Fracture behavior of AlN ceramics with rare earth oxides

The effect of three types of additives, Sm2O3, La2O3 and Y2O3, on the thermal and mechanical properties of AlN ceramics was investigated. Thermal conductivities of AlN ceramics with Sm2O3 and Y2O3 addition were 176 and 173 Wm−1 K−1, respectively. However, in the case of La2O3 addition the value was limited to 101 Wm−1 K−1. The strengths of AlN ceramics with Sm2O3 and La2O3 addition were 455 and 407 MPa, respectively, which are higher than that with Y2O3 addition. Fracture toughnesses of those with Sm2O3 and La2O3 addition were also higher than that with Y2O3 addition. The fracture behavior on the fracture surfaces was found to be a mixed mode of transgranular fracture and intergranular fracture. The specimens with Sm2O3 and La2O3 addition had more transgranular fracture than that with Y2O3 addition. The strengthening mechanism of Sm2O3 and La2O3 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 Sm2O3 and La2O3 addition were mainly achieved by strengthening the grain boundary.