Failure mode of thermal barrier coatings for gas turbine vanes under bending

Crack propagation studies under bending are described which were performed with plasma sprayed zirconia bonded by a MCrAlY layer to Ni -base superalloy. Such thermal barrier composites are currently considered as candidate materials for advanced stationary gas turbine components. The crack propagation behaviour of the ceramic thermal barrier coatings (TBCs) at room temperature, in as received and oxidised conditions reveals that cracks grow linearly in the TBC with increase in bending load until about the yield point of the superalloy is reached. Approaching the interface between the ceramic layer and the bond coat, a high threshold load is required to propagate the crack further into the bond coat. Once the threshold is surpassed, the crack grows rapidly into the brittle bond coat without an appreciable increase in the load. At a temperature of 800 degrees C, the crack is found to propagate only in the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. Effects of bond coat oxidation on crack propagation in the interface regime have been examined and are discussed.