Phase relations in the Si-Al-Yb-O-C system

SiC (containing) ceramics were prepared by pressureless sintering at 1400-1750 °C in an argon atmosphere using SiC, Al2O3, Yb2O3 and SiO2 as starting materials. Phase compositions of the as-sintered materials were investigated by X-ray diffraction. Subsolidus phase relations in the SiC-Yb2O3-Al2O3-SiO2 quaternary system were investigated. The results confirmed the formation of oxide compounds of Yb4Al2O9 (YbAM), Yb3Al5O12 (YbAG), Yb2SiO5, Yb2Si2O7 and mullite, and a solid solution of Yb4Al2(1−x)Si2xO9−x (YbAMss) on the YbAM-Yb2SiO5 tie line. SiC was compatible with each of these phases, as well as the oxides of Al2O3, Yb2O3 and SiO2, whereas YbAG was compatible with Yb2Si2O7, Al2O3 and SiC, but was not compatible with mullite. Based on these observations, six compatible four-phase regions were established in the SiC-Yb2O3-Al2O3-SiO2 phase diagram. Effects of SiO2 on liquid phase sintering and high-temperature mechanical properties of SiC ceramics were discussed.