Reinforced High-Entropy Fluorite Oxide Ceramic Composites for Thermal Barrier Coating Application

High-entropy ceramics hold promise for application as thermal barrier coating materials. However, a key challenge in practical applications lies in the low fracture toughness compared to that of yttria-stabilized zirconia (YSZ). Herein, we designed (Hf,Zr,Ce,M)O_2-δ-Al₂O₃ (M = Y, Ca, and Gd) ceramic composites by following a set of fundamental guidelines. First-principles calculations predicted that the inclusion of Al₂O₃ in compositions containing the other four binary oxides decreased the propensity for single high-entropy phase formation. Instead, it increased the potential for Al₂O₃ to form a second phase within the high-entropy ceramic matrix, compared to compositions without Al₂O₃. Ceramic composites consisting of the Al₂O₃ second phase in a high-entropy fluorite oxide (Hf,Zr,Ce,M)O_2-δ matrix were synthesized in situ via conventional solid-state reactions from the five constituent binary oxides. Both the hardness and fracture toughness of the ceramic composites were enhanced due to toughening mechanisms from the discrete Al₂O₃ particles, microcracks, and crack deflections. Additionally, the ceramic composites exhibited coefficients of thermal expansion and thermal conductivities comparable with those of YSZ. Our findings demonstrated the potential of the high-entropy (Hf,Zr,Ce,M)O_2-δ-Al₂O₃ ceramic composites for advanced thermal barrier coating materials and offered a possible approach to reinforce other high-entropy oxide-based ceramic systems.