Fracture behavior of AlN ceramics with rare earth oxides

The effect of three types of additives, Sm 2O 3, La 2O 3 and Y 2O 3, on the thermal and mechanical properties of AlN ceramics was investigated. Thermal conductivities of AlN ceramics with Sm 2O 3 and Y 2O 3 addition were 176 and 173 Wm −1 K −1, respectively. However, in the case of La 2O 3 addition the value was limited to 101 Wm −1 K −1. The strengths of AlN ceramics with Sm 2O 3 and La 2O 3 addition were 455 and 407 MPa, respectively, which are higher than that with Y 2O 3 addition. Fracture toughnesses of those with Sm 2O 3 and La 2O 3 addition were also higher than that with Y 2O 3 addition. The fracture behavior on the fracture surfaces was found to be a mixed mode of transgranular fracture and intergranular fracture. The specimens with Sm 2O 3 and La 2O 3 addition had more transgranular fracture than that with Y 2O 3 addition. The strengthening mechanism of Sm 2O 3 and La 2O 3 addition was clarified by using a theory on the fracture toughness considering the crack propagation path. As a result, we concluded that the improvement of strength and fracture toughness of AlN ceramics with Sm 2O 3 and La 2O 3 addition were mainly achieved by strengthening the grain boundary.