Thermodynamics of the Al–C–O system and properties of SiC–AlN–Al2OC composites

Based on a recent thermodynamic evaluation of the Al–C–O system, the standard Gibbs free energies of formation of both aluminium oxycarbides Al₄O₄C and Al₂OC are given, and a classical stability diagram is shown at 2100 K. Because Al₂OC is unstable below 1715 °C, the stable würtzite compound 2AlN.Al₂OC has been preferred, and formed in-situ as the second phase in SiC-based composites. Starting with commercial powders of -SiC, AlN, Al₂O₃ and Al₄C₃, dense materials are obtained by pressureless sintering (up to 2020 °C) or hot-pressing (up to 1950 °C), owing to the liquid phase from the Al₂O₃–Al₄C₃ system. The existence of a miscibility gap is shown, and the microstructures are fine grained and equiaxed. Compared with SiC–Al₂OC alloys, the hot-pressed materials with 90 wt% SiC exhibit slightly higher mechanical properties and a good retention nearly up to 1500 °C.