Reinforcement of Hydroxyapatite Bioceramic by Addition of Ni3Al and Al2O3

The effects of Ni 3 Al and Al 2 O 3 additions on the mechanical properties of hydroxyapatite (HAp) were investigated. The addition of Ni 3 Al particles increased the strength as well as the fracture toughness of HAp. However, the improvements in the properties were limited because of the formation of microcracks around the metal particles. The microcracks were formed because of the large difference in the coefficients of thermal expansion between HAp and Ni 3 Al, and because of the relatively large size of Ni 3 Al particles (∼20 µm). The addition of submicrometer Al 2 O 3 powder was also effective in increasing the mechanical properties. The flexural strength and the fracture toughness were increased from about 100 MPa and 0.7 MPam 1/2 , respectively, to 200 MPa and 1.5 MPam 1/2 by the addition of 20 vol% Al 2 O 3 . When Ni 3 Al and Al 2 O 3 were added together, the fracture toughness was further increased to 2.3 MPam 1/2 . This increase in the fracture toughness was attributed to the synergistic effect of matrix strengthening and crack interactions with the metal particles.