Mechanical properties of high-temperature-degraded yttria-stabilized zirconia

In the pursuit of higher operating temperatures and better durability of thermal barrier coating (TBC) materials, the currently used air-plasma-sprayed (APS) 8 wt.% yttria-stabilized zirconia (8YSZ) ceramic is faced with a phase degradation problem regarding its initial metastable T prime (T′) phase, which results in residual stress and cracks during long-term operations. In this study, we focus on the relationship between the long-term phase transformation and mechanical properties, which facilitates the simulation and prediction of properties under operating conditions. Field-assisted sintering specimens of the pure T′ phase were annealed at 1300 °C for different periods of time to obtain various phase compositions. A nano/micro hybrid structure was observed as the phase transformation developed. The Vickers hardness of the materials increased by 8% due to the precipitation of fine T phase grains. On the other hand, the bending strength decreased by more than 60% when the T′ phase decomposed from 70 to 20 vol.%. A noteworthy reduction in the fracture toughness, along with a decrease in the volume percentage of the T′ phase, was also observed during long-term degradation. Effects of the T′ phase decomposition on mechanical properties are discussed in this paper. This systematic research on the mechanical properties and phase degradation of YSZ is of significant importance in predicting and evaluating the performance evolvement of TBCs.