ALLOYING MoS2 WITH Al AND GOLD: EFFECTS OF DEPOSITION PARAMETERS AND ATOMIC PERCENTAGE ON NANOHARDNESS, FRICTION, WEAR, ADHESION AND BULK PROPERTIES E. PFLUEGER, J. D. HOLBERY CSEM Instruments SA, Jaquet-Droz 1, CH-2007, Neuchâtel, SWITZERLAND; e-mail: epf@csem.ch A. SAVAN, Y. GERBIG CSEM Surface Engineering, Jaquet-Droz 1, CH-2007, Neuchâtel, SWITZERLAND Q. LUO, D. B. LEWIS, W.-D. MUNZ

Materials Research Institute, Sheffield Hallam University, Sheffield, UK SUMMARY Mechanical and tribological properties of MoS2 alloyed with Al and Au have been analysed using nanohardness, pin-on-disk, X-ray diffraction and TEM to determine the relationship between coating structure and performance. Physical effects by the alloying partner, atomic percentage, and variation of deposition parameters on the final behaviour of the coatings are evaluated. XRD and TEM analysis indicate two different structures exist in the MoS2 alloys studied: MoS2-Au alloys deposited from a compound target exhibited periodic distributed particles of 2.5 to 6 nm while MoS2-Al coatings deposited from separate, pure targets in a multi-pass, rotating-substrate system evolved with a multi-layer architecture. Lifetime pin-on-disk studies indicate that the tribological performance is enhanced two-fold in the MoS2-Au alloy when the processing pressure is maintained at 12 mT. XRD analysis indicates crystalline peak intensity ratios are highest at the lower deposition pressures, whereas higher processing pressures result in more amorphous coatings. This is supported by Vickers hardness results that are highest for MoS2-Au alloys processed at 12 mT. Pin-on-disk tribological performance has been shown to be highest in MoS2-Al coatings produced at pressures at or near 1 mT, with deposition power having a secondary effect. XRD analysis indicates interfacial mixing and roughness present within the individual layers are greatest near 1 mT depositions and nanohardness results indicate Vickers hardness to be correspondingly higher.

Keywords: Alloyed MoS2, Nanohardness, Friction, Wear, X-Ray Diffraction, TEM

1 INTRODUCTION Processing parameters and the resulting crystallographic structure of alloyed MoS2 have been explored in the past to determine the relationship between mechanical properties, wear life, and overall composition as a basis for understanding the methodology for tailoring dry-lubricant tribological coatings [1 - 4]. These coatings have many applications including the expanding field of MEMS devices [5]. The implementation of MoS2 alloyed systems in industrial applications is based on the material performance inherently influenced by material structure and deposition technique. Coverage of MoS2 coatings a few nanometers in thickness were determined to maintain a first interface layer with crystallites having their (002) planes parallel to the substrate [6]. Sub-sequent analysis determined that the structure of sput-tered thin films was strongly influenced by the growth and crystallization mechanisms which exhibited branching and epitaxial characteristics resulting in a lamellar morphology characterized by the presence of a very thin interfacial region (about 2 - 5) with (002) planes parallel to the surface and of the sample [7].

Alloying materials have been determined to improve the mechanisms governing long-term friction and wear properties of MoS2 thin films [8]. It is believed that dopant materials such as Au, Ni, and Fe densify the thin films thereby increasing hardness, lowering friction, and

reducing crystallite breakage [9]. A detailed analysis of MoS2 film structure revealed that with certain alloying ele-ments (e.g. Ni) long-range crystallographic order was re-duced and resulted in crystallites below a certain size [9]. Subsequent research on sputter-deposited MoS2 films incorporating periodic metal multi-layers of Au-20 % Pd and Ni were developed [10] and characterized using high resolution transmission electron microscopy (HRTEM) in combination with XRD to reveal that the nanostructure of Au-Pd / MoS2 films consisted of discrete islands or to be continuous with higher metal deposition promoting continuous layers [11]. It was found that these three-dimensional islands between 2.5 and 5 nm were comprised of single crystals and twined particles. Others have analysed Pb-Mo-S films produced in an ion-beam assisted deposition process with TEM and found that nano-crystallites 3 – 5 nm in size exist just below a crystalline region [12]. Similar observa-tions were made with TEM in amorphous MoS2 films revealing grains between 3 and 10 nm [13].

Tribological behaviour and layer-structures were ex-amined for MoS2 with interfacial layers of Pb and Ti [14]. Beneficial effects were revealed by layered structures (Ti) but for certain alloys no layer could be detected (Pb) by RBS-analysis. While in the past many groups have investigated the processing effects on MoS2 film performance, our efforts have built upon this work to determine the process parameters that greatly impact

tribological performance in MoS2-metal thin films. In this study, we relate the long-term performance of both Al- and Au-alloyed MoS2 films with the crystallo-graphic structure of the films, concentrating on the correlation between discrete clusters on the order of several nanometers in size and the hardness and long-term wear properties of these films. 2 EXPERIMENTAL PROCEDURE 2.1 Coating Deposition Composite films were grown by magnetron sputtering using two different deposition systems. In the case of MoS2-Au, a 2 inch (50 mm) target of MoS2/Au having a composition of 90 at.% and 10 at.% respectively was used (Cerac Inc., Milwaukee, WI, USA) in a static, sputter-down configuration. In the second case, a Hauzer Techno Coatings HTC-1000/4 was used with one pure Al target located in between two opposing pure MoS2 targets. The fourth target, opposite to the Al, was pure Cr, used for making the adhesion layer. In this machine, the targets and samples are oriented vertically, and with the samples moving continuously in front of the targets with two-fold rotation. Film growth rates for pure films were determined using profilometry and these growth rates were used to calculate the target power required for a specific dopant concentration. Film composition was measured using WDX analysis.

The substrate used for all coatings was AISI 440C stainless steel. In certain cases, (100) silicon and glass microscope slides were also included for the depositions. An adhesion promotion Cr layer has been deposited on each 440C steel disk applying a ion etch and cathodic arc evaporation process in the HTC-1000/4PVD machine. In the case of MoS2-Al coatings, the Cr adhesion layer was made immediately prior to each deposition. For the MoS2-Au coatings, a batch of Cr-coated 440C disks was prepared, and stored in vacuum until the experimental depositions were per-formed.

A sample of natural molybdenite (stoichiometric MoS2) was used as a standard to base the ratio of Mo/S. Analysis revealed that the amount of sulphur in MoSx to be approximately 1.6. Film thickness was kept constant at 0.5 um. System base pressure prior to deposition was better than 1 × 10-5 Pa. A working gas pressure of 0.25 mT to 1.2 mT was applied during sputtering. MoS2-Au depositions were performed at room tempera-ture with the substrate temperature remaining at less than 325 K during deposition. MoS2-Al depositions were performed at 75 °C. Films were grown onto both glass (roughness Ra ~1 nm) and polished 440C steel (Ra < 0.01 mm) substrates at room temperature. In all cases a Cr adhesion layer was included between the substrate and coating. 2.2 Pin-on-Disk Pin-on-disk wear testing was performed with a CSEM Instruments Tribometer on films grown on 440C steel substrates with a normal load of 5 N, velocity of 10 cm/s, air atmosphere with 50 % RH using 100Cr6 balls (diameter 6 mm) resulting in a mean Hertzian

contact pressure of 1 GPa. Reported results for coating lifetime are based on the duration of the test maintaining a friction coefficient below 0.45. Previous studies found that MoS2-TiN was insensitive to changes in thickness in sliding contacts at RH = 60 % [15]. Individual tests indicate the friction coefficient to be within the range of 0.15 - 0.25 during the duration of the test, but we choose to report the lifetime of the coating by the point at which the coating begins to show degradation fails he coefficient of friction value increases beyond 0.45. 2.3 Nanohardness Nanohardness measurements were performed with a CSEM Instruments Nanohardness Test instrument (NHT) fixed with a Berkovich diamond tip. Measure-ments have been performed at the 2 mN, 5 mN, and 10 mN load levels to determine specimen hardness as a function of the resulting depths. The maximum load was applied at twice the rate of the final load per minute, i.e. for a maximum load of 2 mN, the load was applied and removed at 4 mN per minute. We report Vickers hard-ness results of indentations taken at an average depth between 95 – 120 nm. This represents a depth between 20 – 24 % of the total coating thickness, within an acceptable range for comparing similar coatings but perhaps not accurate for comparing to bulk values [16]. 2.4 X-ray Diffraction X-ray diffraction was carried out on both coatings using Cu Kα radiation. Theta/2 theta and glancing angle (1° incidence) x-ray diffraction patterns were collected using a scan step size of 0.04° between 2θ values of 2°-140° and 6° - 140° respectively. The glancing angle technique was used to enhance the signal from the coating. 2.5 Transmission Electron Microscopy Hardened tool steel samples containing a MoS2-Al or MoS2-Au coating were sectioned into blocks 2 mm × 3 mm in size, using a cooled high-speed SiC disc. Two sectioned blocks were glued on a glass plate with the coated side face-to-face for metallographic grinding and polishing to thickness ~50 µm, which was then glued on a copper grid having a slot hole for ion-beam milling. The Ar ion-beam milling was conducted using a Gatan Precision Ion Polishing System (PIPS) at 5 KeV, +/- 5o-10o, to reach electron transparency in the cross-section of the coating region. The prepared cross-section samples were examined using an analytical transmission electron microscope Philips CM-20 STEM (200 kV) having ultra-thin-window energy dispersive X-ray spectroscopy (EDS) and Oxford Link ISIS system for TEM-BF, TEM-SAD and TEM-EDX.

3 RESULTS AND DISCUSSION 3.1 Pin-on-Disk Pin-on-disk tests indicate clear trends for coating wear life as a function of both composition and processing parameters, namely the sputtering pressure and input power. Pin-on-disk testing was performed on two

samples for each pressure level and the two-sample average lifetime (revolutions/nanometer thickness of coating), defined as the point the friction coefficient exceeds 0.45, is reported in Figure 1. 3.1.1 MoS2/Au Depositions made at a pressure of 12 mT have the longest lifetime for coatings made in the range of 5 to 20 mT. Further investigations into the process para-meters for the MoS2/Au alloy indicate that as a function of sample distance to the target, the lifetime of the coating also shows significant pin-on-disk lifetime changes. Figure 2 indicates that at the applied pressure level of 12 mT and the target distance of 80 mm produce lifetime pin-on-disk results that are nearly twice that of samples placed at distances of 60, 100, and 120 mm from the target in the chamber. Again, these results are an average of two samples tested for each target distance. Thus we can see that the effect of pressure and sample distance to target play a dramatic role in the endurance of the MoS2-Au composite coating in a pure sliding test in humid atmosphere.

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Figure 1: Sample MoS2-Au average pin-on-disk lifetime

(revolutions/nm) versus pressure (mT) in room temperature, 50 % RH air.

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Figure 2: Sample MoS2-Au average pin-on-disk lifetime (revolutions/nm) versus target to substrate distance (mm). All depositions produced at the same pressure: 12 mT.

3.1.2 MoS2/Al Lifetime pin-on-disk results of the MoS2-Al samples indicate that as a function of process pressure, favourable results are achieved at or near the process

pressure of 1 mT (Figure 3). These values are four to five times the lifetimes achieved at pressures of 0.5 mT and 1.2 mT.

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Figure 3: Sample MoS2-Al average pin-on-disk lifetime (revolutions/nm) versus pressure (mT). Note the data

has been curve fit . Similarly, there is also an indication that the process power input, an independent variable from the chamber pressure, also has an effect on the overall pin-on-disk lifetime. All samples have been processed in this series at a pressure of 0.5 mT. While the data is somewhat inconclusive and further testing is needed for confirmation, there are clear indications that low power (0.2 to 0.5 kW) produce favourable lifetime averages as compared to power levels above 0.75 kW (Figure 4). The anomalous data point at 0.2 kW may be due to the difficulty in accurately controlling the magnetron power and maintaining a plasma at such a low value of power and pressure for the 12 kW maximum power supply. The changes in coating performance are likely due to the changes in coating structure and will be analysed in the following XRD and TEM sections.

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Figure 4: Sample MoS2-Al average pin-on-disk lifetime (revolutions/nm) versus applied process power (kW). All

depositions produced at the same pressure: 0.5 mT. 3.2 Nanohardness Nanohardness results reveal the resulting mechanical property of a coating as a function of the inherent structure. Indentations were performed at 2 mN load

231 278 212

127

with the resulting hardness value calculated using the widely accepted method of Oliver and Pharr [17]. The analysis method applied is based on depth sensing inden-tation that derives the mechanical properties of the sample from the unloading curve taking into account the plasticity of the material through a power-law relation-ship [17]. An example load-displacement curve of specimen 991006-130 is depicted in Figure 5.

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Figure 5: Nanoindentation Load versus Displacement

curve for MoS2-Au 10 % sample with an average composite coating thickness of 496 nm.

3.2.1 MoS2/Au Alloy structure is affected by processing parameters resulting in distinct differences in Vickers hardness as is evident in the results of MoS2-Au listed in Table 1. Each value is the average of three indentation measurements with standard deviations reported. It appears that the structure of films processed at 12 mT show higher hardness results as compared to those processed at deposition pressures of 5, 16, and 20 mT. These values should be used only for comparison and not as absolute.

Vickers Hardness Specimen

Number

Depo-sition

Pressure (mT)

Coating Thick-

ness (nm)

Depth ofInden-tation (nm) Value Std.

Dev.

991006-130 5 496 119 565 33 991001-128 12 488 104 735 41 991007-134 16 438 95 406 18 991006-132 20 498 107 454 32

Table 1: Nanoindentation results of MoS2-Au.

A plot of MoS2-Au average Vickers hardness versus deposition pressure provided in Figure 6 reveals a trend strikingly similar to average pin-on-disk lifetime versus pressure (mT). A correlation this strong with a deposi-tion pressure of 12 mT indicates in the MoS2-Au alloy favourable processing conditions will achieve structural features that provide superior localized mechanical properties.

0100200300400500600700800

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Figure 6: MoS2-Au average Vickers hardness versus

deposition pressure. 3.2.2 MoS2/Al Nanohardness measurements of the MoS2-Al were performed with the load controlled at 5 mN. This was chosen because this load produced indentation depths of approximately 20 % of the total film thickness. Results reported in Table 2 indicate the differences in the Vickers hardness to be far less pronounced than for the Au-alloyed samples. However, viewing Figure 7 reveals localized mechanical properties are influenced in the MoS2-Al alloy to some degree by deposition pressure. Further evidence of these characteristics is revealed by x-ray diffraction and TEM analysis.

Vickers

Hardness Specimen Number

Depo-sition

Pressure (mT)

CoatingThick-

ness (µm)

Depth of Inden-tation (µm) Value Std.

Dev. 990623-01 0.25 1.112 0.199 552 26 990621-00 0.5 0.736 0.186 565 23 990625-00 0.75 1.071 0.204 594 38 990711-00 0.9 1.212 0.207 581 34 990622-03 1.05 0.841 0.189 494 19 990625-02 1.2 1.392 0.210 401 24

Table 2: Nanoindentation results of MoS2/Al

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Figure 7: MoS2/Al average Vickers hardness versus

deposition pressure.

3.3 X-ray Diffraction 3.3.1 MoS2-Au Figure 8 shows glancing angle X-ray diffraction patterns (XRD) of the MoS2-Au coating, deposited at pressures between 5 mT and 20 mT. There is evidence of broad peaks with 2θ between 12.6° and 13.4° with full width half maximums, β2θ° between 3.6° and 4.5° in addition to a diffuse peak centred around 2θ = 40°. See Table 3.

Figure 8: Glancing angle XRD scans of MoS2-Au.

Sharp peaks in the X-ray diffraction pattern are from the steel substrate material. Peaks measured between 12.6° and 13.4° correspond to the (00.2) reflection from the basal plane of MoS2 with measured ‘d’ spacings between 0.702 and 0.667 nm. The bulk value for the (00.2) reflection from MoS2 is 0.6155 nm [18]. Peaks at larger ‘d’ spacings than that of the bulk value are commonly observed in sputter deposited films and have been ascribed to either substitution of the sulphur by oxygen or self/foreign intercalation [19]. Intensity of the (00.2) reflection and full width half maximums, β2θ°, decreased and increased respectively as the deposition pressure was increased from 5 mT to 20 mT. De-convolution of the diffuse peaks centred at 2θ ≅ 40° for the coatings deposited at pressures between 16 mT and 20 mT, showed them to be composed of two broad peaks at 2θ values and full width half maximums shown in Table 3.

Specimen Number

Deposition Pressure

(mT)

‘d’ spacing (00.2)

nm

FWHM (00.2) ββββ° 2θθθθ

Bi-layer thickness

nm 991006-130 5 0.702 3.6 1.59 991001-128 12 0.670 3.7 1.59 991007-134 16 0.661 4.0 1.41 991006-132 20 0.667 4.5 1.52

Table 3: XRD data of MoS2-Au alloy specimens. Peaks observed between x and y correspond to that of the (10.0) reflection from MoS2 with measured ‘d’ spacings of 0.268 nm, which are smaller than that of the bulk value of 0.273 [20]. The coating deposited at

5 mT showed no detectable evidence of the (10.0) reflection for MoS2. The intensity ratio of the (00.2):(10.0) is observed to increase systematically with decreasing pressure as did the lifetime on the pin-on-disk tests. Thus, films with high (00.2):(10.0) ratio gave the best tribological performance as measured by pin-on-disk in 50 % RH air. The broad diffuse peaks in the coatings measured at 2θ values between 40.0° and 40.2° are clearly indicative of an amorphous structure. The results clearly indicate that the film consists of a mixture of a microcrystalline MoS2 phase and an amorphous gold phase with no evidence of a multi-layer architecture. There is also a minor reflection at 2θ = 59°, which can be identified as the edge oriented (11.0) reflection for MoS2.

SpecimenNumber

‘d’ spacing (10.0)

nm

FWHM (00.2) ββββ° 2θθθθ

2θθθθ ° position diffuse peak

FWHMDiffuse Peak ββββ° 2θθθθ

I(00.2)/I(10.0)

991006-130 - - 41.1 11 - 991001-128 0.262 6.7 41.2 11.40 1.99 991007-134 0.268 5.7 40.0 11.9 1.40 991006-132 0.266 7.4 40.4 12.4 0.75

Table 4: XRD data of MoS2-Au alloy specimens. 3.3.2 MoS2 - Al

Figure 9 shows low angle θ/2θ x-ray diffraction patterns (XRD) of MoS2-Al coatings deposited at pressures between 0.25 mT and 1.25 mT. Unlike the θ/2θ scan for the MoS2-Au coating, all XRD patterns show a distinct low angle peak indicative of a superlattice or multi-layer structure with periods between 1.41 nm and 2.0 nm. However, there were significant differences in the peak height, peak width (FWHM) and peak position in the coatings deposited at different pressures, Table 5. The coatings deposited at the lowest and highest vacuum had the sharpest peaks. In the sharpest peaks there was also indication of a distinct shoulder at the high angle side of the peak.

Figure 9: Low angle XRD scans Al-MoS2

SpecimenNumber

Deposition Pressure

(mT)

FWHM ββββ° 2θθθθ

Peak Height Counts

Bi-layer thickness

nm

990623-01 0.25 0.105 1348 1.59 990621-00 0.5 0.133 775 1.59 990625-00 0.75 0.12 563 1.41 990711-00 0.9 0.38 170 1.52 990622-03 1.05 0.31 350 2.0 990625-02 1.2 0.16 1051 1.59

Table 5: XRD data MoS2-Al specimens

In contrast, the coatings deposited in the middle vacuum range (deposited between 0.75 mT - 1.05 mT) exhibited the broadest peaks. A plot of the peak height vs average pin-on-disk lifetime (revolutions, in 50 % RH air), Figure 10 shows that the lifetime is inversely proportio-nal to the peak height of the low angle peak. It is well established that the presence of peak broadening and low relative intensities indicates that some level of interfacial mixing and roughness is present within the individual layers in the films [20]. On this basis therefore a maximum in roughness between individual MoS2/Al layers would be observed at deposition pressure of 1.05 mT.

Plot of Peak height vs avg., POD Lifetime

y = -1.4092x + 2284R2 = 0.8374

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Figure 10: Plot of the peak height vs average pin on

disk lifetime (revolutions, in 50 % RH air) Figure 11 shows high angle XRD patterns of MoS2-Al coatings deposited at pressures between 0.25 mT and 1.2 mT. In the XRD patterns in Figure 11 all coatings show diffuse peaks centred at 2θ ≅ 41° with full a width half-maximum (FWHM), β2θ ≅ 14.0°, which is close to the position of the {111} reflection for aluminium. Coatings deposited at pressures of 0.25 mT and 1.2 mT exhibit a very weak peak close to the position of the (00.2) reflection for MoS2. The sharp peaks present in the X-ray diffraction pattern are from the steel substrate material. The diffuse peaks present in the XRD pattern are typical of those obtained from an amorphous structure.

Figure 11: High angle XRD scans Al-MoS2

3.4 Transmission Electron Microscopy 3.4.1 MoS2-Au (Sample 991001-128) Figure 12 shows the cross-sectional microstructure of the MoS2-Au coating. The coating was 0.58 µm thick, having plane smooth surface and dense interface. EDX analysis in the MoS2-Au region indicated the presence of Mo, Au, and S. An electron diffraction pattern of the MoS2-Au coating is shown in Figure 12. The pattern is comprised of an inner ring and a diffuse outer halo which is consistent with the findings of the XRD studies, i.e. a broad peak at 2θ = 13.12° and a diffuse peak centred around 2θ = 41.2.

50 nm

Figure 12: Cross-sectional BF micrograph of MoS2-Au coating and an inserted SAD pattern of the MoS2-Au.

Uniformly distributed particles of 2.5 to 6 nm were observed in the MoS2-Au coating, Figures 12 and 13. Particles of these dimensions are consistent with the broad peaks identified as MoS2 in the XRD patterns. Figure 12 also shows a sharp interface between the coating and the substrate. An interfacial band 80nm thick within the coating was free from the particles indicating that during the initial stages the film develops

with an amorphous structure [19]. Using careful de-focus control (Frennel imaging) and a magnification of (750,000×), the MoS2-Au coating exhibited fringes having identical orientation (parallel to the coating surface) and a uniform period 0.89 nm, Figure 13. How-ever, no evidence of a multi-layer structure could be observed by XRD and indeed a multi-layered structure would not be predicted as unlike the MoS2-Al coating the MoS2-Au coating was deposited by static deposition using a composite MoS2+10 at.% Au target.

Figure 13: Bright field high-magnification micrograph

of MoS2-Au fringes. 3.4.2 MoS2 – Al (Sample 990711-00) A cross-sectional microstructure of the MoS2-Al coating is shown in Figure 14.

Figure 14: Cross-sectional BF micrograph of MoS2-Al

coating and an inserted SAD pattern of the MoS2-Al.

The coating was fully dense almost featureless, 1.6 µm thick, and having smooth surface and clear interface with the steel substrate. An inserted SADP (selected area electron diffraction) pattern in Figure 14 shows broad halos indicative of an amorphous structure of the MoS2-Al coating. The coating-substrate interface was further imaged at higher magnification, Figure 15.

Figure 15: Interface between MoS2-Al coating and steel

substrate. An interface layer, of uniform thickness 17 nm, was observed between the normal steel substrate and the coating. This interfacial layer is caused by Cr-ion etching prior to the deposition and is beneficial to the bonding. Energy dispersive analysis (EDX) in the MoS2-Al coating showed it to consist of Al, Mo and S.

Figure 16: BF high-magnification micrograph of MoS2-

Al multilayer fringes. Figure 16 is a bright field (BF) micrograph at the highest magnification, showing de-focussed Frennel fringes parallel to each other and to the coating-

substrate interface. These fringes exhibited a period 1.47 nm. This measurement is clearly consistent with the sharp peak identified in the low angle region of the XRD pattern at 6.23° corresponding to a period of 1.51 nm.

These results clearly show that the MoS2-Al coating evolved with a multi-layer architecture. A multi-layered coating would be predicted considering that the coatings were deposited using 3 separate targets (2 MoS2 and 1 Al) and that the specimens were subjected to con-tinuous rotation during deposition. Thus, alternate layers of MoS2 and Al will be deposited as the sample passes in front of each target. 4 CONCLUSIONS XRD and TEM analysis support the theory that two different structures are believed to exist in the MoS2 alloys studied: MoS2-Au alloys exhibited periodic distributed Au particles of 2.5 to 6 nm while MoS2-Al coatings evolved with a multi-layer architecture. This is believed to account for some of the dramatic differences in the lifetime properties and mechanical performance differences in each alloy.

Lifetime pin-on-disk studies indicate tribological per-formance is enhanced two-fold in MoS2-Au alloy when the processing pressure in a magnetron sputtering process is maintained at 12 mT. Similarly, when pro-cessing at 12 mT, the specimen-to-target distance of 80 mm enhances the tribological properties significantly which we believe is directly due to corresponding microstructural differences. This is supported by XRD analysis that indicates intensity ratios are highest at the lower application pressures apart from the 5 mT sample which showed no detectable evidence of (10.0) reflection from MoS2. The fact that XRD data indicates that higher process pressures result in more amorphous coatings may explain the higher Vickers hardness results for MoS2-Au alloys processed at 12 mT.

Pin-on-disk tribological performance has been shown to be highest in MoS2-Al coatings that have been produced in a multi-target rotation process at pressures 1 mT. This may be explained by the fact that at these levels interfacial mixing and roughness present within the individual layers are greatest. Also, this might aid in the nanohardness results that indicate Vickers hardness to be greatest in the region of 1 mT.

Finally, an important outcome of this study is that application of these materials in industrial practice results high tribological lifetimes at 50 % RH can be obtained by (1) creating nanostructure multi-layers using rotating deposition systems, e.g. MoS2 and Al, and (2) by creating nano-compounds using pre-mixed targets, e.g., MoS2 and Au. 5 REFERENCES [1] Spalvins, T., “Morphological and frictional behavior of sputtered MoS2 films”, Thin Solid Films, 96(1982) 17-24. [2] Spalvins, T., “Frictional and morphological properties of Au-MoS2 films sputtered from a compact target”, Thin Solid Films, 118 (1984) 375-384.

[3] Buck, V., “Preparation and properties of different types of sputtered MoS2 films”, Wear, 114 (1987) 263-274. [4] Christy, R.I., Ludwig, H.R., “R.F: Sputtered MoS2 Parameter Effects on Wear Life”, Thin Solid Films, 64 (1979) 223-229. [5] Shibaike, N., Takeuchi, H., Nakamura, K., Shimizu, N., “Approach to higher reliability in 3D micro-mechanisms” in Micromachining Technology for Micro-Optics, Lee, S. and Johnson, E., Eds., Proc. of SPIE, 4179 (2000) 170-178. [6] Fleischauer, P.D., Bauer, R., “Chemical and structural effects on the lubrication properties of sputtered MoS2 films”, Trib. Trans., 31 (1988) 239-250. [7] Moser, J., Levy, F., “Growth mechanisms and near-interference structure in relation to orientation of MoS2 sputtered thin films”, J. Mat. Res., 7(3) (1992) 734-740. [8] Zabinski, J.S., Donley, M.S., Walch, S.D., Schneider, T.R., Mcdevitt, N.T., “The effects of dopants on the chemistry and tribology of sputter-deposited M Films”, Trib. Trans., 38(4) 1992 894-904. [9] Lince, J.R., Hilton, M.R., Bommannavar, A.S., “Metal incorporation in sputter-deposited films studied by extended x-ray absorption fine structure”, J. Mat. Res., 10(8) (1995) 2091-2105. [10] Hilton, M.R., Fleischauer, P.D., “Structural Studies of Sputter-Deposited MoS2 solid lubricant films”, in L. Pope, L. Fehrenbacher, and W.Winer (eds.), New Materials Approaches to Tribology: Theory and Applications: Mat. Res. Soc. Proc., V. 140, Mat. Res. Soc., 1989 227-238. [11] Jayaram, G., Marks, L.D., Hilton, M.R., “Nano-structure of Au-20% Pd layers in MoS2 multilayer solid lubricant films”, Surf. And Coat. Tech., 76 77 (1995) 393-399. [12] Dunn, D.N., Wahl, K.J., Singer, I.L., “Nano-structural Aspects of Wear in Ion-Beam Deposited Pb-Mo-S Films”, MRS Proc. V. 522, Moody, E., Gerberich, W.W., Burnham, N., Baker, S.P., Eds., (1998) 451-456. [13] Mattern, N., Hermann, H., Weise, G., Teresiak, A., Bauer, H.D., Mat. Sci. Forum V 235-238 8 (1997) 613-618. [14] Simmons, M.C., Savan, A., Pflüger, E., Van Swygenhoven, H., “Microstructure and tribological performance of MoS2 /Au Co-Sputtered Composites” , Surf. and Coat. Tech. 126 (2000) 15-24. [15] Wahl, J.J., Belin, M., Singer, I.L., “A Triboscopic Investigation of the Wear and Friction of MoS2in a Reciprocating Sliding Contact”, Wear, 214, 2 (1998) 212-220. [16] Doerner, M.F., Nix, W.D., J. of Mat. Res. 1, (1986) 601. [17] Oliver, W.C., Pharr, G.M., J. of Mat. Res. 7, (1992) 1564. [18] JCPDS-ICDD, Newtown Sq, PA 19073, USA [19] Lince, J.R., Hilton, M.R., Bommannavar, A.S., J. Mater. Res. 2 (1987) 827. [20] Håkansson, G., PhD. Dissertation, 255, Linköping University, Linköping, (1991).