Enhancing superplasticity of ZrO2 (Y2O3)–Al2O3 composites prepared by spark plasma sintering of metastable powders

Dense ZrO2 (5 wt% Y2O3)–20 wt% Al2O3 composites with an average grain size of the zirconia matrix of about 450 nm were prepared by means of spark plasma sintering of a metastable powder. The powder, consisting of a supersaturated solid solution of Al3+ in a stabilized cubic ZrO2(Y) matrix, was prepared by atmospheric plasma spraying using liquid nitrogen cooled substrates. During sintering, phase separation and precipitation of Al2O3 in a tetragonal ZrO2(Y) matrix occurred. Due to the rapid sintering, thermodynamic equilibrium was not fully achieved and part of the metastable nature was retained in the form of Al3+, Y3+ co-doped ZrO2 grains. High temperature Vickers indentations and creep tests in uniaxial compression were performed to characterize the deformation behavior. Arrhenius plots of the high temperature hardness indicated a transition of the deformation process at 1080 °C. Creep tests above the transition temperature yielded superplastic deformation. The stress exponent n, the grain size exponent p and the activation energy Q of the deformation process were calculated and discussed with respect to the microstructure evolution during deformation. Grain boundary sliding was assumed to be the deformation mechanism accompanied by cation diffusion through the lattice. A strain rate of 3 × 10−3 s−1 at a temperature of 1350 °C was achieved in the absence of a glassy grain boundary phase. Normalized stress–strain rate plots indicated that the alumina doping successfully increases the deformability of this composite with respect to a conventionally processed ZrO2(Y)–Al2O3 material.

Research highlights▶ Spark plasma sintering of metastable powders leads to metastable microstructures. ▶ Ceramics derived from metastable powders show enhanced superplasticity. ▶ Superplasticity due to grain boundary sliding accompanied by diffusional processes.