Oxidation of NiAl/Al2O3 composites for controlled development of surface layers and toughening

The oxidation of NiAl/Al2O3 composites has been performed to develop protective layers with compressive stress for toughening. The dominant formation of NiAl2O4 is observed for NiAl/Al2O3 composites contrary to the consecutive formation of Ni3Al+Al2O3, 2Ni+Al2O3, 2NiO+Al2O3, and NiO+NiAl2O4 for NiAl powder. The NiO formed on the composites immediately diffuses along with the grain boundary, reacts with boundary Al2O3, and provides the metamorphic layer composed of NiAl2O4 and α-Al2O3 with the thickness of 20–50 μm. The volume expansion of the oxidation forming NiAl2O4 induces the compressive stress in the metamorphic layer. The compressive stress for an oxidized 10 vol.% NiAl/Al2O3 composite attains to 127 MPa leading the improvement of the fracture toughness to 6.2 MPa m1/2. The degradation of the strength due to the tensile stress generated against the compressive surface stress is limited by the stress-tolerancy of NiAl/Al2O3 composites.