Liquid Phase Sintering of Silicon Carbide with AlN-Re2O3 Additives

Abstract

Silicon carbide can be pressureless sintered by a solid stated process with the sintering aids of B and C to near full density at temperatures in excess of 2100°C (Prochazka,1974). However, the lower fracture toughness (3 to 4 Mpa·m1/2) limit their use in many potential structural applications. It has been known that sintering of SiC can be achieved at relatively lower temperature (1850°C-2000°C) with the addition of oxides (Al2O3 and Y2O3) via liquid phase sintering(Omori & Takei, 1988; Nitin, 1994). The resulting material obtained with homogeneous and equiaxed fine-grained microstructure. Oxides like SiO2 and Al2O3, which are normally considered as thermodynamically stable, are prone to react with SiC at temperature of about 2000°C, leading to formation of gaseous products such as CO, SiO and Al2O. Al2O3+SiC→Al2O(g)+SiO(g)+CO(g) In order to suppress these reactions, a powder bed is generally required (Tan et al, 1998). Alternatively, the additive system of AlN and rare earth oxides including Y2O3, is used where the decomposition of AlN into Al and N2 can be efficiently controlled by using N2 atmosphere, leading to lower weight lost (Chia et al, 1994; Ye et al, 20002). The AlN –Y2O3 phase diagram indicates that eutectic temperature in this system is about 1850°C (Kouhik, 2002). It might avoid forming a liquid with rather low melting temperature and a coarse surface of ceramic caused by vaporized gases from the reaction of SiO2 and Al2O3-Y2O3. Also in this system the intermediate compositions can offer sufficient amount of liquid with melting temperature higher than 1700°C as sintering aid of LPS-SiC. Some studies have been carried out by using rare-earth oxide containing densification aids (Koushik et al, 2004; Koushik et al, 2005). Our previous study on melting behaviours of SiC and a series of Re2O3 (1:1 mol mixture)has shown that melting temperatures raise with increasing the atomic number of rare earth element (from La to Er and Y) (Wu et al, 2008). The aim of this work was to study the sintering behavior of liquid phase sintered SiC with AlN and Re2O3 (La2O3, Nd2O3 , Y2O3) additive system and their mechanical property in both pressureless sintering and hot press sintering. 21