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. Here we prepare ZYTO TBCs by air plasma spraying (APS) and investigate their thermal cycling performances at 1150 ºC. 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 TaO2.5 induced by limited solubility and rapid quenching during APS process, resulting in a tetragonal (t) to cubic (c) and metastable-tetragonal (tm) phase transformation in ZYTO TBCs. The volume shrinkage (~0.74%) associated with the phase transformation generates many cracks at the c/tm phase boundaries after deposition. On the other hand, 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.