Study On Synthesis Mechanism Of Tungsten Diboride Powder By Boro/carbothermal Reduction And Densification And Properties Of Its Bulks

Tungsten diboride(WB₂)has high hardness,high melting point,good thermal conductivity,wear resistance and chemical inertia.Thus,it exhibits a great application potential in the fields of cutting tools,abrasives,wear-resistant coatings and so on.As seen in the cases of other transition metal borides,WB₂ is generally difficult to be sintered to full density.It has been known that the raw powder quality and densification method applied are the key factors affecting the sintered density of WB₂ ceramics.In order to obtain high density WB₂ ceramics,WB₂ powder was first synthesized by boro/carbothermal reduction(BCTR)process,and the reaction mechanism of WB₂ powder synthesized by BCTR was revealed,and spark plasma sintering(SPS)method was used to densify the synthesized WB₂ powder in this study.Also,the effects of processing parameters such as the type of carbon source,milling time,raw material ratio,powder synthesis temperature,and heat preservation temperature(sintering temperature)on the phase composition,microstructure,and density of WB₂ powder or bulk were studied,and ultimately the optimum processing parameters were determined.In addition,W_0.95TM_0.05B₂(TM=Ta,Nb,Mo,or Cr)solid solution powders were synthesized in situ by adding the third elemental dopant,and the sinterability of the powder was further improved.It could then facilitate the process of applying WB₂ to various industrial fields.The main contents of this research are listed as follows:(1)WB₂ powder was first synthesized by BCTR process successfully,and the reaction mechanism of preparing WB₂ powder by BCTR was revealed,and the synthesized powder was sintered by SPS method.For WB₂ powders synthesized by BCTR process,the particle size of WB₂ powders synthesized from carbon black and graphite is 0.29±0.08?m and0.53±0.26?m,respectively.The two powders and their corresponding bulks are mainly WB₂-WB₂(p6₃/mmc)and contain impurities,such as C,B₂O₃,and WO_3-x.The oxygen content of WB₂ powder synthesized from carbon black and graphite is 1.65 wt.%and 2.04 wt.%,respectively.Between them,the WB₂ powder synthesized with carbon black as carbon source offers the advantages of small mean particle size and narrow particle size distribution,and the grain size of sintered WB₂ ceramics is about 0.5-1.5?m.The densities of WB₂ ceramics synthesized with carbon black and graphite as carbon sources are 91%and 87%,respectively.The vickers hardness of WB₂ bulk prepared with carbon black as carbon source is 24.4±0.4GPa(4.9 N),which is higher than those values reported in open literatures(under non-high pressure),and close to the hardness value(25.5 GPa)of WB₂ prepared under high temperature and high pressure.(2)Some problems existed during the synthesis of powder and sintering(such as low production efficiency,a small amount of impurities in the synthesized powder,density need to be improved,etc.)were effectively optimized next.The effects of different milling time,raw material ratio,synthesis temperature,and heat preservation temperature on the phase composition,microstructure,and density of WB₂ powder and bulk were investigated and compared.The results show that the shorter ball milling time will not have an apparent effect on the phase composition of the synthesized powder,which is beneficial to improve the production efficiency.Compared with WO₃:B₄C:C=2:1.1:5,the impurity content clearly decreased and the density of the prepared WB₂ bulk was increased when the ratio was adjusted to 2:1.075:4.9.The optimum processing parameters are as follows:carbon black as carbon source,ball milling for 1h,raw material molar ratio WO₃:B₄C:C=2:1.075:4.9,powder synthesis temperature 1400?,and heat preservation temperature 1600?.The highest density achieved is 93.0%,and the grain size is 0.91±0.29?m under these processing parameters.This processing condition can not only remove the residual impurities in the synthesized powder and ensure the high purity of WB₂ phase,but also improve the production efficiency.(3)Based on the optimized processing conditions,W_0.95TM_0.05B₂(TM=Ta,Nb,Mo,Cr)solid solution powders were synthesized in situ by adding the third elemental dopant.The effects of different doping elements on the phase composition,microstructure,and density of WB₂ solid solution powders and bulks were systematically studied.The measurements of Vickers hardness,fracture toughness,thermal conductivity,and electrical conductivity were then performed.Results show that the density of W_0.95Cr_0.05B₂ and W_0.95Cr_0.05B₂ bulks is as high as 95.9%and 96.2%respectively,which is the highest density ever reported(under non-high pressure conditions).The highest density obtained in this study is also close to the density of WB₂ ceramic prepared under high temperature and high pressure(97.86%).