A study of processing parameters in thermal-sprayed alumina and zircon mixtures

A method of plasma spraying of alumina and zircon mixture to form ZrO₂mullite composites has been proposed and developed. The feedstock is prepared by a combination of mechanical alloying, which allows formation of fine-grained, homogeneous solid-solution mixtures, followed by plasma spheroidization that yields rapid solidified microstructures and enhanced compositional homogeneity. The effects of ball-milling duration and milling media were studied. It was found that zirconia is a more efficient milling media and that increasing milling duration enhanced the dissociation of zircon. Flame spray and plasma spray processes were used to spheroidize the spray-dried powders. The temperature of the flame spray was found to be insufficient to melt the powders completely. The processing parameters of the plasma spray played an important role in zircon decomposition and mullite formation. Increasing the arc current or reducing secondary gas pressure caused more zircon to decompose and more mullite to form after heat treatment at 1200°C for 3 h. Dissociation of zircon and the amount of mullite formed can be enhanced significantly when using the more efficient, computerized plasma-spraying system and increasing the ball-milling duration from 4 to 8 h.