Heat treatment induced phase and microstructural development in bulk plasma sprayed alumina

The influence of systematically varied heat treatments on the microstructure and phase system of a bulk plasma sprayed alumina has been investigated by applying conventional electron microscopy [scanning electron microscopy (SEM) and transmission electron microscopy (TEM)], X-ray and electron diffraction [X-ray diffractometry (XRD) and selected area electron diffraction (SAED)], Archimedean porosimetry and dilatometry. Plasma sprayed alumina has a quasi-laminated microstructure, consisting of layered “splats” with a pronounced ultra-microstructure which results from rapid cooling. The as-sprayed material consists of transition aluminas and about 35% α-Al2O3. Both transition phases and α-Al2O3 form splats with transversal ultra-fine columnar intra-splat grains. Post deposition exposure to high temperatures causes significant microstructural, phase and dimensional changes. The transition aluminas undergo a continuous order–disorder transformation until the reconstructive phase change to α-Al2O3 is reached. This is accompanied by precipitation of aligned porosity, opening of splat interfaces and the formation of dislocation networks. It is suggested that the increase in porosity is responsible for the directionally dependent and lower than expected shrinkage on transformation. Thermal expansivity at lower temperatures is found to be independent of splat orientation. A block domain structure and extensive twinning form during the transition transformations. Extended exposure to near-sintering temperatures leads to the recrystallisation of the splat substructure whilst maintaining the splat structure.