Ceramic flake formation in the aluminosilicate system by plasma spraying

Alumina and aluminosilicate flakes with compositions Al2O3, 3Al2O3⋅2SiO2 and Al2O3⋅2SiO2 have been produced from commercial raw materials using a direct current plasma spray process in air. The microstructure and phase constitution of the as-sprayed flakes were examined with optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Changes in phase constitution of the flakes with heat-treatment were examined using differential thermal analysis (DTA) and XRD of heat-treated samples. The as-sprayed Al2O3 flakes consisted of α -Al2O3 phase plus a minor γ-Al2O3 phase. The γ-phase could be removed by heat treating the flakes at 1300∘C for 2 h. The aluminosilicate flakes consisted of 3Al2O3:2SiO2 (mullite) and an amorphous phase which crystallised to 3Al2O3:2SiO2 (mullite) after heat treatment at 1100∘C for 2 h. These flakes may find applications as high temperature thermal insulation materials.