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

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.