Mechanical stability of DLC films on metallic substrates Part II — Interfacial toughness, debonding and blistering

Abstract

DLC coatings have been deposited onto four metallic substrates, and also onto silicon, with the interfaces being prepared in various ways. Films with a range of thicknesses have been produced and specimens have been subjected to changes in temperature. From a knowledge of the residual stress levels in these films as a function of temperature (see Part I), the strain energy release rate for interfacial debonding has been monitored during deposition and subsequent temperature changes. The value of the strain energy release rate at a point when interfacial debonding occurred has been taken as the interfacial toughness (fracture energy). It is concluded from these observations that the interfacial toughness is low for DLC films deposited onto mechanically-polished surfaces of titanium, mild steel or stainless steel, but quite high on aluminium. Pre-cleaning by bombardment with energetic argon ions raises the interfacial toughness for the steels and for aluminium, but not for titanium. These observations have been related to the nature of the oxide films on these substrates. A thin sputtered interlayer of aluminium was found to raise the interfacial toughness significantly for the steels and for titanium. However, experiments with silicon substrate specimens showed that the good interfacial adhesion obtained with Al interlayers is severely impaired by heating in vacuum to temperatures of the order of 500°C. It has been established that this impairment of adhesion is a consequence of the release of Ar entrapped in the Al during sputter deposition, which promotes the formation of blisters and detachment of the film.