WC-reinforced AlN-Y₂O₃ composites: Grain coarsening for toughness and thermal performance

Grain coarsening is conventionally considered detrimental to mechanical performance, typically reducing Vickers hardness and indentation fracture toughness in metals, ceramics, and composites. Challenging this paradigm, we demonstrate that controlled grain coarsening, when combined with WC reinforcement, can enhance mechanical toughness of AlN-Y₂O₃ composites without significantly compromising thermal conductivity. The composites were fabricated via hot-press sintering at 1700 and 1800 °C for 2 h under 50 MPa in a nitrogen atmosphere, both achieving near-full densification. The higher temperature promoted substantial grain coarsening, which increased the frequency of crack-WC interactions. These interactions activated fracture energy-dissipating mechanisms such as crack deflection, bridging, branching, and grain pullout. The specimen sintered at 1800 °C with 10 wt% WC exhibited the most balanced properties, achieving a fracture toughness of 6.0 MPa·m^1/2, Vickers hardness of 13.4 GPa, and thermal conductivity of 148.3 W/m·K. Finite element simulation based on experimentally informed microstructures reproduced the composition-dependent thermal conductivity trends and supported the validity of the proposed design strategy. These findings demonstrate that grain coarsening, when strategically combined with well-dispersed reinforcement, offers a viable pathway to simultaneously enhance indentation fracture toughness and retain thermal functionality in ceramic composites.