Reaction and sintering mechanisms of mullite in the systems cristobalite/α-Al 2O 3 and amorphous SiO 2/α-Al 2O 3

Abstract

Powder mixtures of SiO 2 glass and cristobalite with fine α-Al 2O 3 were heat-treated at different temperatures up to 1700 °C. Green samples consisting of SiO 2 glass plus α-Al 2O 3 (RSS) generally were sintered to higher densities than those consisting of cristobalite plus α-Al 2O 3 (RSC). In both powder compacts (RSS and RSC) two different sintering mechanisms were active before and after the onset of mullitization, respectively. Before mullite formation begins, densification occurs in the RSS sample through viscous flow sintering and in the RSC sample by solid-state sintering. After the formation of mullite crystals, transient liquid-phase sintering is assumed for both samples (RSS and RSC). The amount of SiO 2-rich liquid phase, which controls degree and rate of the second-stage sintering mechanism, depends on the amount of residual SiO 2 left unreacted after initial mullitization. The temperature-dependent mullite formation process was complex. At 1700 °C sample RSS formed ≈ 80% mullite, while the degree of mullitization of sample RSC at the same temperature was ≈ 90%. However, this tendency did not hold at higher temperatures, since both samples displayed near-total mullitization at 1750 °C. Four different temperature-dependent steps of mullite formation could be distinguished: a temperature field where mullite nucleation was observed (<1500 °C), a temperature range of high mullitization degree (1500–1550 °C), followed by a low mullitization region (1550–1625 °C). At higher temperatures (>1625 °C) high mullitization rates were observed again.