Liquid/glass immiscibility in yttria doped mullite ceramics

5 wt.% yttria doped mullite ceramics were sintered using nanocomposite powders which consisted of a Y₂O₃–SiO₂ core coated with pseudoboehmite. Firstly, the powders were heated up to 1300 °C; mullite began to form at 1050 °C which is ∼150 °C lower than in previous diphasic precursors. Then, the powders were sintered at 1550 °C for 0.5, 5 and 48 h, and characterized using XRD, SEM and TEM with EDS. The phases present were mullite and two intergranular glasses comprising silica-rich droplets embedded in a Y–Si–Al–O glassy matrix, providing direct evidence of liquid-liquid phase separation at the grain boundaries during sintering. This liquid-liquid separation is facilitated by the prior existence of yttrium disilicate which moves the overall sintering liquid compositions toward the two immiscible liquids region of the Y₂O₃–Al₂O₃–SiO₂ phase diagram. The presence of the silica-rich phase in the grain boundaries will effectively increase their viscosity and thus the resistance of the Y₂O₃ densified mullite to creep.