Effects of various sintering additives on the properties of β-SiAlON–SiC ceramics obtained by liquid phase sintering

Abstract β-SiAlON–SiC ceramics were obtained by the liquid phase sintering method under N2 atmosphere. The effects of various sintering additives (including Y2O3, ZrO2, ZrSiO4, Suzhou kaolin, petalite, and cordierite) on the structure, thermal expansion coefficients (room temperature–1000 °C), and high-temperature (1400 °C) mechanical properties of β-SiAlON–SiC ceramics were studied. The results indicate that 10 wt% Y2O3 promoted the formation of columnar β-SiAlON crystals resulting in samples with the highest room-temperature bending strength of 103.7 MPa, but an increased thermal expansion coefficient of 4.7 × 10−6 °C−1. A 10 wt% addition of ZrO2 (or ZrSiO4) fractured the samples greatly due to the transformation of cubic ZrO2 to monoclinic ZrO2 during the cooling stage, and the use of Y2O3 as a stabilizer was indispensable for the ZrO2-containing sintering additives. The combined use of Y2O3, ZrO2, and ZrSiO4 exhibited a better effect on the densification of the ceramics than the other sintering additives, and the lowest water absorption of 12.1% was obtained. Y2O3-containing additives caused severe oxidation of the β-SiAlON–SiC ceramics at 1400 °C, decreasing the high-temperature bending strength to 10.3–25.3 MPa. Suzhou kaolin, petalite, and cordierite were also helpful for increasing the nitridation ratio by carbothermic or aluminothermic nitridation of SiO2 and endowed the samples with low thermal expansion coefficients of 3.7–4 × 10−6 °C−1. The addition of Suzhou kaolin resulted in the highest high-temperature bending strength of 50.5 MPa by improving the oxidation resistance of β-SiAlON–SiC ceramics.