The structure of thermally sprayed coatings consists of lamellae which are oriented parallel to the substrate surface. Coatings most often consist of a metallic bond coat and a ceramic overlay, although in principle there is no restriction to the number of components in any coating system. The lamellae separate and fracture by distinctive mechanisms which are reflected in the failure morphology, and these may be described as adhesive (between the coating and substrate), cohesive (within the coating), or mixed mode. There is a large variability in the failure stress for any nominally identical group of coatings. This variability in coating strength is initially assumed to arise from the defective nature of the coating. A lower bound for the fracture toughness of alumina coatings can be calculated as 0.2 MNm-3/2. The coating strength values may also be treated as belonging to the statistical distribution of the Weibull function. The Weibull modulus of the coating strength varied from 1.4 to 3.8. This analysis infers that the flaw size within coatings is highly variable and that the flaws are non-uniformly dispersed. The present work focuses on the question of whether tensile adhesion tests are an appropriate testing method for thermally sprayed materials.
Journal of Materials Engineering,
Vol. 12, no. 2 (Jun 1990), pp. 151-158