Adhesion of Thermal Barrier Coating Systems and Incorporation of an Oxidation Barrier Layer

Abstract

Abstract The adhesion of various interfaces in thermal barrier coating systems strongly affects their stability and thermal cycling life. A new spontaneous debonding technique and the four point bend delamination test have been applied to measure the interfacial fracture toughnesses of various interfaces in several thermal barrier coating systems. The spontaneous debonding technique is based on spraying a relatively stiff layer on top of the ZrO2 coating. This raises the strain energy release rate for debonding, the magnitude of which is monitored via modelling of the stress distribution. The critical strain energy release rate for debonding (interfacial fracture energy) was then determined from the stress states before and after debonding. Thermal barrier coatings (TBCs), consisting of a Ni-22wt.%Cr-10wt.%Al-lwt.%Y bond coat and a ZrO2-8%Y2O3 top coat, were deposited on a nickelbased superalloy. Two methods, air (APS) and vacuum (VPS) plasma spraying, were used to produce the bond and the top coats. The corresponding as-sprayed residual stress distributions and the interfacial fracture energies were evaluated. It was found that a VPS bond coat and an APS top coat produced the most mechanically stable structure. A layer of vacuum plasma sprayed AI2O3 was then introduced between the top and the bond coat, designed to act as an oxygen diffusion barrier. The effect on residual stress distributions, and associated crack driving forces for debonding, at different interfaces were determined. The effect of the alumina layer on the oxidation behaviour was also studied. It is shown that the oxidation barrier could significantly enhance the coating life-time.