Experimental and DFT insight into the influence of Yb³⁺ doping on phase structure, thermophysical and mechanical properties of (Sm_1-xYb_x)₂Zr₂O₇ ceramics

In this study, a series of (Sm_1-xYb_x)₂Zr₂O₇ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) ceramics were synthesized by solid state reaction, and their phase structure, thermophysical properties, and mechanical properties were investigated. The results showed that (Sm_1-xYb_x)₂Zr₂O₇ (x = 0, 0.1) exhibited pyrochlore structure, while (Sm_1-xYb_x)₂Zr₂O₇ (x = 0.3, 0.5, 0.7, 0.9, 1) possessed defective fluorite structure. The thermal conductivity of (Sm_1-xYb_x)₂Zr₂O₇ ceramics initially decreased with the increases of Yb³⁺ doping fraction, but subsequently increased when x exceeded 0.3. The lowest thermal conductivity of (Sm_0.7Yb_0.3)₂Zr₂O₇ achieved was 1.341 W m⁻¹ K⁻¹ at 800 °C. Differential scanning calorimetry (DSC) results and thermogravimetric analysis (TGA) confirmed that (Sm_0.7Yb_0.3)₂Zr₂O₇ possessed high thermal phase stability. Moreover, (Sm_0.7Yb_0.3)₂Zr₂O₇ had a Vickers hardness of 9.084 GPa and a fracture toughness of 1.886 MPa m^1/2, exhibiting good resistance to deformation and crack propagation. The density functional theory (DFT) calculation results suggested that the declined thermal conductivity is due to the lattice distortion. And the variation in fracture toughness is related to the reduced lattice parameter and uneven bond strength within RE-O and Zr-O following the substitution of Yb³⁺ on Sm³⁺. These findings indicate that (Sm_0.7Yb_0.3)₂Zr₂O₇ possesses high phase stability and favorable thermophysical and mechanical properties, highlighting its great potential as a thermal barrier coating (TBC) material.