Effects of HfB2 and HfN Additions on the Microstructures and Mechanical Properties of TiB2-Based Ceramic Tool Materials

The effects of HfB₂ and HfN additions on the microstructures and mechanical properties of TiB₂-based ceramic tool materials were investigated. The results showed that the HfB₂ additive not only can inhibit the TiB₂ grain growth but can also change the morphology of some TiB₂ grains from bigger polygons to smaller polygons or longer ovals that are advantageous for forming a relatively fine microstructure, and that the HfN additive had a tendency toward agglomeration. The improvement of flexural strength and Vickers hardness of the TiB₂-HfB₂ ceramics was due to the relatively fine microstructure; the decrease of fracture toughness was ascribed to the formation of a weaker grain boundary strength due to the brittle rim phase and the poor wettability between HfB₂ and Ni. The decrease of the flexural strength and Vickers hardness of the TiB₂-HfN ceramics was due to the increase of defects such as TiB₂ coarse grains and HfN agglomeration; the enhancement of fracture toughness was mainly attributed to the decrease of the pore number and the increase of the rim phase and TiB₂ coarse grains. The toughening mechanisms of TiB₂-HfB₂ ceramics mainly included crack bridging and transgranular fracture, while the toughening mechanisms of TiB₂-HfN ceramics mainly included crack deflection, crack bridging, transgranular fracture, and the core-rim structure.