Study Of Mo₂NiB₂ Based Cermets Produced By Reaction Boronizing Sintering Method

Mo2NiB2-based cermets can be used as a promising candidate for wear, corrosion and thermal resistant applications due to their excellent mechanical properties. In this dissertation, the ternary boride based hard alloy Mo2NiB2-Ni was prepared successfully by using three elementary substances, Mo, Ni, and amorphous B powders as raw materials. The relationship among sintering process, composition, microstructure and properties of Mo2 Ni B2-based cermets has been investigated.The influence of the sintering temperature and heating rate on the microstructure and properties of Mo2NiB2-based cermets has been studied. It was found that the cermets sintered in vacumm at 1280℃ for 30 min with heating rate of 2℃/min near the stage of liquid phase(1230℃-1270℃) showed a homogeneous microstructure with relatively finer grains and better mechanical properties. In addition, the isotropic shrinkage was obtained.Preliminary explored the influence of the Mo/B atomic ratio on the Mo2NiB2-based cermets. The results showed that there is a strong correlation between the Mo/B atomic ratio and properties. When the Mo/B atomic ratio was 1.1, the cermets consisted of an optimal microstructure with relatively finer grains, no third brittles phase was found. The cermets exhibited the highest transverse rupture strength(TRS) and higher hardness. The microstructure of the cermets with Mo/B atomic ratio of 0.9 and 1.0 consists of some coarsened Mo2NiB2 grains. The cermets with Mo/B atomic ratio of 1.2, due to the agglomeration of Mo2NiB2 grains and the existence of a third phase, which resulted in the lowest TRS.The effect of the carbides on the microstructure and properties of Mo2NiB2-based cermets has also been studied. It was found that the temperature of the liquid formation was decreased with the addition of Cr3C2 or VC. The addition of Cr3C2 or VC improved the wettability of hard phase by binder phase, and accordingly resulted in the decrease of porosity and improvement of mechanical properties. The cermets with 2.5wt.% Cr3C2 addition showed the highest TRS and higher hardness of 1.93 GPa and 66 HRC. While the cermets with 2.5wt.% VC addition showed the highest TRS and hardness.