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 x wt.% SiC (x = 5, 10, 15, 20) were milled for 180 min. The milled powders were compacted in uniaxial press at 60 MPa for 30 s 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 5 wt. % 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 24.6 GPa and 5.7 MPa m^1/2, respectively, was obtained in 10 wt. % 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 20 wt.% 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 2–3.6 μm 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.

Resumen

Se estudian materiales compuestos de Al₂O₃–SiC procesados por sinterización convencional a 1500 °C y por microondas. Las mezclas de Al₂O₃ con la segunda fase x% en peso de SiC (x = 5, 10, 15, 20) se prepararon por molienda en molino de bolas durante 180 minutos, secado y prensado uniaxial a 60 MPa durante 30. En ambos procesos de sinterización, el patrón de difracción de rayos X muestra la formación de fases de SiO₂ junto con las fases originales, Al₂O₃ y SiC, siendo mayor su proporción en los materiales preparados por sinterización convencional y mayores proporciones de SiC. Los materiales sinterizados se caracterizaron en términos de densidad, fases cristalinas, microestructura, tamaño grano, dureza y tenacidad a la fractura Vickers. Se realiza una comparación entre los materiales preparados utilizando ambos métodos de sinterización. La sinterización de microondas proporciono materiales de mayores densidad, dureza y tenacidad que la sinterización convencional. El máximo de densidad se obtuvo para 5% en peso de SiC y los valores disminuyeron para mayores proporciones de segunda fase. Los máximos de dureza y tenacidad a la fractura se obtuvieron para 10% en peso de SiC.