ioe

(AlCrMoSiTi)N coatings

stresses for applied biases of 100 V or more. A good correlation between the residual stress and lattice constant under various depositionconditions is found. For the coatings deposited at 100 V, and at temperatures above 573 K, the hardness could attain to the range of 32 to 35 GPa.

systems [17]. However, based on the principle of high-entropyalloys (HEAs), which contain at least five principal elements in

sputtering from HEA targets, namely FeCoNiCrCuAlMn,FeCoNiCrCuAl0.5, FeCoNiCrCuAl2 and SiNiCrTiAl targets, itwas found that the coatings possessed relatively low hardnessesof 15 GPa or less [14,15]. The low hardnesses of these coatings

Available online at www.sciencedirect.com

20near equimolar proportions, and the promising properties ofsuitably designed HEAs, such as (i) good thermal stability,Even after annealing in vacuum at 1173 K for 5 h, there is no notable change in the as-deposited phase, grain size or lattice constant of the coatingsbut an increase in hardness. The thermal stability of microstructure is considered to be a result of the high mixing entropy and sluggish diffusion ofthesemulti-component coatings. For the anneal hardening it is proposed that the overall bonding between target elements and nitrogen is enhanced bythermal energy during annealing. 2007 Elsevier B.V. All rights reserved.

Keywords: Nanostructure; High-entropy alloys; Nitride coatings; Sluggish diffusion; Anneal hardening; Thermal stability

1. Introduction

The majority of work pertaining to vacuum deposited nitridecoatings appears to be limited to ternary or, at most, quaternary

(ii) high hardness and (iii) superior corrosion resistance [813],it is envisaged that such multi-element materials may beadvantageous in the use as protective coatings. In an initialstudy of nitride coatings deposited by reactive magnetrona Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwanb Institute of Materials Science and Manufacturing, Chinese Culture University, Taipei 111, Taiwan

c Nano-Powder and Thin Film technology Center, Industrial Technology Research Institute, Tainan 70955, Taiwan

Received 19 September 2007; accepted in revised form 11 December 2007Available online 23 December 2007

Abstract

Nitride films are deposited from a single equiatomic AlCrMoSiTi target by reactive DC magnetron sputtering. The influence of the substratebias and deposition temperature on the coating structure and properties are investigated. The bias is varied from 0 to 200 V while maintaining asubstrate temperature of 573 K. And the temperature is changed from 300 to 773 K whilst maintaining a substrate bias of 100 V. From X-raydiffraction analysis, it is found that all the as-deposited coatings are of a single phase with NaCl-type FCC structure. This is attributed to the highmixing entropy of AlN, CrN, MoN, SiN, and TiN, and the limited diffusion kinetics during coating growth. Specific aspects of the coating, namelythe grain size, lattice constant and compressive stress, are seen to be influenced more by substrate bias than deposition temperature. In fact, it ispossible to classify the deposited films as large grained (~15 nm) with a reduced lattice constant (~4.15 ) and low compressive residual stressesfor lower applied substrate biases, and as small grained (~4 nm) with an increased lattice constant (~4.25 ) and high compressive residualHui-Wen Chang a, Ping-Kang Huang a, Jien-Wei Yeh a,,Andrew Davison a, Chun-Huei Tsau b, Chih-Chao Yang cInfluence of substrate bias, depositannealing on the structure and prop

Surface & Coatings Technology Corresponding author. Tel.: +886 3 5742621 33868; fax: +886 3 5722366.E-mail address: jwyeh@mx.nthu.edu.tw (J.-W. Yeh).

0257-8972/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.12.014n temperature and post-depositionrties of multi-principal-component

2 (2008) 33603366www.elsevier.com/locate/surfcoatcan be associated to the high content of non-nitride formingelements and their amorphous structures. It was found that thehardness of virtually stoichiometric nitride coatings deposited

energy of the ions flux [19]. As shown in Fig. 1(b), theroughness is seen to be around 0.3 nm and 1.25 nm at the lowerand higher deposition temperatures, respectively. The increasein surface roughness at the higher deposition temperatures issuggested to result from the grain growth effect. Nevertheless,as the original substrate surface roughness is around 1 nm andthe film thicknesses are about 1.5 m, all these measuredroughnesses are low.

3.1.2. XRD

3.1.2.1. Crystal structure. From XRD analysis, shown inFigs. 2(a) and (b), it is found that all of the coatings, which haveclose to the same amount of each target element and stoichiometricnitride compositions, i.e. ~Al0.2Cr0.2Mo0.2Si0.2Ti0.2 N, exhibitonly a single FCC structure of NaCl-type. This indicates that FCCstructure has a large accommodation of other non-FCC binarynitrides since only TiN (a=4.24 ) and CrN (a=4.14 ) are FCCnitrides and AlN (HCP), MoN (HCP) and Si3N4 (hexagonalstructures, which are built up by corner-sharing SiN4 tetrahedra)are non-FCC ones.

It has been determined that as-deposited mixtures of FCCforming ternary nitrides, i.e. ZrxTi1 xN [2], CrxTi1 xN [2,4],MoxTi1 xN [4], NbxTi1 xN [4], and WxTi1 xN [4], have a single

Fig. 1. Surface roughness as a function of (a) substrate bias and (b) depositiontemperature.

ingsfrom HEA targets with only nitride forming elements, that isAlCrTaTiZr [16,17] and AlCrTiSiV [18], had increased hard-nesses of about 35 GPa and 30 GPa, respectively. From X-raydiffraction (XRD) studies, these stoichiometric nitride coatingswere also found to exhibit a single FCC structure [1618].

In the present work, nitride coatings shall be deposited froma single equiatomic AlCrMoSiTi target by reactive DC magne-tron sputtering. The influence of deposition temperature, sub-strate bias, and post-deposition heat treatments in vacuum onthe structure and properties of the coatings shall be assessed.

2. Experiment

The Si (001) wafer substrates were cleaned sequentially inacetone, pure alcohol and de-ionized water. The substrates wereplaced approximately 12 cm from the target on the centre of asubstrate table that was rotated at a speed of 5 rpm. Prior todeposition, both the substrate and target were independentlysputter cleaned by means of a shutter placed between them. Thefilms were deposited from an equiatomic Al20Cr20Mo20Si21Ti19target by DCmagnetron sputtering operated at a power of 150W.The base pressure of the systemwas better than 2.7103 Pa. Theworking pressure was 0.67 Pa, with the nitrogen and argon gasflow rates each fixed at 20 sccm.

When investigating the influence of substrate bias (0(grounded), 50, 100, 150, and 200 V), the depositiontemperature was held at 573 K. The current densities, respectiveto the aforementioned voltages, were recorded to be 0.14, 0.33,0.36, 0.39, and 0.40 mA/cm2. Likewise, the substrate bias wasfixed at 100 V (0.36 mA/cm2) when investigating the affectsof deposition temperature (300, 473, 573, 673, and 773 K).Annealing was carried out for 5 h in an evacuated quartz tube(1.3 Pa).

The film thicknesses, measured by an -step device, werewithin the ranges of 1.1 to 1.4 m and 1.4 to 1.7 m whenexamining the effects of substrate bias and deposition tem-perature, respectively. The structure of the films was character-ized using X-ray diffraction (MAC Science MXP18 XRDspectrometer) with a glancing angle of 1, using a copper target(k1 wavelength=0.15406 nm). The composition of the filmswas determined by an energy dispersive X-ray analyzer(EDXA) (JEOL JSM-5410) and electron microprobe analyzer(EPMA) (JEOL JAX-8800). The hardness and modulus of thecoatings were measured by nano-indention (XP System MTS).The stress was determined using the substratecurvaturetechnique and applying Stoney's equation.

3. Results and discussion

3.1. As-deposited coatings

3.1.1. TopographyIn Fig. 1(a) it is seen that the roughness decreases with

substrate bias until it reaches a minimum of about 0.3 nm

H.-W. Chang et al. / Surface & Coatat 100 V. The decrease in surface roughness with bias isascribed to an increase in the atomic movement and densifica-tion of the coating material as a result of the increased flux and3361Technology 202 (2008) 33603366FCC solid-solution structure across the entire composition range.Additionally, a single FCC solid-solution has been reported toform for approximately 60 to 70% of Al in both TixAl1 xN and

ings3362 H.-W. Chang et al. / Surface & CoatCrxAl1 xN coatings, in which Al atoms are considered to occupyTi lattice sites [2,3]. Single as-deposited FCC structures have infact been predicted in various theoretical works for a highproportion of AlN mixed with an FCC forming nitridecomponent, i.e. TiAlN, CrAlN, ZrAlN, etc. [2024]. Never-theless, there appears to be some inconsistencies for both thetheoretically predicted and experimentally determined solubilitylimits of HCP AlN in an FCC structure. For example, it waspredicted byMakinoa [23] that the solubility limits of AlN withinthe FCC structures of HfN, ZrN, NbN, and WN are only 21, 33,53, and 54%, respectively. These values given for HfN and ZrNare far lower than those predicted by Holleck [20], which was inthe range of about 60 to 70% for both. From deposition studies ofZrAlN by Makinao, it was determined that the solubility limit ofAlN in FCC ZrN was 30 to 35%, which is in agreement to hispredicted value. However Holleck [20] and Sanjines [24] reportedthat even up to 42 and 50%, respectively, of AlN in ZrN a singleFCC was observed these aluminum contents being themaximum studied. The contradiction between these works mayoriginate from the deposition conditions employed; for example,Makinao [25], who reported a particularly low-solubility limit ofAlN in FCC ZrAlN, deposited the films under unconventionallyhighly-enhanced deposition conditions.

Although there appears to be a general lack of informationpertaining to the thermal stability of single-phased FCC as-

Fig. 2. XRD patterns of the coatings deposited at different (a) substrate biasesand (b) substrate temperatures.deposited ternary coatings, phase separation after annealing inthe TixAl1 xN [26] and ZrxTi1 xN [27] systems, at about1200 K, reflects that many of them are likely to be of a meta-stable nature. The tendency to deposit coatings with a singlemetastable FCC nitride phase has been considered to be a resultof limited diffusion kinetics, in which the elements cannot reachtheir stable configurations [20]. However, the single as-depo-sited FCC phase of the present nitride is not simply due tokinetic reason. It is believed to also relate with the high mixingentropy effect since the mixing of five kinds of different binarynitrides to form the FCC nitride solution might give largemixing entropy to counterbalance the positive mixing enthalpydue to lattice distortion and different crystal tendency, and thusinhibit the phase separation. This preference of FCC solutionphase could be supported by its thermal stability with which itdidn't show any decomposition under XRD resolution leveleven after annealed at 1173 K for 5 h, as will be seen in Section3.2. Although mixing entropy effect is expected to be smallerat lower temperatures and favor phase separation, sluggishdiffusion effect due to five different species and lower thermalenergy is involved to retard phase separation. This phenomenondemonstrates that the high entropy and sluggish effects instabilizing the solid-solution phase for high-entropy alloys alsooccur in multi-component nitrides [813].

3.1.2.2. Lattice parameter. The majority of the present coat-ings are determined to have a lattice constant of about 4.25 .Those deposited at biases of less than 100 V, however, have asmaller lattice constant of 4.15 (Fig. 2(a)). As these coatingshave comparable compositions, the change in lattice constantmust be a result of other factors. It has been found that bothTiSiAlN and TiAlN FCC solution phases, with lattice constantsof 4.19 and 4.29 , respectively, can exist within the same as-deposited TiAlSiN films [6]. The formation of the TiAlN solu-tion phase in these coatings was attributed to deposition con-ditions that increased adatom mobility during film growth, i.e. ahigher substrate temperature and/or reduced deposition rate,therefore permitting the segregation of Si to form a separateamorphous Si3N4 phase. Furthermore, the segregation of anamorphous Si3N4 phase was considered to inhibit grain growthin the as-deposited films [6]. With reference to the presentcoating system, it appears that the increase in the lattice constantas well as the reduction in grain size concur with depositionconditions under which adatom mobility is possibly increased,i.e. at higher applied substrate biases for a given depositiontemperature. Nevertheless, for the present coatings, even thougha wide range of substrate conditions have been examined, onlya single FCC nitride phase can be identified in any one indi-vidual coating. Adatom mobility will also be expected to bereduced at the lower deposition temperatures of 300 and 473 K,yet these coatings still possess the larger lattice constant and thesmallest grain sizes (Fig. 2(b)). Both of these factors suggestthat the changes in lattice constant of the present coatings do notconcur with the segregation of Si, as reported in the TiAlSiN

Technology 202 (2008) 33603366system [6].An alternative explanation for the increased lattice constant

at higher substrate biases is the enhanced flux and energy of the

bombarding ions, whereby Frenkel pairs and anti-Schottky de-fects are induced by the ion peening effect [2830]. FromFig. 3, the good correlation between two coating propertieswhich are mutually increased by the presence of Frenkel pairsand anti-Schottky defects, i.e. the lattice constant (determinedfrom the Scherrer equation using XRD data) and the com-pressive residual stress (measured by the curvature technique),implies that the change in lattice constant is a result of the ionpeening effect as opposed to segregation of a Si3N4 phase. Thatis to say, even though the present system contains severalimmiscible nitrides, for example AlN, TiN and Si3N4, it appearsto retain a single solid-solution structure, even at a depositiontemperature of 773 K (maximum permissible in the presentsystem) and an applied substrate bias of 200 V. This in factagrees with the XRD observation that the as-deposited nitridesare single FCC phases.

Fig. 4(b), which is considered to be a result of the fact that they tooare biased at 100 V. The increase in compressive residual stressat higher applied substrate biases is attributed to an increase in theion peening effect that leads to atoms being knocked deeper intothe film, where they become trapped creating Frenkel pairs andanti-Schottky defects [2830]. By applying the rule ofmixtures todetermine the coefficient of thermal expansion (CTE) of thecoating as a whole from each of the constituent binary nitridecomponents, it is determined that the mismatch between the CTEof the coating and silicon substrate would lead to greater tensileresidual stresses at higher deposition temperatures. It is thussurprising that the opposite appears to occur in Fig. 4(b), wherebythe residual stress is seen to become more compressive at highersubstrate temperatures. As all discharge conditions are held con-stant, the energetic species arriving at the growing film from thedischarge will be the same, regardless of the deposition temper-ature. The higher compressive stresses, therefore, is thought to bea result of thermally-enhanced crystallinity and bonding strengthbetween target elements and nitrogen, which aid the grain growth.As the grain boundary fraction decreases, grain boundary slidingeffects are reduced and thus higher compressive stresses can begenerated during film growth. Both the crystallinity and strongerMN bonding is also reflected by the hardness and the elasticmodulus since both increase with increasing substrate tempera-

Fig. 4. Residual stress determined as a function of (a) substrate bias and (b) depositiontemperature.

H.-W. Chang et al. / Surface & Coatings3.1.2.3. Grain size. The decreased grain size at the highersubstrate biases, as shown in Fig. 2(a), can be related to an increasein the nucleation rate at ion-induced surface defects [31,32], asopposed to phase segregation. For the coatings deposited at dif-ferent temperatures, which are all biased at 100 V, it is seen inFig. 2(b) that as long as a sufficiently high substrate temperature isused, i.e. 573 K, the coatings have a grain size of about 4 nm. Thiswould appear to suggest that the grain size is most influenced bythe ion-induced nucleation rate as opposed to the changes in graingrowth kinetics expected at different temperatures. The smallergrain size at the lowest deposition temperatures is considered to bea result of both a reduction in the critical nuclei size and thereduced energy for grain growth [33].

3.1.3. Mechanical propertiesIn Fig. 4(a), it is seen that the residual stress of the coatings

biased at 0 and 50 V is only slightly compressive, with values ofabout 0.3 GPa. The coatings deposited at substrate biasesof 100 V or higher, however, have compressive stresses ofaround 3 GPa. The coatings deposited at different temperaturesalso possess compressive stresses of about 3 GPa, as shown in

Fig. 3. Lattice constant as a function of compressive residual stress. The point

enclosed by the circle is not from the present work, and was deposited at a highernitrogen flow rate ratio of 67% (same total pressure) with a substrate bias andtemperature of 50 V and 573 K, respectively.3363Technology 202 (2008) 33603366ture (see next paragraph).The hardness and modulus are shown as functions of sub-

strate bias and deposition temperature in Figs. 5(a) and (b),

3.2. Vacuum annealing

Based onXRD analysis, it is possible to group the as-depositedcoatings as being (i) [V50] large grained (~15 nm) with a smalllattice constant (4.15 ), and low compressive stresses(b0.5 GPa), or (ii) [V100] as having small grains (b4 nm), alarge lattice constant (~4.26 ), and high compressive stresses(N2.5 GPa). Coatings belonging to each of these groups shalltherefore be annealed in vacuum for 5 h at different temperaturesto assess and compare their thermal stability.

The XRD diffraction patterns of the V50 and V100 filmsannealed at different temperatures in an evacuated quartz tubeare shown in Fig. 6(a) and (b), respectively. It can be seen thatboth the coatings not only don't decompose into other newphases but also retain their original structure in terms of thepreferred orientation, lattice constant, and grain size even afterannealing at 1173 K for 5 h.

The stable FCC solution phase without any XRD detectablephase separation up to 1173 K has been attributed to the highmixing entropy effect as discussed in Section 3.1.2.1. Fromthermodynamics, it is well known that entropy effect becomesmore important at elevated temperatures and thus decreases thefree energy of the FCC solution phase.

ings Technology 202 (2008) 336033663364 H.-W. Chang et al. / Surface & Coatrespectively. In Fig. 5(a) the modulus is observed to initial-ly decrease, but then maintain a constant value for a bias of100 V or greater. The hardness on the other hand initiallyincreases to a maximum of about 33 GPa at 100 V, beyondwhich it decreases. The maximum in hardness attained at100 V is attributed to the slightly greater grain size than thosedeposited with a higher applied bias and greater compres-sive stress of those deposited at the lower biases. The similarhardness of the coatings deposited at the lowest and highestsubstrate biases is credited to the coatings with the increasedcompressive stress, i.e. deposited at higher applied biases, hav-ing their hardness limited by their smaller grain size, i.e. theinverse HallPetch effect [34], i.e. a balance between grain sizeand residual stress on the measured hardness. The reduction inthe hardness of the coatings deposited at the lower substratetemperatures, as shown in Fig. 5(b), is also attributed to theirsmaller grain size, and thus the inverse HallPetch effect [34].The hardnesses of around 34 GPa for the coatings depositedat higher temperatures are ascribed to their greater grain size,denser structures, and increased compressive stress. As a resultof its relatively high hardness and low modulus, the greatestratio between these two mechanical properties, shown in Fig. 5(a) and (b), is obtained for the coating deposited at 100 V and573 K. The hardness to modulus ratio has been found to be agood indicator of the performance of a coating under a numberof wear conditions [35].

The nearly unchanged lattice constants of the V50 and V100as-deposited films after annealing imply that stress relaxation

Fig. 5. Hardness, modulus and (inset) the hardness/modulus ratio as a functionof (a) substrate bias and (b) deposition temperature.Fig. 6. X-ray spectrum of the (a) V50 and (b) V100 films annealed for 5 h atdifferent temperatures in a sealed quartz tube at 1.33 Pa.

due to defect annihilation is very small. It has been reported thatthe compressive stress of the as-deposited coatings is reducedafter annealing, as noted from a shift in the XRD peak position[36]. This is attributable to the diffusion of implanted atoms tothe surface, hence the annihilation of the stress inducing defects[36]. Nevertheless, this is not observed in the present coatingsystem, for which there is an insignificant change in the XRDpeak position, even after annealing for 5 h up to 1173 K. This isreasonable since diffusion in these multi-component coatings issluggish to prevent the migration of defects to the surface, andhence prevents stress relaxation. Similarly, sluggish diffusionmight be also used to explain why the preferred orientation andgrain size are almost unchanged.

Unlike the decrease in hardness found for coatings undergoing

H.-W. Chang et al. / Surface & Coatingsstress relaxation [36,37], there is an increase in the hardness afterannealing of the present coatings, as shown in Fig. 7. Such anincrease in the hardness after annealing has been reported to be aconsequence of phase segregation in as-deposited metastablesingle-phased FCC nitride coatings, such as TiAlN [26], ZrTiN[27], and TiSiN [38]. Based on its greater incompatibility with theother nitride components present, as inferred from theoreticalstudies of segregation in TiAlN [21,22] and TiSiN [39] coatings,the segregation of SiN ismost likely in the present coatings. But asit cannot be resolved in XRD analysis, its volume fraction wouldbe too low to account for the hardening effect. Onemore evidenceruling out the significant segregation of SiN is that obvioushardening during annealing can be obtained even at 773 K whichis too low to form the segregation. Thus, it is proposed that annealhardening phenomenon comes from the enhancement of overallbonding between target elements and nitrogen by thermal energyduring annealing.

4. Conclusions

From the deposition of ~Al0.2Cr0.2Mo0.2Si0.2Ti0.2N0.5 filmsat a range of substrate temperatures and applied substrate biases,and through vacuum annealing treatments, it was found that:

(i) All of the coatings possess a single FCC structure ofNaCl-type. This indicates that the FCC lattice formed byFig. 7. Hardness as a function of annealing temperature for the coatings withdifferent substrate bias.FCC forming nitrides can dissolve a large proportion ofnon-FCC forming nitrides, i.e. Si, Al, within itself.

(ii) As a result of ion-induced defects, the grain size wasreduced at higher substrate biases. Furthermore, as long asa critical deposition temperature was exceeded, the grainsize was apparently dictated by ion-induced nucleation asopposed to temperature dependent grain growth kinetics.

(iii) The lattice constant and residual compressive stress of thevarious coatings, which both increase as a result of en-hanced ion peening, had a close correlation. This suggeststhat the coatings are of a predominantly single as-depo-sited solution phase.

(iv) In contrary to that expected from considerations to theCTE of the coatings, the compressive residual stressesactually increase for higher deposition temperatures. Thisis attributable to the thermally-enhanced crystallinity andincreased bonding strength between target elements andnitrogen, which aid the grain growth and thus reducegrain boundary sliding effects in relaxing compressivestresses.

(v) Even though the coatings deposited at the lowest appliedsubstrate biases have a smaller inherent compressive stresscompared to those deposited at the highest biases, theypossess similar hardnesses. This is considered to be aresult of the larger grain size of the lower biased coatingscompensating for their reduced compressive stresses.

(vi) The coatings deposited at an applied bias of 100 V, anddeposition temperature higher than 573 K, have the high-est hardnesses of about 34 GPa.

(vii) The coatings deposited at the lowest temperatures havethe lowest hardnesses on account of the inverse HallPetch effect due to their very small grain size.

(viii) The lattice constant is not found to significantly change afterannealing at 1173 K for 5 h in vacuum. This is considered tobe a result of the prevention of stress relaxation by thediffusion of defects to the surface due to the sluggish dif-fusion in these multi-component nitride coatings.

(ix) The hardness of the coatings, however, is found to increaseafter annealing, which suggests that the overall bondingbetween target elements and nitrogen is enhanced by ther-mal energy during annealing.

Acknowledgements

The authors gratefully acknowledge the financial support forthis research from the National Science Council of Taiwan undergrants NSC-93-2120-M-007-006 and the ministry of EconomicAffairs of Taiwan under grant 93-EC-17-A-08-S1-0003.

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3366 H.-W. Chang et al. / Surface & Coatings Technology 202 (2008) 33603366

Influence of substrate bias, deposition temperature and post-deposition annealing on the struct.....IntroductionExperimentResults and discussionAs-deposited coatingsTopographyXRDCrystal structureLattice parameterGrain size

Mechanical properties

Vacuum annealing

ConclusionsAcknowledgementsReferences