Coat of Alumina Sheet with Needle-like Mullite

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Abstract
A new alumina substrate coated with porous mullite layer consisting of needle-like crystals was developed for improving the physicochemical properties of the substrate surface. Calcined porous alumina substrates with a 40% porosity and an average pore diameter of 0.1μm were dipped into aqueous suspensions of New Zealand kaolin with particle sizes of below 1μm for 5-20sec to form a kaolin layer 10-40μm thick. The porous alumina substrates coated with kaolin were fired at 1500° to 1650°C to enhance the growth of thick needle-like mullite crystals with lengths of 1-10μm and diameters of about 1μm over the surface of dense alumina substrates. The glassy phase coexisting with mullite crystals over the substrate surface was dissolved in a hydrofluoric acid solution. Pillar-like mullite crystals with diameters of about 3μm were formed at the interface between alumina substrate and mullite layer fired at 1650°C. The pillar-like mullite crystals contained a larger amount of Al2O3 than needle-like mullite crystals with the chemical composition of 3Al2O3⋅2SiO2. No cracks were observed at the interface between the alumina substrate and porous mullite layer after thermal shock test with a temperature difference of 400°-450°C.

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