The retention of thermal barrier coating systems on single-crystal superalloys: Effects of substrate composition

The retention during thermal cycling of three types of thermal barrier coating (TBC) systems by a number of different nickel-based single-crystal superalloy substrates is examined. It is demonstrated conclusively that TBC compatibility depends upon substrate composition; this result implies that considerable chemical effects are at play. The influence of substrate composition on TBC spallation life is at least as strong as that induced by altering the bond coat (platinum-diffused, inward/outward platinum aluminides). A significant improvement was nonetheless displayed by the so-called platinum-diffused bond coat as compared to conventional platinum aluminide systems. The results can be explained only if the fracture toughness parameters controlling decohesion – e.g. the fracture toughness of the thermally grown oxide and the fracture toughnesses of the interfaces bounding it – are influenced strongly by small changes in composition arising from interdiffusion with the bond coat and underlying substrate, such that they are degraded during thermal cycling. As the sulphur content in the substrates was nearly constant at 2–3 ppm, other elements must represent the cause of the differences observed.