Enhancing superplasticity of ZrO 2 (Y 2O 3)–Al 2O 3 composites prepared by spark plasma sintering of metastable powders

Abstract

Dense ZrO 2 (5 wt% Y 2O 3)–20 wt% Al 2O 3 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 Al 3+ in a stabilized cubic ZrO 2(Y) matrix, was prepared by atmospheric plasma spraying using liquid nitrogen cooled substrates. During sintering, phase separation and precipitation of Al 2O 3 in a tetragonal ZrO 2(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 Al 3+, Y 3+ co-doped ZrO 2 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 ZrO 2(Y)–Al 2O 3 material.