Influence of Al₂O₃ Content on the Microstructure and Microhardness of NiAl- Al₂O₃ Composites with Micro-Sized Al₂O₃ Strengthening Particles by Plasma Activated Sintering

Thermal spraying may be a promising approach for the deposition of NiAl-Al₂O₃ composite coatings. To understand the intrinsic mechanical property of NiAl-Al₂O₃ composite, dense NiAl-Al₂O₃ composites were prepared by plasma activated sintering using the ball milled Ni/Al- Al₂O₃ composite powders contain both micro-sized Al₂O₃ strengthening particles and submicron-sized Al₂O₃ dispersoids homogeneously distributed in the NiAl matrix phase. The angular morphology of the Al₂O₃ particles in composite powder was changed to spherical or near spherical morphology after plasma activated sintering. Compared to the NiAl-Al₂O₃ composites with low Al₂O₃ content, NiAl- 60vol.%Al₂O₃ composite consisted of less fine submicron-sized Al₂O₃ dispersoids. Melting induced mechanism was proposed to explain the spheroidization and the change of Al₂O₃ particle size distribution. The hardness of the NiAl-Al₂O₃ composites with both micro-sized Al₂O₃ strengthening particles and submicron-sized Al₂O₃ dispersoids increased with the increase of Al₂O₃ content. The relation was employed to theoretically estimate the hardness using volume fractions of components and corresponding hardness. The estimated hardness was compared with the observed ones. It was found that bimodal sized particles reinforcement results in a higher hardness than the theoretical value, which could be attributed to the dispersion hardening effect.