Formation of NiAl2O4 Layer on High Temperature Oxidation of Ni/Al2O3 Nanocomposites

Abstract

Al2O3 composites dispersed with nano-Ni particles, Ni/Al2O3 nanocomposites, are candidates for high temperature structural applications because of excellent mechanical properties. Those nanocomposites have a self-healing function for surface cracks via high-temperature oxidation. In high temperature oxidation of Ni/Al2O3 nanocomposites, inward diffusion of oxide ions occurs mainly to form oxidation zone consisting of NiAl2O4 and Al2O3. Outward diffusion of cations also occurs to develop NiAl2O4 layer on the surface of Ni/Al2O3 nanocomposites. Surface cracks of Ni/Al2O3 nanocomposites are filled out by the surface NiAl2O4 developed by high temperature oxidation. Kinetics on growth of an NiAl2O4 layer was discussed in order to understand kinetics of oxidation and self-healing of Ni/Al2O3nanocomposites at high temperatures. Starting powder of Ni/Al2O3 nanocomposites was prepared by drying and reducing Al2O3 aqueous slurry with Ni nitrate. Volume fraction of Ni was ranged from 1 to 10 vol%. The powder was densified by pulsed electric current sintering at 1400°C for 5 min under 40 MPa in vac. Grain sizes of Al2O3 matrix and Ni particles were equal to 1 μm and 300 nm, respectively. The Ni/Al2O3 nanocomposites were oxidized at 1300°C in air. Thickness of the NiAl2O4 layer was measured as well as the oxidized zone of Ni/Al2O3nanocomposites. Growth of surface NiAl2O3 developed on Ni/Al2O3 nanocomposites,, as well as the oxidized zone, followed a parabolic manner. The parabolic rate constant on the growth of NiAl2O4 layer was proportional to the Ni volume fraction until 5 vol%. Reaching to 10 vol% in Ni volume fraction, the value of the parabolic rate constant increased remarkably. The fast growth of NiAl2O4 layer on 10 vol% Ni/Al2O3 nanocomposite would be caused by connection of NiAl2O4 grains in the oxidized zone because Ni diffusion in NiAl2O4 is faster than that in Al2O3 matrix.