Microstructure Control And Performance Optimization Of The Third Element Doped Hexagonal Structure OsB₂

Transition metal borides?TMBs?,have attracted attention due to their excellent properties such as high hardness,high wear resistance,high shear strength and electrical conductivity,and the ability to be synthesized under normal pressure.TMBs are considered as potential new superhard materials with very promising application prospects in polishing,cutting tools and wear-resistant coatings,etc,.Re B₂-type Os B₂with a hexagonal structure was predicted to be a superhard material theoretically.However,the hexagonal structure Os B₂is metastable,which transfers to lower hardness orthogonal structure during the sintering process,resulting in a decrease in hardness.Therefore,improving the thermal stability of the hexagonal structure Os B₂is the key to obtain a dense Re B₂type,hexagonal structure Os B₂bulk superhard material.In the present work,firstly,the effects of third element Re,W and Ru doping on thermal stability of the Re B₂-type Os B₂were compared;sencondly,sintering aids:B₄C,Ni and Mg O were added into Os_1-xMT_xB₂for densification,the effects on each sintering aid on phase composition,microstructure and hardness of the samples were studied;finally,the effect of Mg O sintering aid adding content on phase composition,microstructure and hardness of Os_0.9Re_0.1B₂were discussed.The main research contents and conclusions are as follows:1)Using Os,MT?Re,W or Ru?and B powder as raw materials,the hexagonal structure Os_1-xMT_xB₂?MT=Re,W or Ru?powders was synthesized by mechanochemical method.The effects of doping elements,doping amount on thermal stability of hexagonal structure Os B₂were studied.The results show that under the experimental conditions,both Re and W doping can improve the thermal stability of the hexagonal structure Os B₂;the influence of Ru doping on the phase structure and stability of Os B₂is affected by the doping amount;2)The effects of three sintering aids,B₄C,Ni and Mg O,on densification,microstructure and mechanical properties of Os_1-xMT_xB₂were studied.In addition,on the basis a certain amount of sintering aids,B powder buried pressureless sintering was applied for comparison with no B powder buried sintering.The results show that the addition of Ni and Mg O can increase the density of Os_0.9Re_0.1B₂;the addition of B₄C cannot increase the density of the Os_1-xMT_xB₂?MT=Re or W?.B powder buried sintering can suppress the generation of low boride,refine the grain,and improve the mechanical properties;3)Os_0.9Re_0.1B₂ceramics were prepared using spark plasma sintering?SPS?technology with variable amount of sintering aid Mg O?0 wt.%,3 wt.%,6 wt.%And 9 wt.%?.The effects of Mg O amount on microstructure and mechanical properties of the Os_0.9Re_0.1B₂ceramics were studied.The results show that,the best comprehensive performances of the Os_0.9Re_0.1B₂sample were obtained when the amount of Mg O added is 6 wt.%among the four groups of samples:the Vickers hardness value reaches?35±2?GPa and the friction coefficient is about 0.6,Wear scar width and wear rate are 278.5?m and2.387×10^-4mm^-3/N·m,respectively.In conclusion,this paper studied the effects of the doping of the third element on thermal stability,densification,microstructure and mechanical properties of the hexagonal structure Os B₂,and discussed the influence of the sintering aid and its content,as well as the influence of the sintering process and its influence law.The experimental results obtained in this paper will provide an experimental basis for the research on finding,designing and preparing more novel transition metal compound superhard materials.