Recovery of mechanical strength by surface crack disappearance via thermal oxidation for nano-Ni/Al₂O₃ hybrid materials

Recovery of mechanical strength as a function of heat-treatment temperature and time was investigated in α-Al₂O₃-based hybrid materials containing with 5 vol% dispersed Ni nanoparticles (nano-Ni/Al ₂O₃) via fraction of surface crack disappearance. Surface cracks introduced by Vickers indentation disappeared completely because of the oxidation product NiAl₂O₄. Bending strengths of as-sintered and as-cracked samples were 490±58 and 180±15 MPa, respectively. Heat-treatment at 1000 °C for 1 h resulted in recovery of bending strength up to 550±190 MPa with 10% fraction of crack disappearance. The bending strength of samples heat-treated at higher temperatures or for longer durations was comparable with that of the as-sintered samples. Bending strength reached and remained at approximately 604 MPa when the fraction of surface crack disappearance was over 55%. As-recovered samples did not fracture at the surface cracks during the three-point bending test. Thus, recovery of mechanical strength was achieved by partial disappearance of the surface cracks. This observed recovery of mechanical strength was caused by a reduction in stress concentration at crack-tips covered with NiAl ₂O₄ formed from Ni^2- along grain boundaries of the Al₂O₃ matrix.