Premature failure induced by non-equilibrium grain-boundary tantalum segregation in air-plasma sprayed ZrO₂–YO_1.5–TaO_2.5 thermal barrier coatings

ZrO₂–YO_1.5–TaO_2.5 (ZYTO) is a promising top-coat material for thermal barrier coatings (TBCs). The bulk properties of ZYTO have been reported by several studies, but its performances as TBCs are less-well understood. In this work, ZYTO TBCs were prepared by air plasma spraying (APS) and their thermal cycling performances were investigated at 1150°C. Despite of the good bulk properties, APS ZYTO TBCs present an extremely short thermal fatigue life. This is attributed to the non-equilibrium grain-boundary segregation of TaO_2.5 induced by limited solubility and rapid quenching during APS process, resulting in a tetragonal (t) to cubic (c) and metastable-tetragonal (t_m) phase transformation in ZYTO TBCs. The volume shrinkage (~0.74vol%) of phase transformation leads to many cracks at the c/t_m phase boundaries after deposition. On the other hand, the formation of cubic phase with massive grain-boundary Ta segregation induces a large intergranular embrittlement and a weak bonding strength (~5.3 MPa), resulting in the premature failure of the ZYTO TBCs.