Influence of WC/a-C:H Tribological Coating on Micropitting Wear B. Mahmoudi1; B. Tury1; G. L. Doll, 1; C. H. Hager2; R. D. Evans2 1 The Timken Engineered Surfaces Laboratories, The University of Akron, Akron, OH, United States. 2 The Timken Company, Canton, OH, United States. INTRODUCTION Micropitting is a problematic wear mode in rolling/sliding contacts such as gears and rolling element bearings. Micropitting is a surface fatigue phenomenon that progressively wears material and alters geometric profiles. Micropitting occurs in boundary lubrication (0<λ<1) where asperities of the surfaces are directly contacting. The presence of slip and the associated boundary friction shear stress are required for the generation of micropitting [1]. Proposed resolving strategy (WC/a-C:H coating) WC/a-C:H is a diamond-like carbon material comprised of nanometer-size precipitates of β-W2C and an amorphous hydrocarbon matrix. WC/a-C:H provides excellent resistance to adhesive and abrasive wear and has a low friction coefficient in boundary lubrication, so it might be a candidate to mitigate micropitting. This study evaluates the performance of a WC/a-C:H coating in micropitting tests conducted on SAE52100 bearing steel. RESULTS DISCUSSION The behavior of the WC/a-C:H coating on discs versus on the roller can be addressed by two simultaneous phenomena: Hydrostatic strain field under the contact area. Plastic deformation of the surface asperities. In Hertzian contacts, the hydrostatic strain field under the contact depends on the size and geometry of the components. Elastic deformation in each of the bodies can be calculated by using equations below [2]. Therefore, small diameter roller is more vulnerable to micropitting than discs due to higher stress amplitude in larger elastically deformed volume under the contact. WC/a-C:H coated discs abrade the roller surface. At this point, the roller is more vulnerable to fatigue since any asperity on the coated counterpart can act as a stress riser. Meanwhile, considerably harder coated asperities which have maintained initial sharpness are more aggressive than uncoated asperities. WC/a-C:H coated Roller wears the aggressive asperities on the discs and creates plateaus (skewness of discs decreases). Formation of plateaus causes a reduction in the actual contact stress inside the roller within a few cycles before the roller enters the fatigue process. Schematic of Results High internal stress Low internal stress WC/a-C:H Coating Stress above yield strength Roller Disk 𝛿= 𝑎 2 𝑅 ∗ 1 𝑅 ∗ = 1 𝑅 1 + 1 𝑅 2 Plastic deformation of the asperities occurs at the level of the asperities based on hardness, roughness and orientation of contact asperities. Since the difference between the hardness WC/a-C:H (13 GPa) and steel (7 GPa) is considerable, plastic deformation of the coated asperities is negligible compared to the uncoated asperities. Therefore, surface of steel counterpart undergoes abrasive wear. I Surface of Rollers Steel Discs vs. Coated Roller Steel Discs vs. Steel Roller Coated Discs vs. Steel Roller 1) 2) 3) 2 million cycles 10 million cycles 1 million cycle 350 μm 350 μm 350 μm Moderately micropitted Not micropitted Severely micropitted Compare to steel/steel contact: WC/a-C:H coating on the discs decreases micropitting life of roller. WC/a-C:H coating on the roller increases micropitting life of roller substantially. EXPERIMENTAL PROCEDURE Table 1: Material properties and test parameters in micropitting experiments Test Contact H (HRc) Ra (μm) Pmax (GPa) u (m/s) SRR % T (˚C) λ D R 1.1 Steel/Steel 62 57 0.2 0.25 1.5 1 2 40 0.36 1.2 53 0.18 75 2.1 WC/a-C:H /ST 2.2 3.1 ST/ WC/a-C:H 3.2 Three categories has been studied: Steel Discs on Steel Roller (ST/ST) WC/a-C:H coated Discs on Steel Roller (WC/a-C:H /ST) Steel Discs on WC/a-C:H Roller (ST/ WC/a-C:H) Lubricant: PAO (ISO-10) with no AW or EP additives. WC/a-C:H coating deposition: Closed-field unbalanced magnetron sputtering (CFUBMS), A Physical Vapor Deposition/Chemical Vapor Deposition hybrid process. CFUBMS HAADF-STEM HRTEM I Applying the coating II Micropitting test MPR chamber Tested Roller 3D profilometer Micropits II Surface of Discs Steel Discs vs. Steel Roller Steel Discs vs. Coated Roller 1) 3) 350 μm 350 μm Ra: 0.085 μm, Rsk: -0.299 Ra: 0.086 μm, Rsk: -0.877 Abrasive & Adhesive Wear Just Abrasive Wear CONCLUSIONS The volume of the elastically deformed meniscus under the contact has an impact on the micropitting onset of contact components. The larger the hydrostatic strained volume, the greater the strain energy and the proclivity for crack propagation. As a result, in MPR the roller is more susceptible to micropitting than the discs. WC/a-C:H on the roller acts as a protective shield for the vulnerable part (roller) against the discs. WC/a-C:H on the discs aggressively abrades the roller. Similarly with rolling element bearings, applying the coating on the rollers is advantageous, but not on the ring raceways. The wear mechanism of steel/steel contact is a combination of abrasive and adhesive. The wear mechanism of WC/a-C:H/steel is dominantly abrasive due to hardness difference between coating and SAE52100, and low affinity of a-C:H for steel. ACKNOWLEDGEMENTS Prof. Y. Dong, Paul Shiller & R. Fowler University of Akron. Financial Support from TIMKEN Company. REFERENCES [1]. V. Brizmer, G. E. Morales-Espejel, "Micropitting Modelling in Rolling–Sliding Contacts: Application to Rolling Bearings," Tribology Transactions, vol. 54, pp. 625-643, 2011. [2]. One Hundred Years of Hertz Contact, K L Johnson, Proceedings of the Institution of Mechanical Engineers 1982 196: 363.