Abstract
High-purity mullite ceramics are fabricated to improve their high-temperature mechanical properties using an intelligent eco-friendly sintering process with a transient silica-rich liquid phase formed during sintering, followed by the crystallization of the residual glassy phase of SiO2. The fabricated ceramics possess high density and excellent high-temperature flexural strength. The sinterability of the mullite ceramics improved dramatically when silica-rich compositions were used. This can be attributed to the viscous flow of the silica-rich viscous liquid phase formed during sintering at high temperatures. To increase the high-temperature flexural strength without degrading the fracture toughness, the residual boundary phase was crystallized into cristobalite by controlling its amount and size. The crystallite size of the precipitated cristobalite obtained using suitable post-annealing conditions, i.e., at 1500 °C for 120 h or more, was suppressed; very fine crystals of size 33–38 nm were obtained. The flexural strength measured at 1400 °C for the specimens, which were post-annealed at 1500 °C for 120 h, exceeded that of the as-sintered specimen without resulting in any degradation in fracture toughness at room temperature. These results suggest that suitable grain boundary design conditions, including the composition and post-annealing conditions, can improve the sinterability and high-temperature mechanical properties of the highly pure mullite ceramics at low sintering temperatures. Such grain boundary design can lead to the development of eco-friendly processing of low-temperature sintering for high-performance structural ceramics.
