Unveiling Entropy‐Driven Hard‐Tough Synergy Mechanisms in High‐Entropy Carbonitride Ceramics

High‐entropy ceramics manifested promise for extreme environments, yet their phase stability and mechanical synergy mechanisms remain elusive. Herein, entropy‐difference carbonitrides, including (TaTiV)CN, (TaTiVNb)CN, and (TaTiVNbHf)CN, were designed and consolidated via spark plasma sintering. An integrated multiscale approach combining DFT calculations, Bader charge analysis, thermodynamic modeling, and experiments elucidated their electronic origins, lattice distortion, diffusion‐controlled evolution, and mechanical responses. Nb/Hf co‐doping triggered electronic reconstruction, manifested as enhanced d‐p hybridization, homogenized charge transfer, and elevated elastic constants, collectively strengthening intrinsic bond rigidity and shear resistance. Concurrently, sequential diffusion balanced sintering energy with solid‐solution formation, yielding progressive grain refinement and the transition from intergranular to transgranular fracture. Controlled lattice micro‐strain and increased dislocation density raised energy barriers for dislocation glide and crack propagation, while residual HfO 2 particles imparted extrinsic toughening employing crack bridging and deflection. Summatively, the predominant hard‐tough synergy mechanisms include electronic reconstruction, sequential diffusion‐controlled grain refinement, and HfO 2 ‐induced extrinsic toughening. High‐entropy ceramic (TaTiVNbHf)CN achieved exceptional hardness (23.02 GPa), flexural strength (556.98 MPa), and fracture toughness (4.48 MPa m 1/2 ), substantially outperforming medium‐entropy carbonitrides and conventional binary carbonitrides. This work established a multiscale mechanistic framework linking electronic structure, lattice distortion, diffusion kinetics, and mechanical performance, offering a paradigm for entropy‐mediated design of advanced ceramics with tailored property portfolios.