Interfacial fracture toughness and residual stress of thermally sprayed coatings

Residual stresses generated in coatings during thermal spraying plays an important role on the interfacial adhesion. Furthermore, their magnitude and distributions are affected by both materials and processing parameters. Thus, it is of interest to evaluate their evolution and influence on the adhesive behavior of thermally sprayed coatings. Given the complexity in both the process and dimensions, it is difficult to extract both residual stresses and adhesion strength from a single specimen. In this study, the residual stresses were determined using an in situ thermo-elastic curvature method which not only provides a non-destructive method of characterizing stress but also allows for its layer by layer evolution during deposition. Concurrently, the interfacial indentation test is proposed to characterize the adhesion strength of plasma-sprayed coatings. Through a combination of these experiments, the interfacial fracture toughness of the plasma-sprayed ZrO₂ coatings deposited on Al substrates were evaluated. In order to study the effects of the residual stresses, samples were sprayed under various processing conditions while the indentation tests on resultant coatings were also compared to the standard tensile adhesion tests. These test results indicate a certain correlation between the residual stresses and the interfacial fracture toughness of thermally sprayed coatings.