Improved Crack Healing and High‐Temperature Oxidation Resistance of Ni/Al 2 O 3 by Y or Si Doping

Recovery of mechanical strength was investigated for 5 vol% Ni/α‐Al 2 O 3 nanocomposites that had improved resistance to high‐temperature oxidation by doping with Y or Si (Ni/Al 2 O 3 ‐Y and Ni/Al 2 O 3 ‐Si). Surface cracks disappeared completely because of the oxidation product, NiAl 2 O 4 . The fraction of crack disappearance was comparable between Ni/Al 2 O 3 ‐Y and Ni/Al 2 O 3 ‐Si. The apparent activation energy of crack healing is similar to the grain‐boundary diffusion of Ni ions in an Al 2 O 3 matrix. The rate‐controlling process of crack healing is the grain‐boundary diffusion of cations in an internally oxidized zone ( IOZ ) of the Ni/Al 2 O 3 system. The bending strengths of the as‐sintered and as‐cracked Ni/Al 2 O 3 ‐Y samples were 561 and 232 MPa, respectively. Heat treatment at 1200°C for 6 h resulted in a recovery of the bending strength up to 662 MPa for Ni/Al 2 O 3 ‐Y as well as 606 MPa for Ni/Al 2 O 3 ‐Si. Y and Si dopants were segregated into the Al site at the Al 2 O 3 grain boundaries, and then, enhanced covalent bonding occurred with neighboring oxygen. While the flux of Ni ions was retarded slightly by doping with Y and Si, a shorter IOZ provided enough Ni ions to form NiAl 2 O 4 on the surface. Ni/Al 2 O 3 ‐Y and Ni/Al 2 O 3 ‐Si have the desirable properties of crack healing and resistance to high‐temperature oxidation.