Relationships between the mode II fracture toughness and microstructure ofthermal spray coatings

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