Surface and Coatings Technology 116–119 (1999) 36–45 www.elsevier.nl/locate/surfcoat Nanocomposite tribological coatings for aerospace applications A.A. Voevodin *, J.P. O’Neill, J.S. Zabinski Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/MLBT, Wright–Patterson Air Force Base, OH 45433-7750, USA Abstract Challenges in aerospace tribology and composite coatings for aerospace applications are brieﬂy reviewed. Attention is given to nanocomposite coatings made of carbide, diamond-like carbon (DLC ) and transition-metal dichalcogenide phases. The preparation of such coatings within the W–C–S material system using a hybrid of magnetron sputtering and pulsed laser deposition is described. Coatings consist of 1–2 nm WC and 5–10 nm WS grains embedded in an amorphous DLC matrix. These 2 WC/DLC/WS nanocomposites demonstrate low friction and wear in tests performed in high vacuum, dry nitrogen and humid 2 air. Coatings are found to adapt to the test conditions, which results in: (1) crystallization and reorientation of initially nanocrystalline and randomly oriented WS grains; (2) graphitization of the initially amorphous DLC matrix; (3) reversible 2 regulation of the composition of the transfer ﬁlm between WS and graphite with environmental cycling from dry to humid; and 2 (4) possible DLC/WS synergistic eﬀects, providing friction reduction in oxidizing environments. These adaptive mechanisms 2 achieve low friction coeﬃcients of 0.02–0.05 and an endurance above two million cycles in space simulation tests. This also provides stable coating performance and recovery of low friction in tests simulating ambient/space environmental cycling. Correlations among WC/DLC/WS chemistry, structure, hardness, friction and wear are discussed. The tremendous potential of 2 such composites for aerospace tribology is demonstrated. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Aerospace tribology; Carbide; Diamond-like carbon; Dichalcogenide; Nanocomposite coating 1. Introduction assembly or launch, and contact atomic oxygen in low Earth orbit. Another environmental factor is the broad 1.1. Challenges of aerospace tribology temperature range of operation; e.g., −100 to +100°C for space-borne devices and −40 to +300°C for aircraft One life-limiting problem of aerospace systems is components. Shuttle-type operations cause environmen- friction and wear in various movable devices, such as tal cycling, and planetary explorations add the problem reaction wheels, gyroscopes, solar arrays, antenna of abrasive wear by electrostatically attracted dust, such drives, sensor-pointing mechanisms, gears, pumps, actu- as ﬁne dust in the dry CO atmosphere of Mars experi- 2 ators, latches and releases. Together they cover a broad enced during the recent 1997 Pathﬁnder mission . range of contact stresses from 107 to 1010 Pa and sliding The expected life of aerospace devices rapidly pro- speeds from near zero in restraining mechanisms to gresses, which generates new challenges in mechanism above 20 m s−1 in control-moment gyros. They experi- endurance. Examples are the more than two decades ence low-frequency launch vibrations, and some of them service of high-altitude SR-71 ‘Blackbird’ reconnais- operate with dithering motion at frequencies of up to sance planes  and the increase of space station service 500 Hz and high peak loads; e.g., gimbal bearings on from the several weeks of early Apollo and Salyut space platforms capable of rapid re-targeting. programs to the more than 10 years of the MIR station Another challenge is extreme operation conditions. and forthcoming International Space Station . Aircraft are repeatedly subjected to high and low humid- Estimates for 15–30 years of service give about ity with altitude changes. Satellites designed for the high 109–1010 cycles in gimbal bearings of momentum-control vacuum of space can be exposed to moisture during devices or 106–108 oscillations in solar-array joints. The mechanical, environmental and endurance * Corresponding author. Tel.: +1-937-255-9001; requirements of the aerospace applications exceed the fax: +1-937-255-2176. available lubrication and wear-reduction technologies, E-mail address: firstname.lastname@example.org (A.A. Voevodin) demanding novel materials and advanced technologies. 0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 22 8 - 5 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 37 Presently, low outgassing liquids (e.g., mineral oils, posites for the use in tool industry [42,43]. Similar synthetic hydrocarbons, perﬂuoropolyalkylethers) and nanocomposites can be developed for aerospace needs. solid lubricants [e.g., polytetraﬂuoroethylene (PTFE) One relatively new tribological material for aerospace polymers, soft metals, dichalcogenides] are the predomi- applications is diamond-like carbon (DLC ), which has nant materials used for aerospace lubrication [4–6 ]. high hardness, low friction and low wear [44,45]. An Liquids are used when the tribo-assembly can be sealed eﬀort was made to combine superhard unhydrogenated from the outside environment. In other cases, solid DLC with transition-metal carbides in nanocrystalline/ lubrication is preferred. Aerospace solid lubrication and amorphous composites. TiC or WC nanograins have wear reduction with advanced composite coatings is the been successfully embedded in a DLC matrix [46–49]. focus of this paper. Both TiC/DLC and WC/DLC composites demonstrated the desirable combination of hardness, toughness, low friction and wear in tests in ambient environments 1.2. Composite coatings for aerospace applications [47,49]. The use of two-phase carbide/DLC nanocomposites Transition-metal dichalcogenides (MoS , WS , in aerospace applications may create a problem due to 2 2 NbSe , etc.) are the most common solid lubricants for DLC graphitization in friction contacts and the associ- 2 space applications. They are applied as powders mixed ated increase of friction coeﬃcient in the high-vacuum with various organic and inorganic binders, burnished environment . A hydrogenated DLC phase provides to the surface from powders, or deposited by spray and a remedy by hydrogen termination of active carbon vacuum deposition methods . These lubricants are bonds, but not for long owing to hydrogen depletion soft, not abrasion-resistant and oxidize in air. To comply after about 104 cycles . An alternative approach may with endurance requirements of aerospace applications, be incorporation of dichalcogenide space lubricants, more advanced approaches were explored. One of them such as MoS or WS , into a carbide/DLC/ 2 2 was self-lubricating composites, where solid lubricant dichalcogenide composite. One design explored included was pressed into a supporting matrix, such as in formation of MoS reservoirs within a TiC/DLC nano- 2 bronze/lead composites and glass-ﬁber- and polyamade- composite by laser milling and MoS sputtering . 2 reinforced PTFE composites coated with MoS [8–11]. The coating maintained more than a million cycles of 2 The composite approach was also used in high-temper- operation in space simulation tests and with humid/dry ature space lubrication coatings, such as the NASA cycling. The deposition process was, however, quite PS-200 series coatings produced by plasma spraying sophisticated. which consisted of BaF /CaF lubricant, silver lubricant The nanocomposite coating approach using hard and 2 2 and binder, and Cr C support . lubricating phases for both humid and dry environments 3 2 The development of vacuum deposition methods can be more technologically robust. A synergetic eﬀect added new capabilities for controlling the chemistry, of graphite/WS powder lubricants was reported to be 2 structure, morphology and thickness of solid lubricants. beneﬁcial for air/vacuum environmental cycling . This improved their friction, endurance and environmen- Taking into account DLC graphitization in friction tal adaptation, and allowed precision components to be contacts, a WC/DLC/WS composite coating was 2 coated. Examples are textured MoS ﬁlms [13–18], designed, where hard nanocrystalline WC provides wear 2 metal-doped MoS and WS ﬁlms [19–23], resistance and mechanical stability, hard amorphous 2 2 CF -doped WS ﬁlms , metal–MoS multilayers DLC provides low friction and wear in ambient condi- x 2 2 [25–28], temperature-adaptive PbO/MoS , ZnO/MoS , tions, and WS provides lubrication in vacuum. 2 2 2 ZnO/WS and moisture-resistant PTFE/MoS , Structural and tribological characterization of 2 2 LaF /MoS composites [29–33], etc. Vacuum deposition WC/DLC/WS coatings are discussed in this paper. 3 2 2 also provided a variety of advanced hard coatings for wear protection [34–37], which are currently expanding toward aerospace applications due to the endurance 2. Experimental details requirement. For example, surface-treated diamond coatings were developed for space friction and wear WC/DLC/WS coatings approximately 0.5 mm thick 2 reduction [38,39], while TiC and TiN coatings were were grown on 440C stainless steel disks, 25.4 mm in used to delay fatigue wear in ball bearings lubricated diameter, and polished to a surface roughness of with perﬂuoropolyalkylether ﬂuids . The possibility 0.02 mm R . Coatings were produced by either magnet- a to mix hard and lubricious phases in thin nanocomposite ron-assisted pulsed laser deposition (MSPLD)  or coatings is another beneﬁt of vacuum deposition. There by laser ablation of a composite target made of graphite were reports of producing TiN/MoS composites by and WS sectors. The deposition chamber was evacuated 2 2 chemical vapor deposition  and, most recently, on to less than 10−6 Pa. A magnetron with a WS target 2 magnetron-sputtered TiB /MoS and TiN/MoS com- was operated at 8 W cm−2 in 0.2 Pa argon to create a 2 2 2 38 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 40 000 cycles. Coating endurance in space simulating conditions was estimated by long-duration tests of up to 2 000 000 cycles, which were run continuously in vacuum. The cross-sectional area and diameter of the wear tracks were determined with a Dektak proﬁlometer and an optical microscope, respectively. Normalized wear volumes (mm3 N−1 m−1) were then calculated based on these measurements. Raman spectroscopy was used to analyze the micro- structure of triboﬁlms formed on the ball wear scar and disk wear track. The spectra were obtained with a Renishaw Ramascope 2000 equipped with a 514.5 nm laser and an air-cooled charge coupled device (CCD) detector. Spectrum identiﬁcation was based on a Raman Fig. 1. The W–C–S ternary composition diagram with dots represent- database of tribological materials . ing the chemistry of investigated coatings. The line of WS stoichiome- 2 try is also plotted. 3. Results and discussion plasma ﬂux directed at 45° to the substrate. The KrF excimer laser beam (200 mJ, 20 ns, 1–20 Hz) ablated a 3.1. Chemistry and structure graphite target at a 45° incidence angle. The coating composition was controlled by varying the laser pulse The goal of this study was to investigate nanocom- rate when using MSPLD, and by varying the WS sector posites, consisting of WC, DLC and WS . Thus, the 2 2 area on the target when using only laser ablation. elemental composition of the coatings was adjusted to Coating elemental composition was investigated using satisfy two criteria: (1) enough carbon should be avail- a Surface Science Instruments M-Probe X-ray photo- able to form both nanocrystalline b-WC and a free electron spectroscope. Relative peak areas were used for carbon phase with DLC properties; and (2) enough compositional quantiﬁcation, and corrected for a spec- sulfur should be available to form WS . The composi- 2 trometer factor and X-ray cross-section. The coatings tions investigated are indicated with dots in the ternary were not sputtered prior to scanning because sulfur is W–C–S diagram in Fig. 1, neglecting 2–4 at% oxygen preferentially removed. There was some oxygen contam- contamination. ination due to the environment but it was mainly on All coatings had carbon contents within 60 to 90 at% the surface; upon sputtering 5 nm deep, the oxygen ( Fig. 1). From previous studies of WC/DLC nanocom- content was less than 4 at%. Coating structural investi- posites, the region of 65–75 at% was found to provide gations were performed with transmission electron the formation of b-WC nanograins embedded in an microscopy ( TEM ), using a Philips CM200 instrument amorphous DLC matrix . This region produced an operated at 200 keV. optimum combination of hardness and low friction in Hardness and Young’s moduli were determined with ambient environments . Fig. 1 indicates also the a Nanoindenter IIs microprobe. A Berkovich indenter deviation of the W/S ratio from the WS stoichiometry 2 was loaded in the range of 1 mN. Hardness was found line due to the consumption of tungsten in both WC for the maximum penetration, and moduli were calcu- and WS phases. 2 lated from unloading portions of the load–displace- TEM images and electron diﬀraction patterns of W– ment curves. C–S coatings with diﬀerent sulfur contents are compared Coating friction was measured using a ball-on-disk in Fig. 2. The coating without sulfur had WC nanoclus- tribometer. A 100 g load was applied to a 440C stainless ters (more dense dark spots) surrounded by amorphous steel ball, which was about 6 mm in diameter. A lower DLC material (gray areas) [Fig. 2(a)]. The formation limit estimate of Hertzian contact stress was 0.5 GPa, of ﬁne, 1–2 nm sized WC clusters was conﬁrmed by assuming steel-on-steel contact. Tests were conducted in electron diﬀraction patterns, containing largely diﬀuse air with relative humidity (RH ) controlled at 50±1%, rings for the main b-WC planes. The DLC phase was dry nitrogen (<0.1% RH ) and 10−6 Pa vacuum. The detected by characteristic Raman scattering in experi- tests were run over 50 000 cycles (approximately 2500 m) ments described previously for room-temperature depos- for sliding friction investigations, and over 10 000 cycles ition of b-WC/DLC nanocomposites . (approximately 500 m) for wear-rate estimations. Sliding When the coating was doped with 15 at% sulfur, the speeds were close to 10 m min−1. Coating friction was microstructure changed slightly [Fig. 2(b)]. There were also investigated by cycling between dry nitrogen and also some changes in the diﬀraction pattern, where new humid air every 5000 cycles for total test duration of rings appeared, indicating the beginning of WS forma- 2 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 39 tion. For 29 at% sulfur, the microstructure and diﬀrac- spacing of about 0.3 nm, corresponding to the tion pattern changed completely [Fig. 2(c)]. They clearly (10Z) group of planes of a WS standard [56 ]. Z=0,1,2,3 2 demonstrated the presence of hexagonal WS . Two types Some curvature of widely spaced basal planes is visible, 2 of WS crystal plane were identiﬁed: (1) basal planes but few of them extend more than 10 nm in length. 2 perpendicular to the c-axis of WS with a spacing of Most of the WS grains had sizes of about 5–10 nm and 2 2 about 0.6 nm, corresponding to (002) planes of a WS were randomly oriented in the surrounding material, 2 standard; and (2) planes parallel to the c-axis with a consisting of the same dark and gray areas that were (a) (b) (c) Fig. 2. Transmission electron microscope images and selected-area diﬀraction patterns for the W–C–S coatings with various sulfur contents: (a) 0 at%; (b) 15 at%; (c) 29 at%. 40 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 identiﬁed as b-WC and DLC in composites without 3.3. Friction sulfur. Grazing-angle X-ray diﬀraction studies did not reveal Variation of the friction coeﬃcient of the composite well-deﬁned diﬀraction patterns, conﬁrming that the coatings with sulfur content is shown in Fig. 4 for dry grain sizes of WC and WS were on average less than nitrogen, vacuum and moist air environments. The data 2 10 nm. Also, Raman scattering analyses could not are averaged over 50 000 sliding cycles and represent resolve characteristic peaks of free carbon or WS from steady-state wear conditions. The dependence of friction 2 the background noise. This added to the conclusion that coeﬃcients on sulfur content for sliding in dry nitrogen all phases in the WC/DLC/WS composites are either [Fig. 4(a)] and vacuum [Fig. 4(b)] was very similar. 2 nearly amorphous or extremely nanocrystalline with 1– 10 nm dimensions. 3.2. Hardness Fig. 3 plots the variation of the composite hardness as a function of sulfur content. The coating hardness is maintained at the level of about 15 GPa up to 15 at% sulfur. This level of hardness corresponds to the WC/DLC nanocomposite deposited at room temper- ature without sulfur . At 15–20 at% sulfur the hardness decreased, reaching the level of about 7 GPa above 20 at% sulfur ( Fig. 3). The elastic modulus also decreased with sulfur content, from 230–250 GPa for the coatings with less than 15 at% sulfur to 100–130 GPa (a) for the coatings with above 20 at% sulfur. This drop of mechanical chacteristics was associated with the segre- gation of soft WS within the composite. 2 The hardness of WC and other carbide coatings is as high as 24–28 GPa . At the same time, the hardness of WS and other solid lubricants (MoS , graphite, lead) 2 2 is generally below the 3 GPa level [7,58]. Thus, the 7 GPa hardness of WC/DLC/WS composites is not 2 comparable with the hardness of carbides, but is signiﬁ- cantly higher than that of typical solid lubricants. This creates the potential for low wear rates and longer endurance, providing they maintain low friction coeﬃ- cients similar to solid lubricants. (b) (c) Fig. 3. Hardness variation of the coatings as a function of sulfur Fig. 4. Variation of the friction coeﬃcient with sulfur content for tests content. Dotted line provides guidance for the eye. in: (a) dry nitrogen; (b) vacuum; and (c) moist air. A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 41 There was a sharp reduction of friction coeﬃcient in these environments at 15–20 at% sulfur. For coatings with more than 20 at% sulfur, there was formation of thick transfer ﬁlms after tests in vacuum (Fig. 5). This was observed on ball counterparts [Fig. 5(a)] and within wear tracks [Fig. 5(b)], appearing as gray patches along the sliding direction. A reduction of the friction coeﬃ- cient to below 0.1 and third-body formation within friction contacts were connected to the presence of WS . 2 The low friction of WS originates from weak Van 2 der Waals’ forces between sandwiches of SMWMS hex- agonal basal planes in its crystal lattice , providing Fig. 6. Micro-Raman spectra recorded for friction surfaces of a easy intra- and intercrystalline slip in the friction contact WC/DLC/WS composite with about 20 at% sulfur after tests in 2 . These slip mechanisms require the presence of vacuum. developed crystals with basal planes oriented parallel to the substrate surface. However, TEM studies indicated extremely nanocrystalline and randomly oriented WS 2 grains in composites with above 20 at% sulfur. There were several reports about friction-induced crystalliza- tion and basal plane reorientation for vacuum-deposited MoS [15,61–63] and WS ﬁlms [64,65]. Reorientation 2 2 and conglomeration of initially randomly oriented WS nanograins was veriﬁed in this research by micro- 2 Raman analyses of the contact areas indicated in Fig. 5. The spectra clearly indicated the presence of hexagonal WS planes identiﬁed by peaks at about 400 cm−1 2 ( Fig. 6). The contribution of carbon to friction in vacuum was negligible, which was supported by negligi- ble Raman activity in the 1300–1600 cm−1 region, corre- sponding to the scattering from amorphous and/or graphite carbon [66 ]. The composites with sulfur content below 15 at% had higher friction coeﬃcients of 0.5–0.7 in vacuum (a) [Fig. 4(b)]. Raman studies did not reveal the presence of hexagonal WS in friction contacts, but rather indi- 2 cated the formation of amorphous carbon with partial graphitization detected by broad overlapped peaks, cen- tered at about 1380 and 1530 cm−1 ( Fig. 7, upper curve). Graphitization of the DLC phase was suggested as the (b) Fig. 5. Microphotographs of ball wear spot (a) and wear track (b) in the WC/DLC/WS coating with about 20 at% sulfur formed after fric- 2 tion tests in vacuum. Arrows indicate regions used for micro-Raman Fig. 7. Micro-Raman spectra recorded for wear track surfaces of coat- studies. ings with about 15 at% sulfur after tests in vacuum and moist air. 42 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 main process inﬂuencing the tribological characteristics. Friction coeﬃcients of single-phase unhydrogenated DLC in vacuum can be as high as 0.5–0.6 . The dependence of the friction coeﬃcients on sulfur content in humid air was more complicated [Fig. 4(c)]. For composites with lower sulfur content the formation of amorphous carbon in wear tracks was detected (Fig. 7, lower curve). This helped to achieve lower friction coeﬃcients of about 0.1, approaching the friction of unhydrogenated DLC in humid air . For ﬁlms with about 15–20 at% sulfur, phases of WS , WO and 2 3 amorphous carbon were detected inside wear tracks and debris along the sides of wear tracks ( Fig. 8). There were also peaks of Fe O at about 220 cm−1 and some Fig. 9. Coating wear rates as a function of sulfur content for three 2 3 iron and/or chrome oxides peaks at about 900 cm−1 environments: moist air (&), dry nitrogen (#) and high vacuum ($). . These oxide peaks appeared to be due to oxidation Lines represent trend ﬁts. and transfer of the ball material. These processes lead to friction coeﬃcients of 0.40–0.45 for composites with rates increased to about 6×10−6 mm3 N−1 m−1. This about 15–20 at% sulfur with hardness comparable to level of wear rate was comparable to the wear rates of that of the 440C steel balls (10–12 GPa). For softer various ceramic and metal-doped DLC materials tested coatings with more than 20 at% sulfur, friction coeﬃ- in ambient conditions [67–69]. The wear rates in vacuum cients reduced to the level of 0.2, which is typical for for the coatings with 20–30 at% sulfur were as low as WS in moist air . 10−6 mm3 N−1 m−1. 2 Friction tests demonstrated the need for about 20 at% Raman spectra were taken from wear debris formed sulfur in the WC/DLC/WS composite coatings to gen- in vacuum tests of the coating with about 20 at% sulfur 2 erate lubrication. The friction of such composites is ( Fig. 10). The spectra indicated the domination of mainly governed by the WS phase reorientation and/or graphite-like carbon in the wear debris (Fig. 5). There 2 DLC phase graphitization. The resulting friction coeﬃ- was also WO in debris along the wear tracks (Fig. 10, cients depend on both sulfur content and humidity. 3 lower curve). The formation of WO in vacuum tests 3 can be caused by oxygen contamination during coating 3.4. Wear and endurance deposition or by residual water pressure during wear tests. There were no peaks indicating the presence of Fig. 9 shows the variation of coating wear rates as a oxides due to counterpart ball wear. It was concluded function of sulfur content for tests with 10 000 sliding that the wear debris formed in vacuum consisted pre- cycles in three environments. Similar to the friction dominantly of graphitic carbon and some amount of coeﬃcients, the wear rates in dry environments (nitrogen WO ( Fig. 10), while the wear debris formed in moist and vacuum) decreased considerably above 20 at% 3 air consisted predominantly of oxides ( WO and ball sulfur, with initiation of WS lubrication. In moist air, 3 2 metal oxides) and some amount of graphite carbon a reverse dependence was found (Fig. 9), where wear ( Fig. 8). Fig. 8. Micro-Raman spectra recorded inside wear tracks and alongside Fig. 10. Micro-Raman spectra recorded for wear debris of a wear debris for coatings with about 20 at% sulfur after tests in moist WC/DLC/WS composite with about 20 at% sulfur after tests in 2 air. vacuum. A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 43 The tests of WC/DLC/WS coating endurance in 2 high-vacuum environment were particularly important. Fig. 11 shows an example of friction coeﬃcient variation over two million sliding cycles for a coating with 22 at% sulfur. The friction coeﬃcient stayed in the range of 0.03 to 0.06. Some friction reduction with the number of cycles indicated the ongoing process of WS phase 2 reorientation and accumulation in the friction contact. The wear-rate estimate after this long run was about 10−7 mm3 N−1 m−1, which was an order of magnitude lower than the wear rate of similar composites estimated after 10 000 cycles ( Fig. 9). The wear rate decrease with the increase of test duration is typical for solid lubri- cants, since most of the wear occurs initially and is followed by a long period of a very low wear [15,70]. The WS/DLC/WS composites demonstrated low 2 wear rates in diﬀerent environments and excellent endu- Fig. 12. Friction coeﬃcient variation in tests with environment cycled rance in long runs simulating space applications. Their from dry nitrogen to moist air every 5000 cycles. low wear and friction fulﬁl the criteria of self-lubricated wear-resistant coatings for space applications, which was proposed to be 10−6 mm3 N−1 m−1 for wear rates An interesting observation was a slightly lower friction and <0.1 for friction coeﬃcients . coeﬃcient recorded for moist air cycles, in comparison to the tests when the humidity was kept constant 3.5. Friction stability in environmental cycling [Fig. 4(c)]. One explanation can be that 5000 cycles in moist air are not suﬃcient to develop a strongly oxidized A critical requirement of aerospace applications is ﬁlm such as that formed after 50 000 cycles of ﬁxed- stable performance in environments with varied humid- humidity tests. Another can be a synergistic eﬀect of ity. Fig. 12 provides an example of friction coeﬃcient DLC and WS , similar to the one observed for 2 variation for the environment varied every 5000 sliding graphite/MoS mixtures [71–73]. 2 cycles between dry nitrogen (<0.05% RH ) and moist Based on the observed friction variation and the air (50% RH ) for a WC/DLC/WS coating. This test analyses of wear debries in Section 3.3, the following 2 procedure was selected to simulate friction in hypothesis was made about environmental adaptation ambient/space cycling, since friction and wear results of the WC/DLC/WS coatings. In dry conditions, which 2 for dry nitrogen and vacuum were similar, and moist are favorable for WS but severe for graphite, there is 2 air/high vacuum cycling was diﬃcult to implement. a predominant formation of hexagonal WS planes in 2 For all coatings with above 15 at% sulfur, a low the friction contact. Once the humidity is cycled high, friction coeﬃcient of 0.02–0.05 was recorded in low- the WS planes in friction surfaces are destroyed by 2 humidity cycles, corresponding to lubrication by WS . oxidation and replaced with graphite planes formed by 2 In high-humidity cycles, the friction coeﬃcients were the friction-induced transformation of DLC. For graph- about 0.10–0.20, corresponding to lubrication by DLC. ite, the intercalation of basal planes with water molecules is essential for low friction and wear rates [74,75]. In the next dry cycle, the desorbtion of water molecules causes a high wear of graphite sheets and reveals fresh WS grains. The friction-induced reorientation of WS 2 2 basal planes promotes the low friction and wear in dry conditions, and so on. To verify this hypothesis, the tests were stopped in the middle of a dry cycle, and in the middle of a moist cycle, to investigate predominant phases in the friction contacts by micro-Raman spectroscopy (Fig. 13). For the dry cycle, the results found Raman activity in the WS region and almost the absence of carbon-related 2 scattering ( lower curve). For the moist cycle, the signal from graphite-like carbon was clearly identiﬁed in the Fig. 11. Friction variation for the WC/DLC/WS composite with spectra (Fig. 13, upper curve). This supported the adap- 2 about 22 at% sulfur in a long-duration test in vacuum. tation of the coatings to these environments. 44 A.A. Voevodin et al. / Surface and Coatings Technology 116–119 (1999) 36–45 tion of an initially amorphous DLC matrix; (3) reversible regulation of the composition of the transfer ﬁlm between WS and graphite with environmental 2 cycling from dry to humid; and (4) possible DLC/WS 2 synergistic eﬀects, providing friction reduction in oxidiz- ing environments. The activation of these adaptation mechanisms in the WC/DLC/WS nanocomposites 2 helped to achieve low friction coeﬃcients of 0.02–0.05 and an endurance of two million cycles in space simula- tion tests. This also provided stable coating performance and recovery of low friction in tests simulating ambient/space environmental cycling. The low friction and wear as well as unique environ- Fig. 13. Micro-Raman spectra recorded for friction surfaces of a mental adaptation of WC/DLC/WS coatings open new WC/DLC/WS composite with about 20 at% sulfur in tests with cycled 2 2 perspectives for aerospace applications of these or sim- humidity: (a) last cycle was in moist air; (b) last cycle was in dry nitrogen. ilar carbide/DLC/dichalcogenide nanocomposites. The eﬀect on friction regulation with environmental References cycling was similar to the lubrication by WS /graphite 2 powders with an inorganic binder investigated for aero-  The Rover Team, Science 278 (1997) 1765. space applications a decade ago . However, such  R.H. Graham, SR-71 Revealed, Motorbooks Int, Osceda, WI, USA, 1997. 224 pp. powders are soft and easy to wear, and cannot provide  International Space Station, Fact Sheet IS-1995-08-SS004JSC, thickness precision in the deposition of sub-mm coatings. National Aeronautics and Space Administration, Washington, The relatively harder WC/DLC/WS coatings have the DC, August 1995. 2 desirable combination of hardness, low wear rate and  R.L. Fusaro, Lubrication Eng. 3 (1995) 182. low friction in high/low humidity. They can be applied  A. Borrien, in: Proceedings of NATO Advisory Group of Aero- with thickness precision on the nm scale. 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