Preparation Of ZrB₂@zr SiO₄ Composites Using Sol-gel Method And Its Oxidation Resistance

Zirconium diboride（ZrB₂）is widely used in refractory materials and composite materials and other aspects,due to its high melting point,high strength and high thermal conductivity and many other advantages.However,one of the key problems that limits the application of ZrB₂ in high temperature oxidation conditions is its poor oxidation resistance property.The studies show that the oxidation resistance of ZrB₂ can be improved to a large extent by introducing the second phase,mainly by forming a dense glass layer or being coated with a layer of antioxidant powder on the surface.In order to combine the advantages of both,coated ZrSiO₄-ZrB₂ powders were selected to improve the oxidation resistance of ZrB2 in this study.Nowadays the researches on the preparation of ZrSiO₄ by sol-gel method have been mature,while the preparation of ZrSiO₄ by sol-gel method for coating is comparatively weak.In this paper,coated ZrSiO₄-ZrB₂ powders were prepared by sol-gel method.Tetraethoxysilane and zirconium oxychloride octahydrate were selected as raw materials.The hydrolysis products were coated on the surface of ZrB₂ powder during sol-gel process,and then the gel was dried and calcined.The samples obtained were characterized by XRD,SEM,TG-DSC and FTIR.Firstly,the effects of different technological parameters on ZrB₂@ZrSiO₄composites were discussed respectively.ZrB₂@ZrSiO₄ composites with 0.2μm ZrSiO₄ particles were prepared when n_ZrSiO4/n_ZrB2 is 0.7,n_Si/n_Zr is 1.4,hydrolysis pH of tetraethoxysilane is 9,heating rate is 5^oC/min,calcination temperature is 1100^oC and holding time is 2h,the samples were tested for oxidation resistance in the air atmosphere,the results showed that the mass of the coated powder began to increase slowly about 900^oC,and the weight gain rate of oxidation was only 0.88%.In previous preparing pure ZrSiO4 by sol-gel method,nSi/nZr is 1.2,but it is necessary to add more silicon raw materials to neutralize ZrO₂ in this experiment,which is mainly due to the formation of glass phase containing SiO2 and B2O3.After exploring the sol-gel reaction conditions and the calcination conditions,ZrB₂ powders were modified to obtain ZrB₂ modified powders of 36%HCl-3h and18%HCl-30min,three different ZrB₂@ZrSiO₄ composites were prepared at the same conditions.Comparing with unmodified ZrB2 powder,the particle size of ZrSiO4 in ZrB₂@ZrSiO₄ composites obtained by 36%HCl-3h modified powder was also 0.2μm,but the mass fraction of ZrSiO4 relative to ZrO2 increased from 66.75%to 70.75%,when the oxidation resistance test was carried out at the same conditions,the weight gain rate of oxidation decreased from 0.88%to 0.33%.The results showed that the modified ZrB2 powder treated with 36%HCl-3h was more favorable for the formation of coated powder,and the coated powder was more complete and uniform with better oxidation resistance property.The oxidation resistance of ZrB2@ZrSiO4 composites was improved not only due to the ZrSiO4 particles coated on the surface of ZrB2 powder,but also because of the glass phase containing B₂O₃ and SiO₂.Glass phase was filled with the gap and the surface of ZrSiO4 particles,preventing the ZrB2 powder from being oxidized in the air atmosphere of lower temperature,and the ZrSiO4 particles could continue to play the certain antioxidant effect after the glass phase volatilized at high temperature,combining the effect of both,the oxidation resistance of ZrB2 powder was improved to a great extent.