Failure mechanisms of thermal barrier coatings on MCrA1Y-type bondcoats associated with the formation of the thermally grown oxide

摘要

The effect of the thermally grown oxide (TOO) formation on the lifetime of the thermal barrier coatings (TBC) with MCrA1Y-bondcoats (BC) is reviewed. A number of factors affecting the TOO-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 TOO a critical TGO-thickness for TBC-failure can be defined. This TOO-morphology is, however, not necessarily optimum for obtain_ing 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 MCrA1Y_surfaces as well as due to the commonly observed repeated_cracking/re-growth of the TOO 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.