Plasma spraying of Al₂O₃ and ZrSiO₄ to form ZrO₂-mullite composites

Zirconia is effective in improving the fracture toughness of a number of ceramics when introduced as a reinforcement either in the form of particulates, dispersed phase or whiskers because of its unique tetragonal-monoclinic (t→m) transformation. In this paper, the authors attempt to prepare ZrO₂ reinforced mullite by plasma spraying mixtures of zircon and alumina. Pre-mixed powders of zircon and alumina are injected into a dc plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The results indicate that the plasma sprayed powders consist of zirconia, zircon and alumina. It was found that fine grained, even amorphous and chemically homogeneous composite powders could be obtained by ball milling and plasma spraying. Recrystallization of amorphous phases and formation of mullite occurred at about 1000 °C in plasma sprayed powders. This value is more than 500 °C lower than the formation of mullite in as-milled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders. Preheat treatment of the spheroidized powder promoted dissociation of zircon. Zirconia remained in its tetragonal phase below 1000 °C in the plasma sprayed coatings.