Silica‐based ceramic core for aviation applications: Facile pore filling and flexural strength improvement

Abstract

The effect of alumina addition on the pore filling process, crystallization behavior and mechanical properties of silica-based ceramic cores was investigated. The sintered samples at 1250°C were treated at 1550°C for 0.5 hour in order to simulate a casting process condition. The microstructure analysis indicated that an appropriate alumina addition enhanced the pore filling process by supplying a substantial liquid viscous flow. With an increase in the liquid viscous flow, smaller pores were filled first, and larger pores were filled later. The results also indicated that the pore filling process could be enhanced by adding small powder additives to decrease the pore size between the particles in the ceramic material. However, an excessive alumina addition in the silica-based ceramic cores had a negative effect on the flexural strength and leaching rate. As the alumina content increased from 8 to 12 wt%, the flexural strength of the ceramic core decreased from 16.37 to 4.60 MPa, respectively, and the leaching rate also decreased obviously. These results were explained by an acceleration in crystallization trend of the fused silica particle surface and the merging of connected pores in the sintered body. K E Y W O R D Sliquid viscous flow, mechanical properties, pore filling process, silica-based ceramic core 2182 | CHEN Et al.

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

“…The disappearance of pores leads to the shrinkage of ceramic core 26 . The open porosity can be decreased by increasing fine particles, increasing liquid phase content and coarsening sintering necks, 10,21 which precisely matched the results in the present work. Figure 8 shows the densification process of sintered samples (ZM‐0, ZM‐4.6, and ZM‐9.2) and secondary sintered samples (ZM2‐0, ZM2‐4.6, and ZM2‐9.2).…”

“…Such a high open porosity of a silica‐based ceramic core is beneficial to remove the silica‐based ceramic cores use caustic solutions (e.g. NaOH and KOH) during investment casting 10 …”

“…Wilson14 and Kazemi16 reported that the flexural strength of silica-based ceramic cores with only zircon could not exceed 13 MPa, when the D 50 of zircon was 20 μm. Apparently, the micro-sized mullite had a significant influence on the flexural strength of silica-based ceramic samples without micro-sized mullite, the flexural strength was enhanced by 12.32 MPa, when the micro-sized mullite content increased to 2.3 wt%, it exhibited greater promotion on flexural strength than conventional additives,2,3,10,14,16,23 since the fracture mechanism of ceramic samples has been changed.Figure5shows the SEM images of ZM-0 and ZM-2.3 and EDS results of ZM-2.3. As shown in Figure5A,B, the fused silica particles of ZM-0 had more microcracks than…”

“…[7][8][9] It was reported that cristobalite crystallization occurs on the surface of fused silica particles. 10 Cristobalite has a higher strength than the fused silica, and it could inhibit the liquid phase to form. Consequently, controllable crystallization of fused silica to cristobalite is effective to improve the thermal properties of the ceramic cores.…”

“…As an amorphous material, the fused silica will form a liquid phase (glass phase) in silica‐based ceramic cores in the casting process which could soften the ceramic cores and reduce the deformation resistance of the cores at high temperatures 7–9 . It was reported that cristobalite crystallization occurs on the surface of fused silica particles 10 . Cristobalite has a higher strength than the fused silica, and it could inhibit the liquid phase to form.…”