Bio-inspired surface engineering and tribology of MoS2 overcoated cBN–TiN composite coating

Wear 261 (2006) 592–599

Bio-inspired surface engineering and tribology of MoS2 overcoated cBN–TiN composite coating J.-H. Wu a , B.S. Phillips b , Wenping Jiang c , J.H. Sanders b , J.S. Zabinski b , A.P. Malshe a,∗ a

Materials and Manufacturing Research Lab (MRL), MEEG 204, Mechanical Engineering, The University of Arkansas, 863 W. Dickson St., Fayetteville, AR 72701, United States b AFRL/MLBT, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH 45433, United States c NanoMech LLC, 535 W. Research Blvd., Suite 135, Fayetteville, AR 72701, United States Received 19 October 2005; received in revised form 12 December 2005; accepted 12 January 2006 Available online 23 March 2006

Abstract A hybrid deposition technique, i.e., electrostatic spray coating (ESC) followed by chemical vapor infiltration (CVI), was used to synthesize cubic boron nitride and titanium nitride (cBN–TiN) hard composite coatings. The as-prepared coatings have a surface texture similar to that of lotus leaf. The “valleys” embossed inside the biomimetic surface structure served as nano-/micro-reservoirs of solid lubricant particles of MoS2 , sized from nano- to submicron in diameter. MoS2 particles were applied to coating surfaces by a tumbling method. The tumbled solid lubricant particles covered all surface structures, leading to a smoother coating surface (or reduced average surface roughness Ra ). Tribological tests were carried out for the coatings both before and after tumbling. Sliding results suggest that the tumbled cBN–TiN coating has a significantly lower coefficient of friction. Effectiveness of the tumbled lubricant layer decreased with increasing testing temperature, as indicated by sliding results at elevated temperatures. Worn coating surfaces and pin scars were characterized using surface profilometer, scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), focus ion beam, Raman spectroscopy and scanning transmission electron microscopy (STEM). The tumbled MoS2 layer is responsible for friction reduction during dry sliding at different temperature conditions. Sliding induced reorientation of the MoS2 platelets is correlated with the lubrication mechanism. Oxidation caused degradation of MoS2 at higher temperatures, which is consistent with the frictional behavior at elevated temperatures. It is suggested that the biomimetic surface structure is effective for entrapping application-specific solid lubricant particles. © 2006 Elsevier B.V. All rights reserved. Keywords: cBN–TiN coatings; Biomimetic; Tumbling; MoS2 ; Sliding

1. Introduction As opposed to conventional liquid lubrication methods, solid lubrication provides an advantageous alternative for controlling friction and wear in the absence of an external supply of lubricants, and it has been widely used in many modern tribological applications. However, in spite of extensive research and development efforts through the years, there still has been no single engineered solid lubricant that can provide both low friction and wear over broad application conditions, temperatures and environments [1–3]. To meet the requirements of extreme and demanding tribological applications, advanced lubrication

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Corresponding author. Tel.: +1 479 575 6561; fax: +1 479 575 8720. E-mail address: [email protected] (A.P. Malshe).

0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.01.027

concepts and various complex coating structure and chemistry have been developed, using modern deposition technologies. Typical structures include functionally gradient, multilayered, superlattice, nanocomposite and smart or adaptive coatings [1,2,4–9]. Structural incorporation of these designs during coating preparation can significantly improve the hardness, fracture toughness, lubricity and adhesion of the coatings, and therefore result in reduced friction and enhanced wear resistance, leading to an extended wear life. The exceptional tribological properties associated with these advanced coatings are attributed to the ability of those aforementioned structures/designs to stop crack propagation or dislocation movement during operation under severe conditions; on the other hand, solid lubricants embedded in the structures can effectively reduce coefficient of friction when they are subject to frictional contacts [1,5,6,10].

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Recently, as learned from nature, hydrophobic bumpy and waxy surfaces, with surface structures similar to that of a lotus leaf, have been designed and developed for water-repellent applications [11–14]. It is also reported that these bio-inspired surface textures could be exploited to reduce friction for microand nano-devices [15,16]. In this research, based on a multilayered design and surface patterning, a novel bio-inspired self-lubricating surface layer was produced on a cubic boron nitride (cBN) and titanium nitride (TiN) composite hard coating, which has a biomimetic surface texture, by a tumbling method. The cBN–TiN hard coating was deposited on a cemented tungsten carbide substrate by a hybrid coating technology, i.e., electrostatic spray coating of the nano-/micro-particles of cBN followed by chemical vapor infiltration of a TiN binder phase. cBN is very hard, its hardness exceeded only by diamond, and it has very good thermal stability and chemical inertness. The continuous hard TiN phase, combined with the cemented carbide substrate, offers excellent adhesion, toughness and compatibility with cBN. This nanostructured coating layer provides a unique combination of thermal stability, chemical inertness, outstanding wear resistance and high load support capacity. The top surface of the coating layer was textured to give biomimetic morphology by carefully controlling ESC and CVI processing parameters. Nano-/submicron solid lubricant particles, molybdenum disulfide (MoS2 ), were then embedded into the micro-/nano-scale surface valleys, or micro/nano-reservoirs, through tumbling. The bio-inspired surface design can effectively hold solid lubricant particles, and those lubricant particles are expected to be released from the reservoirs to the sliding interface and reduce coefficient of friction during tribological operations. In this paper the coating processes, including electrostatic spray coating (ESC), chemical vapor infiltration (CVI) and tumbling techniques, will be briefly described. Details on tribotests and testing results will be depicted and discussed, respectively. Various temperature conditions in the tribotests were employed to evaluate temperature effects on coating performance. Correlations between tribological performance and structural and/or compositional changes on the sliding interface will be made. Characterization results will be used to determine the effectiveness of the coating design and to understand coating performance and related lubrication mechanism under different sliding conditions. 2. Experimental details 2.1. Materials and coating processes Wear-resistant cBN–TiN composite hard coatings were produced by using a hybrid coating technology combining ESC technology and CVI deposition. Submicron cBN powder (particle size less than 2 ␮m) was applied to the grounded substrates as a coating preform by the ESC spray process. In the ESC deposition process the commercially acquired cBN particles (from Diamond Innovations Inc.) were ejected from the spray gun with negative charges. The charged particles followed the electrical field lines toward the grounded substrates and assembled a uni-

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form coating on the substrate surfaces. Coating thickness and uniformity were controlled and optimized by adjusting process parameters including electrode-substrate distance, applied electrical voltage, pressure of flow gas, etc. The electrical voltage used ranged from −75 to −45 kV, and the deposition distance was about 150 mm. The air pressure used in this deposition was 207 kPa. The resulting coating had a thickness of about 20 ␮m, which has been a significant challenge for conventional chemical vapor deposition (CVD) or physical vapor deposition (PVD) techniques to achieve. CVD or PVD process generally results in very thin films (less than 2–3 ␮m), and they have intrinsic compressive stress problems that can lead to weak bonding between coating and substrate and further delamination when subjected to load [1,17–19]. Details of the ESC process for producing the coatings are given in Refs. [20–22]. The ESC-coated samples on the substrates were porous, which allows for infiltration of vapor phase to stabilize and bond the superhard cBN particles. In addition, the ESC-coated cBN particles were only loosely bound to each other and to the substrate by electrostatic forces. To make the deposited coating functional, a follow-up CVI process was used to consolidate the porous cBN preform to form a composite coating. The CVI process works on the same principle as a CVD process [22]. In this particular application, TiN was uniformly infiltrated on the individual cBN particles throughout the depth of the ESC-coated porous layer and the substrate surface. Also, if necessary, a pure TiN layer can be grown on the top of the composite cBN–TiN layer after the pore volume filling is complete. The precursors used in the CVI process were TiCl4 , H2 and N2 , and the overall chemical reaction is shown in Eq. (1). During the infiltration process, temperature of the CVI furnace chamber was maintained at 1000 ◦ C. Details of the CVI process can be found elsewhere in Ref. [22]. 2TiCl4 + 4H2 + N2 → 2TiN + 8HCl

(1)

The substrates used for deposition of cBN–TiN wear-resistant coatings were industrial grade tool inserts with nominal composition WC + 6 wt.% Co. Hardness of the as-received substrates was 18 GPa, as measured by a micro-hardness tester. Before deposition, the tool inserts were cleaned by ultrasonic washing in acetone, and then in ethanol, followed by drying in hot air. Solid lubricant (molybdenum disulfide) particles, with an average particle size of 700 nm, were acquired from Alfa Aesar. The fresh MoS2 lubricant particles were encapsulated in the chamber of a tumbler (TRU-SQUARE metal products, model B), and were tumbled on the cBN–TiN composite coating surfaces with a tumbling period of 45 min. For each tumbling operation, fresh particles were added in the chamber. 2.2. Pin-on-disk tests Friction tests were performed using a ball-on-disk arrangement. The tribometer was fixed inside a chamber equipped with a centrifugal pump, allowing the control of testing environment. During sliding, friction forces were measured by strain gauges. The system can acquire data at a frequency of 2 kHz. Details

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about the tribometer can be found elsewhere in Ref. [23]. Different test atmospheres including dry N2 and vacuum (∼10−4 Pa) were used to test environmental sensitivity of the lubricant layer. Tests were also carried out at three kinds of temperature conditions: room temperature, 300 and 500 ◦ C. These tests were used to examine the temperature effect on solid lubricant performance. The sliders (balls, with a diameter of 6 mm) were made from Ti–6A1–4V with nominal composition Ti + 5.5–6.5 wt.% Al, 3.5–4.5 wt.% V, 0.08 wt.% C, 0.05 wt.% N, 0.015 wt.% H, 0.13 wt.% O and 0.25 wt.% Fe. The vacuum tribotests were done with 440C stainless steel balls which had nominal composition Fe + 1 wt.% C, 17.3 wt.% Cr, 0.48 wt.% Mn, 0.41 wt.% Si, 0.48 wt.% Mo and 0.14 wt.% V. The Ti–6A1–4V balls had hardness of Rc = 34, and the 440C steel balls had hardness of Rc = 69. A normal load of 1 N was applied to the ball during sliding, which corresponded to an initial Hertzian contact stress of about 0.8 GPa for the steel balls. Sliding speed was 200 rpm (or roughly 0.13 m/s). The sliding process lasted either 1 h or 104 cycles before the contact was disengaged. 2.3. Characterization Structural and compositional changes induced by the sliding process were characterized using complementary research facilities available at the Arkansas Analytical Laboratory (AAL), Air Force Research Laboratory (AFRL) and the Campus Electron Optics Facility (CEOF) at The Ohio State University. Coating hardness was measured using Vickers and Knoop micro-hardness testers (Buehler). Different X-ray, laser and electron techniques were used to identify coating surface morphology before and after tribotests, and phase composition changes induced by the sliding process. The techniques included surface profilometer (Veeco Dektak 3030), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), focused ion beam (FIB), (scanning) transmission electron microscopy (TEM/STEM) and Raman spectroscopy. SEM was done on a Phillips XL-30 ESEM system equipped with an EDS detector for chemical microanalysis. TEM/STEM analysis was conducted on a FEI Tecnai TF20 system with an operating voltage of 200 kV. Raman spectra were acquired from a

Renishaw Raman microscope with an argon laser source (wavelength 514 nm). Complimentary characterization results from these techniques provide insights for understanding the tribological behavior of the tumbled cBN–TiN wear-resistant coatings. 3. Results and discussions 3.1. Coating surface characteristics and properties The as-deposited cBN–TiN wear-resistant coating has a biomimetic surface texture, similar to that of lotus leaf, as shown in Fig. 1(a). The rounded protrusions of TiN, with diameters ranging mainly from 5 to 20 ␮m, but with a few as large as 40 ␮m, on the coating surface are like epidermal papillae on the surface of a lotus leaf as shown in Fig. 1(b). Naturally there is a tremendous number of micro-scale surface “valleys” constructed among those protrusions. This kind of lotus effect has been used to create hydrophobic surfaces for self-cleaning applications, where surface morphology and chemistry are important factors to consider [12,24,25]. In this work, a new application was established for those surface structures. The troughs (or valleys) formed between the epidermal papillae of TiN can become ideal micro-reservoirs for holding lubricant particles within them. These special biomimetic surface features enable us to design and develop a solid lubricant surface layer on the cBN–TiN coating to reduce friction and improve wear life for tribological applications. The hard cBN–TiN composite coating layer is engineered for increasing load supporting capacity, while the top embedded layer of solid lubricant can significantly decrease sliding friction during operation, resulting in reduced energy dissipation and improved wear resistance. Surface valleys were filled with solid lubricant particles after the tumbling process. Fig. 2 shows the surface morphology of a tumbled cBN–TiN coating surface with MoS2 nano-/microparticles, where Fig. 2(b) is an enlarged view of the highlighted area in Fig. 2(a). It is clear that after tumbling MoS2 particles were trapped in the surface micro-reservoirs of the cBN–TiN coating, and consequently the biomimetic feature of epidermal papillae were sealed by the lubricant particles. EDS results suggested that the whole coating surface was covered by molybdenum disulfide. This implies that besides the trough areas the

Fig. 1. SEM images showing the similarity of surface morphologies of (a) cBN–TiN composite coating and (b) lotus leaf [24].

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Fig. 2. Secondary electron (SE) micrographs showing that MoS2 nano-/micro-particles are trapped in the surface reservoirs on cBN–TiN coating after tumbling, (b) is an enlarged view of the highlighted area in (a).

top surfaces of TiN epidermal papillae were also burnished with MoS2 . As a matter of fact, the contrast shown in Fig. 2(a) confirms that a MoS2 layer on the top of TiN protrusions was more sturdily bonded to the coating surface, compared with the loosely trapped MoS2 particles in the trough areas. As measured by a micro-hardness tester, the as-deposited cBN–TiN composite coating has a hardness of 34 GPa. After tumbling with MoS2 nano-/micro-particles, the coating’s average surface roughness (Ra ) was reduced from 4.5 to 3.2 ␮m. The reduction is correlated with the entrapment of MoS2 particles in the empty micro-reservoirs of the coating surface. 3.2. Tribological testing and evaluation Room temperature friction tests (against 440C stainless steel balls) were conducted in vacuum with the same sliding conditions for both the as-deposited and tumbled cBN–TiN composite coatings. Results of associated friction coefficients are shown in Fig. 3. After the run-in period, for the as-deposited cBN–TiN

Fig. 3. Coefficient of friction vs. time during sliding of 440C stainless steel balls against a cBN–TiN composite coating and burnished MoS2 on a cBN–TiN coating in vacuum (∼10−4 Pa). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

coating, the friction coefficient continuously ascended to an average value of about 0.65. SEM examination showed that there was a large wear scar on the ball after the sliding test, but the width of the wear track on the coating surface was narrow and no wear debris associated with cBN or TiN was found on the track. Instead, there were debris particles comprised of Fe, C and O on the wear track, as identified by EDS analysis. Consequently, it is concluded that the superhard epidermal papillae of the cBN–TiN composite coating caused severe abrasion of the relatively soft steel ball during sliding, generating a significant number of debris particles on the track and rapid wear of the ball. The wear particles on the track also played an important role (“third-body” effect) in increasing friction coefficient, as discussed by Singer in Ref. [26]. In this particular study, it is suggested that generation of abrasive wear of the slider dominates the sliding process, and the high coefficient of friction is related to the abrasion and “third-body” processes. In contrast, sliding of the tumbled cBN–TiN composite coating against a 440C steel ball resulted in a relatively low coefficient of friction, as indicated in Fig. 3. The coefficient of friction started high in the run-in period, with an initial value of µ = 0.40,

Fig. 4. SEM image of the wear track (from the vacuum test depicted by the blue curve in Fig. 3), showing the lubricant particles were smeared along the sliding direction and the overall contact region was smoothened after sliding.

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and that value decreased to about 0.28 when the sliding reached a steady state. After sliding, the wear track was smooth as opposed to the original tumbled surface, with MoS2 particles recognizable on the track. Fig. 4 shows features of the wear track after sliding in vacuum. Mo and S peaks dominated the EDS spectra for the chemical analysis on the track, but also there was a significant amount of Fe, C and O. They were transferred from the pin surface and mechanically mixed with MoS2 particles in the contact region, consistent with the observation of materials transfer and mixing in sliding of other coating systems [8,27] and metals [28]. From Fig. 4, it can also be seen that the MoS2 platelets on the wear track were smeared along the sliding direction, and the overall wear track was smoothened after sliding. Further FIB/TEM and STEM/EDS studies on cross-sections of the sliding interface and subsurface confirmed that after sliding

the crystalline MoS2 platelets were reoriented with respect to the sliding direction, i.e., after sliding their basal planes should be parallel to the sliding direction, leading to good lubrication, consistent with observations and discussions from other researchers in Refs. [29–31]. EDS line scanning with a nanoprobe in STEM verified that the needle-like particles are MoS2 , as demonstrated in Fig. 5(a). Fig. 5(b) displays the overall arrangement of the MoS2 nano-/micro-particles with respect to the sliding direction. The reorientation of MoS2 platelets induced by sliding process on the wear track reduces the shear strength of the tribolayer, making sliding between the pin and the tribolayer easier. This is responsible for friction reduction during steady state sliding of the tumbled cBN–TiN composite coating. To comparatively evaluate the performance of the MoS2 overcoated superhard cBN–TiN composite coatings, a commercially

Fig. 5. (a) EDS line scanning result for Mo along the indicated line 1 across the STEM image and (b) STEM image of the sliding interface/subsurface microstructure (TEM foil was prepared by FIB method), showing the reorientation of MoS2 particles with respect to the sliding direction.

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Fig. 6. Coefficient of friction vs. time during sliding of 440C stainless steel balls against a PVD MoS2 coating in vacuum (∼10−4 Pa). For comparison, result from sliding of the burnished MoS2 on a cBN–TiN coating is also plotted in the figure.

achieved MoS2 coating deposited by a PVD method, specifically ion beam deposition, on a tool insert substrate was tested with the same sliding conditions. Results are displayed in Fig. 6. Unexpectedly, the steady state coefficient of friction for PVD MoS2 coating was similar to that for the tumbled cBN–TiN composite coating, except that they had different initial transients. Sliding of PVD MoS2 coating experienced a gradual increase of coefficient of friction before it reached an average steady state value of about µ = 0.28. The as-received coating had a granular surface, as shown in Fig. 7(a). A stripe pattern was found on the coating surface and is believed to be due to the original substrate roughness. Fig. 7(b) shows the SEM image of the wear track after sliding. A uniform tribolayer was identified on the wear track, and local regions of the tribolayer were removed from the contact area. It is suggested that smearing of the rough contacting area is characteristic of the early sliding stage, and formation of the tribolayer is responsible for steady state sliding. The removal and re-formation of local interfacial patches are correlated with friction fluctuations, which have been discussed in Ref. [8]. EDS detected a small amount of C and O in the tribolayer, but no Fe was found, which implies that there was no transfer of pin mate-

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Fig. 8. Coefficient of friction vs. sliding cycles during sliding of Ti–6A1–4V balls against tumbled cBN–TiN composite coatings at 300 and 500 ◦ C, respectively.

rial to the disk during sliding. The relatively high coefficient of friction for the PVD MoS2 coating might be due to the surface roughness originating from the striped substrate surface. The contact stress distribution is not even along the contact region in the wear track. Temperature can significantly affect the sliding behavior of MoS2 solid lubricant particles. For high-temperature tribotests, with the same sliding conditions, results suggest that the MoS2 lubricant layer was only effective for a limited number of sliding cycles. Fig. 8 shows the friction results from tests conducted at 300 and 500 ◦ C, respectively. For these two tests the testing chamber was initially flushed for 10 min with dry nitrogen gas to remove the air retained in the chamber, and then the chamber was closed for testing, however, certain degree of air leakage was expected during sliding tests. In the 300 ◦ C test, the coefficient of friction stayed at a steady state value of about 0.34 before it increased slightly at around 5000 cycles, marking the local or complete failure of the lubricant layer on the sliding interface. For the 500 ◦ C test, the intermediate friction level was about 0.30, but it experienced a sudden increase in less than 3000 cycles of sliding. The fluctuation of friction coefficient

Fig. 7. (a) SEM image of as-deposited PVD MoS2 coating on tool insert substrate, showing granular surface and (b) SEM image of a tribolayer formed on wear track after sliding in vacuum.

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to tribo-oxidation on the wear track for sliding at 500 ◦ C. This is consistent with the worse performance of the MoS2 lubricant layer at high temperatures. The biomimetic cBN–TiN coating surface is effective in entrapping nano-/micro-particles, and application-specific solid lubricant particles can be applied for various tribological applications. Acknowledgments

Fig. 9. Raman spectra for wear tracks tested at different temperature conditions: room temperature, 300 and 500 ◦ C.

after failure might be related to the generation and ejection of wear particles from the sliding interface. Compared with the earlier room temperature test, high-temperature causes earlier degradation of the MoS2 solid lubricant particles. Raman analysis on the wear tracks from different temperature tests detected phase and chemical changes when the sliding test was conducted at high-temperature (500 ◦ C), as indicated in Fig. 9. From both Figs. 8 and 9, it is clear that the MoS2 lubricant layer is effective up to 300 ◦ C. Formation of the MoO3 phase due to tribo-oxidation causes the earlier failure of the lubricant layer, resulting in loss of lubricity of the tribolayer and a dramatic increase of the coefficient of friction. Tumbling of nano-/micro-particles of high-temperature solid lubricants such as CaF2 might be viable for specific high-temperature applications. 4. Summary and conclusions The as-produced cBN–TiN composite coating has biomimetic surface structures similar to that of a lotus leaf, and those surface structures can be applied as nano/micro-reservoirs to effectively entrap application-specific solid lubricant particles. Tribotests showed that sliding of cBN–TiN against a 440C stainless steel ball resulted in a very high coefficient of friction (µ = 0.65), but with tumbled MoS2 particles sliding friction was significantly reduced to an intermediate level (µ = 0.28), and it was comparable to that of a PVD MoS2 coating on a tool insert substrate. It is suggested that abrasion of the steel slider and a “third-body” process dominated the sliding of the superhard cBN–TiN coating. High-temperature sliding resulted in a reduced number of sliding cycles for the tumbled MoS2 lubricant layer. Oxidation of the MoS2 particles at high temperatures is correlated with the degradation of sliding performance. Post-test characterization results indicated the formation of a lubricious tribolayer on the wear track, and reorientation of MoS2 platelets with respect to the sliding direction was responsible for the relatively low coefficient of friction in the sliding of both tumbled cBN–TiN and PVD MoS2 coatings. Raman spectroscopy detected formation of MoO3 due

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