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Tribology of tungsten disulfide films in humid environments:
Wear 230 Ž1999. 24–34
Tribology of tungsten disulfide films in humid environments: The role of a tailored metal-matrix composite substrate S.V. Prasad
a,)
, N.T. McDevitt b, J.S. Zabinski
c
a
c
Research Institute, UniÕersity of Dayton, Dayton, OH 45469-0168, USA b RAMSPEC Research, Dayton, OH 45431, USA Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRLr MLBT), Wright Patterson Air Force Base, OH 45433-7750, USA Received 12 October 1998; received in revised form 14 October 1998; accepted 8 December 1998
Abstract As a result of tribo-induced oxidation, tungsten disulfide ŽWS 2 . loses its lubricating behavior in humid environments. The purpose of this study is to explore the role of a tailored metal-matrix composite ŽMMC. substrate in imparting oxidation resistance to WS 2 films in sliding contact. The substrate is an aluminum MMC disk reinforced with 20 vol.% silicon carbide ŽSiC. particles. The MMC disk was metallographically polished and etched to create SiC particle protrusions. The films were grown on the MMC substrates using a pulsed laser. Friction and wear tests were performed in dry nitrogen and in humid air with 90% relative humidity. The counterface was a 440C steel ball. The wear scars and third-body transfer films were characterized by scanning electron microscopy and Raman spectroscopy. In dry nitrogen, the friction coefficient of WS 2 films on MMC substrates was low Ž0.035–0.050., indicating that the carbide protrusions did not adversely affect the lubricating behavior. In humid air, the friction coefficient of WS 2 films on polished steel substrates increased to 0.4 during the first 1000 cycles of sliding, whereas the ones on MMC substrates lasted for the entire duration of 50,000-cycle tests, with friction coefficients ranging from 0.15 to 0.22. The counterface wear, or scratching of the steel ball by SiC, was practically absent. The role of carbide protrusions in controlling chemically assisted crack propagation is discussed. The implications of this work as a model study for the design of thin film composite coatings is highlighted. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Tungsten disulfide; Carbide protrusions; Pulsed laser deposition
1. Introduction Transition metal dichalcogenides, MX 2 Žwhere M is molybdenum or tungsten, and X is sulfur, selenium or tellurium., are well-known for their solid lubricating behavior. Compared with molybdenum disulfide ŽMoS 2 ., the most popular member of the transition metal dichalcogenide family, tungsten disulfide ŽWS 2 ., provides an approximately 1008C increase in maximum operating temperature w1,2x. The basic mechanisms of lubrication by tungsten disulfide are similar to those of the well-researched molybdenum disulfide w3–6x. In dry environments, WS 2 provides a low friction surface Ž m s 0.03–0.05. and prolonged wear life w7,8x. Transfer films formed in dry environments are typically smooth, tenacious and firmly adherent to the counterface w8x. Like MoS 2 w5,6x, tungsten disulfide loses its lubricating behavior in humid environments. Our previ)
Corresponding author
ous study on the friction behavior of pulsed laser-deposited ŽPLD. WS 2 films showed that in humid environments, WS 2 films oxidize to WO 3 resulting in an increase in friction coefficient Ž0.10–0.15. w7x. Tungsten trioxide, WO 3 , is not lubricious at room temperature. Moreover, its presence in the sliding contact interferes with transfer film formation. In fact, the transfer films formed in humid environments were reported to be patchy and powdery in nature w7x. Hardness and topography of the substrate can also influence the tribological behavior of transition metal dichalcogenide films w9,10x. For example, Fusaro reported that roughened substrates improved the wear life of burnished MoS 2 films w10x. This was attributed to the presence of valleys on the rough substrate which served as reservoirs for MoS 2 . Hiraoka and Sasaki w11x reported that the presence of a discontinuous hard under-coating Že.g., chrome., between an aluminum substrate and a bonded MoS 2 solid lubricant coating, could result in much improved wear life of the lubricant film under extreme
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 0 8 2 - 4
S.V. Prasad et al.r Wear 230 (1999) 24–34
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contact pressure. Lancaster investigated the role of substrate hardness on the endurance of MoS 2 films and showed that the rate of MoS 2 transfer to the counterface increased with softer substrates, while hard substrates facilitated the formation of uniform transfer films w9x. The above studies highlight the significance of an engineered substrate in optimizing the tribological performance of a solid lubricant coating. An ideal substrate should be a dual-phase one which could generate microscopic reservoirs of the lubricant film. In recent years, a number of discontinuously reinforced metal-matrix composites ŽMMCs. have been developed for various engineering Žincluding tribological. applications w12x. The MMCs present a unique opportunity to prepare novel substrates with hard second-phase particles protruding from a relatively softer metal matrix. The hard particle protrusions on the substrate could be effective in deflecting or arresting a propagating crack in a tungsten disulfide film. Secondly, the second-phase particle protrusions provide a unique opportunity to create microscopic reservoirs of the solid lubricant film. The substrate under consideration here is an aluminum MMC reinforced with silicon carbide ŽSiC. particles. The feasibility of depositing WS 2 films on a tailored aluminum MMC substrate by pulsed laser deposition ŽPLD. technique has been previously reported w13x. Under humid conditions, the PLD WS 2 films deposited on a tailored MMC substrate had much longer wear life as compared with those on pure metal substrates w13x. The objective of the current study was to understand the role of carbide protrusions in MMC substrate on the tribological performance tungsten disulfide films in humid environments. The WS 2 films were deposited using a pulsed laser.
2. Experimental 2.1. Preparation of the substrate An aluminum metal-matrix composite ŽMMC. reinforced with 20 vol.% SiC particles Žaverage particle size s 13 mm. was used as the substrate. The matrix of the MMC was a hypoeutectic Al–7.0 wt.% Si alloy. The composite was produced by a casting route and supplied by Duralcan ŽUSA. in as cast condition. Aluminum MMC disks of 30 mm diameter were cut from the cast ingots and polished using standard metallographic procedures. An optical micrograph of a polished MMC surface showing the distribution SiC particles is given in Fig. 1a. The polished surface was etched with Keller’s reagent w15x for 1 min. An optical micrograph of an etched MMC surface is shown in Fig. 1b. Particle protrusions ŽSiC and eutectic Si. can be seen in Fig. 1b. The micropits on primary aluminum ŽFig. 1b. were deliberately created to provide sites for mechanical interlocking of the PLD WS 2 film with the aluminum MMC substrate. A typical profilometer trace of the etched
Fig. 1. Microstructures of the Al–Si alloy MMC reinforced with 20 vol.% SiC Žaverage particle sizes13 mm. particles: Ža. metallographically polished surface; Žb. etched surface showing eutectic silicon and SiC particle protrusions; Žc. a typical profilometer trace of the etched MMC surface.
MMC surface is given in Fig. 1c. The average surface roughness, R a , is 0.2 mm, while the typical carbide protrusion is 1 mm high. 2.2. Pulsed laser deposition The target was fabricated by cold-pressing the WS 2 powder Ž99.8% purity. into 25 mm diameter disks at an
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X-ray spectroscope. After the wear test, the steel ball was dismounted from the tribometer and the transfer film was analyzed by SEM. Wear scars and transfer films were also analyzed by Raman spectroscopy. An intensified 1024 element diode array detector was used to collect the Raman signal.
3. Results
Fig. 2. A typical SEM micrograph of a tungsten disulfide film deposited on an etched Al MMC surface.
applied pressure of 50 MPa. An excimer laser charged with KrF was used to provide 248 nm radiation to the target at a fluence of 15 KJrm2 and a pulse rate of 10 Hz. The deposition chamber was maintained at an initial base pressure of 8 = 10y7 Pa. A calibrated quartz crystal oscillator was used to measure deposition rates and film thickness. The substrate was kept at room temperature during film deposition. Films were grown to a thickness of 1 mm. A more complete description of growing WS 2 solid lubricant films by the PLD technique is given elsewhere w14x. 2.3. Friction tests Friction measurements were made using a ball-on-disk tribometer in which a stationary ball was held on a rotating disk. A more complete description of the tribometer is given elsewhere w7x. A 3.125 mm Ž1r8Y . diameter 440C steel ball was used as the counterface. The normal load on the ball was 1 N; the maximum initial Hertzian contact stress between a 3.125 mm Ž1r8Y . diameter steel ball and SiC works out to be 830 MPa. The ball was held in a lever arm, and the friction force was measured using a strain gauge circuit. For each friction track, the rotational speed was adjusted to get a constant sliding speed of 50 mmrs. For comparison, friction measurements were also made on WS 2 films deposited on polished 440C steel substrates. In order to maintain a constant humidity level in the test environment, the pin-disk assembly was enclosed in a chamber. The relative humidity in the test environment could be controlled from near zero to almost 100% RH by regulating the relative flow rates of dry air and saturated air into the enclosure. The percentage relative humidity was measured by a high-performance humidity sensor inserted into the chamber. Friction measurements were made in dry nitrogen and in humid air with 90 Ž"5.% RH. Measurements were made at room temperature.
An SEM micrograph of a typical WS 2 film on an aluminum MMC substrate is shown in Fig. 2. All the constituent phases of the MMC, namely silicon carbide protrusions, eutectic silicon and aluminum, were covered by the film. A total of six wear tracks, two per film, were made in each environmental condition. In dry nitrogen, the friction coefficient of WS 2 films ranged from 0.035 to 0.050; the substrate had no influence. A typical friction trace on an MMC substrate in dry nitrogen is given in Fig. 3. It can be seen that the friction trace is smooth, and carbide protrusions did not adversely influence the tribological behavior of the film. However, in humid air Ž90% RH., the friction coefficient of the film on a steel substrate increased to 0.4 during the first 1000 cycles of sliding, as can be seen from Fig. 4b. By contrast, the films on MMC substrates lasted the whole duration of 50,000-cycle tests with friction coefficients ranging from 0.18 to 0.22. A typical friction trace of the film on an MMC substrate is given in Fig. 4a. An SEM micrograph of a 10,000-cycle wear scar of the film on MMC substrate is given in Fig. 5a. The test was conducted in humid air Ž90% RH.. The dark particles in the wear scar correspond to SiC protrusions from the MMC substrate. Fig. 5b is the SEM of the wear scar after 50,000 cycles of sliding in humid air. Compared with the 10,000-cycle wear scar, more carbide particles can be seen in the 50,000-cycle wear scar. In fact, the 50,000-cycle
2.4. Analyses of wear surfaces Wear scars on the films were analyzed by a scanning electron microscope equipped with an energy dispersive
Fig. 3. A typical friction trace of a PLD WS 2 film on MMC substrate in dry nitrogen.
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Fig. 4. Friction traces of WS 2 films in humid air Ž90% RH.. Ža. MMC substrate; Žb. 440C steel substrate.
wear scar looks like a composite of SiC particle-reinforced WS 2 . An SEM micrograph of a wear scar on the steel ball from the 10,000-cycle wear test on WS 2 –MMC film in humid air is shown in Fig. 6. The bright portion in the micrograph corresponds to the debris sticking to the ball. It should be noted that carbide protrusions from the substrate did not introduce deep scratches on the steel ball. The steel ball was cleaned ultrasonically in isopropanol to remove the loose debris, and the wear scar on the steel ball was
reexamined in the SEM. Fig. 7a is a backscattered electron image of the cleaned steel ball. There are several dark patches on the wear scar. Fig. 7b is a higher magnification backscattered electron image showing the wear pattern on the steel ball at the microscopic level. It can be seen that the dark island at the center of the micrograph is almost free of scratches, and the surrounding region has shallow microscratches. The dark phase is different in composition from the surrounding matrix, as revealed by the energy dispersive X-ray analysis. Fig. 8a is the EDX spectrum
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Fig. 5. SEM micrographs of wear scars on WS 2 films generated in humid environment: Ža. 10,000-cycle wear scar; Žb. 50,000-cycle wear scar.
collected in spot mode from point ‘a’ on the dark phase. It can be seen that the Cr K a peak is much stronger than the Fe K a peak. Also, the spectrum in Fig. 8a has a small peak corresponding to sulfur K a. Fig. 8b is an EDX spectrum from the surrounding matrix. Here the intensity of Fe K a peak is much higher than that of the Cr K a peak. The tiny Si peak in Fig. 8b may be due to the silicon
Fig. 6. SEM micrograph of the wear scar on steel ball after 25,000 cycles of sliding on a WS 2 film on MMC substrate.
Fig. 7. Higher magnification micrographs of Fig. 6 in backscattered electron image mode.
carbide wear debris. Carbon could not be detected because a beryllium window EDS detector was used. The contrast levels in backscattered electron images are strongly dependent on atomic numbers. Phases with higher average atomic numbers appear brighter than those with lower atomic numbers. This implies that the dark island in Fig. 7b has a lower average atomic number than that of the surrounding matrix. Since the EDX spectrum from the dark island showed much stronger Cr K a peak, the dark phase on the wear scar of the steel ball can be identified as chrome carbide in 440C stainless steel. A typical Raman spectrum of the WS 2 film on MMC substrate is shown in Fig. 9a, while the one in Fig. 9b corresponds to the cold-pressed WS 2 target. The spectrum from the aluminum MMC substrate is shown in Fig. 9c. The spectrum from WS 2 target has clear bands at 355 cmy1 and 420 cmy1 . The spectrum from the film on MMC is featureless ŽFig. 9a.. As reported in our previous study w7x, PLD WS 2 films, in as-deposited condition, do not possess sufficient long-range order to give rise to Raman peaks. However, during the initial stages of sliding, stress-induced crystallization occurs in the wear scar resulting in clear Raman peaks, 355 cmy1 and 420 cmy1 w7x. It should also be noted that the features from the substrate
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Fig. 8. EDX spectra corresponding to Fig. 7b: Ža. from point ‘a’ on the dark island; Žb. a typical spectrum from the surrounding region.
ŽFig. 9c. are absent in the spectrum from the film indicating that the film has completely masked the carbide and silicon protrusions. A typical Raman spectrum of a 25,000-cycle wear scar of PLD WS 2 film on MMC substrate generated in humid air is given in Fig. 10a. The
Fig. 9. Raman spectra: Ža. PLD WS 2 film on MMC substrate; Žb. WS 2 target; Žc. MMC substrate.
peaks from the substrate ŽSi, SiC. can be seen in the spectrum of the wear scar. Also, the bands at 355 cmy1 and 420 cmy1 indicate that sliding induced crystallization of WS 2 on MMC substrate even in humid air. The wear scar on the steel ball showed the transfer of crystallized WS 2 . The Raman spectrum from the wear scar of the film on steel substrate in humid air produced bands at 244, 715 and 809 cmy1 representing tungsten trioxide, WO 3 , as shown in Fig. 11a. The transfer film on steel ball from this
Fig. 10. Raman spectra from wear surfaces of WS 2 film on MMC substrate: Ža. wear scar; Žb. transfer film on the steel ball. Environment: humid air.
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Fig. 11. Raman spectra from wear surfaces of WS 2 film on steel substrate: Ža. wear scar; Žb. transfer film on the steel ball. Environment: humid air.
experiment also produced a similar Raman spectrum with bands at 244, 715 and 809 cmy1 ŽFig. 11b., confirming that when the substrate is steel, the transfer film formed in humid air is comprised of WO 3 . 4. Discussion The current study has shown that the tribological behavior of tungsten disulfide ŽWS 2 . films in humid air can be improved if the films were deposited on tailored metal-matrix composites with hard particle protrusions. Such improvements could extend the scope of their use in groundbased engineering applications operating in humid environments. In space environments, where transition metal
dichalcogenide-based solid lubricants such as MoS 2 are currently being used, humidity may not be a major concern; however, moisture sensitivity could be a serious issue during ground testing and extended periods of storage, and in reusable launch vehicles w16,17x. Before discussing the specific role of the substrate, we shall briefly review the mechanisms of lubrication by transition metal dichalcogenides. It is widely accepted that the lubricating behavior of WS 2 stems from its interlamellar mechanical weakness which is intrinsic to its crystal structure. Tungsten disulfide crystallizes in the hexagonal structure in which a sheet of tungsten atoms is sandwiched between two hexagonally packed sulfur layers. The bonding within the sandwich is covalent, whereas the sandwiches are held together by weak Van der Waals forces, resulting in interplanar mechanical weakness. Under the action of a shear force, intracrystalline slip occurs in the weak interplanar regions. This mechanism is responsible for the formation of smooth transfer films by wear. The new surfaces, created by separating the weakly bonded sandwiches are quite inert. They can easily slide back and forth over one another Žby intercrystalline slip., thereby providing lubrication. The major obstacle to lubrication by WS 2 Žor transition metal dichalcogenides. is the presence of unsaturated or dangling bonds. In layered structures, an edge plane is the most obvious source of unsaturated bonds. This problem is usually suppressed either by adding dopants that give rise to fibrous morphology with no long-range order, or by incorporating metal multilayers yielding dense morphology with strong basal plane orientation w16–19x. The presence of defects and cracks in the film is another source of unsaturated bonds. Even the freshly cleaved Ž0001. basal planes of single crystal transition metal dichalcogenides are known to contain nanometerscale surface defects w20x. Studies on wear and oxidation by atomic force microscopy showed that wear actually
Fig. 12. Ža. Schematic illustration of the tribo-induced oxidation of WS 2 film in humid environments w3x.
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Fig. 13. Ža. Schematic illustration of crack deflectionrarresting by second-phase particles in the film; Žb. cross-section of the film showing carbide protrusions in the MMC substrate. Note that carbide protrusions can also arrest the cracks that nucleate at the substraterfilm interface.
proceeds at these defects w20x. In addition to the nanometer-scale surface defects, most engineering films contain a population of micron and submicron-sized cracks. As depicted schematically in Fig. 12, propagation of a randomly oriented crack during sliding contact will break the covalent bonds within the S–W–S sandwich and create activated surfaces with unsaturated bonds. The activated surfaces can instantly react with moisture in the surrounding environment, forming WO 3 ŽFig. 12.. Also, diffusion of gaseous species along a crack path can accelerate crack propagation and further increase the rate of oxidation. The situation is analogous to the environmental stress corrosion cracking of glass w21x.
and arrest a propagating crack in the lubricating matrix. The film in the current study is a pure single phase WS 2 . However, due to carbide protrusions on the MMC substrate, the initial contact will be between the steel ball and
4.1. Crack arresting One way to impart tribo-oxidation resistance to a tungsten disulfide film is to improve its fracture toughness. In principle, this can be accomplished by arresting a propagating crack, as depicted schematically in Fig. 13a. The film in this schematic illustration has a dual-phase microstructure where the second phase can effectively deflect
Fig. 14. SEM micrograph of a 10,000-cycle wear scar on WS 2 film ŽMMC substrate. generated in humid air.
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Cracks can nucleate along the film–substrate interface, as shown schematically in Fig. 13b. Intersection of such cracks with those within the film ŽFig. 13a. can result in spallation and delamination of the film, thereby reducing the wear life. In a recent study, Hilton w22x investigated the fracture behavior of sputter-deposited MoS 2 films using brale indentation tests. This study showed that in addition to radial and circumferential cracks, delamination could play a role in the overall fracture behavior of MoS 2 films during brale indentation tests. In the current study, the protruding carbides from the substrate can effectively arrest the cracks that nucleate at the film–substrate interface ŽFig. 13b.. 4.2. Microscopic reserÕoirs By judicious control of the size, height and area fraction of carbides, microscopic reservoirs of solid lubricant can be created on the surface of the substrates. Creation of such reservoirs on the substrate facilitates the retention and recirculation of WS 2 particles in the contact zone ŽFig. 16., instead of gradual wearing away of the film as in the case of a polished steel substrate. 4.3. Third-bodies and counterface wear Fig. 15. SEM micrographs of a 50,000-cycle wear of a WS 2 film on MMC substrate showing: Ža. crack arresting; Žb. crack deflection by carbide particles in the substrate. Environment: humid air.
microscopic regions of WS 2 on SiC particle protrusions. Once these regions of WS 2 wear out or get pushed into the valleys between the carbide protrusions, the wear track essentially becomes a composite of carbides in a WS 2 matrix as can be seen from the SEM of a 10,000-cycle wear track in Fig. 14. Typical SEM micrographs of a 50,000-cycle wear scar showing the crack front in the vicinity of carbide particles are shown in Fig. 15 as illustrations of crack deflection and arrest.
In addition to crack arresting, carbides can roughen the steel counterface, as can be seen from the SEM micrograph in Fig. 7b. The fresh surface thus created is ideally suited for improved adhesion of the solid lubricant transfer film. It must be noted that carbides did not cause excessive wear or scratching of the counterface. In fact, the 440C steel resembles a chemically etched surface where the carbide phase in steel is clearly revealed. Mild roughening followed by formation of transfer film on the steel counterface must be responsible for providing lubricated contacts with minimum damage to the counterface. In the absence of a lubricant, carbides can cause excessive damage to steel. As an example, an SEM micrograph of a wear scar
Fig. 16. Schematic illustration of the microscopic reservoir concept.
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Fig. 19. SEM of the wear scar from the friction test shown in Fig. 18. Fig. 17. SEM micrograph of a wear scar on steel ball that was rubbed on an uncoated Al–Si–SiC MMC surface for 1000 cycles of sliding.
on a steel ball that was used to rub an uncoated Al MMC disk for 1000 cycles of sliding is given in Fig. 17. 4.4. The role of an etched MMC substrate A new set of experiments was performed to check whether etching alone could have produced a lubricious layer on the MMC surface. A friction test was conducted on an uncoated MMC disk that was etched with Keller’s reagent for 1 min. Fig. 18 shows that the friction coefficient of the etched MMC disk is high. Similarly, the test
produced a rough wear scar ŽFig. 19., typical of an unlubricated metal-to-metal contact. Similarly, our previous study w13x showed that the tribological behavior of WS 2 films on pure aluminum substrates in humid air was similar to that shown in Fig. 4b. So, the improved tribological performance of the WS 2 film on an etched Al MMC substrate seen in Fig. 4a is not due to etching alone. Although Raman analysis showed that the film in the wear track is unoxidized ŽFig. 10., the friction coefficient of WS 2 on an MMC substrate in humid environment is higher, 0.15–0.22, as compared with the friction coefficient of the same film in dry nitrogen ŽFig. 3.. This may be due to the morphology of transfer films. In addition to
Fig. 18. Friction trace on an etched Al MMC disk without the WS 2 film.
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chemistry, morphology of the transfer films on the counterface plays a significant role in controlling friction. Although the use of the MMC substrate reduced tribooxidation of WS 2 in humid environments, it did not facilitate the formation of smooth transfer films on the steel counterface, similar to the ones seen in dry nitrogen w7x. 4.5. Implications The current work has demonstrated the feasibility of controlling the chemically assisted fracture of tungsten disulfide by using engineered substrates. In addition to the one described here, a number of other metal-matrix composites are commercially available w11,12x; their surfaces can be suitably tailored to derive optimum tribological behavior of the transition metal dichalcogenide films deposited on them. The MMC technology offers additional advantage of reinforcing the materials with high strengthrhigh modulus fibers w23x. Thus, a substrate can be engineered in such way that the bulk meets the mechanical and physically property requirements of the intended tribocomponent, while the surface is enriched with hard particle protrusions to optimize the tribological behavior of the solid lubricant films. Finally, the results of this study provide insights for the design of thin film composite coatings. 5. Conclusions Ž1. Tungsten disulfide solid lubricant films can be grown on tailored aluminum metal-matrix composite ŽMMC. substrates by pulsed laser deposition. Ž2. In dry nitrogen, the tribological behavior of the WS 2 films on MMC substrates is very similar to the behavior of pulsed laser-deposited films on polished steel substrates. Ž3. In humid air Ž90% RH., the films on steel substrates failed during the first 500–1000 cycles of sliding, whereas the ones on MMC substrates lasted for the entire duration of a 50,000-cycle wear test with friction coefficients ranging from 0.15–0.22. Ž4. The carbide protrusions were found to influence the crack propagation in WS 2 films during sliding contact without causing excessive counterface wear or scratching. References w1x S.F. Murray, S.J. Calabrese, Effect of solid lubricants on low speed sliding behavior of silicon nitride at temperatures to 8008C, Lubr. Eng. 49 Ž1993. 955–964.
w2x H.E. Sliney, Decomposition kinetics of some solid lubricants determined by elevated temperature X-ray diffraction techniques, Proc. of the US Air Force Aerospace Fluids Lubr. Conf., 1963, pp. 350–367. w3x W.O. Winer, Molybdenum disulfide as a lubricant: a review of the fundamental knowledge, Wear 10 Ž1967. 422–452. w4x A.W.J. DeGee, G. Solomon, J.H. Zaat, On the mechanisms of MoS 2 film failure in sliding friction, ASLE Trans. 8 Ž1965. 156–163. w5x J.K. Lancaster, A review of the influence of environmental humidity and water on friction, lubrication and wear, Tribology International 23 Ž1990. 371–389. w6x C. Pritchard, J.W. Midgley, The effect of humidity on the friction and life of unbonded molybdenum disulfide, Wear 13 Ž1969. 39–50. w7x S.V. Prasad, J.S. Zabinski, N.T. McDevitt, Friction behavior of pulsed laser deposited tungsten disulfide films, Tribology Transactions 38 Ž1995. 57–62. w8x S.V. Prasad, J.S. Zabinski, Tribology of tungsten disulfide: characterization of wear-induced transfer films, J. Mater. Sci. Lett. 12 Ž1993. 1413–1415. w9x J.K. Lancaster, The influence of substrate hardness on the formation and endurance of molybdenum disulfide films, Wear 10 Ž1967. 103–107. w10x R.L. Fusaro, Effect of substrate finish on the lubrication and failure mechanisms of molybdenum disulfide films, ASLE Trans. 25 Ž1982. 141–156. w11x N. Hiraoka, A. Sasaki, Effect of discontinuous hard under-coating on the life of solid film lubricant under extreme contact pressure, Tribology International 30 Ž1997. 344–429. w12x S.V. Prasad, P.K. Rohatgi, Tribological properties of Al alloy particle composites, J. Met. 39 Ž1987. 22–26. w13x S.V. Prasad, J.S. Zabinski, V.J. Dyhouse, Pulsed laser deposition of tungsten disulfide on aluminum metal-matrix composites, J. Mater. Sci. Lett. 11 Ž1992. 1282–1284. w14x J.S. Zabinski, M.S. Donley, S.V. Prasad, N.T. McDevitt, Synthesis and characterization of tungsten disulfide films grown by pulsed laser deposition, J. Mater. Sci. 29 Ž1994. 4834–4839. w15x Metallography, structures and phase diagrams, ASM Metals Hand Book, Vol. 8, Metals Park, OH, 1973, p. 124. w16x M.R. Hilton, P.D. Fleischauer, Applications of solid lubricant films in spacecraft, Surface and Coatings Technology 54r55 Ž1992. 435– 441. w17x T. Spalvins, Lubrication with Sputtered MoS 2 Films: Principles, Operation, Limitations, NASA TM-105292, 1991. w18x G. Jayaram, L.D. Marks, M.R. Hilton, Nanostructure of Au–20% Pd layers in multilayer solid lubricant films, Surface and Coatings Technology 76r77 Ž1995. 393–399. w19x J.S. Zabinski, M.S. Donely, N.T. McDevitt, Mechanistic study of the synergism between Sb 2 O 3 and MoS 2 lubricant system using Raman spectroscopy, Wear 165 Ž1993. 103–108. w20x Y. Kim, J.L. Huang, C.M. Leiber, Characterization of nanometer scale wear and oxidation of transition metal dichalcogenide lubricants by atomic force microscopy, Appl. Phys. Lett. 59 Ž26. Ž1991. 3404–3406. w21x B.R. Lawn, T.R. Wilshaw, Fracture of Brittle Solids, Chap. 8, Cambridge Univ. Press, Cambridge, 1975. w22x M.R. Hilton, Fracture in MoS 2 solid lubricant films, Surface and Coatings Technology 68r69 Ž1994. 407–415. w23x S.V. Prasad, K.R. Mecklenburg, Friction behavior of ceramic fiberreinforced aluminum metal-matrix composites against a 440C steel counterface, Wear 162r164 Ž1993. 47–56.
Tribology of tungsten disulfide films in humid environments: The role of a tailored metal-matrix composite substrate S.V. Prasad
a,)
, N.T. McDevitt b, J.S. Zabinski
c
a
c
Research Institute, UniÕersity of Dayton, Dayton, OH 45469-0168, USA b RAMSPEC Research, Dayton, OH 45431, USA Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRLr MLBT), Wright Patterson Air Force Base, OH 45433-7750, USA Received 12 October 1998; received in revised form 14 October 1998; accepted 8 December 1998
Abstract As a result of tribo-induced oxidation, tungsten disulfide ŽWS 2 . loses its lubricating behavior in humid environments. The purpose of this study is to explore the role of a tailored metal-matrix composite ŽMMC. substrate in imparting oxidation resistance to WS 2 films in sliding contact. The substrate is an aluminum MMC disk reinforced with 20 vol.% silicon carbide ŽSiC. particles. The MMC disk was metallographically polished and etched to create SiC particle protrusions. The films were grown on the MMC substrates using a pulsed laser. Friction and wear tests were performed in dry nitrogen and in humid air with 90% relative humidity. The counterface was a 440C steel ball. The wear scars and third-body transfer films were characterized by scanning electron microscopy and Raman spectroscopy. In dry nitrogen, the friction coefficient of WS 2 films on MMC substrates was low Ž0.035–0.050., indicating that the carbide protrusions did not adversely affect the lubricating behavior. In humid air, the friction coefficient of WS 2 films on polished steel substrates increased to 0.4 during the first 1000 cycles of sliding, whereas the ones on MMC substrates lasted for the entire duration of 50,000-cycle tests, with friction coefficients ranging from 0.15 to 0.22. The counterface wear, or scratching of the steel ball by SiC, was practically absent. The role of carbide protrusions in controlling chemically assisted crack propagation is discussed. The implications of this work as a model study for the design of thin film composite coatings is highlighted. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Tungsten disulfide; Carbide protrusions; Pulsed laser deposition
1. Introduction Transition metal dichalcogenides, MX 2 Žwhere M is molybdenum or tungsten, and X is sulfur, selenium or tellurium., are well-known for their solid lubricating behavior. Compared with molybdenum disulfide ŽMoS 2 ., the most popular member of the transition metal dichalcogenide family, tungsten disulfide ŽWS 2 ., provides an approximately 1008C increase in maximum operating temperature w1,2x. The basic mechanisms of lubrication by tungsten disulfide are similar to those of the well-researched molybdenum disulfide w3–6x. In dry environments, WS 2 provides a low friction surface Ž m s 0.03–0.05. and prolonged wear life w7,8x. Transfer films formed in dry environments are typically smooth, tenacious and firmly adherent to the counterface w8x. Like MoS 2 w5,6x, tungsten disulfide loses its lubricating behavior in humid environments. Our previ)
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ous study on the friction behavior of pulsed laser-deposited ŽPLD. WS 2 films showed that in humid environments, WS 2 films oxidize to WO 3 resulting in an increase in friction coefficient Ž0.10–0.15. w7x. Tungsten trioxide, WO 3 , is not lubricious at room temperature. Moreover, its presence in the sliding contact interferes with transfer film formation. In fact, the transfer films formed in humid environments were reported to be patchy and powdery in nature w7x. Hardness and topography of the substrate can also influence the tribological behavior of transition metal dichalcogenide films w9,10x. For example, Fusaro reported that roughened substrates improved the wear life of burnished MoS 2 films w10x. This was attributed to the presence of valleys on the rough substrate which served as reservoirs for MoS 2 . Hiraoka and Sasaki w11x reported that the presence of a discontinuous hard under-coating Že.g., chrome., between an aluminum substrate and a bonded MoS 2 solid lubricant coating, could result in much improved wear life of the lubricant film under extreme
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 0 8 2 - 4
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contact pressure. Lancaster investigated the role of substrate hardness on the endurance of MoS 2 films and showed that the rate of MoS 2 transfer to the counterface increased with softer substrates, while hard substrates facilitated the formation of uniform transfer films w9x. The above studies highlight the significance of an engineered substrate in optimizing the tribological performance of a solid lubricant coating. An ideal substrate should be a dual-phase one which could generate microscopic reservoirs of the lubricant film. In recent years, a number of discontinuously reinforced metal-matrix composites ŽMMCs. have been developed for various engineering Žincluding tribological. applications w12x. The MMCs present a unique opportunity to prepare novel substrates with hard second-phase particles protruding from a relatively softer metal matrix. The hard particle protrusions on the substrate could be effective in deflecting or arresting a propagating crack in a tungsten disulfide film. Secondly, the second-phase particle protrusions provide a unique opportunity to create microscopic reservoirs of the solid lubricant film. The substrate under consideration here is an aluminum MMC reinforced with silicon carbide ŽSiC. particles. The feasibility of depositing WS 2 films on a tailored aluminum MMC substrate by pulsed laser deposition ŽPLD. technique has been previously reported w13x. Under humid conditions, the PLD WS 2 films deposited on a tailored MMC substrate had much longer wear life as compared with those on pure metal substrates w13x. The objective of the current study was to understand the role of carbide protrusions in MMC substrate on the tribological performance tungsten disulfide films in humid environments. The WS 2 films were deposited using a pulsed laser.
2. Experimental 2.1. Preparation of the substrate An aluminum metal-matrix composite ŽMMC. reinforced with 20 vol.% SiC particles Žaverage particle size s 13 mm. was used as the substrate. The matrix of the MMC was a hypoeutectic Al–7.0 wt.% Si alloy. The composite was produced by a casting route and supplied by Duralcan ŽUSA. in as cast condition. Aluminum MMC disks of 30 mm diameter were cut from the cast ingots and polished using standard metallographic procedures. An optical micrograph of a polished MMC surface showing the distribution SiC particles is given in Fig. 1a. The polished surface was etched with Keller’s reagent w15x for 1 min. An optical micrograph of an etched MMC surface is shown in Fig. 1b. Particle protrusions ŽSiC and eutectic Si. can be seen in Fig. 1b. The micropits on primary aluminum ŽFig. 1b. were deliberately created to provide sites for mechanical interlocking of the PLD WS 2 film with the aluminum MMC substrate. A typical profilometer trace of the etched
Fig. 1. Microstructures of the Al–Si alloy MMC reinforced with 20 vol.% SiC Žaverage particle sizes13 mm. particles: Ža. metallographically polished surface; Žb. etched surface showing eutectic silicon and SiC particle protrusions; Žc. a typical profilometer trace of the etched MMC surface.
MMC surface is given in Fig. 1c. The average surface roughness, R a , is 0.2 mm, while the typical carbide protrusion is 1 mm high. 2.2. Pulsed laser deposition The target was fabricated by cold-pressing the WS 2 powder Ž99.8% purity. into 25 mm diameter disks at an
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X-ray spectroscope. After the wear test, the steel ball was dismounted from the tribometer and the transfer film was analyzed by SEM. Wear scars and transfer films were also analyzed by Raman spectroscopy. An intensified 1024 element diode array detector was used to collect the Raman signal.
3. Results
Fig. 2. A typical SEM micrograph of a tungsten disulfide film deposited on an etched Al MMC surface.
applied pressure of 50 MPa. An excimer laser charged with KrF was used to provide 248 nm radiation to the target at a fluence of 15 KJrm2 and a pulse rate of 10 Hz. The deposition chamber was maintained at an initial base pressure of 8 = 10y7 Pa. A calibrated quartz crystal oscillator was used to measure deposition rates and film thickness. The substrate was kept at room temperature during film deposition. Films were grown to a thickness of 1 mm. A more complete description of growing WS 2 solid lubricant films by the PLD technique is given elsewhere w14x. 2.3. Friction tests Friction measurements were made using a ball-on-disk tribometer in which a stationary ball was held on a rotating disk. A more complete description of the tribometer is given elsewhere w7x. A 3.125 mm Ž1r8Y . diameter 440C steel ball was used as the counterface. The normal load on the ball was 1 N; the maximum initial Hertzian contact stress between a 3.125 mm Ž1r8Y . diameter steel ball and SiC works out to be 830 MPa. The ball was held in a lever arm, and the friction force was measured using a strain gauge circuit. For each friction track, the rotational speed was adjusted to get a constant sliding speed of 50 mmrs. For comparison, friction measurements were also made on WS 2 films deposited on polished 440C steel substrates. In order to maintain a constant humidity level in the test environment, the pin-disk assembly was enclosed in a chamber. The relative humidity in the test environment could be controlled from near zero to almost 100% RH by regulating the relative flow rates of dry air and saturated air into the enclosure. The percentage relative humidity was measured by a high-performance humidity sensor inserted into the chamber. Friction measurements were made in dry nitrogen and in humid air with 90 Ž"5.% RH. Measurements were made at room temperature.
An SEM micrograph of a typical WS 2 film on an aluminum MMC substrate is shown in Fig. 2. All the constituent phases of the MMC, namely silicon carbide protrusions, eutectic silicon and aluminum, were covered by the film. A total of six wear tracks, two per film, were made in each environmental condition. In dry nitrogen, the friction coefficient of WS 2 films ranged from 0.035 to 0.050; the substrate had no influence. A typical friction trace on an MMC substrate in dry nitrogen is given in Fig. 3. It can be seen that the friction trace is smooth, and carbide protrusions did not adversely influence the tribological behavior of the film. However, in humid air Ž90% RH., the friction coefficient of the film on a steel substrate increased to 0.4 during the first 1000 cycles of sliding, as can be seen from Fig. 4b. By contrast, the films on MMC substrates lasted the whole duration of 50,000-cycle tests with friction coefficients ranging from 0.18 to 0.22. A typical friction trace of the film on an MMC substrate is given in Fig. 4a. An SEM micrograph of a 10,000-cycle wear scar of the film on MMC substrate is given in Fig. 5a. The test was conducted in humid air Ž90% RH.. The dark particles in the wear scar correspond to SiC protrusions from the MMC substrate. Fig. 5b is the SEM of the wear scar after 50,000 cycles of sliding in humid air. Compared with the 10,000-cycle wear scar, more carbide particles can be seen in the 50,000-cycle wear scar. In fact, the 50,000-cycle
2.4. Analyses of wear surfaces Wear scars on the films were analyzed by a scanning electron microscope equipped with an energy dispersive
Fig. 3. A typical friction trace of a PLD WS 2 film on MMC substrate in dry nitrogen.
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Fig. 4. Friction traces of WS 2 films in humid air Ž90% RH.. Ža. MMC substrate; Žb. 440C steel substrate.
wear scar looks like a composite of SiC particle-reinforced WS 2 . An SEM micrograph of a wear scar on the steel ball from the 10,000-cycle wear test on WS 2 –MMC film in humid air is shown in Fig. 6. The bright portion in the micrograph corresponds to the debris sticking to the ball. It should be noted that carbide protrusions from the substrate did not introduce deep scratches on the steel ball. The steel ball was cleaned ultrasonically in isopropanol to remove the loose debris, and the wear scar on the steel ball was
reexamined in the SEM. Fig. 7a is a backscattered electron image of the cleaned steel ball. There are several dark patches on the wear scar. Fig. 7b is a higher magnification backscattered electron image showing the wear pattern on the steel ball at the microscopic level. It can be seen that the dark island at the center of the micrograph is almost free of scratches, and the surrounding region has shallow microscratches. The dark phase is different in composition from the surrounding matrix, as revealed by the energy dispersive X-ray analysis. Fig. 8a is the EDX spectrum
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Fig. 5. SEM micrographs of wear scars on WS 2 films generated in humid environment: Ža. 10,000-cycle wear scar; Žb. 50,000-cycle wear scar.
collected in spot mode from point ‘a’ on the dark phase. It can be seen that the Cr K a peak is much stronger than the Fe K a peak. Also, the spectrum in Fig. 8a has a small peak corresponding to sulfur K a. Fig. 8b is an EDX spectrum from the surrounding matrix. Here the intensity of Fe K a peak is much higher than that of the Cr K a peak. The tiny Si peak in Fig. 8b may be due to the silicon
Fig. 6. SEM micrograph of the wear scar on steel ball after 25,000 cycles of sliding on a WS 2 film on MMC substrate.
Fig. 7. Higher magnification micrographs of Fig. 6 in backscattered electron image mode.
carbide wear debris. Carbon could not be detected because a beryllium window EDS detector was used. The contrast levels in backscattered electron images are strongly dependent on atomic numbers. Phases with higher average atomic numbers appear brighter than those with lower atomic numbers. This implies that the dark island in Fig. 7b has a lower average atomic number than that of the surrounding matrix. Since the EDX spectrum from the dark island showed much stronger Cr K a peak, the dark phase on the wear scar of the steel ball can be identified as chrome carbide in 440C stainless steel. A typical Raman spectrum of the WS 2 film on MMC substrate is shown in Fig. 9a, while the one in Fig. 9b corresponds to the cold-pressed WS 2 target. The spectrum from the aluminum MMC substrate is shown in Fig. 9c. The spectrum from WS 2 target has clear bands at 355 cmy1 and 420 cmy1 . The spectrum from the film on MMC is featureless ŽFig. 9a.. As reported in our previous study w7x, PLD WS 2 films, in as-deposited condition, do not possess sufficient long-range order to give rise to Raman peaks. However, during the initial stages of sliding, stress-induced crystallization occurs in the wear scar resulting in clear Raman peaks, 355 cmy1 and 420 cmy1 w7x. It should also be noted that the features from the substrate
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Fig. 8. EDX spectra corresponding to Fig. 7b: Ža. from point ‘a’ on the dark island; Žb. a typical spectrum from the surrounding region.
ŽFig. 9c. are absent in the spectrum from the film indicating that the film has completely masked the carbide and silicon protrusions. A typical Raman spectrum of a 25,000-cycle wear scar of PLD WS 2 film on MMC substrate generated in humid air is given in Fig. 10a. The
Fig. 9. Raman spectra: Ža. PLD WS 2 film on MMC substrate; Žb. WS 2 target; Žc. MMC substrate.
peaks from the substrate ŽSi, SiC. can be seen in the spectrum of the wear scar. Also, the bands at 355 cmy1 and 420 cmy1 indicate that sliding induced crystallization of WS 2 on MMC substrate even in humid air. The wear scar on the steel ball showed the transfer of crystallized WS 2 . The Raman spectrum from the wear scar of the film on steel substrate in humid air produced bands at 244, 715 and 809 cmy1 representing tungsten trioxide, WO 3 , as shown in Fig. 11a. The transfer film on steel ball from this
Fig. 10. Raman spectra from wear surfaces of WS 2 film on MMC substrate: Ža. wear scar; Žb. transfer film on the steel ball. Environment: humid air.
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Fig. 11. Raman spectra from wear surfaces of WS 2 film on steel substrate: Ža. wear scar; Žb. transfer film on the steel ball. Environment: humid air.
experiment also produced a similar Raman spectrum with bands at 244, 715 and 809 cmy1 ŽFig. 11b., confirming that when the substrate is steel, the transfer film formed in humid air is comprised of WO 3 . 4. Discussion The current study has shown that the tribological behavior of tungsten disulfide ŽWS 2 . films in humid air can be improved if the films were deposited on tailored metal-matrix composites with hard particle protrusions. Such improvements could extend the scope of their use in groundbased engineering applications operating in humid environments. In space environments, where transition metal
dichalcogenide-based solid lubricants such as MoS 2 are currently being used, humidity may not be a major concern; however, moisture sensitivity could be a serious issue during ground testing and extended periods of storage, and in reusable launch vehicles w16,17x. Before discussing the specific role of the substrate, we shall briefly review the mechanisms of lubrication by transition metal dichalcogenides. It is widely accepted that the lubricating behavior of WS 2 stems from its interlamellar mechanical weakness which is intrinsic to its crystal structure. Tungsten disulfide crystallizes in the hexagonal structure in which a sheet of tungsten atoms is sandwiched between two hexagonally packed sulfur layers. The bonding within the sandwich is covalent, whereas the sandwiches are held together by weak Van der Waals forces, resulting in interplanar mechanical weakness. Under the action of a shear force, intracrystalline slip occurs in the weak interplanar regions. This mechanism is responsible for the formation of smooth transfer films by wear. The new surfaces, created by separating the weakly bonded sandwiches are quite inert. They can easily slide back and forth over one another Žby intercrystalline slip., thereby providing lubrication. The major obstacle to lubrication by WS 2 Žor transition metal dichalcogenides. is the presence of unsaturated or dangling bonds. In layered structures, an edge plane is the most obvious source of unsaturated bonds. This problem is usually suppressed either by adding dopants that give rise to fibrous morphology with no long-range order, or by incorporating metal multilayers yielding dense morphology with strong basal plane orientation w16–19x. The presence of defects and cracks in the film is another source of unsaturated bonds. Even the freshly cleaved Ž0001. basal planes of single crystal transition metal dichalcogenides are known to contain nanometerscale surface defects w20x. Studies on wear and oxidation by atomic force microscopy showed that wear actually
Fig. 12. Ža. Schematic illustration of the tribo-induced oxidation of WS 2 film in humid environments w3x.
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Fig. 13. Ža. Schematic illustration of crack deflectionrarresting by second-phase particles in the film; Žb. cross-section of the film showing carbide protrusions in the MMC substrate. Note that carbide protrusions can also arrest the cracks that nucleate at the substraterfilm interface.
proceeds at these defects w20x. In addition to the nanometer-scale surface defects, most engineering films contain a population of micron and submicron-sized cracks. As depicted schematically in Fig. 12, propagation of a randomly oriented crack during sliding contact will break the covalent bonds within the S–W–S sandwich and create activated surfaces with unsaturated bonds. The activated surfaces can instantly react with moisture in the surrounding environment, forming WO 3 ŽFig. 12.. Also, diffusion of gaseous species along a crack path can accelerate crack propagation and further increase the rate of oxidation. The situation is analogous to the environmental stress corrosion cracking of glass w21x.
and arrest a propagating crack in the lubricating matrix. The film in the current study is a pure single phase WS 2 . However, due to carbide protrusions on the MMC substrate, the initial contact will be between the steel ball and
4.1. Crack arresting One way to impart tribo-oxidation resistance to a tungsten disulfide film is to improve its fracture toughness. In principle, this can be accomplished by arresting a propagating crack, as depicted schematically in Fig. 13a. The film in this schematic illustration has a dual-phase microstructure where the second phase can effectively deflect
Fig. 14. SEM micrograph of a 10,000-cycle wear scar on WS 2 film ŽMMC substrate. generated in humid air.
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Cracks can nucleate along the film–substrate interface, as shown schematically in Fig. 13b. Intersection of such cracks with those within the film ŽFig. 13a. can result in spallation and delamination of the film, thereby reducing the wear life. In a recent study, Hilton w22x investigated the fracture behavior of sputter-deposited MoS 2 films using brale indentation tests. This study showed that in addition to radial and circumferential cracks, delamination could play a role in the overall fracture behavior of MoS 2 films during brale indentation tests. In the current study, the protruding carbides from the substrate can effectively arrest the cracks that nucleate at the film–substrate interface ŽFig. 13b.. 4.2. Microscopic reserÕoirs By judicious control of the size, height and area fraction of carbides, microscopic reservoirs of solid lubricant can be created on the surface of the substrates. Creation of such reservoirs on the substrate facilitates the retention and recirculation of WS 2 particles in the contact zone ŽFig. 16., instead of gradual wearing away of the film as in the case of a polished steel substrate. 4.3. Third-bodies and counterface wear Fig. 15. SEM micrographs of a 50,000-cycle wear of a WS 2 film on MMC substrate showing: Ža. crack arresting; Žb. crack deflection by carbide particles in the substrate. Environment: humid air.
microscopic regions of WS 2 on SiC particle protrusions. Once these regions of WS 2 wear out or get pushed into the valleys between the carbide protrusions, the wear track essentially becomes a composite of carbides in a WS 2 matrix as can be seen from the SEM of a 10,000-cycle wear track in Fig. 14. Typical SEM micrographs of a 50,000-cycle wear scar showing the crack front in the vicinity of carbide particles are shown in Fig. 15 as illustrations of crack deflection and arrest.
In addition to crack arresting, carbides can roughen the steel counterface, as can be seen from the SEM micrograph in Fig. 7b. The fresh surface thus created is ideally suited for improved adhesion of the solid lubricant transfer film. It must be noted that carbides did not cause excessive wear or scratching of the counterface. In fact, the 440C steel resembles a chemically etched surface where the carbide phase in steel is clearly revealed. Mild roughening followed by formation of transfer film on the steel counterface must be responsible for providing lubricated contacts with minimum damage to the counterface. In the absence of a lubricant, carbides can cause excessive damage to steel. As an example, an SEM micrograph of a wear scar
Fig. 16. Schematic illustration of the microscopic reservoir concept.
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Fig. 19. SEM of the wear scar from the friction test shown in Fig. 18. Fig. 17. SEM micrograph of a wear scar on steel ball that was rubbed on an uncoated Al–Si–SiC MMC surface for 1000 cycles of sliding.
on a steel ball that was used to rub an uncoated Al MMC disk for 1000 cycles of sliding is given in Fig. 17. 4.4. The role of an etched MMC substrate A new set of experiments was performed to check whether etching alone could have produced a lubricious layer on the MMC surface. A friction test was conducted on an uncoated MMC disk that was etched with Keller’s reagent for 1 min. Fig. 18 shows that the friction coefficient of the etched MMC disk is high. Similarly, the test
produced a rough wear scar ŽFig. 19., typical of an unlubricated metal-to-metal contact. Similarly, our previous study w13x showed that the tribological behavior of WS 2 films on pure aluminum substrates in humid air was similar to that shown in Fig. 4b. So, the improved tribological performance of the WS 2 film on an etched Al MMC substrate seen in Fig. 4a is not due to etching alone. Although Raman analysis showed that the film in the wear track is unoxidized ŽFig. 10., the friction coefficient of WS 2 on an MMC substrate in humid environment is higher, 0.15–0.22, as compared with the friction coefficient of the same film in dry nitrogen ŽFig. 3.. This may be due to the morphology of transfer films. In addition to
Fig. 18. Friction trace on an etched Al MMC disk without the WS 2 film.
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chemistry, morphology of the transfer films on the counterface plays a significant role in controlling friction. Although the use of the MMC substrate reduced tribooxidation of WS 2 in humid environments, it did not facilitate the formation of smooth transfer films on the steel counterface, similar to the ones seen in dry nitrogen w7x. 4.5. Implications The current work has demonstrated the feasibility of controlling the chemically assisted fracture of tungsten disulfide by using engineered substrates. In addition to the one described here, a number of other metal-matrix composites are commercially available w11,12x; their surfaces can be suitably tailored to derive optimum tribological behavior of the transition metal dichalcogenide films deposited on them. The MMC technology offers additional advantage of reinforcing the materials with high strengthrhigh modulus fibers w23x. Thus, a substrate can be engineered in such way that the bulk meets the mechanical and physically property requirements of the intended tribocomponent, while the surface is enriched with hard particle protrusions to optimize the tribological behavior of the solid lubricant films. Finally, the results of this study provide insights for the design of thin film composite coatings. 5. Conclusions Ž1. Tungsten disulfide solid lubricant films can be grown on tailored aluminum metal-matrix composite ŽMMC. substrates by pulsed laser deposition. Ž2. In dry nitrogen, the tribological behavior of the WS 2 films on MMC substrates is very similar to the behavior of pulsed laser-deposited films on polished steel substrates. Ž3. In humid air Ž90% RH., the films on steel substrates failed during the first 500–1000 cycles of sliding, whereas the ones on MMC substrates lasted for the entire duration of a 50,000-cycle wear test with friction coefficients ranging from 0.15–0.22. Ž4. The carbide protrusions were found to influence the crack propagation in WS 2 films during sliding contact without causing excessive counterface wear or scratching. References w1x S.F. Murray, S.J. Calabrese, Effect of solid lubricants on low speed sliding behavior of silicon nitride at temperatures to 8008C, Lubr. Eng. 49 Ž1993. 955–964.
w2x H.E. Sliney, Decomposition kinetics of some solid lubricants determined by elevated temperature X-ray diffraction techniques, Proc. of the US Air Force Aerospace Fluids Lubr. Conf., 1963, pp. 350–367. w3x W.O. Winer, Molybdenum disulfide as a lubricant: a review of the fundamental knowledge, Wear 10 Ž1967. 422–452. w4x A.W.J. DeGee, G. Solomon, J.H. Zaat, On the mechanisms of MoS 2 film failure in sliding friction, ASLE Trans. 8 Ž1965. 156–163. w5x J.K. Lancaster, A review of the influence of environmental humidity and water on friction, lubrication and wear, Tribology International 23 Ž1990. 371–389. w6x C. Pritchard, J.W. Midgley, The effect of humidity on the friction and life of unbonded molybdenum disulfide, Wear 13 Ž1969. 39–50. w7x S.V. Prasad, J.S. Zabinski, N.T. McDevitt, Friction behavior of pulsed laser deposited tungsten disulfide films, Tribology Transactions 38 Ž1995. 57–62. w8x S.V. Prasad, J.S. Zabinski, Tribology of tungsten disulfide: characterization of wear-induced transfer films, J. Mater. Sci. Lett. 12 Ž1993. 1413–1415. w9x J.K. Lancaster, The influence of substrate hardness on the formation and endurance of molybdenum disulfide films, Wear 10 Ž1967. 103–107. w10x R.L. Fusaro, Effect of substrate finish on the lubrication and failure mechanisms of molybdenum disulfide films, ASLE Trans. 25 Ž1982. 141–156. w11x N. Hiraoka, A. Sasaki, Effect of discontinuous hard under-coating on the life of solid film lubricant under extreme contact pressure, Tribology International 30 Ž1997. 344–429. w12x S.V. Prasad, P.K. Rohatgi, Tribological properties of Al alloy particle composites, J. Met. 39 Ž1987. 22–26. w13x S.V. Prasad, J.S. Zabinski, V.J. Dyhouse, Pulsed laser deposition of tungsten disulfide on aluminum metal-matrix composites, J. Mater. Sci. Lett. 11 Ž1992. 1282–1284. w14x J.S. Zabinski, M.S. Donley, S.V. Prasad, N.T. McDevitt, Synthesis and characterization of tungsten disulfide films grown by pulsed laser deposition, J. Mater. Sci. 29 Ž1994. 4834–4839. w15x Metallography, structures and phase diagrams, ASM Metals Hand Book, Vol. 8, Metals Park, OH, 1973, p. 124. w16x M.R. Hilton, P.D. Fleischauer, Applications of solid lubricant films in spacecraft, Surface and Coatings Technology 54r55 Ž1992. 435– 441. w17x T. Spalvins, Lubrication with Sputtered MoS 2 Films: Principles, Operation, Limitations, NASA TM-105292, 1991. w18x G. Jayaram, L.D. Marks, M.R. Hilton, Nanostructure of Au–20% Pd layers in multilayer solid lubricant films, Surface and Coatings Technology 76r77 Ž1995. 393–399. w19x J.S. Zabinski, M.S. Donely, N.T. McDevitt, Mechanistic study of the synergism between Sb 2 O 3 and MoS 2 lubricant system using Raman spectroscopy, Wear 165 Ž1993. 103–108. w20x Y. Kim, J.L. Huang, C.M. Leiber, Characterization of nanometer scale wear and oxidation of transition metal dichalcogenide lubricants by atomic force microscopy, Appl. Phys. Lett. 59 Ž26. Ž1991. 3404–3406. w21x B.R. Lawn, T.R. Wilshaw, Fracture of Brittle Solids, Chap. 8, Cambridge Univ. Press, Cambridge, 1975. w22x M.R. Hilton, Fracture in MoS 2 solid lubricant films, Surface and Coatings Technology 68r69 Ž1994. 407–415. w23x S.V. Prasad, K.R. Mecklenburg, Friction behavior of ceramic fiberreinforced aluminum metal-matrix composites against a 440C steel counterface, Wear 162r164 Ž1993. 47–56.