Thermal Cycling Induced Bond-Coat Rumpling as a Precursor to TBC Failure

Microstructural observations of TBCs failed under thermal cycling conditions reveal that failure is associated with extensive local separations between either the thermally grown oxide and the TBC or within the TBC itself close to the thermally grown oxide. Based on extensive microstructural characterization and measurements of concentration profiles within the bond-coat, we present a new model for the cause of these separations based on local increases in the density of the bond coat associated with the beta-NiAl to gamma-prime Ni3Al phase transformation. The phase transformation, driven by aluminum depletion required to form the protective alumina thermally grown oxide, is constrained by the overlying TBC thereby generating tensile stresses across the TBC/TGO interface and its vicinity. The observations and evidence for the new model will be described together with the role of thermal cycling.