Phase relations in the Si–Al–Gd–O–C system

SiC (containing) ceramics were fabricated by pressureless sintering or hot pressing using SiC, Al2O3, Gd2O3 and SiO2 as starting materials. Phase compositions of the ceramics were detected by X-ray diffraction (XRD). The results showed that SiC was compatible with Al2O3, SiO2 and Gd2O3 at <1700 °C. Six binary oxide compounds, i.e. Gd4Al2O9 (GdAM), GdAlO3 (GdAP), Gd2SiO5, Gd2Si2O7, Gd9.33(SiO4)6O2 (GdH) and mullite, and two solid solutions, i.e. Gd4Al2(1−x)Si2xO9+x (GdAMss) on the GdAM–Gd2SiO5 tie-line and the Gd9.33+2x (Si1−xAlxO4)6O2 (GdHss) on the Gd9.33 (SiO4)6O2–GdAM tie-line formed. The GdAP phase was compatible with each of GdH, Al2O3 and SiC, but was incompatible with Gd2Si2O7 or mullite. Based on this information, seven four-phase compatible regions were established in the subsolidus phase diagram of the Gd2O3–Al2O3–SiO2–SiC quaternary system. Effects of the presence of SiO2 on liquid phase sintering and high-temperature strength of the SiC ceramics were clarified.