistance

Corrosion resistance of alloy deposits is a function of their compo-sition and microstructure. Corrosion study of NieCo deposits withdifferent Co content in 3 wt% NaCl revealed that by increasing theamount of Co in the coatings, corrosion potential (Ecorr) shifts towardsless noble values, and the corrosion current density (jcorr) is increased[80,254] contrary to other results [24,255]. However, it has been re-ported that Ni-17% Co coating possess the highest corrosion resistanceamong the coating with 0, 17 and 42 wt% Co [220,256].

Corrosion behaviour of NieCo deposits is influenced by severalimportant parameters: (1) Co content, (2) phase structure, (3) or-ientation, (4) grain size and (5) surface morphology. Ni is nobler thanCo, therefore it is expected that by adding Co to Ni coatings, the activityof alloy coatings becomes greater than Ni coating. However, the mi-crostructure is another important factor affecting the corrosion beha-viour. Corrosion resistance of a single-phase structure is higher than adual-phase structure due to the formation of galvanic cells. Yet thephase structure of NieCo coatings in the range of 0–49 Co is notchanged and maintains a steady effect on corrosion behaviour ofcoatings. Another important factor affecting the corrosion resistance isthe preferred orientation. As mentioned earlier (Fig. 16h), by increasingthe Co content, RTC percentage of deposits is altered. Therefore, it canbe concluded that the higher resistance of Ni-17%Co is because of (111)orientation. Grain size is another explanation for corrosion behaviour ofNieCo coatings. NieCo coatings with higher Co content exhibit finer

Fig. 20. (a) Internal stress in the deposit as a function of Co content in NieCo coatings [239]. (b) Effect of nanoparticles and electrodeposition technique on internalstress of nanocomposite coatings [205].

Fig. 21. The effect of (a) bath composition, (b) pH [137] and (c) heat treatment [2] on the saturation magnetisation and coercivity.


483

grains resulting in a high density of grain boundaries. Ni-17% Co had48 nm grain size compared to 15 nm for Ni-42% Co. Consequently, themoderate grain size of Ni-17% Co is another reason for higher corrosionresistance of 10.08 kΩ.cm2. In another study [234], it was similarlyreported that that NieCo with moderate Co content (Ni-20%Co) ex-hibited lowest icorr value in 3.5% NaCl solution. The lowest corrosionrate of this coating was related to its high, smooth and dense surface.The impact of deposition current on corrosion behaviour in 3.5% NaClindicates that the increase in deposition current density leads to anincrease in icorr due to variations in surface structure and alloy com-position [3]. Comparison of steel and brass as a substrate for NieCocoatings showed that Nie20Co had better corrosion resistance in bothcases, yet the corrosion efficiency was improved in at the brass sub-strate [234].

Corrosion behaviour of coatings is affected by media [257,258].NieCo coatings displayed different corrosion behaviour in basic solu-tions such as NaOH. EIS studies demonstrated that there was a flat apexin Bode diagrams at moderate frequencies, indicating the passivation ofNieCo deposits in 10% NaOH. It has been proposed that the passivationoccurs due to the bilayer structure of MO and M(OH)2 [185]. However,cyclic voltammetry experiments have proven that the passivation hasbeen a result of Co(OH)2 and CoOOH. A small oxidation peak exists at−0.20 V in the first cycle which belongs to Co(OH)2. The second peak

observed at 0.12 V is corresponding to CoOOH. The difference betweenthe second cycle and the first is due to the formation of Co oxides [72].It has been proposed that the presence of Co in the deposit hinders thegrowth of oxide layer [193].

In some cases, a third element is added to the NieCo coatings forvarious purposes which can affect corrosion behaviour. It has beenreported that icorr was lowered by reducing the Fe content and in-creasing the Ni content in Ni-Co-Fe coatings (for magnetic goals), re-sulting in denser and smoother ternary films [259]. In case of Ni-Co-Wcoatings, it was shown that Co and W had a different impact on icorr [3].The addition of P to NieCo coatings leads to an amorphous structureand deterioration of corrosion resistance [9].

The corrosion resistance of NieCo coating can be improved inpresence of micro- or nanoparticles. Various particles such as Al2O3,SiC, TiO2, WC and MWCNT have been added to the coatings. Immersioncorrosion studies in 3.5% NaCl and 5% HCl indicated that adding SiCnanoparticles to NieCo coating resulted in lower corrosion rates.According to potentiodynamic polarisation tests, in Ni-Co-SiC nano-composite coatings, Ecorr shifts towards more positive values and icorr isdecreased compared to NieCo coatings [258], which is in agreementwith the previous results [22]. SiC nanoparticles fill defects such asholes, gaps, and crevices, as well as reducing the size of the defects,resulting in a more compact microstructure. In this case, the size of

Fig. 22. Wear track of (a) pure Ni, (b) Nie58Co, (c) Nie75Co and (d) Nie83Co against an AISI-52100 6 mm diameter, stainless steel ball with a hardness of 700 HVin a reciprocating wear test at a humidity of 40–50% with a load of 14 N, sliding frequency of 1 Hz, sliding stroke 2.69 mm and 5.38 mm s−1 sliding speed over a timeof 25 min [262].


484

particles is of great importance. Ni-Co-SiC coatings with nanoparticlesexhibited higher corrosion resistance compared to the coatings con-taining micro-size particles [260].

In addition, nanoparticles are similar to inert corrosion barriers,effectively limiting their growth. In other words, they decrease the ef-fective surface area exposed to the electrolyte by uniform distributionin the deposit, so that the Ecorr shifts to more positive values. However,incorporation of CeO nanoparticles reduced the corrosion resistance ofcoatings. Lower corrosion resistance can be attributed to CeO agglom-erations in the coating. Adding high amounts of TiO2 to NieCo elec-trodeposition bath leads to lower corrosion resistance resulting fromthe agglomeration of nanoparticles [261], similar to Ni-Co-SiC coatings[262]. The nanoparticles lead to the formation of numerous micro-galvanic cells (with nanoparticles and NieCo matrix acting as cathodeand anode, respectively). Thus, uniform distribution of nanoparticles iscapable of changing the mechanism of corrosion, unlike the negativeeffect of agglomeration. Corrosion associated with erosion has a sy-nergistic effect on the corrosion behaviour of materials in pipelines. Ithas been demonstrated that Ni-Co-SiC/Al2O3 nanocomposite coatingshave a positive effect on erosion-enhanced of API X-65 pipes in oil sandslurry consisting NaCl, NaHCO3, silica sands and other additives[153,263].

13. Tribological properties

Addition of Co to Ni has a positive impact in decreasing the wearrate and coefficient of friction (COF) of NieCo coatings. In Ni-rich al-loys, the effect of Co on COF is negligible. Co content higher than 49 wt% leads to a decrease in COF, for instance, Ni- 81 wt% Co deposit has alower friction coefficient. Moreover, alloys with high Co content exhibitmore stable friction behaviour in comparison to Ni-rich alloys [154].This behaviour can be attributed to the different phase structure of thecoatings. In alloys with high Co content, crystal transformation from fccto hcp is the reason for the decreased coefficient of friction. COF can beinfluenced by deposition parameters affecting the alloy content such ascurrent density [152,264]. By adding 5 wt% Co to pure Ni, the wearrate is increased from 12 × 10−5 to 25 × 10−5 mm3 N−1 m−1 (drysliding condition and force of 14 N against a steel ball, according to theASTM G99 standard) [13].

The wear rate of these coatings is also reduced further by increasingthe amount of Co in agreement with other studies [264]. However, ithas been reported that Ni-58 at. % Co alloy exhibited lower wear re-sistance (wear rate of 30 × 10−5 mm3 N−1 m−1) than Ni-rich alloys,resulting from mismatching and internal stresses in the coating [13].This results in formation of cracks in a deep wear track. According tothe Archard equation [265], the wear rate (Δw/t) in a pin on disk test isdirectly related to parameters such as (1) the applied load (L), (2) wear

distance (x) and (3) hardness (H):

w K L xH

= (35)

K is the coefficient of friction. According to this equation, wear rateis inversely related to hardness. In a coating containing by up to 49 wt%of Co, increasing the microhardness is the responsible for lower wearrates. Contrary to the Archard law, at Co contents higher than 49 wt%,decreasing the microhardness resulted in a lower wear rate. This mightbe due to the hcp structure and lower grain size of the coatings. Theworn surface of Ni-rich alloys demonstrated higher severe plastic de-formation and adhesive wear compared to that of Co-rich alloys [154].Increasing the Co2+/Ni2+ ratio in the bath via various additives (suchas saccharin) has a significant effect on tribological properties. NieCocoatings obtained from the bath containing saccharin exhibited lowerwear rates compared to the ones obtained from baths without saccharin[188,266]. This is probably due to the reduced grain size and residualstresses, and enhanced adhesion of coatings. Surface morphology is acritical factor in wear behaviour of coatings. It has been attempted toproduce NieCo coatings with enhanced physical and mechanicalproperties through modification of electrodeposition methods, such ascarbon dioxide assisted deposition, leading to lower surface roughnessand improved wear resistance [267]. The wear mechanism was affectedby the type of deposition current. The coatings produced by PRC cur-rent exhibited abrasive wear behaviour rather than adhesive wearwitnessed in DC coatings [204].

Tribological studies have shown that tribofilm and wear debris actas a third body in the tribo-system. Fig. 22 demonstrates the wear trackof NieCo coatings with different Co contents. Microhardness, COF andwear rate of the coatings are also presented in the SEM micrographs. Ascan be seen in Fig. 22d), the worn surface of Nie83Co is shallower thanthe other coatings. The EDS analysis of the highlighted areas in theFig. 22 and wear debris is listed in the Table 4. The Fe content in area Ais high (20 at.%), having been transferred from the steel ball. Thissuggests that the wear mechanism of pure Ni includes severe adhesion.The presence of oxygen and iron confirms the formation of tribofilm inthis coating due to the absence of these elements in the depositedcoating. According to Fig. 22b), the wear track of Nie58Co includesthree zones: (1) a smooth zone B with 7 at.% Fe, (2) a transition zonewith small cracks and (3) a debris covered zone with 2 at.% Fe. Thehigher wear rate of Nie58Co can be related to the internal stressesleading to the formation of cracks in the coating. Nie75Co contains noiron on the highlighted area C in Fig. 22c) or from the resultant debris(Table 4).

The absence of iron and oxygen in wear track confirms that notribofilm was created on the worn surface of Nie75Co. Nie83Co con-tains the lowest amount of debris in Fig. 22d), indicating the lowest

Table 4EDS analysis of demonstrated areas in Fig. 29 and debris collected from the coatings [13].

Content (at.%) Worn surface

Pure Ni (Area A) Ni-58Co (Area B) Ni-75Co (Area C) Ni-83Co (Area D)

O 63 48 0 0Ni 17 18 23 16Co 0 27 77 84Fe 20 7 0 0

Content (at.%) Debris

Pure Ni Ni-58Co Ni-75Co Ni-83Co

O 40 39 36 49Ni 50 28 16 4Co 0 31 48 47Fe 10 2 0 0


485

http://www.astm.org/Standards/G99

Table5

Sum

mar

yof

trib

olog

ical

stud

ies

ofN

i-Co

depo

sits

and

thei

rco

mpo

site

sin

diffe

rent

wea

rte

sts.

coat

ing

Test

Pin

orba

llm

ater

ial

Forc

e(N

)Sp

eed

(mm

.s-1

)CO

FCo

nditi

onO

bser

vatio

nW

ear

mec

hani

smRe

f.

Ni-C

oPi

n-on

-di

skA

l 2O

30.

12-0

.24

Dry

slid

ing

Del

amin

atio

nan

dpl

astic

defo

rmat

ion

Adh

esiv

e-ab

rasi

ve[2

64]

Ni-C

oBa

ll-on

-di

skA

ISI5

2100

stee

l3

550.

25-0

.69

Dep

endi

ngon

Coco

nten

tD

rysl

idin

gSe

vere

plas

ticde

form

atio

nad

hesi

ve[1

54]

Ni-C

oBa

ll-on

-di

skG

Cr15

stee

l2

188

0.4-

0.5

Dry

slid

ing

Plas

ticde

form

atio

n,sl

ight

scra

tch

and

dela

min

atio

nA

dhes

ive-

abra

sive

[267

]

Ni-C

oBa

ll-on

-pl

ate

Al 2

O3

9-

-Tr

ibo-

corr

osio

nSl

idin

ggr

oove

s,re

mov

edpa

ssiv

eox

ide

and

deep

scra

tche

sab

rasi

ve[2

66]

Ni-C

oPi

n-on

-di

skA

ISI1

040

stee

l3

50.

13D

rysl

idin

gSt

able

COF,

dens

ew

orn

surf

ace

and

narr

owsc

ars

Abr

asiv

e-le

ssad

hesi

ve[2

84]

Ann

eale

dN

i-Co

Ball-

on-

disk

Si3N

410

25-

Dry

slid

ing

Plou

ghs,

plas

ticde

form

atio

nan

dsm

alla

bras

ive

part

icle

sA

bras

ive

[285

]

Ann

eale

dN

i-Co

Ball-

on-

disk

Si3N

430

025

-Lu

bric

ated

slid

ing

Hig

hw

ear

rate

with

plou

ghs

atw

orn

surf

ace

Abr

asiv

e[2

85]

Ni-C

o-A

l 2O

3Ri

ng-o

n-bl

ock

GCr

15st

eel

9.8

480

-D

rysl

idin

gD

Cco

atin

gs:P

last

icde

form

atio

nPR

Cco

atin

gs:

Slid

ing

groo

ves

Adh

esiv

eA

bras

ive

[205

Ni-C

o-Si

CPi

n-on

-di

skEN

31ha

rden

edSt

eel

9.8

628

0.49

-0.8

3D

rysl

idin

gLo

wan

dst

able

COF,

fine

groo

ves,

oxid

ela

yer

form

atio

nA

bras

ive

[271

,272

]

Ni-C

o-CN

TsBa

ll-on

-di

skA

ISI5

2100

stee

l0.

5-4

3-5

Hz

freq

uenc

yA

ndam

plitu

deof

5m

m0.

15-0

.65

Dep

endi

ngon

CNT

cont

ent

Dry

slid

ing

Low

COF,

plas

ticde

form

atio

n,sc

uffing

and

atr

ansf

erla

yer

ofca

rbon

onth

eba

llA

dhes

ive

[279

]

Ni-C

o-Zn

OPi

n-on

-di

skA

ISI5

2100

stee

l50

47-

Dry

slid

ing

Hig

hCO

Fflu

ctua

tions

,afe

wsc

ratc

hes

and

mor

epl

astic

defo

rmat

ion

Adh

esiv

e-ab

rasi

ve[2

30]

sol-e

nhan

ced

Ni-C

o-Ti

O2

Pin-

on-

disk

?5

33.3

0.58

-0.6

2D

rysl

idin

gSa

llow

plou

ghlin

esan

dla

rge

debr

isA

dhes

ive

[261

]

Ni-C

o-Si

3N4

Ball-

on-

disk

AIS

I521

00st

eel

15

mm

Am

plitu

dean

d5

Hz

freq

uenc

y0.

33-0

.72

Dry

slid

ing

Low

plas

ticde

form

atio

n,sc

uffing

,gro

oves

and

scra

tche

sA

bras

ive-

less

adhe

sive

[276

]

Ni-C

o-M

oS2

Ball-

on-

disk

AIS

I521

00st

eel

1-4

5m

mA

mpl

itude

and

5H

zfr

eque

ncy

0.15

-0.2

3D

rysl

idin

gSe

vere

plas

ticde

form

atio

n,sc

uffing

and

larg

ede

bris

Adh

esiv

e[2

83]

Func

tiona

llygr

aded

Ni-C

oBa

ll-on

-di

skA

ISI5

2100

stee

l-

-0.

20-0

.28

Dry

slid

ing

Low

COF,

incr

easi

ngth

ew

ear

rate

byra

isin

gth

ean

neal

ing

tem

pera

ture

,sm

ooth

surf

ace

Polis

hing

wea

r[2

39]

Func

tiona

llygr

aded

Ni-

Co/C

oOBa

ll-on

-di

skA

ISI5

2100

stee

l3

110

0.1

Dry

slid

ing

Low

wea

rra

te,n

oev

iden

ceof

rem

oved

mat

eria

l,sm

ooth

wea

rtr

ack

and

CoO

debr

isA

nti-a

dhes

ion

[286

]


486

wear rate compared to the other coatings. There are no traces of iron oroxygen in the wear track (area D). The iron content in the debris wasalso zero. These findings confirm the lack of tribofilm or oxidation inthis coating. The wear tracks without tribofilm related to Nie75Co andNie83Co led to significant reduction of friction and improvement ofwear resistance. This can be related to the hcp structure of Co-richcoatings [12]. The amount and type of additives has a great effect ontribological properties of NieCo coatings. Co-rich coating resulted froma sulfate bath containing 0.5 g dm−3 BD and 2 g dm−3 saccharin [14]exhibited lower wear rate compared to Co-rich coating deposited froman additive-free sulfate bath [154] at same sliding wear condition. Thisis due to the higher hardness of former coating.

Several studies [268,269] have attributed the higher wear re-sistance of Co-rich coatings to the low-stacking fault energy (SFE) ofthese coatings. It has been demonstrated that materials with low SPFpossess higher galling resistance than the materials with higher SFE andequal hardness [269]. In fact, substitution of Ni in Co-based alloys leadsto a decreased SFE and lowers the rate of strain hardening, thereforeimproving plasticity. This makes the alloy susceptible to galling andreduces its wear resistance.

Tribological properties of NieCo coatings can be improved by in-corporation of particles in the alloy and production of micro/nano-composite coatings. NieCo composite coatings are electrodepositedusing different types of micro/nanoparticles. Incorporation of Al2O3

nanoparticles into the NieCo deposits resulted in an overall decrease inwear weight loss and higher microhardness due to dispersion andparticle hardening [139]. An optimal value of current density led to themaximum amount of Al2O3 nanoparticles in NieCo coatings, resultingin higher microhardness value of 555 Hv and minimum wear weightloss in dry conditions [270]. The effect of SiC nanoparticles on wearbehaviour of Ni-rich and Co-rich coatings has been investigated[271,272]. SiC nanoparticles had little effect on the COF of Nie28Cocoating (2% increment in COF). However, the COF of Nie77Co wasapproximately two orders of magnitude compared to Ni-77Co-SiC. In-troduction of lateral stresses from SiC nanoparticles into the counter-part resulted in a higher COF [271]. Similar observations have beenreported for Ni-SiC nanocomposite coatings [273]. Investigating thesize of SiC particles in the micro/nm range indicated that the COF wasnot affected by particle size [272], unlike the effect of ZnO micro/na-noparticles on COF of NieCo coatings [230]. In addition, the number ofSiC particles had an impact on COF and wear loss of NieCo coatings[231]. Ni-Co-SiC nanocomposite coatings exhibited better wear re-sistance in comparison to NieCo alloy coatings. Anti-wear properties inNi-rich nanocomposite coatings are in accordance with Archard's lawdue to higher microhardness. The higher wear resistance of Ni-70Co-SiC (Co-rich nanocomposite coating) has a reverse relationship with theArchard's law [271]. This reverse relation can be ascribed to the changein orientation of grains in SiC nanocomposite coatings. The wear be-haviour correlation with the orientation of crystals was reported inother systems [274]. The nature of wear can be determined based onthe wear coefficient, according to Rabinowicz's classification [275]. It isin order of 10−6 for Ni-Co-SiC nano/micro-composite coatings, in-dicating the wear mechanism as adhesive. Raman spectroscopy andsurface morphology proved the adhesive wear of NieCo coatings inpresence of SiC particles [271,272].

Incorporation of the optimal amount of TiO2 nanoparticles caused adecrease in COF from 0.61 to 0.55. This may be due to the lubricant roleof TiO2 particles. The high amount of nanoparticles resulted in ag-glomeration and formation of a rough surface. TiO2 nanoparticles had apositive impact on wear resistance of coatings [261]. Adding Si3N4

nanoparticles content from 0 to 6.1 wt% to NieCo coating decreasedthe wear rate from about 7.1 × 10−6 to 2.1 × 10−6 mm3 N−1 m−1 dueto silicon oxide which was hydroxylated during the test. The COF wasalso reduced from 0.70 to 0.33 [276]. This compound can be formed inpresence of relative humidity in the range of 52–56% [277,278].

Using solid lubricants is an important way of improving the

tribological properties and lowering the COF of materials. Variousparticles such as carbon nanotubes (CNTs), graphene, MoS2, and WS2

have been utilised as solid lubricants in nanocomposite coatings. TheCOF of Ni-Co-CNTs nanocomposite coatings is reduced by increasing ofthe normal force and sliding speed in the range of 0.20–0.64 [279]. Thelower COF may be because of a lubricious transfer layer which isformed on the surface of counterpart [280].

For coatings containing MoS2, increasing the load from 1 to 4 N ledto a drop in COF of Ni-Co-MoS2 coating, unlike the coating withoutMoS2. This behaviour is due to the formation of transfer layers withlubricating characteristics. A similar behaviour has been reported inother systems [281,282]. Increasing the normal force resulted inchanging of the morphology of worn surface from slight plastic de-formation under low loading of 1.0 N to severe adhesive wear withscuffing and heavy plastic deformation and debris [283]. Tribologicalstudies of NieCo deposits and their composites in different conditionsof wear testing are listed in Table 5.

14. High-temperature oxidation and corrosion resistance

NieCo alloys have a combination of properties such as high hard-ness, formation of conductive oxides and high resistance to corrosionand wear in different media. Moreover, NieCo alloys are hardened bythe formation of a solid solution which is ductile after oxidation heattreatment [287,288]. Therefore, they can serve as suitable candidatesfor high-temperature applications. There have been numerous studieson the oxidation on pure Ni and pure Co, yet the oxidation behaviour ofNieCo alloys is scarcely investigated. The oxidation behaviour of pureNi and NieCo coatings containing 7, 15 and 30 wt% Co were comparedby a thermogravimetric method in open air at 960 °C for 18 h. XRDpatterns of oxidised coatings indicate that the oxide scales of Ni-7, 15Cocoatings were the only solid solutions of Ni(Co)O, but the scale ofNie30Co was a mixture of Ni(Co)O solid solution and (Ni, Co) Co2O4

spinel phase. According to Raman spectra, the top oxide layer ofNie30Co consists of Co3O4 phase. Study of thermal oxidation kineticsdemonstrated that the oxidation rate of alloy coatings was increased byraising Co content, and that the oxide growth followed a parabolic law[289].

Efforts have been made to improve high-temperature oxidation ofNieCo coatings through the addition of various nanoparticles such asZrO2. The oxidation study of Ni-Co-ZrO2 composite coating at 573, 873and 1173 K revealed lower weight gain of the coating compared toNieCo alloy coating [290]. In fact, ZrO2 nanoparticles decrease theeffective area for diffusion of oxygen, thus leading to lower oxidationrate. The oxide scale on the surface of Ni-Co-ZrO2 nano compositecoating was uniform and free of cracks [291]. High-temperature oxi-dation is often used as an expletory method for fabrication of spinelcoatings such as (Ni, Co) Fe2O4. This phase exhibits exceptional thermalstability and high-temperature conductivity. These coatings were pro-duced by composite electrodeposition of Ni-Co-Fe2O3 coating followedby the high-temperature oxidation process [292,293]. XRD patterns ofNi and Ni-Co-Fe2O3 coatings oxidised at 1000 °C for 3600 min showedthat the pre-oxidised Ni- Co-Fe2O3 coatings displayed higher resistanceto hot corrosion at 960 °C in molten salts of Na3AlF3-AlF3-CaF comparedto pure Ni [294]. However, it has been reported that for these coatings,the corrosion rate under sodium chloride at 800 °C and air conditionwas high due to weak adhesion of corrosion products [31].

However, the high-temperature resistant coatings such as diffusioncoatings and thermal barrier coatings are better than NieCo deposits.There have been numerous attempts in order to enhance the propertiesin Ni-Co-Al coatings by formation of Ni and Co aluminides [295]. Theproperties can be improved by additional treatments such as alumi-nizing to form aluminides, as reported in other systems [296].


487

15. Heat treatment of NieCo coatings

15.1. Physical properties

NieCo coatings were heat treated in the temperature range of200–800 °C. The important factors in the heat treatment of coatingsinclude annealing temperature, treatment duration and a coatingcomposition which impacts physical and mechanical properties. XRDpatterns of Ni-rich and Co-rich alloys annealed at different tempera-tures revealed that no change in the phase structure of Ni-rich coatingby increasing the heat treatment temperature. This is in agreement withthe previous studies [2]. In addition, by increasing the annealing tem-perature, FWHM is narrowed, which indicates grain growth within thecoating.

The grain size of Nie20Co was increased from 13 nm to 20, 47 and57 nm by annealing at 523, 623 and 823 K, respectively [297]. Athigher temperatures, crystallites grow randomly and non-uniformly. Byincreasing the temperature, the grains become larger, however only asmall change was observed in the lattice parameter of fcc structure[298]. This has also been demonstrated by SEM and TEM micrographsand bright field images [2,285]. Structural changes in Co-rich alloys aredifferent from Ni-rich alloys, as shown in Fig. 23). According to XRDpatterns, Co-5.5Ni has an hcp phase structure with 20 nm grain size.The volume percent of this phase is decreased by increasing the heattreatment temperature. Pure hcp phase was transformed to a mixedphase of fcc and hcp by annealing at 823 K. By increasing the tem-perature, fcc grains grow whereas hcp grains size was reduced to 7 nm.The austenitic transformation from hcp to fcc phase may be due tochemical composition variations in the fcc phase [297]. According toNieCo binary phase diagram, the allotropic austenitic transformationoccurs almost at the same temperature of approx. 700 K [299].

15.2. Thermal stability

Thermal stability of NieCo coatings is greatly affected by chemicalcomposition. Thermal stability is studied by differential scanning ca-lorimetry (DSC). Fig. 24a) shows DSC diagrams for monocrystallineNieCo coatings with different Co contents. All coatings exhibit a widerange of temperature from the beginning up to the peak temperature(Tp) with low heat flow. Values of Tp for NieCo alloys containing 37, 52and 74 wt% Co are 557, 591 and 596, respectively. The release of heatenergy in Tp is maximum, thus it can be used as a representative ofthermal stability. It can be concluded that the thermal stability of thecoatings is increased by raising Co content in the alloys. Scanning ofsamples in different heating rates is a kinetic resolution for growing of

grains in NieCo coatings. Nie74Co exhibited maximum Tp at variousscan rates including 0.17, 0.33 and 0.67 K.s−1 [298]. The activationenergy needed for the growing of grains is obtained from the re-lationship between Tp and heating rate, according to the Kissingerequation [300]:

bT

ERTp

Clnp2 = +

(36)

In which b is heating rate, E and R are activation energy of depositsfor growing of grains and universal gas constant respectively, and C is aconstant. The values of activation energy of Nie37Co, Nie52Co, andNie74Co were measured as 1.61, 2.02 and 2.08 eV, respectively [298].Consequently, NieCo coatings with higher Co content are more re-sistant to grain growth. This can be ascribed to segregation of Co atomsin grain boundaries, resulting in slower migration of boundaries.Figs. 24b) and 2 cc) compare the average grain size (dAV) before andafter heat treatment. Heating has resulted in larger grain size andcreation of twins.

In addition to the initial microstructure, other parameters such asimpurities lead to fluctuations in thermal stability. It has been reportedthat increasing the concentration of impurities such as sulfur increasedthe thermal stability of NieCo coatings [298]. This is due to an effecttermed solution drag effect. Sulfur is supplied from additives such assaccharin. It is suggested that sulfur impurities limit the grain growth.Thermal stability of NieCo coatings can be improved by adding a thirdalloying element. It is expected that adding alloying elements such asphosphorus and boron enhance grain growth resistance of NieCocoatings. However, investigations in this field are limited and the im-pact of the alloying element should be further studied.

15.3. Effect of heat treatment on mechanical properties

Studying grain growth via methods such as XRD and TEM is difficultsince grain size cannot be accurately measured. Changes in grain sizedue to heat treatment lead to a variety of properties such as hardness,toughness, electrical and magnetic properties. Annealing of NieCocoating containing 22–50 wt% Co in the temperature range of575–728 K for 60 min resulted in lower yield and ultimate strength, aswell as higher ductility and fracture toughness resulting from the in-creased grain size [62,301,302].

The microhardness of Nie50Co and Nie85Co is increased afterannealing in the range of 30–200 °C. This is due to the release of in-ternal stresses [30]. By heat treatment in the temperature range of200–400 °C, microhardness of NieCo alloy and Ni-Co-CeO2 composite

Fig. 23. XRD patterns of (a) Nie20Co coating and (b) Ni-94.5Co at different temperatures [297].


488

coatings is decreased as a result of recrystallization. At annealingtemperatures higher than 400 °C, microhardness is rapidly reducedwhich is attributed to the growth of grains in Nie25Co and Nie50Cocoatings. The unchanged microhardness of Nie85Co is due to thehigher thermal stability of this coating. The decrease in microhardnessof the coatings containing CeO2 is subtler than NieCo alloy coatings[303]. Nanoparticles have a pinning effect on grain boundaries. A si-milar trend has been observed in NieCo composite coatings with othernanoparticles [304]. In addition to mechanical properties, magneticproperties [2] and catalytic properties [28] are also impacted by heattreatment.

16. Electrocatalytic properties of NieCo surfaces

NieCo coatings are utilised as an electrocatalyst in the water elec-trolysis for two reactions [28]: (1) hydrogen evolution reaction (HER)and oxygen evolution reaction (OER). Investigations have shown thatamong active metals, Ni, Co, and their alloys are among the best cat-alysts for HER in alkaline medium. The effect of chemical compositionon HER has been studied in previous studies [305,306]. Fig. 25a)

illustrates HER activities of the coatings as a function of Co content byCV. NieCo coatings exhibit better HER activity compared to pure Ni.Alloys containing Co within the range of 28–95 wt% showed highercurrent density of HER. According to Fig. 25b), the current density isdecreased as a function of the cyclic number. It may be due to deal-loying [307] and delamination [308] occurring after several cycles. Infact, hydrogen bubbles are generated during the test and cover theactive sites. This leads to deterioration of catalyst performance. All thecoatings showed similar trends, and degradation behaviour is notchanged by Co content. In addition, the different decay ratios between−30 and −18% can be influenced by surface morphologies of coatings[26]. Study of discharge potentials for NieCo coatings with different Cocontent at room temperature and 40 °C have revealed that the coatingscontaining 41–64 wt% Co exhibited lower overvoltage, resulting inoptimal catalytic properties [309,310].

Tafel slope is essential in the determination of the controlling step inhydrogen discharge. According to the Tafel curves of Fig. 26, the slopeis changed from about −30 mV to −124 mV at 6 A dm−2. This in-dicates that hydrogen discharge is controlled by two reactions, as fol-lowing [306,309]:

Fig. 24. DSC plots showing the exothermal signal of grain growth as a function of scanning temperature (b = 0.17 K s−1) of the three NieCo alloys [283], (b) and (c)TEM dark field of Co-rich alloy with average grain size (dave) before and after annealing, respectively [218].

Fig. 25. Electrochemical characterisation of NieCo deposited on Cu from electrolytes containing 0.50 M NiSO4.6H2O and 0.01–0.50 M CoSO4.7H2O in 0.50 MH3BO3. (a) Cyclic voltammograms of Nie51Co alloy in 6.0 mol dm−3 KOH at 298 K as a function of cycle number at a sweep rate of 50 mV s−1. (b) Recorded currentdensities of Ni100, NiCo alloys, and Co100 at −1.6 V vs. SCE as a function of cycle number [26].


489

I) Electrochemical desorption (Volmer-Heyrovsky route):

M H O e MH OHads2+ + + (37)

MH H O e H M OHads 2 2+ + + + + (38)

II) Chemical desorption (Volmer-Tafel route):

M H O e MH OHads2+ + + (39)

MH MH H M2ads ads 2+ + (40)

Much effort has been made in order to produce NieCo deposits withimproved catalytic properties [311,312]. HER activity is directly re-lated to the surface area. NieCo coating with macroporous structurefabricated by electrodeposition at a high current density of 60 A.cm−2

exhibited remarkable electrocatalytic activity compared to the coatings

with a smooth structure [313]. Recently, porous NieCo coatings with asea cucumber-like structure have been fabricated through the evolutionof hydrogen-assisted electrochemical deposition process with superiorelectrocatalytic activity and stability [314].

17. NieCo microstructured and nanocomposite coatings

Incorporation of micro/nanoparticles in the metallic matrix resultsin a combination of properties which cannot be achieved independentlyby any other metallurgical methods. Uniform distribution of the na-noparticles such as alumina, SiC, diamond, TiO2, and WC can improvethe microhardness, wear resistance, corrosion resistance, high-tem-perature inertness, and anti-oxidation properties. As was previouslymentioned, nano- and micro-particles enhance microhardness, wearresistance and mechanical behaviour as a result of dispersion hard-ening. Furthermore, incorporation of nanoparticles increases the cor-rosion resistance. According to other researches, the corrosion re-sistance improvement of coatings containing nanoparticles is attributedto the trapped ceramic particles inside the coating [315]. Nanosizedceramic particles within the coating are electrochemically neutral andact as a physical barrier against penetration of corrosive ions, such aschloride ions, thus improving the corrosion behaviour. In other words,the neutral particles fill the grooves, cavities, and gaps, effectivelypreventing the start and the spreading of corrosion defects. For NieCocoatings, the effect of different particles is summarised in Table 6.

18. NieCo alloys with additional alloying elements

Various ternary alloying elements such as W, Fe, Cu, P, B, Mo andZn can be added to NieCo alloy coatings. Each of these elements pro-vides specific properties. Electrodeposition of ternary or quadruple al-loys is more complicated than electrochemical deposition of binaryalloys or individual pure metals. Various third elements utilised inNieCo coatings are summarised in Table 7. The most prominent thirdelements added to NieCo coatings are considered below.

Fig. 26. Tafel plots for corrosion of NieCo coatings of various compositions onan 8 cm2 aluminium mesh in (30 wt% KOH at 60 °C) [309].

Table 6Various micro/nano particle inclusions in NieCo coatings along with their effects.

Class Particle type Electrolyte Particle size(nm)

Particle amount in bath(g dm−3)

Present age incoating

Goal Ref.

Oxide particles Al2O3 Watts-type 500 20–60 – Improving corrosion/erosion resistance of pipesteels

[316]

SiO2 Watts-type 20–40 10–25 Improving corrosion resistance and microhardness [317]TiO2 Acetate nm 2–8 0–3.3 wt% The coatings with optimum amount of 2.5 wt%

TiO2 has higher corrosion resistance[76]

YZA Sulfamate μm 25 2–20 wt% Zr Improving thermal stability [304]CeO2 Sulfamate 20–30 25–100 – Improving thermal stability [303]Y2O3 Watts-type – 4 – Improving electrocatalytic activity [318]ZrO2 Sulfamate 20–30 25 12–24 vol% Improving thermal stability [319]

Carbide particles SiC Watts-type 40 0–8 0–6.21 wt% Lowering corrosion rate [258]Watts-type 50 30 6–10 vol% Investigating the effect of duty cycle and

frequency on SiC content[197]

Watts-type 50 50 SiC reduces the efficiency of deposition [320]Watts-type 50 15 Gradient (0.5–5.5 Improving wear resistance of functionally graded

NieCo coatings[321]

Watts-type 20 0–20 4.4–8.1 vol% Increasing the corrosion resistance [220]WC Watts-type < 1 μm 2–8 – Improving microhardness [322]

Nitride particles Si3N4 Sulfamate 1 μm 20 – Improving microhardness [323]TiN Watts-type < 3 μm 4–12 Increasing corrosion resistance and microhardness [324]

Other particles MoS2 Watts-type 10 1 – Enhancing wear resistance and lowering COF [283]CNT Watts-type nm 1 – Improving hardness and wear resistance [325]

Watts-type nm 1 – Lowering COF and enhancing wear resistance [279]Diamond Watts-type 6–12 μm 1–15 15.5–48.4 vol% High diamond particles by sediment co-deposition

led to lower wear rates[326]

Al Sulfamate 1–3 μm 25–75 12–23 wt% Improving thermal stability and high temperatureoxidation

[295,327]

Mixed particles Al2O3 + SiC Watts-type Al2O: 45SiC: 40

0–40 (Al2O3:SiC:1:1) SiC:1.2–2.0Al2O3:2.2–4.4

wt%

Mixed nanoparticles led to higher corrosionresistance compared to separate nanoparticles

[328]


490

Recently, Ni-Co-Fe ternary alloys have attracted a lot of attention asmagnetic materials since they exhibit improved magnetic and magne-toresistance properties, such as higher saturation induction and mag-netisation and lower coercivity compared to binary magnetic alloys[329]. Therefore, Ni-Co-Fe films have been introduced as a candidatefor MEMs [330–332]. Increasing the Fe content in the coatings led to anincrease in saturation magnetisation [331]. In addition, Ni-Co-Fecoatings have attracted attention due to their notable physical andchemical properties. It has been reported that these coatings possessedsuitable corrosion resistance and desirable electrocatalytic properties[259,333].

Tungsten, similar to P, Mo, and Ge is unable to be electrodepositedfrom the electrolyte on its own. In other words, W is a reluctant metalwhereas Ni and Co are inducing metals [21]. NieCo coatings containingW are deposited from acidic and alkaline baths. The acidic baths arepractically not worthwhile due to unsuitable deposits [21,334]. Thepresence of Co and W has beneficial effects on microstructure, corrosionresistance and mechanical properties [335]. Depending on the deposi-tion conditions, the microstructure of NieW and Ni-Co-W is eitheramorphous or crystalline, with a grain size in the range of nanometer[3,336–338]. High thermal stability of this coating results in the grainsize being maintained at high temperatures [339]. Increasing the Wcontent has been reported to enhance the microhardness [3,340].

NieP and Ni-Co-P films have been suggested as a replacement forhard chrome coatings, owing to their high wear resistance and hardness[15,341,342]. Ni-Co-P coatings are used in fuel cell [343,344] andredox flow cell [345] applications owing to their high corrosion re-sistance and good mechanical properties. However, phosphorus-con-taining coatings exhibit high COF in the range of 0.5–0.7 under drysliding, depending on the counterparts [346,347]. Previous reportshave stated that NieCo deposits have low friction coefficient (COF) andexcellent wear resistance [15,154]. Thus, it can be concluded that analloy combination of Ni, Co, and P (Ni-Co-P) will exhibit low COF andreduced wear weight loss. The structure of the coatings, depending on Pcontent, can be either amorphous, semi-crystalline or nanocrystalline[348]. The mechanical properties and tribological behaviour of Ni-Co-Pcan be improved following heat treatment. It has been reported[342,349,350] that heat treatment of NieP deposits at 400 °C con-siderably improves the hardness. Heat treatment changes the structureof the coatings from amorphous/semi-amorphous to the crystallinestructure. Furthermore, dispersed intermetallic compounds (such asNi3P, Ni12P5) are precipitated in the matrix. These compounds hinderthe movement of dislocation, resulting in higher hardness. The decreaseof hardness at higher temperatures (T > 400 °C) can be related tooxide formation, grain coarsening and phase transition [15].

Fig. 27a) demonstrates the microhardness of Co-Ni-P coating as afunction of heat treatment temperature for 1 h. Fig. 27b) shows the COFof Co-Ni-P coating heat treated at various temperatures. COF of as-de-posited and annealed coatings were measured to be about 0.3 comparedto 0.6 for hard Cr coating. The subtle COF of this coating can be relatedto the presence of Co in the coating. Microhardness and COF affect thewear rate of Co-Ni-P coatings (Fig. 27c). The lower wear rate of coatingannealed at 400 °C is a result of the highest hardness resulting fromprecipitation hardening by Ni12P5 intermetallic particles. Lower hard-ness and grain coarsening lead to higher wear rate of Co-Ni-P coatingannealed at 500 °C. However, it has been reported [351] that Ni-P-SiCcomposite coating heat treated at temperatures ≥400 °C demonstratedlower COF and wear rate. This is related to the formation of H3PO4

during wear test, creating a solid lubricant between the coating andcounterpart. According to literature [352–354], Ni3P reacts withoxygen and forms H3PO4, which has a great influence on sliding be-haviour. The annealing of the coating at higher temperatures leads tohigher amount of Ni3P, and thus formation of more H3PO4 [351].

Ni-Co-P coatings have been widely used as magnetic recording de-vices and hard magnetic coatings [355,356], corrosion resistance ma-terials [9,357], anti-wear coatings [15], microwave absorbers [358],Ta

ble7

Sum

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Sour

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nsity

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Ref.

Ni-C

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g.dm

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0g.

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and

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tives

FeSO

41.

865

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(DC)

[301

]N

i-Co-

W11

0g.

dm-3

NiS

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g.dm

-3N

iCl 2

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2O,2

0g.

dm-3

CoSO

4.7H

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0g

dm-3

Na 3

C 6H

5O7.

2H2O

,25

gdm

-3N

a2W

O4 .2

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,1g

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sacc

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SDS

Na 2

WO

46.

050

12.0

(PRC

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Ni-C

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135

g.dm

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6g.

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gdm

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240

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l 2.6

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H3B

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(CH

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Ni-C

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120

g.dm

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0g

dm-3

CoSO

4.7H

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44g

dm-3

ZnSO

4,30

gdm

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SO4

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25-

[306

]N

i-Co-

Sn83

g.dm

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iCl 2

.6H

2O,3

5g

dm-3

CoCl

2.6H

2O,1

1g

dm-3

SnCl

2.2H

2OSn

Cl2

(Ion

icliq

uid)

253.

0-5.

0(D

C)[7

3]N

i-Co-

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g.dm

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.5g

dm-3

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2.6H

2O,9

0g

dm-3

Cr2(

SO4)

3.6H

2O,2

0g

dm-3

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O3,

10g

dm-3

Na 3

C 6H

5O7,

10g

dm-3

C 6H

8O7

Cr2(

SO4)

32.

4-2.

825

6(D

C)[3

07]

Ni-C

o-Cu

158

gdm

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iSO

4.6H

2O,4

.465

6g

dm-3

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4.7H

2O,6

gdm

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SO4.

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,78

gdm

-3N

a 3C 6

H5O

7Cu

SO4

625

-2V

[308

]N

i-Co-

S1.

4g

dm-3

CoSO

4.7H

2O,1

.3g

dm-3

NiS

O4.

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,57

gdm

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thio

urea

-25

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to0.

2V

(CV)

[309

]


491

electrocatalytic films for hydrogen generation and water electrolysis[359,360].

19. Fabrication of novel NieCo deposits

The NieCo coatings studied so far have been simple monolayercoatings with a uniform chemical composition throughout the thicknessprofile. Recently, coatings with specific structural patterns, such asmultilayer and functionally-graded patterns, have been developedwhich exhibit excellent properties such as high corrosion resistance,low wear rate, and exceptional mechanical properties.

19.1. Functionally graded NieCo alloys and nanocomposite coatings

In these coatings, the Co or nanoparticles content is gradually variedfrom the substrate/coating interface to the surface. This is carried outby changing the parameters including current density, stirring rate,injection of nanoparticles or Co source in the bath, and changing ofpulse parameters such as pulse frequency and duty cycle [361]. GradedNieCo coatings were successfully fabricated by controlling cobalt sul-fate during the coating process and showed a gradual change in theamount of Co and an a phase transformation of fcc to hcp towards thesurface of the deposit. This coating demonstrated lower internal stressesand coefficient of friction, as well as higher wear resistance at hightemperatures [239].

Nanocomposite coatings with gradual structures have various ad-vantages which prevent degradation of various mechanical properties,including decreased ductility due to the difference in mechanicalproperties between the hard particles and the soft substrate.Functionally graded Ni-Co-SiC nanocomposite coatings were electro-plated by gradual variation at peak current density [321]. Fig. 28 showsthe chemical composition, SiC nanoparticles content and grain sizethroughout the thickness profile. It can be observed that by increasingthe current density from 2 to 15 A dm−2 during the coating process,grain size and Co content are reduced along the thickness profile.However, the SiC nanoparticles content is increased towards the sur-face.

The peeling test on these coatings indicated that they exhibitedenhanced adhesion to the substrate compared to uniform coatings[321]. In fact, the value of mismatching at the coating/substrate in-terface is reduced and less stress is created at the interface due to thegradual change in the number of nanoparticles. Due to higher micro-hardness and better adhesion of these coatings, the wear resistance andcorrosion resistance are enhanced [239,286,321,362–365]. Thus, lowinternal stresses and high adhesion of the coating to the substrate aretwo important factors in corrosion and tribological behaviour of coat-ings for prolonged working conditions.

19.2. NieCo multilayer coatings

Electrodeposition has various advantages over electroless techniqueincluding low cost, higher deposition rate, more stable baths, high ef-ficiency and flexibility (single layer or multilayer deposition as inter-mittent or gradient). NieCo multilayer coatings are able to be elec-trodeposited from one bath containing both Co and Ni ions by thealternative change of current density or pulse parameters such as dutycycle. The properties of the coatings are a function of layer thicknessand composition [366]. Fig. 29 shows the cross-sectional micrograph of

Fig. 27. Values of (a) microhardness, (b) coefficient of friction and (c) wear rate(under dry sliding wear conditions) of a Co-Ni-P coating as a function of heattreatment temperature [15].

Fig. 28. Variation of Co, SiC content and grain size of functionally graded Ni-Co-SiC coatings near the surface [321].


492

a multilayer NieCo coating with layers containing different Co content.The coating was electrodeposited by a periodic change in on-time andoff-time duration throughout electrodeposition.

For NieCo multilayer coatings having up to 150 layers, corrosionresistance was improved by increasing the number of layers. However,corrosion resistance was decreased in coatings having > 150 layers[367]. The layers act as a barrier to diffusion of corrosive ions such aschloride [368–370], in the coating with more layers, diffusion of in-terlayers has a negative impact on corrosion behaviour [366,371–373].In addition, it has been reported that increasing the number of layers inNi-Fe-Co multilayer coatings leads to higher microhardness and lowerfriction coefficient [366].

19.3. Electrochemical additive manufacturing

Electrochemical additive manufacturing (ECAM) is the combinationof localised electrodeposition and additive manufacturing principles forcreating novel additive manufacturing processes with several ad-vantages over conventional additive manufacturing [374]. Electro-deposition of NieCo alloys can be used in 3D-printing for two reasons:(1) these alloys are the most easily deposited of all alloys due to closestandard potential and high polarisation of deposits, and (2) electro-deposition is able to produce films at sub-micrometer and nanometervertical resolution. Fig. 30 shows a schematic of the equipment neededfor ECAM. MEMs and sensors are the most prominent devices that havebeen produced by additive manufacturing.

20. Modelling and simulation of NieCo electrodeposition

As mentioned in Section 3, NieCo deposits anomalously, i.e., theatomic fraction of Co in the coating exceeds the Co2+/(Co2+ + Ni2+)ratio in the bath. This system has been analysed in several studies, re-sulting in several models for deposition of iron group metals[86,92–94,96–98]. Steady-state models have been particularly suc-cessful.

According to the Zech model [86], the diffusion layer method isused in the absence of convection. It is assumed that there is an inter-mediate reaction involving adsorption of Ni and Co ions in their par-tially reduced form. Under these conditions, the material balance for Ni(II), Co(II), H+ and OH– ions is as follows:

N· 0Ni II( ) = (41)

N· 0Co II( ) = (42)

N N· · 0H OH =+ (43)

According to the Maltz model [97], inhibition of the more noblemetal is due to surface blocking by the adsorbed species.

Sasaki et al. [94] presented a diffusion model by using the compe-titive adsorption impact and blockage act of hydrogen atoms. Accordingto this model, the included metal mono-hydroxide adsorption from theelemental bath is based on a Langmuir isotherm, whereas from a binarypoint of view, NieCo baths obey competitive Langmuir adsorption[94]:

K CK C1i

i i

jn

j j1=

+ = (44)

By defining θo and kθo as the number of adsorption sites and a factorof pre-exponential kinetics, respectively, the rate constant is correctedas the following equation owing to the changing number of the surfacesites [94]:

k kK

K( ) ( )o alloy o elemental

H alloy

H elemental

,

,=

(45)

In this relationship, KH is the overall rate constant for a reactioninvolving two molecules. Surface coverage of the metals in elementalelectrodeposition is lower than that of metals in alloy electrodepositiondue to the adsorption deference between competitive Langmuir andsingle metal.

As recognised by the investigators, theoretical predictions of thesemodels are a function of the numerical values chosen for the para-meters. There is no quantitative fitting to the experimental result andonly numerical comparisons are presented. Vazquez-Arenas [106] havesuggested a model based on statistical data that fits the model to ex-perimental results for NieCo codeposition. In this model, the 1-D flux ofspecies Ni(II), Co(II), H+ and OH−, as follows:

N D Cy

v Cj ji

y j= +(46)

The solution velocity (vy) can be expressed by:

v v y0.51023y3/2 1/2 2= (47)

Under steady-state conditions, the balance of materials within thislayer are determined as below [106]:

N· 0Ni II( ) = (48)

N· 0Co II( ) = (49)

N N N· · · 0H OH B O OH( )3 3 4+ + =+ (50)

N N· 3 · 0B OH B O OH( ) ( )3 3 3 4 = (51)

The calculations indicate that the B(OH)3 flux controls the pH at thesurface during NieCo alloy deposition.

Fig. 29. Cross-sectional micrograph of NieCo multilayer coating with 10 al-ternative layers with various Co contents [367].

Fig. 30. Schematic showing the ECAM technique to produce layers.


493

21. Conclusions and future outlook

This review has considered NieCo electrodeposited coatings andtheir scope, the mechanism of deposition, deposition techniques,parameters affecting microstructure and alloy composition, depositproperties (physical, mechanical, magnetic, corrosion, high tempera-ture oxidation features and thermal stability of NieCo deposits) as wellas possible structural patterns in NieCo coatings. The unique and sui-table properties of NieCo alloys and nanocomposite coatings renderthese coatings a desirable choice in corrosion, wear, mechanical andelectrocatalytic applications. Modification of NieCo coatings by struc-tural patterning, such as functionally graded and multilayer NieCocoatings, improves the corrosion and tribological properties. This re-view has endeavored to provide a summary of collated information onimportant parameters, electroplating baths, the mechanism of NieCodeposition, methods of electrodeposition and the resultant properties ofNieCo alloys and composite coatings. Aspects requiring further R & Dcan be listed:

1. As discussed in Sections 8.2 and 8.3, pulse and pulse current reversalelectrodeposition can produce coatings with uniform microstructureand chemical composition. Applying anodic current in pulse reverseplating lowers hydrogen reduction and internal stresses. However,there is scarce and contradictory information regarding the de-position mechanism in PRC methods and its effect on anomalousdeposition of Co in these coatings.

2. Incorporation of nanoparticles in the coatings leads to enhancedcharacteristics such as better corrosion resistance, higher hardnessand wear resistance, suitable mechanical properties, better thermalstability and oxidation resistance. Development of strategies such as

the deployment of horizontal electrodes to increase the nanoparticlecontent in the coatings may improve the properties of NieCo coat-ings. Usage of a mixture of nanoparticles with various purposes(such as hard particles with self-lubricating particles) in the elec-trolyte facilitates the deposition of multi-purpose coatings.

3. Electrocatalytic properties. As seen in Section 14, NieCo coatingsexhibit better electrocatalytic properties than pure Ni. Hydrogenproduction using NieCo coatings can be improved by the im-plementation of additional steps, and adding various additives in theNieCo electrolytes such as (1) producing special morphologies, suchas nanocones, via additional crystal modifiers such as polyethyleneglycol, and adding sulfur and phosphorus sources to the electrolytefor electroplating Ni-Co-sulfide and Ni-Co-phosphide coatings. Me-tallic sulfides and phosphides are excellent electrocatalysts[375,376]. These characteristic can be improved further with theaddition of nanoparticles to the coatings and employing PRC elec-trodeposition.

4. Modifying structural patterns. As seen in Sections 19.1 and 19.2,functionally graded and multilayer structures have been successfullyfabricated. Electrodeposition of NieCo multilayer coatings with thinlayers in the range of nanometer will improve corrosion resistanceand mechanical properties such as toughness, hardness and wearresistance.

5. Thermal stability and high-temperature applications. Thermal sta-bility of NieCo coatings can be improved with the addition of athird alloying element such as phosphorus and boron. However, thehigh-temperature resistant coatings such as diffusion coatings andthermal barrier coatings are much better than NieCo electro-deposited coatings. The properties can be improved further by ad-ditional treatments such as aluminising, resulting in nickel and

Fig. 31. Possible future applications of NieCo alloy coatings.


494

cobalt aluminides.6. Additive manufacturing. The electrodeposition of iron-group metals

including Fe, Ni, and Co is probably the easiest alloy system.Therefore, NieCo coatings are a suitable candidate for production ofspecial members via additive manufacturing. Electrodeposition ad-ditive manufacturing is now routinely used for the development ofcertain printed circuit boards, integrated circuits, and hard drivecomponents. Fig. 31 indicates probable future applications of NieCodeposited alloys across diverse industrial sectors.

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