Failure mechanisms of thermal barrier coatings on MCrAlY-type bondcoats associated with the formation of the thermally grown oxide

The effect of the thermally grown oxide (TGO) formation on the lifetime of the thermal barrier coatings (TBC) with MCrAlY-bondcoats (BC) is reviewed. A number of factors affecting the TGO-formation and TBC-failure are discussed including the coating microstructure, geometrical (coating roughness and thickness) and processing parameters. Under given testing conditions for a specific EB-PVD-TBC-system forming a flat, uniform alumina TGO a critical TGO-thickness for TBC-failure can be defined. This TGO-morphology is, however, not necessarily optimum for obtaining long TBC-lifetime, which can be extended by formation of TGO’s with an uneven TGO/BC interface. In contrast, APS-TBC-systems are prone to formation of intrinsically inhomogeneous TGO-morphologies. This is attributed to non-uniform depletion of Y and Al underneath rough MCrAlY-surfaces as well as due to the commonly observed repeated-cracking/re-growth of the TGO during temperature cycling. The latter phenomenon depends on the exposure temperature and the mechanical properties of the APS-TBC. In both types of TBC-systems the TGO-formation and TBC-lifetime appear to be very sensitive to the manufacturing parameters, such as vacuum quality during bondcoat spraying and temperature regime of the bondcoat vacuum heat-treatment.