Fabrication and mechanical properties of functionally gradient TiCN–HfC–WC ceramics

The inherently low fracture toughness of TiCN-based ceramics significantly limits their practical engineering applications. To address this, functionally gradient TiCN–HfC ceramics (FGTHCs) with a WC gradient were successfully fabricated using vacuum hot-pressing sintering, and they exhibited high toughness without sacrificing strength and hardness. As sintering temperature increased, the flexural strength and surface Vickers hardness initially increased and then slightly decreased, whereas the fracture toughness gradually increased. At 1600 °C, FGTHC possessed optimal comprehensive mechanical properties, with a flexural strength of 1619.7 ± 32.6 MPa, a surface Vickers hardness of 20.13 ± 0.53 GPa, and a fracture toughness of 13.88 ± 0.59 MPa m^1/2. Under identical sintering conditions, FGTHC demonstrated superior mechanical properties compared to homogeneous TiCN–HfC ceramics (HTHCs), particularly in terms of fracture toughness (13.88 ± 0.59 vs. 8.53 ± 0.37 MPa m^1/2). From the surface to the core of FGTHC, the residual stress gradually changed from residual compressive stress (RCS) to residual tensile stress (RTS). RCS effectively inhibited crack propagation and enhanced crack deflection, whereas RTS induced crack deflection with a zigzag pattern.