CONTROLLED CRYSTALLISATION OF GRAIN BOUNDARY-TYPE Y-SIALON GLASS TYPICAL OF THOSE FOUND IN SILICON NITRIDE CERAMICS

Following liquid phase sintering of silicon nitride ceramics with yttria and alumina densification additives, a Y-SiAlON glass remains at the grain boundaries. Crystallisation of such a glass in bulk form has been studied using both differential thermal analysis (DTA) and separate isothermal heat treatments in a tube furnace under nitrogen. The activation energy for the crystallisation process was determined by DTA. The nucleation temperature, Tg + 40 degrees C, which corresponded to the maximum volume fraction of crystalline phases, agreed closely with the optimum nucleation temperature of Tg + 35 degrees C, found from DTA. The optimum crystal growth temperature was observed to be 1210 degrees C and yielded the alpha- and beta-polymorphs of yttrium disilicate. The activation energy for crystallisation was observed to be similar to that for viscous flow of Y-SiAlON glasses. Heat treatments over a range of temperatures resulted in formation of different polymorphs of yttrium disilicate in addition to silicon oxynitride. Careful analysis of DTA crystallisation exothermic peaks were carried out in order to clearly identify polymorphic transformations of yttrium disilicate. Partial substitution of yttrium by lanthanum stabilises the alpha-polymorph of yttrium disilicate over a wider temperature range.