Relationships between the mode II fracture toughness and microstructure of thermal spray coatings

A novel test specimen geometry was used to determine the critical mode II strain energy release rate, G(IIc), of nine batches of air- and low-pressure plasma-sprayed coatings. These data will supplement those from existing mode I fracture toughness tests and serve as a basis for understanding the more complex mixed mode loading conditions experienced by operational coatings. It is proposed that coating toughness is dominated by microporosity, macroporosity, unmelted particles, and microcracks parallel and perpendicular to the coating surface. The concept of Quality Indices was introduced, where each batch of coatings was ranked according to the prevalence and severity of these microstructural defects. The Quality Indices, G(IIc) and failure morphology were plotted on a failure map. This showed that an improved coating microstructure, as indicated by rising Quality Indices, was associated with an increasing G(IIc) and a change in failure morphology. (C) 1999 Published by Elsevier Science S.A. All rights reserved.