Plasma Spraying of Silica-Rich Calcined Clay Shale

Silica-rich clay shale is a viable candidate for replacement of mullite in many applications, especially when outstanding refractoriness and chemical resistance to various agents are desirable. In this contribution, instead of the commonly used synthetic mullite feedstock, the thermal stability of inexpensive calcined natural raw clay shale sprayed using water stabilized plasma system is reviewed. Phase stability and phase changes at elevated temperatures up to 1500 °C were studied by an array of experimental techniques ranging from measurements of thermal conductivity and the heat flow as functions of temperature, scanning electron microscopy, x-ray diffraction (XRD) of the annealed samples, and in situ high temperature XRD. The mostly amorphous as-sprayed coatings with less than 10 wt.% of mullite are temperature stable up to 800 °C and rapid crystallization occurs between 920 and 940 °C. Performed analyses gave evidence about the increase of mullite grain sizes for temperatures higher than 1200 °C and, moreover, certain saturation of crystallinity, not surpassing the threshold of 60 wt.% even for 1500 °C, is observed. The microstructure after annealing at 1500 °C is notable by clusters of fine needle-like mullite crystallites with sizes within the range of tens of nanometers in Si-rich amorphous matrix.