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Material removal on lubricated steel gears with W-DLC-coated surfaces
Surface and Coatings Technology 173 (2003) 122–129
Material removal on lubricated steel gears with W-DLC-coated surfaces C. Mercera,*, A.G. Evansb, N. Yao, S. Allameh, C.V. Cooperc a
Materials Department, University of California, Room 1355, Santa Barbara CA 93106, USA b Princeton Materials Institute, Princeton University, Princeton NJ 08544, USA c United Technologies Research Center, East Hartford CT 06108, USA Received 26 April 2002
Abstract The wear process that occurs in lubricated steel gears with thin, metal-containing diamond-like-carbon (Me-DLC) films deposited on the surface has been characterized using a variety of techniques that include the atomic force microscope and the focused ion beam imaging system. The profile of the tungsten-containing DLC (W-DLC) has been found to duplicate that of the original steel surface with peaks and valleys having amplitude up to 1 mm, superposed on a very thin (50 nm) Cr adhesion layer. Beneath the surface, imperfections are embedded in the steel. Wear occurs through the removal of the peaks by a polishing mechanism, leaving the valleys intact. When the peaks have been fully removed to create a plateau, the RMS roughness is approximately 35 nm. There are still pits corresponding to the original valleys up to 200 nm deep. When the remnant DLC becomes smaller than the amplitude of the peaks on the steel surface, the steel and the Cr adhesion layer become polished, causing the peaks to be eliminated, as well as the imperfections present in the subsurface. The resulting surface has RMS roughness with amplitude 25 nm, as well as small protuberances associated with the carbide particles in the steel. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon; Wear; Focused ion beam; Atomic-force microscope
1. Introduction The wear life of steel gears and bearings has been enhanced by deposition of a thin (approx. 1 mm thick) layer of diamond-like carbon (DLC), co-deposited with transition metals such as W and Cr (Me-DLC) w1–9x. The performance improvement has been related to the ability of the Me-DLC to form a smooth surface during initial wear-in. Such smoothing has a beneficial effect on the hydrodynamics of the lubrication layer. The enhanced durability requires that the removal rate of the DLC be small and that it remains intact, with no largescale delamination w10–15x. The purpose of the present study is to characterize the mechanisms governing MeDLC removal in actual gears, drawing upon insights gained from prior assessments w15–22x. Contact conditions comparable to those expected in gears have been explored using a rotating contact simulator w22x. These results demonstrated that the Cr *Corresponding author. Tel.: q1-805-893-5930; fax: q1-805-8938486. E-mail address: [email protected] (C. Mercer).
adhesion layer between the DLC and the steel is sufficiently robust to inhibit interface de-adhesion. Accordingly, delaminations occur internal to the Me-DLC, extending parallel to the interface, consistent with its relatively low toughness (Gf20 Jmy2 w15x). Delamination and spalling have been observed in Cr-DLC w22x, originating at ridge-imperfections on the steel surface. Conversely, W-DLC deposited on the same substrate resisted spalling and, instead, exhibited gradual smoothing of the original rough surface. The difference has been attributed to a combination of residual stress and toughness w22x. In the present study, the corresponding processes that occur in the actual W-DLC coated steel gear are characterized. For this purpose, tested gears are examined in the vicinity of the regions subject to the most extreme material removal rates. In particular, the sub-surface response has been characterized by using the focused ion beam (FIB) imaging system. 2. Material system and gear testing Spur test gears, having 28 teeth and conforming to
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 4 6 7 - 5
C. Mercer et al. / Surface and Coatings Technology 173 (2003) 122–129
AGMA class 12 tolerances, were fabricated from the Carpenter Technology alloy, Pyrowear 53 in vacuuminduction-melted, vacuum-arc-remelted (VIMyVAR) condition. The nominal composition of the Fe alloy, in mass percent, was 3.3Mo – 2.0Ni – 2.0Cu – 1.0Cr – 1.0Si – 0.35Mn – 0.1V – 0.1C. Following rough machining, blanks were surface carburized to a maximum hardness, RC;62. After subsequent surface grinding to restore the gear involute profile, the effective case (the depth at which the hardness decreases to RC;50) was approximately 1.1 mm. Thereafter, the gears were subjected to a vapor honing process, in which Al2O3 particles ;10 mm in diameter were entrained in compressed air and delivered to the gear involute surfaces under pneumatic pressure. Test gears were cleaned and coated at Balzers, Inc. with W-DLC via magnetron sputtering w23,24x at a substrate temperature ;200 8C. The primary coating constituents included W, C and H, with Cr used as a thin adhesion layer. Secondary ion mass spectrometry (SIMS), applied to the outermost coating layer, revealed a composition of 70 at.% C, 15 at.% H, 12 at.% W and 3 at.% Ni, the Ni being used as a binder for the WC sputtering targets. The coated gears were tested in a ‘four-square’ testing machine at NASAyGlenn Research Center at a contact stress of 2.1 GPa and a pitch-line velocity of 46.6 msy1 in a polyol ester oil having 5 cSt kinematic viscosity conforming to MIL-L-23699. The gear testing procedures are described in more detail elsewhere w25– 27x. With an oil outlet temperature of 77 8C, the resulting l (the ratio of the oil film thickness to the surface roughness) was 0.55. The tests proceeded for 3=108 cycles prior to removal and characterization. 3. Characterization methods The gear teeth were examined using a Philips XL-30 FEG scanning electron microscope (SEM), in order to obtain a general overview of the extent of wear along the tooth profile. Backscattered electron imaging and energy-dispersive spectroscopy (EDS) element mapping were employed to distinguish between the areas where the W-DLC coating remained and areas where it had been completely removed. The surfaces of the material in the as-deposited and partially worn conditions have been characterized using the atomic force microscope (AFM) in the tapping mode. The instrument is used to obtain images as well as surface profiles that can be quantified in terms of dominant wavelengths and amplitudes. The cross-sections of W-DLC layers deposited on steel gears were micro-machined and imaged using the FEI Strata DB 235 focused ion beam (FIB) workstation, which combines a FIB and an SEM into a single instrument. For the FIB mode, this instrument uses a Gaq liquid metal ion source, with a 5 nm minimum
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probe diameter and a maximum ion current of 20 nA, operated at a standard beam energy of 30 keV. Gallium ions at this energy ablate any material, with sputter yields of order ten, corresponding to a material removal rate ;1 mm3 nAy1 sy1. The electron column in this instrument uses a Schottky field-emission gun operated at energy range between 1 and 30 keV, providing an image resolution of 3 nm. In order to protect the surface features on the area of interest, a Pt layer was deposited prior to ion beam micro-machining. This deposition was accomplished by introducing Pt atoms very close to the sample surface where they collide with Ga ions of the primary ion beam and distribute over the sample surface. Once this initial layer has been deposited, a high ion beam current (approx. 5000 pA) is used to cut a stair-step trench ;30 nm long, 10 nm wide and 6 nm deep (at the deepest step) along the region of interest. The front surface of the trench was polished using a low ion beam current (50 pA) and then imaged with the electron beam. Following the FIB studies, EDS analysis (using the XL-30 SEM) was used to determine the composition, and hence, the origin of defects between the W-DLC coating and the steel substrate. In order to accomplish this, selected area scans were performed at various positions and EDS spectra were collected and compared. 4. Observations and measurements of material removal 4.1. General characteristics Low magnification optical images of the gears have indicated two regions of preferential material removal: one near the root and the other at the tip. Detailed analysis has been performed in the root region. Within this region there are three domains. (i) One experienced complete W-DLC removal, exposing the underlying steel (Figs. 1 and 2). (ii) The adjacent zone exhibits partially worn W-DLC. (iii) Outside this zone, the W-DLC appeared to be unaffected. Chemical analyses of these zones are summarized in Fig. 2 (b). Most notable is the zone with strong Cr content between the steel and the partially worn W-DLC, highlighting the remnants of the Cr adhesion layer. Otherwise, the W in the DLC is apparent, as are the steel alloy constituents of Fe, Mo, Cr and Ni. 4.2. Cross sections Scanning electron images of sections created with the FIB are summarized in Fig. 3 and Figs. 5–7. In the region unaffected by wear, Fig. 3, several morphological features are apparent. The W-DLC has a columnar structure, with gaps between some of the columns. The
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Fig. 1. (a) Amplitude and (b) contrast scans performed in the Atomic Force Microscope visualizing the three basic domains. The as-deposited region with relatively large amplitude roughness evident at the bottom left of (a) is comprised of peaks and deep valleys. The zone of partially removed W-DLC coating consists of peaks planarized by a polishing mechanism. The zone where the W-DLC coating has been completely removed to expose the underlying steel, located at the top right of (a), has been polished during the removal process.
surface has a roughness reflecting the columnar structure, which will be quantified below. The underlying steel surface has a topography characteristic of a grit blast metal surface. The roughness is appreciable. There are folds of metal indicative of plastic extrusion adjacent to the impacting particles. Small particles embedded in the substrate beneath the surface exhibit EDS spectra (Fig. 4), with prominent Al peaks, indicating that they are particles composed of Al2O3. The thin (approx. 50 nm thick), uniform layer with lighter contrast, outlining the steel surface, is the Cr adhesion layer. Upon entering the partially worn areas, where the W-DLC is retained (Fig. 5), the surface changes without affecting any of the sub-surface features. The W-DLC now has a smooth surface with fewer gaps between the columnar domains. The roughness is characterized in the next section. In regions where the W-DLC has been completely removed, as in Fig. 6, the steel surface is smooth, with no evidence of either the folds or the embedded particles. In the transition region, where the removal of the WDLC begins to expose the Cr adhesion layer and the steel, Fig. 7, the following features are apparent. (i) The surfaces of the worn W-DLC and the fully exposed steel have similar appearance. (ii) In regions where the Cr and steel are partially exposed, the W-DLC resides exclusively within the valleys of the original steel surface, while the tops of the protruding steel peaks have been flattened. The Cr interlayer wears uniformly wherever it intersects the surface plane. The general implication is that wear occurs in accordance with a
chemomechanical material removal process with no involvement of delamination, either in the W-DLC coating or at the interface or in the steel. 4.3. Surface topography The atomic force microscope has been used to characterize the preceding topographical features, augmented by measurements of the profiles imaged on the cross sections (Figs. 3, 5–7). The typical image of the three predominant regions obtained at low (in-plane) spatial resolution (Fig. 1) provides a visual perspective. The evolution in smoothness from the as-deposited surface to the worn W-DLC to the exposed steel is vividly displayed. At this resolution, it is evident that the exposed peaks of the rough, as-deposited surface are systematically removed by a polishing mechanism. Moreover, the underlying valley topology is retained, establishing that the excess W-DLC is removed from the system. Consequently, as the peaks are eliminated and replaced by a smooth plateau, a distribution of isolated pits remains, coincident with the deepest valleys. Amplitude scans across the as-deposited surface and along trajectories within the partially worn W-DLC (Figs. 8 and 9), establish the predominant amplitudes and wavelengths. As deposited, the surface has peaks and valleys symmetric about the median plane, with RMS amplitude, ARMSs0.18 mm. The largest amplitudes are: Amaxf1.1 mm, with wavelength, Lmaxf3 mm.
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Fig. 2. (a) Scanning electron micrograph of the tooth region showing the as-deposited DLC (left), the region of partially worn W-DLC coating (center), as well as the underlying steel exposed after complete removal of W-DLC coating (right) and (b) EDS element map corresponding to the same region.
These amplitudes are consistent with those measured directly from the cross sections (Figs. 3, 5–7). There do not appear to be any dominant wavelengths (Fig. 10). For the partially worn W-DLC, the amplitudes systematically decrease as wear proceeds. It is apparent from the profiles that the peaks are removed while the valleys are retained. Once a plateau has been created, the RMS amplitude is about a factor of 5 smaller,
ARMSf35 nm, with the largest amplitudes, Amaxf200 nm, representing the remnant valleys. These average amplitudes are below the resolutions achievable on the cross sections shown in Fig. 5. Nevertheless, these SEM images reveal the presence of valleys having depth consistent with the 200 nm ascertained using the AFM. In the regions where the W-DLC has been removed entirely, the exposed steel surface is even smoother. The
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Fig. 3. Scanning electron micrograph showing a cross-section made through the as-deposited W-DLC coating using the FIB.
amplitude scans indicate ARMSf25 nm. It is noteworthy, however, that the largest amplitudes in the distribution now protrude from the surface. The valleys have been eliminated. The protrusions are spatially dispersed over the surface with amplitude, Adf200 nm, and diameter, Df2 mm. They correlate with the carbide particles used for strengthening, which polish more slowly than the matrix of tempered martensite. 5. Assessment The material removal mechanisms found for thin WDLC on steel gears appear to be consistent with those found in other tests. Namely, the W-DLC adheres well
to the steel because of the Cr adhesion layer. Moreover, it is sufficiently resistant to delamination and spalling that the material removal occurs by a gradual polishing mechanism. The polishing occurs preferentially at the most prominent ridges to form a plateau, leaving the original valleys intact. This process reduces the RMS roughness, improving the performance of the lubricant layer. It also increases the contact area and diminishes the ensuing wear rate. This process continues until the Cr interlayer and the ridges on the original steel surface are exposed. At this stage, instead of activating a new wear mechanism, the presence of the DLC plateau allows material removal to proceed by polishing, enabling the RMS roughness to diminish even further.
Fig. 4. EDS spectrum from particle of Al2O3 embedded in the steel substrate beneath the W-DLC layer.
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Fig. 5. Scanning electron micrograph showing a FIB cross-section through the partially worn zone.
Accordingly, the benefit of the DLC is retained even after it has been removed from much of the surface. It remains to ascertain whether other M-DLCs will perform in similar manner. 6. Summary The sequence of observations has revealed the following five basic steps governing the wear of steel gears with W-DLC layers. i. The original steel surface after vapor honing with Al2O3 particles is rough and highly defective. Some Al2O3 particles, having a diameter of approximately 500 nm or less, are embedded into the sub-surface to a depth of approximately 1 mm. Some regions are extruded, upward, by approximately 1 mm, indicative of plastic pile-up. Other regions are folded over, resulting in embedded crack-like imperfections. The
Cr adhesion layer, approximately 50 nm thick, covers these imperfections with remarkable spatial uniformity. ii. As the W-DLC coating is deposited, it develops intercolumnar gaps at the larger imperfections on the steel surface, resulting in an eventual roughness with peakto-valley amplitudes as great as 1 mm, with RMS amplitude of approximately 0.2 mm. iii.Partial wear of the W-DLC coating causes a flattening and shearing of the peaks on the W-DLC surface. This occurs by the systematic polishing of the peaks to create a plateau that broadens as wear proceeds, until a continuous flat plateau is created. At the plateau stage, the RMS roughness has amplitude ;35 nm with pits corresponding to the original valleys down to 200 nm. iv. When the wear progresses down to the Cr layer, the systematic polishing process continues with the steel
Fig. 6. Scanning electron micrograph of a cross-section through the region where the W-DLC coating has been removed completely.
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Fig. 7. Scanning electron micrographs showing cross-section through transition region between the partially worn W-DLC coating and the exposed steel surface (the lower micrograph is a continuation of the cross-section towards the tooth root).
Fig. 8. Surface topography line scans in three regions: (top) underlying steel, (middle) partially worn W-DLC and (bottom) as-deposited W-DLC. The locations of the line scans are specified in the 2-D surface scans on the left.
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peaks being removed in a manner similar to the WDLC in the valleys. v. When the W-DLC and the Cr interlayer have been removed entirely, the remnant steel surface is highly polished, with RMS roughness of approximately 25 nm, less than that found on the partially worn WDLC, but with small protuberances related to the carbide particles used for strengthening. References
Fig. 9. The amplitudes of the surface features measured using the AFM in the three domains identified in the images.
Fig. 10. Number of peaksyvalleys with heightsydepths exceeding the specified amplitudes.
w1x H. Dimigen, H. Hubsch, R. Memming, Appl. Phys. Lett. 50 (1987) 1056. w2x K.R. Lee, K.Y. Eun, K.M. Kim, Surf. Coat. Technol. 77 (1995) 786. w3x A.A. Voevodin, S.V. Prasad, J.S. Zabinski, J. Appl. Phys. 82 (1997) 855. w4x C. Donnet, Surf. Coat. Technol. 101 (1998) 180. w5x A. Grill, Diamond Rel. Mater. 8 (1999) 428. w6x F.Z. Cui, D.J. Li, Surf. Coat. Technol. 131 (2000) 481. w7x R. Wei, P.J. Wilbur, M.J. Liston, Diamond Rel. Mater. 2 (1993) 898. w8x A. Grill, Surf. Coat. Technol. 94–95 (1997) 507. w9x J.A. Heimberg, K.J. Wahl, I.L. Singer, A. Erdemir, Appl. Phys. Lett. 78 (2001) 2449. w10x G. Gille, B. Rau, Thin Solid Films 120 (1984) 109. w11x D. Nir, Thin Solid Films 112 (1984) 41. w12x J. Seth, R. Padiyath, S.V. Babu, J. Vac. Sci. Technol. A10 (1992) 284. w13x N.B. Thomsen, A.C. Fischer-Cripps, M.V. Swain, Thin Solid Films 332 (1998) 180. w14x X.L. Peng, T.W. Clyne, Thin Solid Films 312 (1998) 219. w15x J.S. Wang, Y. Sugimura, A.G. Evans, W.K. Tredway, Thin Solid Films 325 (1998) 163. w16x S.J. Harris, A.M. Weiner, W.-J. Meng, Wear 211 (1997) 208. w17x S.J. Harris, A.M. Weiner, Wear 213 (1997) 200. w18x S.J. Harris, A.M. Weiner, Wear 223 (1998) 31. w19x K. Jia, Y.Q. Li, T.E. Fischer, B. Gallois, J. Mater. Res. 10 (1995) 1403. w20x J. Takadoum, H.H. Bennani, M. Allouard, Surf. Coat. Technol. 88 (1996) 232. w21x T.O. Mulhearn, L.E. Samuels, Wear 5 (1962) 478. w22x R. Wang, C. Mercer, A.G. Evans, C.V. Cooper, H.K. Yoon, Diamond Rel. Mater. 11 (2002) 1797. w23x K. Bewilogua, C.V. Cooper, C. Specht, J. Schroeder, R. Wittorf, M. Grischke, Surf. Coat. Technol. 127 (2000) 224. w24x W.J. Meng, B.A. Gillispie, J. Appl. Phys. 84 (1998) 4314. w25x H.W. Scibbe, D.P. Townsend, P.R. Aron, Effect of lubricant extreme pressure additives on surface fatigue life of AISI 9310 spur gears, NASA Technical Paper 2408 (1984). w26x D.P. Townsend, Surface fatigue life and failure characteristics of EX-53, CBS 1000M, andAISI 9310 gear materials, NASA Technical Paper 2513 (1985). w27x D.P. Townsend, J. Shimski, Evaluation of advanced lubricants for aircraft applications using gear surface fatigue tests, NASA Technical Memorandum 104336 (1991).
Material removal on lubricated steel gears with W-DLC-coated surfaces C. Mercera,*, A.G. Evansb, N. Yao, S. Allameh, C.V. Cooperc a
Materials Department, University of California, Room 1355, Santa Barbara CA 93106, USA b Princeton Materials Institute, Princeton University, Princeton NJ 08544, USA c United Technologies Research Center, East Hartford CT 06108, USA Received 26 April 2002
Abstract The wear process that occurs in lubricated steel gears with thin, metal-containing diamond-like-carbon (Me-DLC) films deposited on the surface has been characterized using a variety of techniques that include the atomic force microscope and the focused ion beam imaging system. The profile of the tungsten-containing DLC (W-DLC) has been found to duplicate that of the original steel surface with peaks and valleys having amplitude up to 1 mm, superposed on a very thin (50 nm) Cr adhesion layer. Beneath the surface, imperfections are embedded in the steel. Wear occurs through the removal of the peaks by a polishing mechanism, leaving the valleys intact. When the peaks have been fully removed to create a plateau, the RMS roughness is approximately 35 nm. There are still pits corresponding to the original valleys up to 200 nm deep. When the remnant DLC becomes smaller than the amplitude of the peaks on the steel surface, the steel and the Cr adhesion layer become polished, causing the peaks to be eliminated, as well as the imperfections present in the subsurface. The resulting surface has RMS roughness with amplitude 25 nm, as well as small protuberances associated with the carbide particles in the steel. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon; Wear; Focused ion beam; Atomic-force microscope
1. Introduction The wear life of steel gears and bearings has been enhanced by deposition of a thin (approx. 1 mm thick) layer of diamond-like carbon (DLC), co-deposited with transition metals such as W and Cr (Me-DLC) w1–9x. The performance improvement has been related to the ability of the Me-DLC to form a smooth surface during initial wear-in. Such smoothing has a beneficial effect on the hydrodynamics of the lubrication layer. The enhanced durability requires that the removal rate of the DLC be small and that it remains intact, with no largescale delamination w10–15x. The purpose of the present study is to characterize the mechanisms governing MeDLC removal in actual gears, drawing upon insights gained from prior assessments w15–22x. Contact conditions comparable to those expected in gears have been explored using a rotating contact simulator w22x. These results demonstrated that the Cr *Corresponding author. Tel.: q1-805-893-5930; fax: q1-805-8938486. E-mail address: [email protected] (C. Mercer).
adhesion layer between the DLC and the steel is sufficiently robust to inhibit interface de-adhesion. Accordingly, delaminations occur internal to the Me-DLC, extending parallel to the interface, consistent with its relatively low toughness (Gf20 Jmy2 w15x). Delamination and spalling have been observed in Cr-DLC w22x, originating at ridge-imperfections on the steel surface. Conversely, W-DLC deposited on the same substrate resisted spalling and, instead, exhibited gradual smoothing of the original rough surface. The difference has been attributed to a combination of residual stress and toughness w22x. In the present study, the corresponding processes that occur in the actual W-DLC coated steel gear are characterized. For this purpose, tested gears are examined in the vicinity of the regions subject to the most extreme material removal rates. In particular, the sub-surface response has been characterized by using the focused ion beam (FIB) imaging system. 2. Material system and gear testing Spur test gears, having 28 teeth and conforming to
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 4 6 7 - 5
C. Mercer et al. / Surface and Coatings Technology 173 (2003) 122–129
AGMA class 12 tolerances, were fabricated from the Carpenter Technology alloy, Pyrowear 53 in vacuuminduction-melted, vacuum-arc-remelted (VIMyVAR) condition. The nominal composition of the Fe alloy, in mass percent, was 3.3Mo – 2.0Ni – 2.0Cu – 1.0Cr – 1.0Si – 0.35Mn – 0.1V – 0.1C. Following rough machining, blanks were surface carburized to a maximum hardness, RC;62. After subsequent surface grinding to restore the gear involute profile, the effective case (the depth at which the hardness decreases to RC;50) was approximately 1.1 mm. Thereafter, the gears were subjected to a vapor honing process, in which Al2O3 particles ;10 mm in diameter were entrained in compressed air and delivered to the gear involute surfaces under pneumatic pressure. Test gears were cleaned and coated at Balzers, Inc. with W-DLC via magnetron sputtering w23,24x at a substrate temperature ;200 8C. The primary coating constituents included W, C and H, with Cr used as a thin adhesion layer. Secondary ion mass spectrometry (SIMS), applied to the outermost coating layer, revealed a composition of 70 at.% C, 15 at.% H, 12 at.% W and 3 at.% Ni, the Ni being used as a binder for the WC sputtering targets. The coated gears were tested in a ‘four-square’ testing machine at NASAyGlenn Research Center at a contact stress of 2.1 GPa and a pitch-line velocity of 46.6 msy1 in a polyol ester oil having 5 cSt kinematic viscosity conforming to MIL-L-23699. The gear testing procedures are described in more detail elsewhere w25– 27x. With an oil outlet temperature of 77 8C, the resulting l (the ratio of the oil film thickness to the surface roughness) was 0.55. The tests proceeded for 3=108 cycles prior to removal and characterization. 3. Characterization methods The gear teeth were examined using a Philips XL-30 FEG scanning electron microscope (SEM), in order to obtain a general overview of the extent of wear along the tooth profile. Backscattered electron imaging and energy-dispersive spectroscopy (EDS) element mapping were employed to distinguish between the areas where the W-DLC coating remained and areas where it had been completely removed. The surfaces of the material in the as-deposited and partially worn conditions have been characterized using the atomic force microscope (AFM) in the tapping mode. The instrument is used to obtain images as well as surface profiles that can be quantified in terms of dominant wavelengths and amplitudes. The cross-sections of W-DLC layers deposited on steel gears were micro-machined and imaged using the FEI Strata DB 235 focused ion beam (FIB) workstation, which combines a FIB and an SEM into a single instrument. For the FIB mode, this instrument uses a Gaq liquid metal ion source, with a 5 nm minimum
123
probe diameter and a maximum ion current of 20 nA, operated at a standard beam energy of 30 keV. Gallium ions at this energy ablate any material, with sputter yields of order ten, corresponding to a material removal rate ;1 mm3 nAy1 sy1. The electron column in this instrument uses a Schottky field-emission gun operated at energy range between 1 and 30 keV, providing an image resolution of 3 nm. In order to protect the surface features on the area of interest, a Pt layer was deposited prior to ion beam micro-machining. This deposition was accomplished by introducing Pt atoms very close to the sample surface where they collide with Ga ions of the primary ion beam and distribute over the sample surface. Once this initial layer has been deposited, a high ion beam current (approx. 5000 pA) is used to cut a stair-step trench ;30 nm long, 10 nm wide and 6 nm deep (at the deepest step) along the region of interest. The front surface of the trench was polished using a low ion beam current (50 pA) and then imaged with the electron beam. Following the FIB studies, EDS analysis (using the XL-30 SEM) was used to determine the composition, and hence, the origin of defects between the W-DLC coating and the steel substrate. In order to accomplish this, selected area scans were performed at various positions and EDS spectra were collected and compared. 4. Observations and measurements of material removal 4.1. General characteristics Low magnification optical images of the gears have indicated two regions of preferential material removal: one near the root and the other at the tip. Detailed analysis has been performed in the root region. Within this region there are three domains. (i) One experienced complete W-DLC removal, exposing the underlying steel (Figs. 1 and 2). (ii) The adjacent zone exhibits partially worn W-DLC. (iii) Outside this zone, the W-DLC appeared to be unaffected. Chemical analyses of these zones are summarized in Fig. 2 (b). Most notable is the zone with strong Cr content between the steel and the partially worn W-DLC, highlighting the remnants of the Cr adhesion layer. Otherwise, the W in the DLC is apparent, as are the steel alloy constituents of Fe, Mo, Cr and Ni. 4.2. Cross sections Scanning electron images of sections created with the FIB are summarized in Fig. 3 and Figs. 5–7. In the region unaffected by wear, Fig. 3, several morphological features are apparent. The W-DLC has a columnar structure, with gaps between some of the columns. The
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Fig. 1. (a) Amplitude and (b) contrast scans performed in the Atomic Force Microscope visualizing the three basic domains. The as-deposited region with relatively large amplitude roughness evident at the bottom left of (a) is comprised of peaks and deep valleys. The zone of partially removed W-DLC coating consists of peaks planarized by a polishing mechanism. The zone where the W-DLC coating has been completely removed to expose the underlying steel, located at the top right of (a), has been polished during the removal process.
surface has a roughness reflecting the columnar structure, which will be quantified below. The underlying steel surface has a topography characteristic of a grit blast metal surface. The roughness is appreciable. There are folds of metal indicative of plastic extrusion adjacent to the impacting particles. Small particles embedded in the substrate beneath the surface exhibit EDS spectra (Fig. 4), with prominent Al peaks, indicating that they are particles composed of Al2O3. The thin (approx. 50 nm thick), uniform layer with lighter contrast, outlining the steel surface, is the Cr adhesion layer. Upon entering the partially worn areas, where the W-DLC is retained (Fig. 5), the surface changes without affecting any of the sub-surface features. The W-DLC now has a smooth surface with fewer gaps between the columnar domains. The roughness is characterized in the next section. In regions where the W-DLC has been completely removed, as in Fig. 6, the steel surface is smooth, with no evidence of either the folds or the embedded particles. In the transition region, where the removal of the WDLC begins to expose the Cr adhesion layer and the steel, Fig. 7, the following features are apparent. (i) The surfaces of the worn W-DLC and the fully exposed steel have similar appearance. (ii) In regions where the Cr and steel are partially exposed, the W-DLC resides exclusively within the valleys of the original steel surface, while the tops of the protruding steel peaks have been flattened. The Cr interlayer wears uniformly wherever it intersects the surface plane. The general implication is that wear occurs in accordance with a
chemomechanical material removal process with no involvement of delamination, either in the W-DLC coating or at the interface or in the steel. 4.3. Surface topography The atomic force microscope has been used to characterize the preceding topographical features, augmented by measurements of the profiles imaged on the cross sections (Figs. 3, 5–7). The typical image of the three predominant regions obtained at low (in-plane) spatial resolution (Fig. 1) provides a visual perspective. The evolution in smoothness from the as-deposited surface to the worn W-DLC to the exposed steel is vividly displayed. At this resolution, it is evident that the exposed peaks of the rough, as-deposited surface are systematically removed by a polishing mechanism. Moreover, the underlying valley topology is retained, establishing that the excess W-DLC is removed from the system. Consequently, as the peaks are eliminated and replaced by a smooth plateau, a distribution of isolated pits remains, coincident with the deepest valleys. Amplitude scans across the as-deposited surface and along trajectories within the partially worn W-DLC (Figs. 8 and 9), establish the predominant amplitudes and wavelengths. As deposited, the surface has peaks and valleys symmetric about the median plane, with RMS amplitude, ARMSs0.18 mm. The largest amplitudes are: Amaxf1.1 mm, with wavelength, Lmaxf3 mm.
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Fig. 2. (a) Scanning electron micrograph of the tooth region showing the as-deposited DLC (left), the region of partially worn W-DLC coating (center), as well as the underlying steel exposed after complete removal of W-DLC coating (right) and (b) EDS element map corresponding to the same region.
These amplitudes are consistent with those measured directly from the cross sections (Figs. 3, 5–7). There do not appear to be any dominant wavelengths (Fig. 10). For the partially worn W-DLC, the amplitudes systematically decrease as wear proceeds. It is apparent from the profiles that the peaks are removed while the valleys are retained. Once a plateau has been created, the RMS amplitude is about a factor of 5 smaller,
ARMSf35 nm, with the largest amplitudes, Amaxf200 nm, representing the remnant valleys. These average amplitudes are below the resolutions achievable on the cross sections shown in Fig. 5. Nevertheless, these SEM images reveal the presence of valleys having depth consistent with the 200 nm ascertained using the AFM. In the regions where the W-DLC has been removed entirely, the exposed steel surface is even smoother. The
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Fig. 3. Scanning electron micrograph showing a cross-section made through the as-deposited W-DLC coating using the FIB.
amplitude scans indicate ARMSf25 nm. It is noteworthy, however, that the largest amplitudes in the distribution now protrude from the surface. The valleys have been eliminated. The protrusions are spatially dispersed over the surface with amplitude, Adf200 nm, and diameter, Df2 mm. They correlate with the carbide particles used for strengthening, which polish more slowly than the matrix of tempered martensite. 5. Assessment The material removal mechanisms found for thin WDLC on steel gears appear to be consistent with those found in other tests. Namely, the W-DLC adheres well
to the steel because of the Cr adhesion layer. Moreover, it is sufficiently resistant to delamination and spalling that the material removal occurs by a gradual polishing mechanism. The polishing occurs preferentially at the most prominent ridges to form a plateau, leaving the original valleys intact. This process reduces the RMS roughness, improving the performance of the lubricant layer. It also increases the contact area and diminishes the ensuing wear rate. This process continues until the Cr interlayer and the ridges on the original steel surface are exposed. At this stage, instead of activating a new wear mechanism, the presence of the DLC plateau allows material removal to proceed by polishing, enabling the RMS roughness to diminish even further.
Fig. 4. EDS spectrum from particle of Al2O3 embedded in the steel substrate beneath the W-DLC layer.
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Fig. 5. Scanning electron micrograph showing a FIB cross-section through the partially worn zone.
Accordingly, the benefit of the DLC is retained even after it has been removed from much of the surface. It remains to ascertain whether other M-DLCs will perform in similar manner. 6. Summary The sequence of observations has revealed the following five basic steps governing the wear of steel gears with W-DLC layers. i. The original steel surface after vapor honing with Al2O3 particles is rough and highly defective. Some Al2O3 particles, having a diameter of approximately 500 nm or less, are embedded into the sub-surface to a depth of approximately 1 mm. Some regions are extruded, upward, by approximately 1 mm, indicative of plastic pile-up. Other regions are folded over, resulting in embedded crack-like imperfections. The
Cr adhesion layer, approximately 50 nm thick, covers these imperfections with remarkable spatial uniformity. ii. As the W-DLC coating is deposited, it develops intercolumnar gaps at the larger imperfections on the steel surface, resulting in an eventual roughness with peakto-valley amplitudes as great as 1 mm, with RMS amplitude of approximately 0.2 mm. iii.Partial wear of the W-DLC coating causes a flattening and shearing of the peaks on the W-DLC surface. This occurs by the systematic polishing of the peaks to create a plateau that broadens as wear proceeds, until a continuous flat plateau is created. At the plateau stage, the RMS roughness has amplitude ;35 nm with pits corresponding to the original valleys down to 200 nm. iv. When the wear progresses down to the Cr layer, the systematic polishing process continues with the steel
Fig. 6. Scanning electron micrograph of a cross-section through the region where the W-DLC coating has been removed completely.
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Fig. 7. Scanning electron micrographs showing cross-section through transition region between the partially worn W-DLC coating and the exposed steel surface (the lower micrograph is a continuation of the cross-section towards the tooth root).
Fig. 8. Surface topography line scans in three regions: (top) underlying steel, (middle) partially worn W-DLC and (bottom) as-deposited W-DLC. The locations of the line scans are specified in the 2-D surface scans on the left.
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peaks being removed in a manner similar to the WDLC in the valleys. v. When the W-DLC and the Cr interlayer have been removed entirely, the remnant steel surface is highly polished, with RMS roughness of approximately 25 nm, less than that found on the partially worn WDLC, but with small protuberances related to the carbide particles used for strengthening. References
Fig. 9. The amplitudes of the surface features measured using the AFM in the three domains identified in the images.
Fig. 10. Number of peaksyvalleys with heightsydepths exceeding the specified amplitudes.
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