High-entropy diboride (HEB) ceramics constitute a novel class of ultrahigh-temperature ceramics that are appealing for applications in extreme environments. The relative density and grain size play important roles in tailoring the mechanical properties and wear resistance of HEBs, affecting their applications, such as high-temperature structural parts and thermal protection systems. In this study, highly dense (HfZrTaVNb)B₂ ceramics with size-tunable microstructures were successfully synthesized by spark plasma sintering combined with an ingenious two-step strategy. The effects of grain size on the mechanical properties and wear resistance of (HfZrTaVNb)B₂ ceramics were comprehensively investigated. The results indicated that the smaller grain size led to higher hardness and fracture toughness, and the relationship between hardness and grain size fitted the Hall–Petch equation well. In particular, the sample featuring a grain size of 1.64 µm and 97.6% density had the highest hardness and fracture toughness, 26.7 GPa and 4.6 MPa·m^1/2, respectively. Notably, it also demonstrated optimal wear resistance, displaying a minimal wear rate of only 2.53×10⁻⁶ mm³/(N·m) under a 20 N load. Microstructure analysis revealed that the primary wear mechanism observed in (HfZrTaVNb)B₂ was oxidative wear under a 5 N load. Under a 10 N load, the wear mechanism comprised both oxidative and fracture wear. The wear mechanism became more complex and involved oxidation wear, fracture wear, abrasive wear, and fatigue wear at a 20 N load.

High-entropy MXenes, as a new emerging class of materials, possess diverse compositions, unexpected physicochemical characteristics, and great potentials for electromagnetic (EM) wave absorption. Herein, two single-to-few-layer high-entropy MXenes, (Mo_0.25Cr_0.25Ti_0.25V_0.25)₃C₂T_x and (Mo_0.2Cr_0.2Nb_0.2Ti_0.2V_0.2)₄C₃T_x, were synthesized for the first time. During the exfoliation and delamination processes, the structural, morphological, and compositional evolutions were analyzed, verifying the successful formation of single-to-few-layer two-dimensional MXene nanosheets. Investigations indicate that with the filling content of only 35 wt%, MXene powder filled composites exhibit high-efficiency EM wave absorption performances. The f-(Mo_0.25Cr_0.25Ti_0.25V_0.25)₃C₂T_x possesses the minimum reflection loss (RL_min) of −45.0 dB with the matching thickness of 1.52 mm and the maximum effective absorption bandwidth (EAB) of 5.6 GHz at 1.65 mm thickness. Also, f-(Mo_0.2Cr_0.2Nb_0.2Ti_0.2V_0.2)₄C₃T_x can attain an RL_min of −52.8 dB with the thickness of 1.58 mm and an optimum EAB value of 3.6 GHz at 1.50 mm. The satisfactory EM wave absorption efficiency and bandwidth, thin matching thickness, and low filling content prove the lightweight advantage and great application potential of high-entropy MXenes in EM wave absorption. In this work, the high-entropy strategy is applied to tune the EM wave absorption performances for MXenes. Furthermore, high-entropy engineering is expected to provide control and tunability of many other properties, such as electrochemical, catalytic, and mechanical behaviors.