Strengthening and toughening of TiN-based and TiB2-based ceramic tool materials with HfC additive

Effects of HfC addition on the microstructures and mechanical properties of TiN-based and TiB2-based ceramic tool materials have been investigated. Their pore number decreased gradually and relative densities increased progressively when the HfC content increased from 15 wt% to 25 wt%. The achieved high relative densities to some extent derived from the high sintering pressure and the metal phases. HfC grains of about 1 µm evenly dispersed in these materials. Both TiN and TiB2 grains become smaller with increasing HfC content from 15 wt% to 25 wt%, which indicated that HfC additive can inhibit TiN grain and TiB2 grain growth, leading to the formation of a fine microstructure advantageous to improve flexural strength. Especially, TiB2-HfC ceramics exhibited the typical core-rim structure that can enhance flexural strength and fracture toughness. The toughening mechanisms of TiB2-HfC ceramics mainly included the pullout of HfC grain, crack deflection, crack bridging, transgranular fracture and the core-rim structure, while the toughening mechanisms of TiN-HfC ceramics mainly included pullout of HfC grain, fine grain, crack deflection and crack bridging. Besides, HfC hardness had an important influence on the hardness of these materials. Higher HfC content increased Vickers hardness of TiN-HfC composite, but lowered Vickers hardness of TiB2-HfC composite, being HfC hardness higher than for TiN while HfC hardness is lower than for TiB2. The decrease of fracture toughness of TiN-HfC ceramic tool materials with the increase of HfC content was attributed to the formation of a weaker interface strength.