Superplasticity of zirconia–alumina–spinel nanoceramic composite by spark plasma sintering of plasma sprayed powders

Zirconia 3 mol% yttria–alumina–alumina magnesia spinel nanoceramic composite was synthesized by spark plasma sintering of plasma sprayed particles. For compacts sintered from high energy ball milled powders, superplasticity was observed at temperatures between 1300 and 1450 °C and at strain rates between 10−4 and 10−2 s−1, while for those without high energy ball milling, deformation at the same temperature and strain rate range did not show superplastic behavior. Also, the apparent activation energy (945 kJ/mol) of the high energy ball milled batch was much higher than that of the same composite processed from nanopowder mixtures (621 kJ/mol). The flow stresses were also higher at the same temperatures and strain rates. The difference may be related to the unique low angle grain boundaries in the grains that nucleated and grew from the metastable phase inside the plasma sprayed agglomerate at elevated temperatures. Such boundaries were not intrinsically easy to slide.