Effect of chemical vapour infiltration technique on the gradient microstructure and high-temperature strength of gel-cast Al₂O₃-based ceramic moulds

A new approach for the preparation of Al₂O₃ ceramic moulds with gradient microstructure using a chemical vapour infiltration (CVI) technique is proposed. A ceramic mould with a gradient microstructure was designed for casting hollow turbine blades. Ceramic moulds with a homogeneous microstructure were formed by gel casting. With the introduction of SiC by CVI, the gradient microstructure was finally formed in the ceramic mould. Owing to its dense interior and loose exterior structure, the ceramic mould resisted the impact of liquid alloys and showed good deformability at high temperature. After CVI and sintering at 1450 °C in air, the high-temperature strength of the ceramic mould was improved via the reduction of porosity and the generation of mullite, a high-temperature strengthening phase. For the ceramic mould with 60 vol% solid loading, the high-temperature strength at 1500 °C increased from 3.61 to 36.9 MPa and the thermal conductivity increased as well. The microstructures and phase compositions of the sintered ceramic samples were investigated by scanning electron microscopy and X-ray diffraction, respectively. The combination of gel-casting and the CVI technique helped fabricate a ceramic mould with a gradient microstructure and good high-temperature properties.