Nanocomposite Tribological Coatings for Rolling Element Bearings
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Nanocomposite Tribological Coatings for Rolling Element Bearings Ryan D. Evans, Elizabeth P. Cooke, Carl R. Ribaudo, Gary L. Doll Materials R&D, The Timken Company, Canton, OH 44706-0930, U.S.A. ABSTRACT The performance of rolling element bearings is enhanced by the application of nanocomposite coatings that are composed of metal carbides incorporated into an amorphous hydrogenated carbon matrix (MC/a-C:H). When applied to the rolling elements in tapered roller bearings, MC/a-C:H coatings were found to help increase fatigue life, rib-roller end scuffing resistance, and false brinelling resistance in poorly lubricated environments. This series of performance tests were conducted with both coated and uncoated rollers. The results are attributed to the minimization of adhesive interactions and desirable counterface micro-polishing due to the presence of the thin hard coatings on rollers in the bearings. INTRODUCTION Reducing friction and wear, and increasing power density to improve fatigue life are some of the primary reasons for improving the surfaces of precision mechanical components, including rolling element bearings. 1 Examples of beneficial surface improvements include “superfinishing” the surface topography and the application of thin solid films. In general, surface initiated failures are considered to be the dominant factor in limiting the life of modern rolling element bearings, and their occurrence may be minimized by the use of thin film coatings and finishes. 2 One hundred years of research work to optimize steel cleanliness and improve bearing geometries have shifted the primary bearing failure modes from the bulk material to the raceway surfaces. Therefore, the modern bearing designer often turns to enhanced surface finishes and coatings to improve performance and increase bearing life. In demanding applications with low lubrication, debris, high load, low rotational speed, or oscillatory motion, enhanced surfaces are critical and necessary for proper bearing function. The use of nanocomposite metal carbide incorporated amorphous hydrogenated carbon (MC/a-C:H) thin film coatings 3,4 on rolling element surfaces is one such method for enhancing bearing performance and power density. These films typically contain ~2-10 nm metal carbide crystals dispersed in an amorphous hydrogenated carbon (a-C:H) matrix. For example, Meng et al. 5 have used high resolution transmission electron microscopy to image the TiC/a-C:H structure, and have identified TiC nanocrystals in an a-C:H matrix for various Ti compositions > 2.5 at.%. Nanocrystalline/amorphous composite structures can be very hard and wear resistant. Veprek 6 suggested that the enhanced mechanical properties of such films might be related to the lack of dislocations in nanocrystals and the inability of nano-cracks to propagate through the amorphous matrix. Rolling element bearings are comprised of rolling elements such as balls, rollers, or needles that separate inner and outer rings. The type of rolling element is determined by the application dem
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