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

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

Dmitry Naumenko, Vladimir Shemet, Lorenz Singheiser, Willem Josef Quadakkers
Journal of Materials Science

Abstract
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 specic EB-PVD- TBC-system forming a at, uniform alumina TGO a critical TGO-thickness for TBC-failure can be dened. This TGO- morphology is, however, not necessarily optimum for obtain- ing longTBC-lifetime,which can beextendedbyformation of TGO s with an uneven TGO/BC interface. In contrast, APS-TBC-systems are prone to formation of intrinsically inhomogeneousTGO-morphologies. This isattributedtonon- 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 ofTBC-systemstheTGO-formation and TBC-lifetimeappear to be very sensitive to the manufacturing parameters, such as vacuum quality during bondcoat spraying and temperature regime of the bondcoat vacuum heat-treatment.

keywords:thermal barrier coatings; underneath; grown oxide; Failure mechanisms; MCrAlY-bondcoats; TGO-formation; TBC-failure; critical TGO-thickness

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