Wear mechanism operating in W-DLC coatings in contact with machined steel surfaces

The lubricated wear of a steel couple in which one member is coated with W-DLC is contrasted with that found when both surfaces are coated. The protrusions on the uncoated steel surface are sufficiently hard that they penetrate the W-DLC coating on the mating surface, forming plastic grooves that align along the translation direction. The contact severity exceeds the cracking threshold for the W-DLC coating. The lateral cracks create spalls that remove material at a high rate. The median cracks adversely affect the fatigue life, since some penetrate into the steel substrate. The implication is that a W-DLC coating on only one of the mating surfaces does not enhance durability: both surfaces must be coated to realize the durability benefit. The observed spalling and cracking have been rationalized using established indentation results derived for monolithic brittle solids.