Effects of HfB(2) and HfN Additions on the Microstructures and Mechanical Properties of TiB(2)-Based Ceramic Tool Materials

The effects of HfB(2) and HfN additions on the microstructures and mechanical properties of TiB(2)-based ceramic tool materials were investigated. The results showed that the HfB(2) additive not only can inhibit the TiB(2) grain growth but can also change the morphology of some TiB(2) 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 TiB(2)-HfB(2) 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 HfB(2) and Ni. The decrease of the flexural strength and Vickers hardness of the TiB(2)-HfN ceramics was due to the increase of defects such as TiB(2) 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 TiB(2) coarse grains. The toughening mechanisms of TiB(2)-HfB(2) ceramics mainly included crack bridging and transgranular fracture, while the toughening mechanisms of TiB(2)-HfN ceramics mainly included crack deflection, crack bridging, transgranular fracture, and the core-rim structure.