Preparation And Property Study Of ZrB₂-SiC Anti-oxidation Coating

Graphite exhibits excellent heat resistance,good corrosion resistance,good electrical and thermal conductivity and other characteristics,which is often used in metallurgical,electronic,aerospace and other fields.But the oxidizing potential limits its applications,while Si C anti–oxidation coatings can effectively protect graphite at high–temperature.However,the Si O₂,generated by Si C at 1200℃,has poor fluidity and cannot fill the defects in the coating promptly,thus anti–oxidation performance of Si C coatings decreases sharply at 1200℃.Zr B₂ is often used as a second phase to modify Si C coatings for its good stability,high melting point and excellent oxidation resistance,which give Zr B₂–Si C coatings more excellent anti–oxidation properties.Zr B₂ can oxidize at 1200℃to form fluid B₂O₃,which can partially react with Si O₂ to produce borosilicate glass with lower oxygen permeability,and effectively improve the anti–oxidation performance of Zr B₂–Si C coating at 1200℃.In order to study influences of composition and morphology of the synthesized Zr B₂ powders on the anti-oxidation performance of Zr B₂–Si C coating,Zr B₂ powder was synthesized by carbon thermal reduction method with Zr O₂,B₂O₃ and C.The influence of raw material contents and sintering parameters on phase composition and microscopic morphology of the synthesized Zr B₂ powder were investigated.After that,Zr B₂,Si,C and Al₂O₃ were used as raw materials,polyvinyl butyral（PVB）–ethanol solution was used as binder and Ar gas as protective gas,Zr B₂–Si C coating was prepared on graphite surface by slurry method.The effects of raw material contents and heat treatment parameters on microstructure and anti–oxidation properties of the coatings were investigated.Finally,self–produced and purchased Zr B₂ powders were used respectively to prepare Zr B₂–Si C coatings based on exploration of the process parameters.The main research contents and results are as follows:（1）During carbon thermal reduction reaction of Zr B₂ powder,liquid B₂O₃ provides condition for rapid atomic diffusion,which facilitates Zr B₂ nucleation and deposition growth.The content of reducing agent C affects degree of the carbon thermal reaction and thus affects purity of the prepared powders.The content of zirconium source Zr O₂affects purity of the prepared powder.Sintering temperature affects sizes of the generated Zr B₂ particles by influencing degree of the reaction proceeding.Sintering time affects the dominant orientation of the powder growth,which further influences the particle size of the generated Zr B₂ particles.（2）Zr B₂–Si C coatings with 220-270μm thicknesses were prepared by slurry brushing method.Zr B₂ content in the coating could affect amount of the glass generation,as the Zr B₂ content increased from 30.0 wt%to 45.0 wt%,the oxidized coating densified and the mass loss ratios gradually decreased.The concentration of PVB–ethanol solution could affect uniformity of the coating and components contained in the coating,thus affect density of the coating.As the concentration increased from0.025 g/m L to 0.050 g/m L,the glass layer after oxidation was uniform and the mass loss ratios decreased.Sintering time could affect the crystallinity of Si C generated by in–situ reaction.As sintering time increased from 1 h to 4 h,Si C particles coarsened,and distribution of the glass phase increased in oxidized coating with the mass loss ratios gradually decreasing and then slightly increasing.（3）Zr B₂–Si C coating prepared by self–produced Zr B₂ powder was denser.The coating components had higher reactivity during oxidation,more glass phase and oxidized particles were generated.More glass phase can improve anti–oxidation performance of the coating,and more oxidized particles can improve thermal stability of the coating,while mass loss ratio at 1200℃for 120 min was-2.42%.（4）At 1200℃,Zr B₂–Si C coating oxidized to B₂O₃,Si O₂,borosilicate,Zr O₂,Zr Si O₄.B₂O₃ and borosilicate glass can flow to fill pores in the coating which left by escape gas,and form a physical barrier to isolate oxygen.Zr O₂ and Zr Si O₄ particles can improve stability of the coating at high temperature through the pinning effect.