Toughening of NiAl–alumina composites by self-constructed compressive surface layers under oxidation

Self-constructing protective layers have been fabricated on the surface of NiAl–alumina composites by the oxidation of NiAl to NiAl2O4. Dense NiAl–alumina materials for these tri-layer composites can be fabricated by a pressure-less sintering of the fine spherical powders derived from the chemically precipitated precursors. The sintered NiAl–alumina materials indicate high fracture toughness (5–6 MPa·m1/2) suitable as the inner layer material sustained in the tensile stress against the compressive surface stress. The volume expansion under the oxidation of NiAl forming NiAl2O4 provides the compressive surface stress exhibiting the resistance to crack propagation. The determined compressive surface stress attaining to 128 MPa agrees with the estimation from the crack propagation, 170 MPa. The fracture toughness of the tri-layer materials containing fine-grained NiAl exceeds 8 MPa·m1/2. However, the excess oxidation for the 10 vol.%NiAl composition causes the formation of voids around the layer boundary to degrade the toughness with the relaxed residual stress.