Sintering of industrial mullites

Two mullite powders, different in chemical and morphological characteristics, were compared with respect to their sinterability. For the development of a low-cost sintering step, MgO was added as sintering aid and its content was optimized for both mullites: the enhancement in sintering was imputed to a liquid phase formation. The two mullites presented a strongly different sinterability; the one gave rise to high density sintered bodies while the other, even after a grinding step which made its particle size distribution similar to that of the first, did not reach high final density. In both mullites, there was a preexisting glassy phase: the strong influence of its content, composition and spatial distribution on the sintering behaviour was demonstrated. For better clarifying the action of the sintering aid, the glassy phase was eliminated by HF washing and the sintering behaviour of washed and unwashed samples with added MgO was compared. The contribution of MgO during sintering as well as the new phase formation in the sintered bodies seem to be linked to the alumina and impurity content of the mullites.