Microstructure And Mechanical Properties Of Mo ₂FeB₂ Based Cermets By Reaction Boronizing Sintering

Mo₂FeB₂-based cermets by reaction boronizing sintering has a high strength, toughness and a lower cost. Hence, Mo₂FeB₂-based cermets is a promising candidate for wear resistant applications. In this dissertation, the relationship among composition, preparation process, microstructure, post-treatment technique and properties of Mo₂FeB₂-based cermets has been investigated by thermal analysis(DSC), scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD), energy dispersive X-ray analysis(EDS).The characteristic of phase transformation and microstructure evolution of Mo₂FeB₂ based cermets has been studied. Some conclusions were drawn as follows: four different phase reaction occurred with an increase of sintering temperature, which corresponded to the formation of the Fe₂B, Mo₂FeB₂, liquid phase L1 and liquid phase L2. The hard phase Mo₂FeB₂ was produced in the compact prior to liquid formation by the reaction Mo and Fe₂B. Almost full densification was achieved in the stage of liquid phase L1. With an increase of the liquid phase sintering temperature, the microstructure became homogeneous gradually, and the hard phase got elongated. The development of elongated Mo₂FeB₂ grains was shown to improve the fracture toughness of the cermets.The mechanism of liquid phase sintering and abnormal grain growth (AGG) of Mo₂FeB₂ based cermets has been studied. The grain coarsening was controlled by dissolution and precipitation, and the coarsening rate was decided by interface reaction. When the sintering temperature was too high, abnormally large Mo₂FeB₂ grains appeared. The observed abnormal grain growth (AGG) was explained in terms of a two-dimensional nucleation controlled growth mechanism. Coalescence was suggested to the key factor which resulted in the abnormal grain growth.The process of vacuum sintering and hot isostatic press (HIP) for Mo₂FeB₂ based cermets has been studied. The cermets sintered at 1250℃for 40 minutes exhibited a relatively finer grain size, homogeneous microstructure and better mechanical properties. HIP treatment increased the transverse rupture strength and fracture toughness when the sintering temperature or soaking time was not sufficient.The influence of the alloying element and SiCw on the microstructure and properties has been studied. The conclusions were obtained as follows: when the amount of carbon was 0.5wt%, the cermets had a relatively finer grain size and better mechanical properties. When carbon content was 0.5wt%, an optimum content of Mn, V or SiCw could further decreased the grain size and increased the mechanical properties. The refinement mechanism of Mn, V or SiCw was the increase of wetting of the ceramic grains by liquid metal, restraint the dissolution and precipitation and retarder grain boundary migration, respectively. When V content was 2.5wt%, the cermets showed the highest TRS, hardness and fracture toughness of 2400MPa, HRA90.6 and 15.1MPa·m^1/2, respectively.The valence electron structure of Mo₂FeB₂, (Mo,V)₂FeB₂ and Mo2(Fe,Mn)B2 hard phase was calculated. Some conclusions were deduced from the calculated results: the amount of∑I a na could be used as a reference to compare to the plasticity of hard phase. The (Mo,V)2FeB2 phase had the best plasticity, Mo2(Fe,Mn)B2 phase took second place, and the Mo₂FeB₂ phase was the worst. V addition improved the hard phase plasticity and increased coordinate deformation of the binder phase and hard phase, resulting in an increase of TRS.The MoFe powder replaced all of Fe and the majority of Mo powder, the effect of which on the microstructure and properties of the cermets has been studied. The cermets fabricated by MoFe powder exhibited the same phase and a similar microstructure compared with that by Mo and Fe powder. The cermets fabricated by MoFe powder had a better TRS, toughness and lower hardness compared to that by Mo and Fe powder. The cermets could be fabricated by MoFe powder in a lower cost.