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

Thermal and mechanical properties of yttrium tantalate (YTaO 4 ), a top coat ceramic of thermal barrier coatings (TBCs) for aeroengines, are enhanced by synthesizing Y 1− x Ta 1− x M 2 x O 4 (M = Ti, Zr, Hf; x = 0.06, 0.12, 0.18, 0.24) medium-entropy ceramics (MECs) using a two-step sintering method. In addition, the thermal conductivity, thermal expansion coefficients (TECs), and fracture toughness of MECs were investigated. An X-ray diffraction study revealed that the Y 1− x Ta 1− x M 2 x O 4 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 improving the phonon scattering and reduced thermal conductivity, with Y 1− x Ta 1− x M 2 x O 4 MECs ( x = 0.24) exhibiting the lowest thermal conductivity of 1.23 W·m −1 ·K −1 at 900°C. The introduction of MO 2 increased the configurational entropy and weakened the ionic bonding energy, obtaining high TECs (10.4 × 10 −6 K −1 at 1400°C). The reduction in the monoclinic angle β lowered the ferroelastic domain inversion energy barrier. Moreover, microcracks and crack extension toughening endowed Y 1− x Ta 1− x M 2 x O 4 MECs ( x = 0.24) with the highest fracture toughness of (4.1 ± 0.5) MPa·m 1/2 . The simultaneous improvement of the thermal and mechanical properties of the MO 2 (M = Ti, Zr, Hf) co-doped YTaO 4 MECs can be extended to other materials.