Preparation Of Tungsten Boride And Regulation Of Its Composition,Structure And Properties

The transition metal borides are widely used in cutting tools,abrasives,wear-resistant layers,geological drilling,oil exploitation,etc.,owing to their high hardness,high rigidity and good chemical inertness,and.It is found that the high hardness of transition metal boride could be attributed to the three-dimensional network structure constructured by short and strong metal-boron bond and boron-boron bond.Therefore,by increasing the concentration of boron in such transition metal borides crystal lattices to obtain high hardness materials has become a research direction for material researchers.However,most transition metals can only form low borides,while tungsten is one of the few transition metals that can form high borides.In the present work,WB₂powder was synthesized by high-energy ball miling.The as-synthesize powders were then consolidated by using pressureless sintering,hot pressing sintering and spark plasma sintering,respectively.Phase composition,element distribution,microstructure,and hardness of the samples were studied by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,transmission electron microscopy（TEM）,and Vickers hardness tester.The density of the bulk material was measured by boiling.Effects of high energy ball milling time,W-B molar ratio and other factors on phase composition of the end-product were studied;the sintering process,phase composition and mechanical properties of as-sintered WB₂bulk samples with or without the addition of Mg O were compared;the relatiohship between B content in the raw materials and decomposition temperature WB₄was built;and the effects of doping element on thermal stability of WB₄was investigated.The main conclusions can be drawn as follows:The WB₂powders synthesized by high-energy ball milling has a Al B₂（P6/mmm）structure.After heat treatment the Al B₂-type WB₂transformed into the WB₂（P63/mmc）structure.With 5wt.%Mg O as a sintering aid,the WB₂bulk samples sintered by SPS has increased density and decreased grain size while compared with pure WB₂,which therefore increases the hardness of the sample.With the increase of B content in the raw material,the decomposition temperature of WB₄is also increased gradually.But when the molar ratio W:B≧1:9,the decomposition temperature of WB₄cannot be increased by the increase of B content anymore.However,the content of WB₄in the sample gradually is increased from 81%to 97%with the increase of B content.With 5 at.%and 10 at.%of Re,10 at.%of Cr and 3 at.%of Mo doping,the thermal stability of WB₄can be imcreased to some extent,but 2 at.%Ta doping has no effect on the thermal stability of WB₄.