Fabrication and mechanical properties of Si3N4/(W, Ti)C/Co graded nano-composite ceramic tool materials

Si3N4/(W, Ti)C/Co nano-composite ceramic tool materials with multilayered structure were fabricated by hot pressing technology. The composites without Co were used as the surface layers and the composites containing Co were used as the inner layers. The flexural strength, fracture toughness, and hardness of the composites were tested, and the fracture surfaces and indention cracks were observed. The experimental results showed that the flexural strength and fracture toughness were largely increased by the graded structure and the high hardness was still maintained. The five-layer graded nano-composites, with a thickness ratio of 0.2, which were sintered under a pressure of 30 MPa at 1700 °C in vacuum condition for 45 min, had the optimum comprehensive mechanical properties with a flexural strength of 992 MPa, a hardness of 17.83 GPa, and a fracture toughness of 10.5 MPa·m1/2. The formation of elongated β-Si3N4 grains and the residual compressive stress in the surface layer induced by the graded structure contributed to the improved properties. A synergistic effect of intergranular and transgranular fracture, crack bridging, and deflection contributed to the strengthening and toughening mechanisms.