Effects of Nitrogen Partial Pressure on the Microstructure and Mechanical Properties of High-Entropy Ti(C,N)-Based Gradient Cermets

Titanium carbonitride (Ti(C,N))-based ceramics are widely utilized in mechanical machining, aerospace, and electronics, particularly in cutting tools and wear-resistant components. Two single-phase solid solution powders, non-high-entropy (Ti0.83,W0.07,Mo0.04,Nb0.03,Ta0.04)(C0.7,N0.3) and high-entropy (Ti0.6,W0.1,Mo0.1,Nb0.1,Ta0.1)(C0.78,N0.22), were synthesized via the carbothermal reduction–nitridation (CRN) method. Gradient-structured non-high-entropy (C-TiCN) and high-entropy (HE-TiCN) cermets were fabricated at 1450 °C by tailoring the nitrogen partial pressure in the range of 1–8 kPa. The effect of nitrogen partial pressure on the microstructure and mechanical properties of both materials was thoroughly analyzed. Both materials exhibited a three-layer gradient structure comprising a hard-phase-enriched surface layer, a binder-rich subsurface layer, and a chemically uniform core. Optimal performance was achieved at 4 kPa nitrogen partial pressure, at which both HE-TiCN and C-TiCN exhibited a desirable combination of surface hardness and fracture toughness. Compared with C-TiCN, HE-TiCN showed improvements in surface hardness and fracture toughness at subsurface and core regions (40 µm from the surface) by 4.9%, 11.2%, and 12.0%, respectively. The enhanced surface hardness of HE-TiCN is attributed to the significant lattice distortion and the synergistic effects associated with its high-entropy configuration. The improved toughness of the binder-rich layer is primarily ascribed to mechanisms such as crack deflection, crack branching, and the formation of tear ridges. These findings offer a promising strategy for developing gradient Ti(C,N)-based cermets with enhanced mechanical performance.