Interfacial fracture toughness of vacuum-plasma-sprayed coatings

Abstract

A test is described in which a crack is artificially introduced into the interface between a substrate and coating produced by thermal spraying. This crack is then caused to propagate under heavily mixed mode conditions by means of a four-point-bending arrangement. Computations are outlined which allow evaluation of the interfacial fracture energy and critical stress intensity factor from load-displacement data. These calculations incorporate the effect of residual stresses, which are often large in thermally sprayed coatings. Experiments are described involving titanium coatings deposited on titanium alloy substrates under a single set of spraying parameters. The residual stress states employed in the calculations were those obtained from a previously developed computer programme, describing stress build-up during spray deposition. Data from the modelling indicate that the magnitude and distribution of the residual stresses can exert a significant influence over the interfacial toughness and that their omission from the analysis changes the values obtained.