Some Observations of the Contact Failure Response of DLC Coated Steels under Severe Tribological Conditions
A.M. Elwafi, Algonquin Centre for Construction Excellence, Algonquin College, Ottawa, Canada

Abstract

Diamond-like 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 has achieved variable success. The rationale for this work was to investigate the 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. A thrust bearing rig was deployed for this purpose. The behaviour of the three types of the DLC coated materials was compared when using base (additive free) oils and typical auto-engine formulated oils. Durability of the coated rollers was governed by wear/failure of their coatings. The non-doped DLC coatings (ta-C and a-C) were principally damaged via delamination and tearing whereas the tungsten doped coatings (W-DLC) were worn progressively and uniformly via micro-abrasion. The type of oil used made no real difference to the dynamic friction and oil temperature for all the material and oil combinations used. However, the durability to delamination and tearing of the ta-C coating which had intermediate durability was slightly lowered in formulated oil compared to tests made in base oil. The failure of the non-doped coating materials 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 coating system.