Origins of high fracture toughness and glass-like thermal conductivity in Zr–Ta–O composites

Screening multifunctional thermal/environmental barrier coatings (T/EBCs) for ceramic matrix composites (CMCs) and alloys contribute to developing highly efficient hypersonic vehicles and aeroengines. This work elucidates that Zr–Ta–O composites are T/EBCs for Al₂O₃ fiber-reinforced Al₂O₃ (Al₂O_3f/Al₂O₃) CMCs and alloys. Zr–Ta–O composites have superiorities of glass-like thermal conductivity (1.4 W m^–1 K^–1), thermal expansion coefficients (8.6–12.3 × 10^–6 K^–1) matching to Al₂O_3f/Al₂O₃ and alloys, and high toughness (3.4 MPa m^1/2). The origins of high toughness are elucidated from crack-propagation models and formation of micro-cracks, which consume lots of crack fracture energy and relieve the stress concentration of indentation tips. The long-range-ordered and short-range-disordered arrangements of cations and the highly twisting lattices of Ta₂Zr₆O₁₇ significantly contribute to their glass-like thermal conductivity. This work motivates upcoming study and application of Zr–Ta–O composites as T/EBCs with working temperature reaching 1500°C.