Possibility of enhanced strength and self-recovery of surface damages of ceramics composites under oxidative conditions

Oxidation of NiAl/α-Al2O3 composites provides the metamorphic surface layers consisting of boundary NiAl2O4 and Al2O3 grains with compressive stress. Strengthening and toughening from 515 MPa and 4 MPa m1/2 (sintered) to ∼655 MPa and ∼6 MPa m1/2 (oxidized) are enabled by a controlled development of the metamorphic layers at 1200–1300 °C. The reactivity of an oxidation product, NiO, with the matrix-Al2O3 and the rapid diffusion along the grain boundary repairs the damages on surfaces by filling and re-bonding the defects and cracks. The oxidation further progresses via the repaired defects to develop the metamorphic layers supporting them to enhance the recovery of the strength. The degraded strength of the material with defects, 150 MPa, is recovered to 531 MPa by the reparation against the limited recovery to 343 MPa by a thermal healing in Ar.