Preparation of zirconia mullite flakes via a plasma rapid solidification process using starting materials derived from a sol–gel technique

Flakes of zirconia–mullite with different zirconia contents varying from 3 to 24 wt% were produced from sol–gel derived raw materials via a plasma melting method followed by a rapid solidification process using a rotating copper roll. The morphology, phase constitution and microstructure development of the as-prepared flakes and of the flakes after various heat treatments were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). It was found that the starting materials could be transformed from the initial irregular-shaped powders into flakes which consisted of mullite, zirconia phase, a relatively large amount of glassy phase and pores. Using TEM, it was shown that the crystalline phases consisted of zirconia polymorphs and mullite. The glassy phases consisted of Al2O3–SiO2 solid solution supersaturated with zirconia. Firing at 1500 °C or 1700 °C resulted in full crystallisation of the flakes and a fairly homogeneous distribution of zirconia particles in terms of size and shape dispersed in the mullite matrix, which contained both intra-granular and inter-granular precipitates. The microstructural characteristics of the flakes may provide promising physical properties for applications in high temperature thermal insulation materials.