SINTERING AND CRYSTALLIZATION OF A GLASS POWDER IN THE MGO-AL2O3-SIO2-ZRO2 SYSTEM

The sintering and crystallization behaviour was studied of a glass powder in the MgO-Al2O3-SiO2-ZrO2 System in which the main crystal phases to form are clino-enstatite (MgSiO3) and cubic zirconia (c-ZrO2). During isothermal, atmospheric sintering of the glass powder, a fine dispersion of c-ZrO2 particles, 50-100 nm diameter, was observed to form, but this did not appear to inhibit the sintering process. Nucleation of the main crystal phase, clino-enstatite, occurred both within the original glass powder particles and at the former particle surfaces, but the rate of crystallization was greater at the former particle surfaces. The c-ZrO2 precipitates are thought to act as nucleation sites for the crystallization of the clino-enstatite. Relative densities of up to 98% were attainable during sintering, and were reached at a stage where a significant degree of crystal phase development had already taken place, proving that completion of sintering prior to the commencement of crystallization is not always a pre-requisite for the attainment of high final densities. In the material studied, the large volume contraction (approximately 11%) on crystallization and the possible release of dissolved gases led to a decrease in relative density as crystallization proceeded. The relative density after complete crystallization was found to be 94% +/- 1%, irrespective of the temperature and duration of the initial sintering stage of heat-treatment, and it appeared that most of the residual porosity was a result of the volume contraction on crystallization rather than poor densification during sintering.