Thermal/mechanical properties of MO2 (M = Ti, Zr, Hf) co-doped YTaO4 medium entropy ceramics

To improve the thermal and mechanical properties of yttrium tantalate (YTaO4) as a top coat ceramic of thermal barrier coatings (TBCs) for aeroengines, Y1-xTa1-xM2xO4 (M=Ti, Zr, Hf, x = 0.06, 0.12, 0.18, 0.24) middle entropy ceramics (MECs) were synthesized using a two-step sintering method, and their thermal conductivity, thermal expansion coefficients (TECs), and fracture toughness were investigated. An X-ray diffraction study revealed that the Y1-xTa1-xM2xO4 MECs were monoclinic, and the Ti, Zr, and Hf doping elements replaced Y and Ta. The variations in atomic weights and ionic radii led to disturbed atomic arrangements and severe lattice distortions, resulting in improve the phonon scattering and reduced thermal conductivity, with Y1-xTa1-xM2xO4 MECs (x = 0.24) exhibiting the lowest thermal conductivity of 1.23 W·m−1·K−1 at 900°C. The introduction of MO2 increased the configurational entropy and weakened the ionic bonding energy, leading to high thermal expansion coefficients (10.4 × 10−6 K−1, at 1400 °C). The reduction in the monoclinic angle β lowered the ferroelastic domain inversion energy barrier, and microcracks and crack extension toughening endowed Y1-xTa1-xM2xO4 MECs (x = 0.24) with the highest fracture toughness of 4.1 ± 0.5 MPa·m1/2. The simultaneous improvement of the thermal and mechanical properties in the MO2 (M = Ti, Zr, Hf) co-doped YTaO4 MECs, can be extended to other materials.