A pre-requisite for extended and reproducible lifetimes of thermal barrier coating (TBC) systems is the use of oxidation-resistant metallic bondcoats with optimized performance. Whereas in aircraft engines electron-beam physically vapour deposited (EB-PVD) TBCs with Ni-aluminide type bondcoats are used, in land based gas-turbines MCrAlY-type (M=Ni,Co) bondcoats are applied, typically in combination with a ceramic topcoat produced by atmospheric plasma spraying (APS).
Failure mechanisms and parameters, which influence lifetime of the TBC-systems with MCrAlY-bondcoats will be discussed. The performance of MCrAlY-bondcoats will be shown to depend on the contents of the major alloying elements Co, Ni, Cr and Al as well as minor additions of Y and Hf. In addition, the role of manufacturing related properties such as coating thickness, porosity, surface roughness profile and oxygen content in determining TBC-system lifetime will be emphasized.
The requirements of high bondcoat oxidation (corrosion) resistance and good chemical compatibility with the base material are frequently contradictory with respect to the bondcoat chemistry. One of the possible solutions to the latter problem is using multilayered bondcoats with an outer layer optimized for formation of a slowly thickening thermally grown oxide (TGO) and the bottom layer optimized for suitable mechanical properties and reduced interdiffusion with the base material. It will be shown that successful development and application of such complex, multilayer coating systems can be substantially facilitated using thermodynamic/kinetic modeling, the vast potential of which has become apparent in recent years
