Passive and active oxidation of Al in the Al–Al2O3 composite refractory at high temperatures in air

The active and passive oxidation of Al in Al-Al2O3 composites was investigated in air at high temperatures. As fresh air impinged on the surface of the Al-Al2O3 composite sample, a thin Al2O3 layer was preferentially formed via passive oxidation of Al, which consumed most of the oxygen and lowered the local Po2 inside the sample. At low Po2, active oxidation of Al occurred and predominated in the interior transition layer, which further lowered the local Po2 to extremely low values within the non-oxide stability field. The local equilibrium PAlxOy generated by the active oxidation of Al reached a micro-positive pressure state and served as a gas barrier, hindering the inward diffusion of O2. The Po2 inside the composite was lowered by Al in the outer layers via a self-gettering process. Subsequently, Al4O4C and Al2OC-AlN solid solution were formed by both the direct reaction of Al(l) and the indirect reaction of AlxOy(g). After sintering in air, the Al-Al2O3 composite exhibited a functional gradient structure with a thin oxide layer and an interior non-oxide-reinforced corundum. A reaction mechanism model was established.