SOME SPECIAL TRIBOLOGICAL FEATURES OF DLC COATINGS DEPOSITED ON SOFT SUBSTRATES

Sliding wear of composite DLC coatings deposited on two types of alumi nium substrates, 2618 alloy and 6063 aluminium extrusion, with differe nt surface finishes have been investigated. A transition load was usua lly observed, above which the whole coating layer was completely scrap ed off the wear surface within a short sliding time/distance. The tran sition load for coatings on polished substrate surfaces was significan tly lower than that on the rougher machined substrate surfaces. Wear m echanisms varied with substrate surface finish and load. Close to the transition load, the dominant mechanism for failure of coatings on pol ished substrates was fragmentation and spallation in a large scale wit h the nature of fatigue. However, for coatings on relatively rough sub strate surfaces, the coating was first fractured/fragmented and then t he comminuted coating fragments and wear debris particles were mechani cally mixed into the substrate to develop a mechanically alloyed wear- protective layer on the wear surface. The higher transition loads for coatings on the rougher substrate surface compared with those on the p olished substrate surface mainly resulted from the development of such mechanically alloyed layers on the wear surface. A physical model des cribing wear processes during the sliding wear of hard coatings deposi ted on a soft substrate with a rough initial surface finish has been d eveloped which can explain the observed experimental results. (C) 1997 Elsevier Science S.A.