High-performance multifunctional (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)C high-entropy ceramic reinforced with low-loading 3D hybrid graphene–carbon nanotube

There has been growing interest in the high-entropy ceramic (HEC) recently owing to its tailorable compositions and microstructures, versatile properties, together with promising structural and functional applications. However, inferior fracture toughness (K_IC) and damage tolerance restricted many practical applications of the HEC. Herein, we addressed this challenge by incorporating a three-dimensional graphene–carbon nanotube (3D G–CNT) as toughening agent in (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)C. The resulting enhanced 3D G–CNT/(Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)C featured an outstanding toughness of 8.23 MPa·m^1/2, while remaining superior strength (763 MPa) and hardness (24.7 GPa). An ultralow friction coefficient (0.15) coupled with an ultralow wear rate (w, 2.6×10⁻⁷ mm³/(N·m)) in the 3D G–CNT/(Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)C was obtained primarily as a function of lubricating scrolls, in which two-dimensional (2D) graphene acted as a tribolayer, and one-dimensional (1D) carbon nanotubes acted as nano ball bearings embedded inside. Strikingly, the 3D G–CNT/(Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)C exhibited rather low thermal conductivity (κ) yet excellent electrical conductivity (σ, 1.3×10⁶ S/m) in comparison with the pure (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)C. This study provided great potential for maximizing the physical and functional properties of the HEC for various applications.