Friction and wear characteristics of electrodeposited nanocrystalline nickel–tungsten alloy films

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Wear 264 (2008) 106–112

Friction and wear characteristics of electrodeposited nanocrystalline nickel–tungsten alloy films A.S.M.A. Haseeb a,∗ , U. Albers b , K. Bade a a

Institut f¨ur Mikrostrukturtechnik (IMT), Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b Institut f¨ ur Materialforschung I (IMF I), Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Received 16 August 2006; received in revised form 1 February 2007; accepted 1 February 2007 Available online 19 March 2007

Abstract Electrodeposited nanocrystalline nickel–tungsten alloys are being investigated as an attractive alternative to electrodeposited nickel for applications involving fabrication of micro electro mechanical systems (MEMS). Ni–W alloys are also being considered as an environmentally friendly alternative to hard chrome plating in some cases. In applications involving sliding contacts such as in micro-gears in MEMS, mould inserts, etc., tribological properties of Ni–W alloys would be of relevance. In this work, the sliding friction and wear characteristics of Ni–W alloys with different tungsten contents were investigated and compared with that of nickel film deposited from sulphamate bath commonly used in microfabrication. For wear tests, Ni–W alloy films of about 5–7 ␮m, deposited from ammonia-citrate baths on copper substrates were employed. The alloy films possessed W contents in the range of 8.4–12.7 at.% and had an average grain size of about 20 nm. Wear tests were conducted in a pin-on-disc type tribometer under un-lubricated conditions. All the wear tests were carried out at room temperature in air with a controlled relative humidity of 50 ± 5% at a normal load and linear sliding speed of 1 N and 3 cm s−1 , respectively. Hardened steel balls were used as the counter body. Friction force was recorded online during the wear test. Wear damage on Ni and Ni–W alloy films was estimated from the width of the wear track and the wear rate of the counter body was calculated from the worn volume. Results show that Ni–W alloys have somewhat lower friction coefficient against steel counter body as compared with that of the nickel–steel pair. Addition of tungsten to nickel is also seen to result in an improvement in wear resistance. Friction and wear mechanisms operative in Ni–W alloys sliding against steel are discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Ni–W alloys; Ni; Electrodeposition; Friction coefficient; Adhesive wear; Oxidative wear

1. Introduction Micro electro mechanical systems (MEMS) have been emerging as practical real life devices from mere scientific curiosity. MEMS can roughly be divided into two main categories: silicon based and non-silicon based. Although a wide range of applications are envisaged for MEMS such as in the field of microoptics, micromachines, microreactors, etc., the range of materials available for the fabrication MEMS seems to be rather limited so far. In silicon-based MEMS, Si is obviously the main

∗ Corresponding author. Present address: Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +63 7967 5266; fax: +63 7967 5317. E-mail address: [email protected] (A.S.M.A. Haseeb).

0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.02.004

construction material, while in non-silicon MEMS, e.g., particularly in MEMS fabricated by LIGA (a German acronym for lithography, electrodeposition and forming) the dominant material has been electrodeposited Ni. While these materials have their own advantages, the range of properties they can offer is rather limited. This may limit the wider application and reliability of MEMS in practical devices. Friction and wear characteristics are critical reliability issues in MEMS actuators or devices like micromotor, engines, turbines, etc. where sliding contacts are involved. Tribological issues are also involved in micro-mould inserts which are used to mass produce polymer based microcomponents for MEMS. Poor friction and wear characteristics are considered as lifelimiting factors for these devices. In this respect, both nickel and silicon are known to have poor characteristics. For silicon based [1,2] and nickel based [3] MEMS, ways to improve the friction

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and wear characteristics, for instance by applying surface coatings, are being investigated by different researchers. Ni has been found to have high friction coefficient during the wear testing of LIGA fabricated microrotors [4]. High friction coefficient and adhesion between nickel mould inserts and thermoplastic material also lead to problems in obtaining dimensionally accurate microcomponents with high aspect ratio during the demoulding step in LIGA fabrication [5,6]. In an effort to improve the performance of LIGA based MEMS, alternative to Ni is being sought currently. One particular direction has been to introduce alloying element into nickel during electrodeposition. Electrodeposited nanocrystalline nickel–tungsten alloys are being considered as an attractive alternative to electrodeposited nickel for applications involving fabrication of MEMS components [7]. Introduction of W into nickel has been found to improve properties like strength, hardness and high temperature stability. How the addition of W to Ni affects its tribological properties is also of great interest in many applications. Besides its potential applications in MEMS, Ni–W alloys are being actively considered as an environmentally friendly alternative to hard chrome plating in some applications [8]. Tribological properties are also relevant in the latter case. A number of studies have appeared in the literature on the tribological performance of electrodeposited nickel–tungsten based alloys in recent years. Abrasive nano-scratch tests were performed by Schuh et al. [9] on nanocrystalline Ni–W alloys. They observed that Ni–W alloys had better abrasion resistance than nanocrystalline electrodeposited nickel. Sriraman et al. [10] studied the friction and wear behavior of electrodeposited Ni–W alloys deposited at various current densities. They observed that the wear rate of Ni–W alloys decreased as the grain size decreased to a certain limit. The wear rate then increased with a further decrease in grain size. A similar trend in the relationship between friction coefficient and grain size was observed. Jie et al. [11] reported on the wear behavior of brush plated Ni–W alloy against mild steel. Papachristos et al. [12] investigated the sliding wear behavior of compositionally modulated Ni–P–W coatings produced by electrodeposition. The coating consisted of alternate layers of low and high tungsten content with each layer ranging from 6 to 200 nm. They observed an improved wear resistance of the coating with a decrease in individual layer thickness, particularly at higher normal load. Krishnan et al. [13] studied the effect of deposition temperature on the abrasion wear resistance of electrodeposited Ni–W alloys. They reported that the abrasion wear resistance improved with the increase of electrodeposition temperature from 30 to 70 ◦ C. Ni–W alloy coatings are also being considered as friction material in brake applications [14]. Other W containing electrodeposited alloys such as Co–W alloys were also found [15] to possess improved wear and frictional properties when applied as coating to hot forging dies. This led to increased die life by up to 100%. The aim of the present work is to compare the friction and wear characteristics of Ni–W alloys with that of nickel film electrodeposited from sulphamate bath, which is commonly used in LIGA-MEMS fabrication. Ni–W alloys with different W contents were deposited at the same current density from baths with different ratio of nickel to tungsten ion concentra-

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tion. The effect of W content on the tribological performance is described. 2. Experimental Ni–W was electrodeposited from an ammonia-citrate bath suggested in the literature [7,16]. The composition of the electrodeposition solution is essentially that used by Yamasaki et al. [7]. In the present study, however, the concentration of nickel sulphate in the bath was varied in order to vary the W content of the deposit, without changing the deposition current density. Details of the solution and deposition parameters used are given in Table 1. The electrodeposition cell consisted of a cylindrical glass vessel with a PMMA cover. A few holes in the cover allowed the access of substrate, counter electrode and thermometer. Adequate o-ring seals were used in different junctions in the cover to limit the loss of ammonia as well as water vapor. Polycrystalline copper disc of diameter 20 mm and thickness 2 mm were used as substrates. A solution volume of 3 l was used for electrodeposition. Deposition was continued in a particular solution as long as the depletion of the nickel ion in the bath remained below 5% of the original concentration. When the depletion exceeded this limit, a new bath was prepared and used. Deposition was carried out for 2 h which yielded a thickness of 5–7 ␮m. The characteristics of the deposits were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). For comparison purposes, Ni film of 5 ␮m was electrodeposited from sulphamate bath [17]. Ni–W alloy films with three different W contents deposited on copper substrate were tested for tribological performance. Tribological investigations were carried out in a CSEM pin-ondisk Tribometer under un-lubricated conditions. All the wear tests were carried out at room temperature in air with a controlled Table 1 Bath composition and parameters for electrodeposition of nickel–tungsten alloy Bath constituents

pH Deposition temperature Stirring condition Bath volume Cathodic current density Substrate Anode Deposition time

Nickel sulphate (NiSO4 ·6H2 O) Sodium tungstate (Na2 WO4 ·2H2 O) Sodium citrate (Na3 C6 H5 O7 ·2H2 O) Ammonium chloride (NH4 Cl) Sodium bromide (NaBr)

0.06, 0.10 and 0.14 mol l−1 0.14 mol l−1

8.5 (adjusted at room temperature) 75 ◦ C Mild stirring using magnetic stirrer 3000 ml 10 mA cm−2 Cu disc: 2 mm thickness × 20 mm dia Platinised titanium mesh 2h

(pH adjusted with H2 SO4 )

0.50 mol l−1 0.50 mol l−1 0.15 mol l−1

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relative humidity of 50 ± 5% at a normal load and linear sliding speed of 1 N and 3 cm s−1 , respectively. A hardened steel (carbon chromium steel 100 Cr 6) ball of 6 mm diameter was used as the counter body. During the tests, the Ni–W coated disc sample was rotated for 9000 revolutions which give a sliding distance of 226 m for the wear track diameter (8 mm) used. Under the present experimental conditions, wear damage was limited to within the films in all cases. Friction force was recorded online during the wear test. Wear damage was represented by the width of the wear track for Ni and Ni–W alloy films and the wear rate of the counter body was calculated from worn volume. After the wear tests, the damaged areas were investigated by SEM, EDX and optical microscopy.

Table 3 Microhardness of Ni and Ni–W alloy samples Material

Microhardnessa (HV)

Ni Ni–12.7 at.% W Ni–9.4 at.% W Ni–8.4 at.% W

277 536 535 567

a

Measured on samples having a thickness of 100 ␮m, 50 g, 10 s.

3. Results The W contents of the deposits obtained from baths with three different concentrations of nickel sulphate are shown in Table 2. Grain sizes of the samples measured by the X-ray diffraction technique are also given in Table 2. It is seen that by varying the bath nickel sulfate concentration from 0.06 to 0.14 mol l−1 , a range of W concentration from 8.4 to 12.7 at.% is obtained. X-ray diffraction revealed that the grain size did not show a systematic variation with deposit W content under the present experimental conditions. An average grain size in the neighborhood of 20 nm is obtained for all the Ni–W alloy films. This was corroborated by transmission electron microscopy [18]. It may be mentioned that other researchers [7,10] varied deposition current density in order to vary the deposit W content. However, deposition current density, which is in effect the deposition rate, can also have significant effect on grain size. Deposition under variable current densities leads to samples with variable grain size as well as variable W content. It is thus difficult to isolate the effect of a single variable using such samples. In the present study, nickel ion concentration in the bath was varied, keeping the deposition current density constant, in order to obtain Ni–W alloys with different W contents but having a more or less constant grain size. The present work thus allows the study of the effect of W content in the range of 8.4–12.7 at.% on the tribological properties of Ni–W film. The microhardness of the Ni–W alloy samples together with that of nickel is shown in Table 3. Ni–W alloys have hardness in the range of 535–567 HV, while nickel deposit has a hardness of 277 HV. Thus, the microhardness of Ni–W alloy deposits is nearly twice that of the nickel deposit. The microhardness of Ni–W alloy samples does not appear to vary with tungsten content within the range investigated. Table 2 W content and grain size of the Ni–W alloy deposits obtained from baths with different concentration of nickel sulphate NiSO4 ·6H2 O concentration in bath (mol l−1 )

W content in deposit (at.% W)

XRD grain size (nm)

0.06 0.10 0.14

12.7 9.4 8.4

20 22 21

Fig. 1. Friction traces for: (a) pure Ni and (b) Ni–12.70 at.% W alloy films.

Typical friction traces for Ni and a Ni–12.70 at.% W alloy films are shown in Fig. 1. It is seen in both cases that the friction coefficient (μ) increases sharply at the beginning. The rate of change of μ then reaches a steady state at longer sliding distances. Similar behavior was observed for other Ni–W alloy films. The friction coefficient calculated from the last 50 m is shown in Table 4. Steady state friction coefficient obtained for nickel against steel is seen to be 0.87. On the other hand, the friction coefficient for Ni–W alloys sliding against the same steel counter body is in the range of 0.73–0.82. The addition of W to nickel is thus found to have resulted in a slight decrease in friction coefficient. No clear trend in the variation of friction coefficient of Ni–W alloys with W content is seen within the range investigated. Fig. 2 shows the morphology of wear track on Ni and Ni–12.70 at.% W alloy films. At low magnification, the wear track on Ni is found to exhibit faint sliding marks. Some heaps of debris are seen, especially at the middle of the track (Fig. 2a). Occurrence of such debris heaps was widespread on the wear track on Ni. Table 4 Steady state friction coefficient of Ni and Ni–W alloys against steel Material

Steady state friction coefficient (average over last 50 m)

Ni Ni–8.39 at.% W Ni–9.43 at.% W Ni–12.70 at.% W

0.87 0.80 0.73 0.82

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Fig. 2. Morphology of wear track on Ni and Ni–W alloy films: (a) Ni at low magnification, (b) Ni–W alloy at low magnification, (c) Ni at high magnification, (d) Ni–W alloy at high magnification.

Fig. 2c shows a typical view of an area with a heap of debris material on Ni. The debris appears as ‘add-on’ material at high magnification. EDX analysis of the heaps reveals that they contain a high percentage of iron (as high as 40 mass%) and oxygen (close to about 20 mass%), in addition to Ni. Analysis at the cleaner sliding marks (bottom left) in Fig. 2c also reveals a considerable amount of iron and oxygen, although their percentage is lower here as compared with that on the heaps of debris. Iron on the wear track obviously came from the steel counter body. It thus appears that a massive transfer of material from steel counter body to nickel film took place during the wear test. Round powdery oxidized debris particles which appear bright under SEM owing to their low conductivity were also seen on the heap of transfer material. The presence of a considerable amount of oxygen on the wear track on Ni indicates the occurrence of oxidation during the wear process. The wear track on Ni–12.70 at.% W film shows sliding marks and appears smoother at low magnification (Fig. 2b). The width of the wear track on Ni–W alloy films was smaller when compared with the same on Ni film. At high magnification the track revealed smooth and rough bands running parallel to each other. The rough areas showed the presence of patches, dark in color (Fig. 2d), while the smooth areas appeared as smeared sliding track. EDX analysis revealed the presence of Fe and O (in addition to Ni, and W) on both these areas. However, the amount of Fe found on the dark patches of debris on Ni–W alloy was about 15 mass% or less, which is much lower than that on Ni. Oxygen content of the debris patches was considerable, about 20 mass%. The smooth areas on the track contain much lower percentages of Fe and O as compared with that of the debris

patches. The presence of iron on the wear track of Ni–W alloys indicates that transfer of materials from steel to Ni–W films also took place. However, the extent of transfer to Ni–W was much less than that to Ni. The presence of oxygen on the wear track reveals that oxidation also took place during the wear of Ni–W films against steel. Typical surface profiles across the wear tracks on Ni and Ni–W alloys are presented in Fig. 3. Both profiles contain irregular peaks and valleys. The damaged area, indicated by a horizontal arrow, is seen to be wider on Ni. In spite of the presence of irregular peaks, the profile on Ni reveals the formation of a trough on the track due to the wear process. No such clear trough or depression was observed on Ni–W alloys. The peaks represent the presence of debris material on the track. The irregular characteristics of the wear track profile do not allow the accurate calculation of worn volume for Ni and Ni–W alloy films. The width of the wear track was therefore taken as a measure of wear damage on these films. Table 5 presents the wear track width on Ni as well as Ni–W alloy films. It is seen, based on wear track width, that nickel sustained more wear damage when compared with Ni–W alloys. The wear track width on the films Table 5 Wear track width on Ni and Ni–W alloy films Material

Width of wear track (mm)

Ni Ni–8.39 at.% W Ni–9.43 at.% W Ni–12.70 at.% W

1.05 0.90 0.72 0.64

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rate is highest for steel sliding against Ni. A decreasing trend is observed in the wear rate of steel counter body sliding against Ni–W films with increasing W content. The wear rate of the steel counter body sliding against Ni–12.70 at.% W film is about 70% lower as compared with that of counter body sliding against Ni film. 4. Discussion

Fig. 3. Wear track profiles on (a) Ni and (b) Ni–12.70 at.% W alloy films.

decreased by about 40% when 12.70 at.% W is added to Ni. The surface profile on both sides of the wear track (Fig. 3) shows that Ni film has a surface roughness higher than that of Ni–12.70 at.% W alloy film. The surface roughnesses of all Ni–W alloy films were similar. Fig. 4 shows the wear scar on steel counter body sliding against Ni and Ni–12.70 at.% W alloy films. Both scars appear similar, particularly at low magnification, mainly showing sliding marks (Fig. 4a and b). The bright areas on the scars suggest the presence of non-conductive material, presumably oxide. The wear scar on steel ball sliding against Ni is larger when compared with that on steel ball sliding against Ni–W alloys. Chemical analysis showed the presence of mainly iron and oxygen on the scar on steel ball sliding against Ni film. Nickel was not generally detected on the scar on steel sliding against Ni, with the exception of one place where some loose debris showed the presence of Ni. The wear scars on steel ball sliding against Ni–W alloys revealed the presence of considerable amount Ni and W in addition to iron and oxygen. The wear rates of steel counter body sliding against different films are shown in Table 6. Wear Table 6 Wear rate of steel counter body sliding against Ni and Ni–W alloy films Film sliding against steel ball

Wear rate of steel counter body (mm3 N−1 m−1 × 10−5 )

Ni Ni–8.39 at.% W Ni–9.43 at.% W Ni–12.70 at.% W

3.92 2.27 1.88 1.14

It is seen that a substantial transfer of material took place from steel counter body to Ni film. But the reverse transfer is scarce. The transfer to Ni took place in the form of massive heaps of material. The characteristics of the worn surface indicate a strong adhesive interaction between nickel and steel. Such interaction is thought to be linked to good mutual solubility of nickel and iron—the major constituent in steel [19], as well as the higher reactivity of the transition metal, Ni due to its higher d-bond character [20]. Similar interaction between nickel and steel was also observed under fretting wear conditions [21]. Strong adhesive interaction between nickel and steel leads to a higher friction coefficient. In the case of Ni–W alloys, transfer of material from steel to Ni–W film was seen to be much less as compared with that to Ni film. Transfer to Ni–W films took place in the form of small patches, rather than massive heap as was the case for Ni. It seems that the incorporation of W into Ni reduced the adhesive interaction with steel. Based on the above results on wear scar morphology and EDX analysis of the scars, it appears that in the case of Ni/steel pair, the transfer of material primarily takes place from steel ball to nickel film. In the case of Ni–W/steel pair, transfer of materials takes place both ways, i.e. steel is transferred to Ni–W and Ni–W in transferred to steel. It can thus be suggested that the presence of W allows the back transfer, i.e., transfer from Ni–W film to steel. A considerable amount of oxygen is found in the transfer material on all film samples. Spherical oxide particles were also present in the transfer material on the wear track of the films. Oxide film was observed on the worn surface of steel counter body. These indicate that oxidative wear played a significant role during the wear process in the present study. It can therefore be suggested that both adhesive and oxidative wear are operative for Ni as well as Ni–W alloys under the experimental conditions prevailing in the present study. Sriraman et al. [10] studied the wear behavior of Ni–W alloys at loads (5 and 10 N) and speed (50 cm s−1 ) higher than that used in the present study, where a load of 1 N and sliding speed of 3 cm s−1 have been used. The test configuration of Sriraman et al. was opposite to what has been used in the present study. They used Ni–W alloys in the form of a coating on steel pin, which slid against a flat steel disc. They reported that for alloys with grain size at or above 20 nm, adhesive wear mechanism was operative. However, for alloys with smaller grain size and hence having higher hardness, brittle fracture was the dominant mode of wear damage. It may be noted that no brittle fracture was noticed in Ni–W alloys under the present experimental conditions. The wear track on Ni film is found to be wider than that on Ni–W films. Thus, the incorporation of W into nickel reduces

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Fig. 4. Wear scar on steel counter body against: (a) Ni (low magnification), (b) Ni–12.70 at.% W (low magnification), (c) Ni (high magnification), and (d) Ni–12.70 at.% W (high magnification).

the extent of wear damage. The presence of W also reduced the wear rate of the steel counter body. Two possible reasons can be suggested for the lower wear damage in Ni–W/steel pairs as compared with Ni/steel pair. One is the higher hardness of Ni–W alloys (Table 3) which reduces the contact area between the films and ball counter body. Secondly, W, facilitating back transfer to steel ball, helps form a protective transfer film on the latter. The track width also decreased with increasing W content in the films. However, the hardness of the Ni–W alloy films (Table 3) cannot account for this improvement in wear resistance with W content. It is suggested that increased W content in the film led to the formation of a more stable and protective transfer film on

steel ball thereby contributing to the decreased wear damage of the tribo-system. Although friction and wear properties are system properties and direct comparison among data obtained by different researchers can not be made, it is nonetheless useful to have an overview of different sets of data available in the literature. In Table 7 are shown some published friction data for Ni, W and Ni–W alloys. It can be seen that Ni shows a high friction coefficient, about 0.7–0.8 (up to as high as 1.3 in air and 2.0 in vacuum) against itself as well as against steel. Friction data on W is relatively scarce. Self-mated W was found to have a high friction coefficient 0.8 [22]. Friction coefficient of sputter deposited

Table 7 Some friction coefficient data on Ni, W and Ni–W alloys available in the literature Material

Sliding pair

Friction coefficient

Reference

Ni

LIGA-Ni against alumina (microrotor)

[24]

Ni against steel Ni and Ni based composites (Ni/SiC, Ni/graphite, Ni/CNT) against steel Nickel against steel Ni against steel

0.68–1.18 (static) 0.52–0.79 (kinetic) 2.0 (vacuum) 0.6–0.85 (air) 0.8 (fretting) 0.7–1.3 0.18–0.65 (fretting) 0.7

Tunsten against tungsten Tungsten against HD-17D alloy (90% W, 7% Ni, 3% Fe) W against Si3 N4

0.8 0.6 0.14–0.9

[22]

Ni–P–W multilayer against steel Ni–W alloys against steel

0.7–0.9 0.4–1.2

[12] [10]

LIGA-Ni against Ni (microrotor)

W

Ni–W

[4] [21] [25] [26] [27]

[23]

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W film was observed to lie in a wide range, 0.14–0.9 depending upon deposition conditions [23]. Studies [10] reported the friction coefficient values of Ni–W base alloys against steel in the range of 0.4–1.2. The value of friction coefficient for Ni/steel pair obtained in the present study thus lies within the range reported in the literature. The values presently obtained for Ni–W alloys 0.73–0.87 also lie within the range reported in the literature. Friction coefficient of a sliding system depends on the mechanism involved in the friction and wear process. As has been noted earlier, higher friction coefficient of Ni against itself or steel is generally interpreted as being caused by a strong adhesive interaction between Ni and Ni or Ni and Fe. In the present case, the incorporation of W into Ni is found to result in a slight decrease in friction coefficient. This is probably linked to the slight decrease in adhesive interaction brought about by W. 5. Conclusions Friction and wear characteristics of electrodeposited Ni–W alloys with W content in the range of 8.39–12.70 at.% have been investigated and compared with that of electrodeposited Ni. Tribo-tests were conducted under un-lubricated, relatively low load (1 N) and low velocity (3 cm s−1 ) conditions against hardened steel counter body. Based on wear track width, nickel film is found to sustain more wear damage when compared with that on Ni–W alloy films. The wear track width on the film decreased by about 40% upon the addition of 12.70 at.% W to Ni. The wear rate of the steel counter body sliding against Ni–12.70 at.% W film is about 70% lower as compared with that of the counter body sliding against Ni film. Ni–W alloys possessed slightly lower friction coefficient than Ni. Adhesive and oxidative wear mechanisms have been found to be mainly operative under the present experimental conditions. It is suggested that W in the Ni–W alloy films led to the formation of a more stable and protective transfer layer on the steel counter body, thereby contributing to the decreased wear damage of the tribo-system. Acknowledgement One of the authors (ASMAH) would like to thank Alexander von Humboldt Foundation for a fellowship. References [1] P.J. Resnick, S.S. Mani, Surface preparation for selective tungsten deposition on MEMS structures. Reliability, testing and characterization of MEMS/MOEMS, in: R. Ramesham (Ed.), Proc. SPIE 4558 (October) (2001) 181–188. [2] D. Gao, C. Carraro, R.T. Howe, R. Maboudian, Polycrystalline silicon carbide as a substrate material for reducing adhesion in MEMS, Tribology Lett. 21 (2006) 226–232. [3] D. Kim, D. Cao, M.D. Bryant, W. Meng, F.F. Ling, Tribological study of microbearings for MEMS applications, J. Tribology 127 (2005) 537–547.

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