Microwave versus conventional sintering: Microstructure and mechanical properties of Al2O3–SiC ceramic composites

Abstract

In this report, Al2O3–SiC ceramic composites were produced at 1500°C by conventional and microwave sintering. For preparing samples, Al2O3 with the second phase xwt.% SiC (x=5, 10, 15, 20) were milled for 180min. The milled powders were compacted in uniaxial press at 60MPa for 30s and sintered by both the conventional and microwave sintering methods. After sintering, densification, grain size, hardness, fracture toughness, phase variation and microstructure of the samples were examined, and comparisons were made for both the sintering methods. The experimental results revealed that there was an increase in density in the microwave sintering method when compared to conventional sintering. However, it was found that the density decreased in both the conventional and microwave sintering methods when there was an increase in SiC content. The highest relative density of 99.7% was obtained in 5wt. % SiC composite produced by microwave sintering. With regard to hardness and fracture toughness, in both the microwave and conventional sintering methods, though they increased initially and they decreased when there was an increase in SiC content. The maximum hardness and fracture toughness of 24.6GPa and 5.7MPa m1/2, respectively, was obtained in 10wt. % SiC composite sintered by microwave sintering. In both the sintering processes, X-ray diffraction pattern shows the formation of a SiO2 phase in all four compositions along with Al2O3 and SiC phases in conventional sintering, but in microwave sintering only negligible amount of SiO2 phase formed in 15 and 20wt.% SiC composites. The crystalline size decreases in microwave sintering than conventional sintering due to shorter sintering time. Uniform agglomeration and fine grains in the range of 2–3.6μm were formed in microwave sintering, whereas grain size decreases with an increase in the SiC content due to grain boundary pinning due to the intergranular SiC particle.