Wear and friction of diamondlike-carbon coated and uncoated steel roller bearings under high contact pressure oil lubricated rolling/sliding conditions

Diamondlike-carbon (DLC) coatings have received a lot of research attention by physicists and engineers, especially in the past 25 years. Attempts to use such materials in tribological applications have achieved variable success. The rationale for this work was to investigate the wear durability of three types of DLC coatings applied to hardened and tempered bearing steel and subject them to realistic high pressure cyclic loading under oil lubricated conditions for long duration. A thrust bearing design was deployed for this purpose. The wear and friction behavior of the DLC coated materials relative to uncoated materials was compared when using base (additive free) oils and typical autoengine formulated oils. The type of oil used made no difference to the dynamic friction and oil temperature for all the material and oil combinations used. Durability of the coated and uncoated roller bearings was determined by the type of material. For the uncoated bearings, life was limited after very many test cycles (approaching a billion) via classical rolling contact fatigue pitting. For all the DLC coated rollers life was governed by wear of their coatings. In the case of the tungsten doped DLCs (a-C:H:W), these were worn progressively and uniformly via microabrasion, whereas the nondoped ta-C and a-C DLC coatings were principally worn via delamination and tearing. The latter effect was relatively rapid and was considered to be initiated by blistering of the coating, a process that was probably driven by the high elastic energy/internal stress within the nondoped coating materials. The durability to delamination and tearing of the ta-C coatings was slightly lowered in formulated oil compared to tests made in base oil. Overall, for the test conditions used here, there was no apparent benefit in using DLC coatings.