Preparation And Properties Of ZrB₂-SiC Based Protective Coatings Against High/ultrahigh Temperature Oxidation Of Graphite Materials

With the development of aerospace technology,the service condition of aircraft becomes more and more severe.To serve in the ultrahigh temperature environment(over 1600?)for a long time(more than 1800 s),advanced ultrahigh temperature structural materials are needed.Carbon materials are considered to be ideal ultrahigh temperature structural materials because of their high melting point,low density,good thermal shock resistance,and excellent high-temperature mechanical properties.However,the oxidation/ablation resistance of carbon materials in a high temperature oxygen-containing environment is very poor,which severely limits their practical applications in the aerospace field.It's greatly urgent to improve the oxidation/ablation resistance of carbon materials.It has been well known that applying protective coatings is an effective and practical approach to achieve such purpose.With the rapid development of hypersonic vehicles,the sharp structures of nose cones and leading edges lead to rapid temperature rise during flight.Vehicles also suffer complex stress and thermal shock during the re-entry process.New challenges appear for thermal protective coatings due to the severe ultrahigh temperature environment.To apply carbon materials to hypersonic vehicles,it is urgent to develop a protective coating with good oxidation/ablation resistance,as well as thermal shock resistance in ultrahigh temperature environments.The protective performances of the protective coatings on carbon materials are determined mainly by their compositions and microstructures.In this work,high strength graphite was used as substrate material,and a SiC transition layer was prepared on graphite by siliconizing,finally,a ZrB2-SiC based coating was prepared on the siliconized graphite.To improve the protective performances of the ZrB2-SiC based coating,the composition of the coating was modified by transition metal silicide and rare earth oxide,and the microstructure of the coating was modified by various coating preparation methods.A TaSi2 modified ZrB2-SiC coating was prepared on siliconized graphite by slurry brushing.The oxidation resistance of the as-prepared coating at 1700? in air was investigated in detail.The results showed that the addition of TaSi2 promoted the formation of an oxide scale with low oxygen permeability,which is beneficial to the ultrahigh temperature oxidation resistance of the ZrB2-SiC coating.Because of the volatilization of oxidation products formed during the oxidation of coatings,it's more suitable to evaluate the oxidation resistance of the coatings based on the consumption of the coating rather than the weight gain of the coating.A dense ZrB2-SiC-TaSi2-Si coating was prepared on siliconized graphite by slurry brushing and subsequent vapor silicon infiltration(VSI).The oxidation resistance of the as-prepared coating at 1500? in air for a long time and its thermal shock resistance during thermal cycling between 1500? and room temperature were investigated in detail.The results showed that the physical defects in the pristine coating system were filled with silicon during vapor silicon infiltration.The good oxidation resistance and thermal shock resistance of the coating were attributed to the presence of Si,which decreased the short-way channels for the inward diffusion of oxygen,reduced the integral coefficient of thermal expansion(CTE)of the coating,increased the compactness,adhesion and cohesion strength of the coating system,and acted as the self-healing agent.A La2O3 modified ZrB2-SiC coating was prepared on siliconized graphite by slurry brushing and subsequent pack cementation.The as-prepared coating exhibited a double-layer structure(ZrB2-SiC-La2O3 inner layer and SiC outer layer).The oxidation resistance of the as-prepared coating at 1800? in air and its thermal shock resistance during thermal cycling between 1500? and room temperature were investigated in detail.The results showed that the addition of La2O3 promoted the formation of a continuous oxide scale with low oxygen permeability,which is beneficial to the ultrahigh temperature oxidation resistance of the ZrB2-SiC coating.And the addition of La2O3 reduced the thermal stress in the coating during thermal shock,which is beneficial to the thermal shock resistance of the coating.The oxidation resistance of the double-lyer ZrB2-SiC-La2O3/SiC coating at 1800? in air with low pressures was investigated in detail.The results showed that with the decrease of air pressure,the oxidation resistance of the coating became worse.With the decrease of air pressure,the transformation from passive to active oxidation for SiC occurred,the volatilization of SiO was intensified,and the oxide layer formed on the coating surface changed from continuous,stable and protective to discontinuous and porous.The ablation resistance of the double-lyer ZrB2-SiC-La2O3/SiC coating under different ablation conditions was investigated in detail.The results showed that the effective protection time of the SiC outer layer was shortened with the increase of ablation temperature,and the further ablation resistance was provided by the ZrB2-SiC-La2O3 inner layer.With the increase of temperature,the vapor pressure of SiO2 increased,and the viscosity of SiO2 decreased.High vapor pressure resulted in the severe volatilization of SiO2.Low viscosity SiO2 was scoured away easily by flame.