Intrinsic toughness of interfaces between SiC coatings and substrates of Si or C fibre

Abstract

The critical energy release rate for separation of SiC coatings from single crystal Si substrates or surfaces of carbon fibres, along their well-defined interfaces can be determined quite accurately from analysis of the spontaneous delamination of coatings under bi-axial stress, when such coatings exceed a critical thickness. Direct evaluations have been made of the specific work of delamination along the interface for SiC coatings from single crystal Si substrates, for both the case of coatings under bi-axial compression, as well as under bi-axial tension. The critical energy release rate for coatings in tension was 5.1 J m², and that for coatings under compression was 5.9 J m². The higher value of the latter is attributed to relative slippage between coating and substrate before lift-off of the former. Corresponding determination of the critical energy release rate for delamination of SiC coatings under bi-axial tension from surfaces of anisotropic Pitch-55 carbon fibres gave an answer of 5.5 J m². These values compare very well with expectations from surface energies of strong solids.