Effect of TiO ₂ doping on the fracture toughness and thermal conductivity of HfO ₂ ceramic

Abstract Hafnia‐based ceramic coating is considered one of the candidates to substitute yttria partially stabilized zirconia material as thermal barrier coating working above 1200°C. In this work, a series of TiO ₂ ‐doped HfO ₂ ceramics (Hf _{1– x} Ti _x O ₂ , x = 0%, 5%, 10%, 15%, 20%, and 40%) were fabricated by solid‐state reaction. The microstructure, fracture toughness, and thermal conductivity were systematically investigated. The results show that Hf _{1– x} Ti _x O ₂ ceramics are comprised of monoclinic HfO ₂ and orthorhombic HfTiO ₄ phases when Ti ⁴⁺ doping amount is between 5% and 20%, and the weight fraction of HfTiO ₄ phase linearly increases with the Ti ⁴⁺ content and finally reaches 100% for Hf _0.6 Ti _0.4 O ₂ . The fracture toughness of TiO ₂ ‐doped HfO ₂ ceramic is higher than 1.75 MPa m ^.5 when TiO ₂ content is no more than 15% and decreases below 1.5 MPa m ^0.5 if TiO ₂ content further increases. The thermal conductivity is significantly decreased by TiO ₂ doping attributed to the precipitated HfTiO ₄ phase. Considering the mechanical properties and thermal insulation capacity, the doping of 15% TiO ₂ would be beneficial for the HfO ₂ ‐based ceramics.