Wear mechanisms of steel roller bearings protected by thin, hard and low friction coatings

The present work is a contribution towards the development of new and novel low friction coatings for automotive engine cam and cam follower components. A series of hardened and tempered 100Cr6 steel (a 1.0% C, 1.5% Cr bearing steel) rollers, coated with three different coating materials: (i) Cr2N; (ii) CrN + nC (where nC refers to nano-/micrometer-sized carbon particles embedded in the CrN matrix) and (iii) Cr + W–C:H were compared to uncoated 100Cr6 steel using a specially adapted thrust bearing test. The counterface materials, in all tests, comprised two uncoated 100Cr6 steel washers. Tests were conducted in two oils: FVA-3 and SAE10W40. The latter contained various additives whilst the former was additive free. In both lubricants, the uncoated test rollers failed through pitting caused by rolling contact fatigue and a very similar roller life was obtained. Six modes of failure mechanism were observed for the coated rollers. Only two of these resulted in the premature pitting of the substrate via rolling contact fatigue. The rates of degradation of all the coatings was strongly influenced by the type of lubricant used. In the FVA-3 oil, the durability of the Cr2N > Cr + W–C:H > CrN + nC coatings, whereas in the SAE10W40 oil, the durability of the Cr + W–C:H > Cr2N > CrN + nC coatings. The CrN + nC coatings were the least durable of all the coatings and failed through exfoliation (adhesive failure) along or near the coating-substrate interface. The Cr + W–C:H gave the best overall coating durability and were degraded via a combination of polishing wear with micro-/nano-delamination and micro-pitting.