Residual stress and debonding of DLC films on metallic substrates

Abstract

Hydrogenated diamond-like carbon (DLC) films have been prepared using 13.56 MHz capacitively coupled RF glow discharge in a methane atmosphere. Film structure was characterised by FTIR and Raman spectroscopy. Deposition temperatures, which were measured using irreversible self-stick thermax, varied from 40 to 182 °C. Stress levels were measured by monitoring the curvature changes of thin (125 μm) Ti foil after depositing thick (several μm) DLC films, using an Al bonding layer. This intrinsic (deposition) stress was compressive and varied from −0.3 to −2.0 GPa. It increased initially with the average energy of the bombarding ions, reached a maximum and then decreased. Debonding behaviour of DLC coatings on four metallic substrates, with the interfaces being prepared in various ways, has been investigated. Films with a range of thicknesses have been prepared 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, 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. Precleaning 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.