Self-Healing of Ni/Mullite Hybrid Materials via High-Temperature Oxidation

Abstract

Fully-densified mullite nanocomposites dispersed with 5 vol% nano-Ni particles were fabricated by the pulsed electric current sintering technique to investigate their self-healing function. The investigation on thermal oxidation for healing the mullite composites was conducted at temperatures ranging from 1000 to 1200℃ for 1–24 h in air. Surface cracks with a total length of 180 µm completely disappeared by heat-treatment at 1100℃ for 6 h in air. Bending strength of the as-cracked samples which were prepared with three Vickers indentations on the tensile surface only achieves 113 MPa. Disappearance of surface cracks realizes a recovery of bending strength up to 434 MPa, which is comparable with that of as-sintered samples (405 MPa). With heat-treatment in Ar-1%H₂ at 1100℃ for 6 h, the samples show neither surface crack-disappearance nor strength recovery. It is clear that the oxidation of Ni particles at high temperatures induces the self-healing mechanism. The surface crack-disappearance in Ni/mullite is available at lower heat-treatment conditions than the other metal/oxide composites such as Ni/Al₂O₃ and Ni/(ZrO₂+Al₂O₃). Additional information on mechanical properties such as hardness and fracture toughness of the composite are also provided. Dispersion of Ni particles in mullite matrix does not give effect to their hardness and fracture toughness.