Abstract
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.
Highlights
In recent years, with the widespread use of difficult-to-machine materials in engineering, cutting tools have faced the challenge of machining these materials under high speed, which requires that the tools have high hardness, excellent wear resistance, oxidation resistance, and so on
The results showed that the HfB2 additive can inhibit the TiB2 grain growth and can change the morphology of some of the TiB2 grains from bigger polygons to smaller polygons or longer ovals, which is favorable for the formation of a relatively fine microstructure, while the HfN additive tends to agglomerate
The poor wettability between HfB2 and Ni resulted in the formation of weak grain boundary strength and the complex solid solution of TiB2 -HfB2 is a brittle phase, which led to the decrease of fracture toughness of the TiB2 -HfB2 ceramics
Summary
With the widespread use of difficult-to-machine materials in engineering, cutting tools have faced the challenge of machining these materials under high speed, which requires that the tools have high hardness, excellent wear resistance, oxidation resistance, and so on. In order to reverse this tendency and improve the mechanical properties of TiB2 ceramic, reinforcements such as hard phases, metal phases, and whiskers have been employed to fabricate TiB2 -based ceramic materials through spark plasma sintering, vacuum hot-pressed sintering, or reactive hot-pressed sintering. HfB2 and HfN have high hardness, high melting point, and high oxidation resistance, and as reinforcements they can enhance the mechanical properties of ceramics such as. Because HfB2 and HfN have better thermal stability to resist deformation and decomposition at elevated temperature, they may improve the cutting performance and working life of TiB2 -based ceramic tools. TiB2 , HfB2 , HfN, Mo, and Ni by vacuum hot-pressed sintering The characteristics of these composites are analyzed according to their microstructures and mechanical properties
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