Study On Microstructure And Properties Of Mo₂FeB₂ Based Cermet Prepared By Vacuum Sintering

Mo₂FeB₂-based cermets could be used as a promising candidate for high hardness,wear and thermal resistant applications due to their high hardness,high wear resistance,good high temperature stability and excellent electrical conductivity.It had a wide application prospect in the fields of high hardness,high wear resistance and high temperature materials.In this dissertation,the ternary boride based hard alloy Mo₂FeB₂-Fe was prepared successfully by using three elementary substances,Fe,low cost MoFe and BFe powders as raw materials.The relationship among composition,preparation process,microstructure,post-treatment technique and properties of Mo₂FeB₂ based cermets had been investigated by scanning electron microscopy（SEM）,X-ray diffraction（XRD）,energy dispersive X-ray analysis（EDS）.etc.The main findings of this study were as follows:1、It was found that the cermets sintered in vacumm at 1300℃ for 30 min with heating rate of 4℃/min near the stage of eutectic liquid phase 1150℃,where after heat preservation 1 hour,a large amount of liquid phase is generated,and the sample is rapidly densified,showing a homogeneous microstructure with relatively finer grains and better mechanical properties.In addition,the isotropic shrinkage was obtained.2、The effect of the alloying elements and grain growth inhibitors on the microstructure and properties of Mo2NiB2-based cermets had also been studied.The conclusions were obtained as follows:lwt.%C addition could eliminate the adsorbed oxygen completely,and no free carbon and brittle phase occurred.With the content of Ni increased,the Fe based binder changed from ferrite to austenite gradually.When the content of Ni was 2.5wt%,the cermets has the highest bending strength.When the content of Cr was 3wt%,the cermets had better microstructure and mechanical properties.Remarkable decrease of grains size occurred with TaC or TiC addition.TiC addition resulted in the aggregation of ceramic grains and an increase of porosity,so the mechanical properties decreased greatly.When TaC content was 4wt%,a new needle like phase appeared in cermets.3、The influence of the grain growth inhibitor composed of NbC and V on the microstructure and properties of the Mo₂FeB₂-based cermets had been studied.The conclusions were obtained as follows:NbC was mainly distributed in the ceramic phase boundary to prevent the boundary expansion and migration,and the V was mainly distributed in the hard phase,changing the crystal structure of the Mo₂FeB₂ phase and transforming from the columnar crystal to the equiaxed crystal.When the mass fraction of NbC and V were 2%and 4%respectively,the mechanical properties of cermet was the best,the flexural strength reached 1503 MPa and the hardness reached 70.1HRC,respectively.When the content of NbC and V were both 3%,the effect of inhibiting grain growth was the best,and there were still fine grains under long time thermal insulation.4、The Mo₂FeB₂ based cermet was prepared by water-atomization powders,which was amorphous materials.The powder could be compacted with a small amount of binder and pressed to sinter,and densification could be achieved within 1150 ℃.The lower temperature would lead to inhomogeneous microstructure.Exorbitant temperature or too long holding time would destroy the structure of amorphous material and reduce the mechanical properties of cermets.Therefore,it was necessary to control the sintering temperature accurately.