Improved fracture toughness and multiple toughening mechanisms of NdPO4/NdYbZr2O7 composites

The low fracture toughness of NdYbZr2O7 limits its application as a novel thermal barrier coating (TBC) material. In the present work, NdPO4 was selected to toughen NdYbZr2O7 and the NdPO4/NdYbZr2O7 composites were synthesized by solid-state reaction sintering. Microstructures, fracture toughness and associated toughening mechanisms of the composites were investigated. The fracture toughness of the composite with 20 mol% NdPO4 was similar to 1.6 MPa m(1/2), nearly 25% higher than that of NdYbZr2O7 ceramic. With the addition of NdPO4, the fracture mode transformed from intragranular fracture to a mixed mode of transgranular fracture and intergranular fracture. Besides, several crack propagation paths such as crack bridging, deflection and bifurcation were observed in the NdPO4 grains, indicative of enhanced fracture toughness. Compressive stress in the NdYbZr2O7 matrix, weak NdPO4/NdYbZr2O7 interface and tensile stress in the layered NdPO4 second phase are responsible for the improvement of fracture toughness.