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 ~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.