The effect of substrate surface roughness on the wear of DLC coatings

Abstract

The effect of substrate surface roughness on the wear behaviour of diamond-like carbon (DLC) coatings deposited on M42 tool steel substrate with composite interlayers has been investigated on a ball-on-disk wear rig in dry air. After deposition, the surface roughness of the coating was approximately half of the original substrate surface roughness. While the frictional behaviour was not apparently affected, the wear rate of the coatings increased significantly with increase in the substrate surface roughness. Wear rate increased rapidly when the substrate surface roughness exceeded R a 0.93 μm. Above this substrate roughness, the dominant wear mechanism also changed from adhesion to chip/flake formation and fragmentation of the coatings. Chipping/flaking of the coatings initially occurred mainly at the tops of asperities of the surface texture. The Archard specific wear rate increased with increase in total load for coatings on the rough substrate surfaces; however, this was almost invariant with increase in load for coatings on the smoother substrate surfaces, nominally following the Archard's wear law. Contact pressure distributions over the real area of contact between the ball and the rough coating surfaces have been analysed by applying the elastic foundation model of contact mechanics. It has been shown that the contact pressures increase significantly with increase in surface roughness of the coatings. Plastic yielding is highly possible in the coatings deposited on rough substrate surfaces above R a 0.93 μm. The observed apparent effect of surface roughness on wear and wear mechanism transitions of the DLC coatings can be explained according to the contact mechanics analysis results.