deposition of high wear resistance of ni-composite coatings

7/29/2019 Deposition of High Wear Resistance of Ni-Composite Coatings

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As new mater ials, composite coatings offer

many attractive proper ties that can b e very

useful for various engineering applications

such as machine construction and aero-

engines. C omposite coatings are produ ced by

direct entrapm ent of solid particles dur ing the

build-up of the m etal matrix by electro- or

electroless deposition[1].

In ord er to reinforce the mechanical or

antifrictional properties of the coating, the

foreign par ticles can be of refractor y[2-4] or

dry-lubr icant nature[5,6]. T he properties of

the coat ings are govern ed by the type and size

of the part icle, its content in the coating and

the mode of distribution. T he addition of

other alloying elements such as phosphoru s to

the metal matr ix (N i) adds to the improve-

ment of properties. Electroless Ni-P coatingsare characterized by their superior hardn ess

and corrosion resistance. G enerally, the elec-

troless plating process is ham pered by its slow

rate of deposition, complicated op eration and

high cost. Instead, electrodeposition of Ni-P

comp osite coatings is faster, th e solution is

more stable and has fewer replenishment

problems.

T he present work was under taken to eluci-

date the mechanism and principal factors

affecting the incorporation of the solid parti-

cles into a growing deposit. T he aim was to

improve the plating process technology and to

modify the coating properties such as wear

resistance. Most of the work was carried ou t

on N i-P-SiC composite coatings.

Experiment al procedure

Plating solution (Watt’s bath)

(1) Composition

NiSO4.6H 2O 300 g/l

NiCl2.6H 2O 60 g/lH 3BO3 40 g/l

NaH 2PO2 5 g/l

Dispersed particles:

• Mater ial SiC , Al2O

3, sand, quar tz

• Content in solution 20-100 g/l

• Particle size < 7 µm

(2) Operating conditions:-

Temperature 60°C

pH 4

Current density 2.5-10 A/dm2

Mechan ical st ir ring 150 rpm

Wear measurements

For m easurement of wear resistance, the

coated samples (20 × 50 × 1 mm ) were rotated

17 8

Ant i-Corrosion M ethods and M ater ials

Volume 44 · Number 3 · 1997 · pp. 178–185

© M CB Un iv ersi ty Press · I SSN 00 03 -5 59 9

Saher Shawki and

Z. A bdel Hamid

The authorsSaher Shaw ki an d Z. Abdel Hamid are at the Central

M etallurgical R&D Institute, Helwan, Cairo, Egypt.

Abstract

Electrodeposited Ni-P composite coatings incorporating a

variety of inorganic particles w ere obtained from Watt ’s

nickel bath containing sodium hypop hosphite. The mecha-

nism of co-deposit ion of various particles (SiC, Al2O3,

quartz and sand) was studied in view of the electro-kinetic

charge characterizing th e solid particles. M eans to

improve the mob ility of the particles in the plating solution

w ere investigated u sing sodium oleate as surface active

agent. The purpose was to increase particle content in t he

coating t o att ain high hardness values. Special attention

w as given to the deposit ion process using SiC particles.

The surface morp holo gy, hardness and w ear resistance of

the com posite coatings w ere determined. Hardness values

w ere maximized by simple heat treatm ent in air atmos-

phere which led to the precipitation of the hard Ni3P

phase. Sound, coherent and h igh w ear resistance coatings

could be produced.

Depo sit ion of highw ear resist ance of Ni-

com posit e coat ings

Contribut ed pape rs



tangentially in a sand/water (ratio 4:2)[7].

T he rotation speed was kept constant at 150

rpm and the wear effect was measured in

term s of loss in weight expressed as µ /hr.T he

wear measurements were performed to be

within the th ickness of the coating without

reaching to the base metal.

Zeta-potential

T he zeta meter was used to determ ine the

direction and speed of the solid p articles

moving in a dilute plating solution between

two electrodes under an applied electric

potential difference. T he sign and magnitude

of the potent ial (ζ ) of the solid part icles are

related to the mobility of the par ticles in

solution.

Results and d iscussion

Ni-P - particle com posites

Preliminary experiments showed that N i-P

composites containing 4.5 wt. per cent ph os-

phorus could be deposited from Watt’s nickel

bath with the addition of 5 g/l sodium

hypophosphite. T he solid particle inclusion in

the coating (2-4.5 wt per cent for SiC) was

found to depend on t he amount of the parti-

cles in the electrolyte (20-80 g/l).

T he effect of current density on the

amount of particle inclusion is illustrated in

Figure 1, which depicts that th e part icle

content in the coating increases with the

increase of applied current. While deposition

is very slow at cur rent density (c.d) less than 2

A/dm2, deposits with c.d. higher than 7.5

A/dm2 are brittle and can be spalled off at the

edges of the substrate becau se of the bu ilt-in

high internal stress in the coating. T he results

indicated that the mobility of solid par ticles in

the electrolyte is increased by the appliedelectric curren t.

T he growth of the N i-P-SiC coating is

almost regular during th e deposition process,

as shown in F igure 2. T he deposition rate at a

certain current d ensity is not affected by the

amount of solid part icles in plating solution as

indicated b y the results given in Figure 3 for

SiC and Al2O3. T he deposition rate is known

to be controlled by the composition and

operat ing conditions of the plating electrolyte

such as c.d, pH value and temp erature.

Mechanism of deposition

T he precise nature of the process of co-

deposition and the arr ival to the cathode of

the solid par ticle was subject to con troversy.

T hree main mechanisms were previously

suggested[1] to explain the difference in the

ability to deposit various types of solid par ti-

cles: electrophoresis, mechanical entrapment

and physical adsorption

In this research it was initially thought that

the inclusion of solid par ticles depends largely

on their mobility and electrokinetic natu re in

the plating solution. T he work described

below has been designed to elucidate the

aspect of the deposition process on the basis

of the electrokinetic mechanism.

Solid par ticles in solutions are electrolyti-

cally charged by adsorbing ions on their sur-

faces. Th e sign and magnitude of the elec-

trolytic charge is known as zeta potent ial (ζ ).

T he values of zeta potential for different

particles were determ ined in d ilute solution of

pH 4 containing all the plating species. T he

results indicated that the various particles are

negatively charged with a magnitude ranging

from –35 to –60 mV. F igure 4 demon stratesthe dependence of the amount of incorporat-

ed part icles on the magnitude of ζ potential.

T he m echanism of solid par ticle electrode-

position cou ld be suggested as follows: posi-

tively charged N i ions in solution are adsorbed

on negatively charged solid par ticles. T he

particle with adsorbed ions m igrates to the

cathode where metal ions are reduced to N i

atoms forming the coating with th e entrapped

solid par ticle occupying a place in the metal

(or alloy) matr ix. A schemat ic diagram illus-

trating the mechanism is shown in F igure 5.

T he results shown in Figure 4 indicated that

the higher the magnitude of the charge on the

part icle (ζ potential), the lower the am ount

179

Deposit ion of high wear resistance of Ni-composite coatings

Saher Shaw ki and Z. Abdel Ham id


Volume 44 · Number 3 · 1997 · 178–185

Figure 1 Effect of current density on SiC content of t he composite (particle

content in plating solution, 80g/l)



deposited in the coating. This can be explained

on the basis of the following assumptions:

• Particle/ion mobility: with high ζ potentiala greater number of Ni ions are adsorbed

on the surface of the solid particle. Th e

aggregates of Ni ions surround ing a part i-

cle behave as one large charged body sus-

pended in the electrolyte. T he mob ility of

the later formation is, therefore, expected

to decrease under the heavy weight of the

coupled formation. T he result will be a

smaller number of solid particles reaching

the cathode.

• Relative amount occluded: since Ni ions

surround ing each particle are immediately

reduced to N i atoms at the cathode surface,

the relative amount of par ticles of high ζ

potential (quartz, sand) incorporated in the

18 0


Saher Shaw ki and Z. Abdel Hami d


Volume 44 · Number 3 · 1997 · 178–185

Figure 3 The effect of particle content in the electrolyte on the deposit ion rat e of composite coatings

Figure 4 Effect of Zeta potential on the p ercentage of particle in th e composite

Figure 2 Effect of plating t ime on the coating t hickness of composite coatings



coating will be less than the amount of

par ticles of lower potential (SiC, Al2O3). In

solution, the later type of part icles are

surrou nded by fewer Ni ions; the result is

high content of particles in the coating.

Although both assumptions may be valid

together, the aut hors are in favour of the

particle/ion m obility. For confirmation of that

view a number of experimen ts were carried

out u sing additives of anionic surfactant

(sodium oleate) in variable concent rations.

T he aim was to overcome the gravitational

effect of the assumed heavy aggregates of ions

entrapping the solid part icle. T he results of

surfactant addition showed an increase in th e

amoun t of part icles deposited in the coating

by the increase of the concentration of sodium

oleate up to an op timum value of 5 × 10 -3

mole/l (F igure 6) . H igher concentrations of

oleate did not yield a correspond ing increase

in the SiC in the coating.

Na oleate is known to be p resent in acidic

solutions of pH 3-7 in a polarized form[8]. In

a plating solution of pH 4 N a oleate is

adsorbed on the surface of SiC par ticles. T he

following mechan ism can be suggested as an

interpretation for the results shown in

Figure 6.

T here exists in solution a competition

between N i and N a ions (both positively

charged) to cover the n egatively charged SiC

par ticle. M eanwhile, the oleate group , with

its -CO O - part, are attached to the adsorbed

Ni ions. T he surface of the solid par ticle is

then par tly covered with th e oleate while

the tail (-R) han gs in t he solution bulk

(Figure 7).

With increased amou nts of Na oleate a

larger area of the particle’s surface is coveredwith N a+ at the expense of the area covered

with N i++. O n reaching the cathode surface,

nickel (and phosphor us) will be deposited

with a higher content of SiC. T he mecha-

nism can thu s justify the ascending par t of

the relation shown in Figure 6. H owever, the

optim um values of SiC con tent are explained

as being du e to adsorption of only one

mon olayer of the oleate, which hard ly com-

petes with N i ions.

To confirm the later idea, SiC part icles

were treated separately with the surfactant

(withou t Ni ions) prior to addition to the

18 1




Volume 44 · Number 3 · 1997 · 178–185

Figure 5 Schematic diagram of t he mechanism of solid particle mobilit y in

the plating electrolyte

Figure 6 The effect of surfactant concentration on th e content of SiC in the

deposit

Figure 7 Schematic diagram of the mechanism of

adsorption of surfactant (Na-oleate) on a solid particle



plating solution (cond itioning). T he result

(shown in F igure 6) was the deposition of SiC

in a smaller amoun t than that obtained with

optimum value of the surfactant.

Propert ies of compo site coatingMorphology and structure

T he deposited Ni-P-SiC coatings were dark

grey in colour, rough to the touch an d firm ly

adherent to th e base metal. T he roughness

was affected by the amoun t and size of SiC

particles.

Plates 1-3 gives scanning electron micro-

scopic views of the surface of Ni-P composites

with SiC, sand and Al2O3 inclusions. T he fine

par ticles were dispersed hom ogeneously in

the matr ix.Plate 4 is a photomicrograph of unetched

cross section of the as-plated N i-P-SiC coat-

ing (4.3 wt per cent SiC) which shows that the

SiC part icles (less than seven microns) are

evenly distributed in the deposit.

Hardness and wear

T he properties of the coatings are generally

related to and controlled by the amount of

solid part icles in th e coating. T he microhard-

ness tests on the coatings are affected by the

solid par ticles’ inclusion. T he resulting inden-

tation is not a m easure of the p lastic deforma-

tion of the alloy material but the indenter

force is diverged by the embed ded r igid and

uncom pressible particles. For th is reason the

examination of hardn ess of various coatings

was restricted to the assessment of surface

hardness and n ot to the coating profile struc-

ture (cross-section). T he effect of SiC content

in the electrolyte (20-100 g/l) on the hard ness

of two types of deposits (N i-SiC and Ni-P-

SiC) is shown in F igure 8, cur ves (a) and (b )respectively. T he hardn ess values of the

comp osites increase with the increase of SiC

in the coating.

In th e as-deposited N i-P coating, P exists

as super-saturated solid solution in the n ickel

matr ix. On heat treatment at 400°C for one

hour the coating hardn ess was increased from

400 to 500 VHN as a result of the precipita-

tion of Ni3P phase[9]. T he values, however,

depend on the P content in the alloy.

T he incorporation of SiC into Ni-Pdeposits led to further increases in the hard -

ness values when subjected to the same heat

treatment cycle (Figure 8 cu rve (c)). A value

up to 1,200 VHN could be reached with SiC

182




Volume 44 · Number 3 · 1997 · 178–185

Plate 1 Surface morpholo gy of composite coatings,

Ni-P-SiC


Ni-P-Sand


Ni-P-Al2O3

Steelsubstrate

Sampleholdingresin

Composite

coating

Plate 4 The phot omicrograph of cross section of the N i-P-SiC coating



content in the coating of 4.3 wt per cent. T he

drastic increase is due to formation of N i3P

phase as ident ified by X-ray diffraction analy-

sis. Figure 9 shows a compar ison between the

X-ray pattern for the as-plated and heat t reat-

ed N i-P-SiC coatings.

T he wear reistance and h ardn ess of elec-

trodeposited composite coatings were found

to be related to each ot her as well as to the

particle content in the coating. C oatings with

maximum hardness exhibited highest wear

resistance. A tangential mot ion wear test in

sand/water mixture was used to assess the

resistance to sliding and abrasive wear. Th e

weight loss of the coated sample was deter-

mined and the relation between the test time

and loss in coat ing thickness ( µ /hr) was

drawn. Figure 10 shows a group of the

straight line relationships obt ained with

various coatings. T he abrasion resistance

was estimated by calculating the reciprocal

of the slope of the straight line. T he results

are shown in the collective diagram, F igure

11, which indicates the following generalfeatures:

• Phosphorus deposition: slightly increased the

wear resistance of Ni electrod eposits (coat-

ing B compared to A);

• SiC inclusion; considerably increased the

wear resistance of the as-plated N i-P coat-

ings (coatings D and E ); and

• Heat treatment: further imp roved the wear

resistance of the composite coatings (coat-

ings C and F ).

183




Volume 44 · Number 3 · 1997 · 178–185

Figure 8 The effect of SiC (wt . per cent) in the coating on th e hardness of

deposits

Figure 9 The X-ray diffraction patt ern for Ni-P-SiC compo site: withou t heat treatm ent and w ith heat treatm ent



T he highest values of wear resistance could beachieved by heat-treated N i-P-SiC coatings.

Conclusions

• Ni-P composite coatings can be electrode-

posited from Watt’s Ni solution containing

Na hypophosphite and u ltrafine particles of

SiC, Al2O3, sand or quartz.

• T he particle content in the plating solution

and the applied c.d. were found to be

important factors in cont rolling the inclu-

sion of the solid part icles into the deposit.

T he amount was, however, limited by the

volume conten t of the part icles that can be

accommodated in the m etallic matrix of the coating.

• T he co-deposition of solid particles with

Ni-P coatings was explained on the basis of

the m obility and electrokinetic natu re of

the part icles in the electrolyte.

• Under the same operating conditions, the

amou nt of part icles deposited in the coating

could be increased from 4 to 9 wt per cent

by the addition of surface active agents of

anionic type to the plating solution.

• It has been shown that the coating proper-

ties such as hardness and wear resistance

were related and controlled by the am ounts

and distribution of the solid par ticles in the

coating matrix. T he hardn ess of Ni-P-SiC

18 4




Volume 44 · Number 3 · 1997 · 178–185

Figure 1 0 The relationship betw een the test t im e and loss in coating th ickness

0 50 100 150 200 250

0 250 500 750 1,000 1,250

Wear resistance X10 2 ( m/hr) –1

VHN

µ

Ke y

Wear resistance

VHN

A

B

C

D

E

F

Ni

Ni-P as plated

Ni-P heat treated

Ni-SiC

Ni-P-SiC as plat ed

Ni-P-SiC heat t reated

Figure 1 1 Hardness and w ear resistance of the com posite coatings compared t o ot hers electrodeposited



coating could be increased by heat treat-

ment t o values over 1,200 VHN . T hese

coatings are coherent and characterized by

high wear resistance which can be of

benefit in several industr ial applications.

Attention should be given to th e internal

stresses and the m eans of releasing these

stresses, which may hamper the coating

performance if they are subjected to post-

mechanical working operations.

References

1 Celis, J.P., Roos, J.R., Buelen s, C. and Fransaer, J.,

“ M echanism of e lectro lyt ic composite p lat ing: survey

and t rends” , Transactions of th e Institu te of M etal

Finishers, Vol. 69 No. 4, 1991, p. 133.

2 Poeton, A.R., “ Composite coat ings for advancedperformance” , M etals & M ater ia ls, No. 702, 1988.

3 Per iene, N. , Cesuniene, A. and M atul ionis, E. , “ Code-

posit ion of mixt ures of dispersed particles wit h nickel-

phosphorus electrodeposits” , Plating & Surface

Finishing , Vol. 81 No. 10, 1994.

4 Roos, J.R., Celis, J.P., Fransaer, J. and Buelens, C., “ The

development of com posite plating for advanced

mater ia ls” , Jouirnal of M etals , No. 11, 1990, p. 60.

5 Cowieson, D.R., Sadowska-Mazur, J. and Warw ick,

M .E., “ Codeposit ion of non-metall ic particles w ith t inand t in-n ickel a l loy” , Proceedings of 3rd Int ernational

Congress for Surface Technolog y , Berlin, 1985.

6 Lipp, L.C.” Sol id lubr icants – their advantages and

l im i tat ions” , Lubrication Engineering , Vol. 32 No. 11,

1976, p. 574.

7 Soror, T.Y., “ Electroless deposit ion of nickel as related

to th e structure, morphology and properties of the

coati ngs” , PhD Thesis, Cairo University, 1995 .

8 Arbi ter, N. and Wil l iams, E.K.C., “ Condit ioning in o le ic

acid flotat ion” , in Fine Particle Processing , SME-AIME,

USA, Vol. 1, 1980 , pp. 802-31.

9 Changgeng, X. , Zonggeng, D. and Li jun, Z. , “ The

properties of electrodeposited Ni-P-SiC compo site

coat ing ” , Pating & Surface Finishing, Vol. 75 No. 10,

1988.

185




Volume 44 · Number 3 · 1997 · 178–185

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deposition of high wear resistance of ni-composite coatings

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