Effects of micro‐sized mullite on cristobalite crystallization and properties of silica‐based ceramic cores

Abstract Silica‐based ceramic cores are extensively used in investment casting process, during which they must exhibit sufficient flexural strength and deformation resistance. In this study, micro‐sized mullite was used as an additive to silica‐based ceramic cores to optimize their high temperature properties. To investigate the effects of micro‐sized mullite on cristobalite crystallization, mechanical and thermal properties of silica‐based ceramic cores, ceramic cores with different amounts of micro‐sized mullite were fabricated. The XRD results showed that additional micro‐sized mullite diminished the crystallization of cristobalite at high temperatures, primarily caused by the mullite related compressive stresses on the surface regions of fused silica particles. Three‐point bending tests and SEM results showed that micro‐sized mullite had a more significant effect on the flexural strength of ceramic cores compared with conventional additives. Particularly, the fracture mechanism of silica‐based ceramic cores had been changed from intergranular fracture into a mixed fracture consisting of both intergranular and transgranular fracture. The mechanical and thermal properties of ceramic cores were all reduced slightly as the mullite content exceed 4.6 wt%. Hence, to optimize the properties of silica‐based ceramic cores, the micro‐sized mullite content should not exceed 4.6 wt%.