Some special tribological features of DLC coatings deposited on soft substrates

Sliding wear of composite DLC coatings deposited on two types of aluminium substrates, 2618 alloy and 6063 aluminium extrusion, with different surface finishes have been investigated. A transition load was usually observed, above which the whole coating layer was completely scraped off the wear surface within a short sliding time/distance. The transition load for coatings on polished substrate surfaces was significantly lower than that on the rougher machined substrate surfaces. Wear mechanisms varied with substrate surface finish and load. Close to the transition load, the dominant mechanism for failure of coatings on polished substrates was fragmentation and spallation in a large scale with the nature of fatigue. However, for coatings on relatively rough substrate surfaces, the coating was first fractured/fragmented and then the comminuted coating fragments and wear debris particles were mechanically 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 polished substrate surface mainly resulted from the development of such mechanically alloyed layers on the wear surface. A physical model describing wear processes during the sliding wear of hard coatings deposited on a soft substrate with a rough initial surface finish has been developed which can explain the observed experimental results. (C) 1997 Elsevier Science S.A.