High-temperature strength of silicon carbide ceramics sintered with rare-earth oxide and aluminum nitride

The effect sintering additives comprising AlN and Re2O3 (Re = Sc, Lu, Yb, Er and Y) in a 2:3 molar ratio on the high-temperature strength of liquid phase-sintered and subsequently annealed SiC ceramics was investigated. Clean SiC–SiC boundaries and clean SiC-junction phase boundaries without the amorphous inter-granular phase (IGP) were observed in Sc2O3-, Lu2O3- and Yb2O3-doped SiC ceramics. Clean SiC–SiC boundaries and SiC-junction phase boundaries with the amorphous IGP were also observed in Er2O3-doped SiC ceramics. The amorphous IGP was present in both SiC–SiC and SiC-junction phase boundaries in Y2O3-doped SiC ceramics. The high-temperature strength was not dependent on the crystalline nature of the IGP and junction phases, but was dependent on the chemistry of these phases. Lu2O3-, Er2O3-, Sc2O3- and Y2O3-doped SiC maintained their room-temperature strengths up to 1600, 1500, 1400 and 1400 °C, respectively. However, Yb2O3-doped SiC manifested a drastic degradation in strength at 1400 °C. Thus, clean boundaries and/or crystallization of junction phases did not always lead to the hoped-for increase in strength at temperatures above 1400 °C.