Colloidal processing and mechanical properties of silicon carbide with alumina

Submicrometer-sized SiC coated with SiO2 of 0.4-1.8 wt.% and α-Al2O3 powder of median size 0.2 μm were mixed in aqueous solutions in the pH range 3.0-10.0. The SiC/Al2O3 (4.3-6.9 wt.%) powders were consolidated by filtration through gypsum molds and hot-pressed at 1600°-2040 °C under a pressure of 39 MPa. These compacts were densified to near the theoretical density at 1700°-1800 °C. The sintering mechanisms are discussed based on the analysis of shrinkage curves of SiC/Al2O3 compacts during hot-pressing. The equiaxed SiC grains grew with low aspect ratios below 1800 °C and changed to plate-like grains at 1900 °C. The fracture toughness of SiC as a function of average grain size reached a maximum of 5 MPa&middotm0.5 at 2.5 μm grains of low aspect ratios of 1-2. The flexural strengths at room temperature were 230-430 MPa in the SiC above 98% of the theoretical density and showed a similar grain size dependence.