Effect of processing parameters on MCrAlY bondcoat roughness and lifetime of APS–TBC systems

In the present work it is shown that the lifetime of air plasma sprayed thermal barrier coatings systems with high velocity oxyfuel bondcoats can be substantially extended by application of a thin air plasma sprayed “flashcoat” layer onto the base, high velocity oxyfuel bondcoat. This approach allows improvement of the bondcoat roughness profile to the extent typically obtained by optimized vacuum plasma spraying. The oxidation resistance of the flashcoat strongly depends on the spray parameters used. Deviation from the set of the optimized spray parameters was found to result in early catastrophic oxidation of the flashcoat and corresponding very rapid thermal barrier coating-failure. In order to explain the thermal barrier coating lifetime variations between the various bondcoat morphologies, the roughness profiles of high velocity oxyfuel, vacuum plasma sprayed and air plasma sprayed bondcoats were evaluated using fractal analysis. It is suggested that such an approach provides better correlation between the bondcoat morphology and thermal barrier coating-lifetime than the calculation of the arithmetic mean roughness (Ra) of the bondcoat frequently used in industrial practice.