Delamination and spalling of diamond-like-carbon tribological surfaces

The material removal mechanisms accompanying the rotating contact wear of metal (Cr and W)-containing diamond-like-carbon (DLC) thin coatings on steel have been characterized by using a focused ion beam microscope. The W-DLC wears by gradual recession, in accordance with a polishing mechanism. The Cr-DLC has inferior wear resistance, associated with extensive spalling along prominent machining ridges on the steel substrate. The spalls are generated by cracks in the Cr-DLC coating that tunnel beneath the surface, along the ridges. A mechanics analysis identifies a material parameter that governs the incidence of cracking and spalling. The parameter includes the residual compression and the elastic modulus of the metal-containing DLC coating, as well as its fracture toughness. For W-DLC, this parameter is well below the critical level required for spalling, consistent with the observations. For the Cr-DLC, additional property measurements are needed before completing the spalling assessment. The implication is that the Cr-DLC is subject to either a larger residual stress or a lower toughness than the W-DLC.