Silicon nitride (Si3N4) ceramics have been widely applied in modern industry due to its excellent properties including thermal shock resistance, outstanding anti-corrosion and good wear resistance [1–3]. However, the shortcomings of the ceramics such as brittleness, unworkableness and high sensitivity to internal flaws of materials are the main hindrances to its applications [1, 3, 4]. It has been revealed that adding second phase (TiC, TiO2, SiC, BN, etc.) with high hardness and chemical resistance, as the reinforcing phase, could improve the properties of ceramics, such as flexural strength, fracture toughness and creep resistance. For example, silicon carbide (SiC) as reinforcing phase includes whiskers and particles [1, 8, 9]. Kai and Yang, et al.  obtained the Si3N4/SiC composite ceramics with high density and excellent mechanical properties by using the SiC whiskers as a reinforcing phase in Si3N4 matrix. Moreover, many researchers have reported that the SiC nanoparticles were widespread on the grain boundary. The phenonmen has caused interlocking of neighboring Si3N4 grains and impede grain-boundary sliding, therefore, the hardness, strength and high-temperature properties such as creep behavior of ceramics had been improved [9–11]. In order to industrilise Si3N4/SiC ceramics, pressureless sintering was selected as a possible method because it was practical to control the size and shape of the product along with temperature schedule .
Si3N4 and SiC belong to covalent compounds. Both the covalent bonding Si-N and Si-C are so strong, and it is difficult to sinter for Si3N4/SiC ceramics since the sintering temperature is very high [12, 13]. The oxides are always added to ceramics as sintering additives to promote the process of liquid-phase sintering by forming eutectic liquids at the low sintering temperature. The liquid-phase finally could be reserved as glassy or partially crystalline after cooling, and it usually locates in the junctions of three or two grains [14, 15]. The common sintering additives including MgO, Y2O3, Yb2O3, Al2O3, CeO2 and so on have been reported [12–16]. In addition, different types and quantities of additives have different effects on the materials. Lojanová and Tatarko [10, 17, 18], et al. investigated the effects of different rare-earth oxides RE2O3 (RE = La, Nd, Lu, etc.) on the bending strength and microstructure of Si3N4-SiC and Si3N4 nanocomposites, and it also analyzed the role of SiC grains inter the matrix. It indicated that the additives and surfaces oxidation layer could form the liquid-phase showing different the viscosity, which influenced the mechanical properties of ceramics and the location of SiC grains. Y2O3 and CeO2 have been successfully used as sintering additives for Si3N4. The density and bending strength of the materials were improved obviously within certain range of contents [13, 19].
In this study, in order to investigate the effect of both SiC and rare-earth oxides on mechanical properties of Si3N4/SiC composite ceramics, using Si3N4 and SiC particles as raw materials, 3 wt.% rare earth oxides Y2O3 and CeO2 as additives respectively to prepare samples by pressureless sintering method. Furthermore, the microstructure and phase compositions were also analyzed, which might provide evidences to reveal the relationships between the sintering additives and the mechanical properties.