Effects of attrition milling and post-sintering heat treatment on fabrication, microstructure and properties of transformation toughened ZrO2

Abstract

The effect of attrition milling and post-sintering heat treatment on the fabrication, phase relations, microstructure and properties of ZrO2 (+2.3vol% Y2O3) powder used to produce a transformation toughened material was examined. Powder used to fabricate the unmilled material was treated and consolidated by a colloidal method. The same powder, treated and consolidated by the same method, but ball milled in a commercial alumina mill before consolidation, was used to fabricate the milled material. Both materials were sintered at 1400° C for 1 h and then heat treated at higher temperatures. Milling introduced Al2O3 inclusions ( 1450° C produced detectable amounts of cubic ZrO2 consistent with previously reported phase studies of the ZrO2-Y2O3 system. The development of a bimodal grain structure was concurrent with the formation of detectable cubic phase. The larger grains in this bimodal distribution were primarily observed on the external surface and co-ordinating pores produced during the post-sintering heat treatments which were responsible for the bloating phenomenon. It is hypothesized that the pores were produced by the release of high pressure oxygen during cubic phase formation. Both fracture toughness (Kc) and hardness of the as-sintered materials were unaffected by milling. Hardness decreased with bloating and the decrease was more pronounced for the milled material which exhibited more bloating.