LAO Xinbin, ZHAO Yali, XU Xiaoyang, MAO Zhihuan, DENG Yujie, LIANG Jian, LIAO Jurong

(National Engineering Research Center for Domestic and Building Ceramics, Jingdezhen Ceramic University, Jingdezhen 333001, Jiangxi, China)

Extended Abstract:[Background and purpose] SiC ceramics are widely used in nuclear, aerospace and electronics industries. However, the sintering of SiC ceramics is still difficult, because of the strong Si-C covalent bond. Therefore, reaction sintering method is generally used to prepare self-bonded SiC ceramics. The preparation of self-bonded SiC ceramics by using buried powder sintering method has low requirements for equipment, low sintering temperature and high mechanical properties, but poor density. In order to increase the density, liquid-phase sintering was developed by adding Y₂O₃ as a high-temperature sintering additive. V₂O₅ is a commonly used low-temperature sintering additive, because of its low melting point (690 ℃). However, most reports focused on single high-temperature sintering additives or low-temperature sintering additives. In this paper, two typical high-temperature and low-temperature sintering additives (Y₂O₃ and V₂O₅) were used simultaneously. The effect of the content of the composite sintering additives on composition and properties of self-bonded SiC ceramics prepared by using buried powder sintering was studied, while the room-temperature and high-temperature strengths of the SiC ceramics were characterized.[Methods] SiC powder with different grades, Si, Al, Y₂O₃ and V₂O₅ powders were used as raw materials, while 9 wt.% PVA solution was added as binder and the powders were mixed and then die-molded. After drying for 24 h, the green bodies were calcined by using the buried powder sintering method, where the powder is graphite. Porosity, water absorption, bulk density and bending strength of the samples were characterized. High-temperature bending strength of the samples was tested by using a high-temperature bending tester. Phase composition and microstructure of the samples were analyzed by using XRD and FE-SEM. Microstructure of the SiC whisker was analyzed by using HRTEM.[Results] After sintering at 1380–1420 ℃, the water absorption and porosity increased with increasing content of Y₂O₃ and V₂O₅. This is because, in the environment of buried powder sintering, the lack of oxygen promotes the transformation of V₂O₅ to VO₂ at high temperatures, resulting in gas volatilization, which is accelerated with increasing temperature, thus leading to increase in porosity. Due to the increase in porosity, the room-temperature bending strength decreases. When the sintering temperature is higher than 1420 ℃, the water absorption and porosity decrease with increasing temperature. Because Y₂O₃ promotes the formation of liquid phase, the pore formation behavior of V₂O₅ is compensated. As the porosity is reduced, the room-temperature bending strength is increased. The highest bending strength was obtained in VY1 sintered at 1380 ℃, which was 78.8 MPa. Combined with SEM images, the sample with lowest content of V₂O₅ had small size and small number of pores. The gas volatilization of V₂O₅ at high temperature had the smallest influence on structural loosening, while the liquid phase was generated to promote the elimination of pores, so the porosity of VY1 sample was relatively small. Besides porosity, the content of liquid phase had also the effect on the high temperature strength. At higher sintering temperatures, more liquid phase was formed, so the high temperature strength was reduced. According to XRD results, O'-Sialon was formed in the sample after sintering at 1460 ℃, but the high temperature mechanical strength was not increased, indicating that the liquid phase has a greater negative impact on the high temperature strength. The majority of Si has been converted to β-SiC. When the sintering temperature was increased to 1460 ℃, due to the presence of N₂ in the environment, a small part of Si reacted with N₂ and O₂ to form Si₂N₂O. Si₂N₂O then reacted with Al₂O₃ to form O'-Sialon. A large number of nano-SiC whiskers were observed inside the pores that were not filled with liquid phase. HRTEM images revealed that the surface of the whiskers is smooth and the lattice fringe spacing is 0.25 nm. O was also detected on surface of the whisker, suggesting that SiC whiskers were grown through the vapor deposition of SiO.[Conclusions] Self-bonded SiC ceramics were prepared by using the buried powder sintering method, with α-SiC, Si and Al as the main raw materials, with V₂O₅ and Y₂O₃ as the composite sintering additives. The effect of the content of the composite sintering additives on composition, structure and properties of the ceramics was discussed. Due to the volatilization behavior of V₂O₅ at high temperatures, porosity of the sample increased with the increasing temperature and increasing content of V₂O₅, leading to degradation of mechanical performances. However, when the sintering temperature was higher than 1420 ℃, Y₂O₃ promoted the formation of liquid phase, which compensated the pore formation of V₂O₅, thus enhancing the densification. The sample VY1 sintered at 1380 ℃ showed the highest room-temperature and high-temperature bending strengths, with values of 78.82 MPa and 39.31 MPa, respectively. High firing temperature (1460 ℃) led to excessive liquid phase, reducing the high-temperature strength. Most of Si was carbonized to form β-SiC, while a small part reacted with Al₂O₃ to form O'-Sialon, at high sintering temperatures. A large number of nano-β-SiC whiskers were generated in-situ in the samples.

Key words: composite sintering additives; self-bonded SiC; liquid sintering; SiC whisker; high-temperature bending strength