Preparation Of Yttrium Silicate Environmental Barrier Coatings By Slurry Process

Silicon-based structural ceramics possess good high-temperature stability,which makes them a promising hot-section component material for aeroengine.However,silicon-based structural ceramics suffer corrosion from water vapor and silicate deposits,and lead to the failure of the components,which seriously restricts the application of ceramics in high temperature combustion environment.Therefore,it is necessary apply environmental barrier coating on silicon-based ceramic structural components.Environmental barrier coating materials used in extreme environments need to meet following requirements,including:(1)thermal expansion coefficient matching with ceramic substrate;(2)good oxidation resistance and corrosion resistance;(3)low thermal conductivity;(4)good high temperature stability.Environmental barrier coatings have undergone three generations of development up to now.At present,?-Y2Si2O7 and Y2SiO5 are considered as candidate materials for environmental barrier coatings with great research prospects.However,the current research focus on processing of rare earth silicate environmental barrier coatings.In this paper,two research routes have been launched parallelly.In Route 1,Y2SiO5 nano-powder were prepared by sol-gel method.The effects of raw material composition,catalyst and sintering temperature on particle size and phase composition of powders were studied.The Y2SiO5 coating skeleton prepared by spin-coating.Moreover,Y2SiO5 nano-powder slurry with good dispersion was prepared with optimized parameters.The influence of solid content and dropping amount of Y2SiO5 nano-powder slurry on the pre-coated Y2SiO5 skeleton was discussed.Furthermore,the optimum parameters of compact,uniform and continuous Y2SiO5 coating prepared by rotating coating method were obtained.In Route 2,based on the in-situ reaction mechanism and the principle of dipping-drawing process,the effects of the reaction system and process conditions on the phase composition and micro-morphology of Y2Si2O7 coating were studied.By adjusting the proportion of raw materials,acidity,alkalinity,solid content,the selection and dosage of additives,the optimum preparation parameters of suspension stability and rheology of slurry were achieved.Moreover,the process parameters of dipping and drawing,solidification and sintering were optimized,and the preparation process of dense,uniform and continuous Y2Si2O7 coating by in-situ reaction of dipping and drawing was explored.The main results are as follows:1.The Y2SiO5 nano-particles with high purity were successfully prepared by sol-gel method,with an average particle size of 100-200nm.Through the analysis and optimization of the phase composition and particle size of the powder obtained by sol-gel method,the optimum ratio of raw material six hydrated yttrium nitrate and four tetraethyl orthosilicate is 2:1,the best ratio of solvent ethyl alcohol and four tetraethyl orthosilicate is 7:1,the optimum dosage of catalyst concentrated hydrochloric acid(mass fraction 36?38%)is 0.08mL,and the best sintering temperature is 1300?.2.Y2SiO5 coating was successfully prepared by twice-rotating coating method.The density of Y2SiO5 coating can reach more than 90%.By comparing the micro-morphology and calculating the porosity of Y2SiO5 coating,the optimum solid content of nano-powder slurry used by twice-rotating coating method is 30vol%,and the optimum dropping coating amount is 400?L.3.Pure phase YZSi2O7 was successfully prepared by in-situ reaction method.The optimum ratio of raw material Y2SiO5 to silica sol was 1:1.06,the optimum acidity and basicity of mixed slurry was pH=9,and the optimum in-situ reaction sintering temperature was 1550?.4.Y2Si2O7 coatings with dense,continuous and homogeneous pure phase were successfully prepared by in situ reaction method of dipping coating.The optimum solid content of slurry was 24 wt%,the optimum amount of binder was 4 wt%of dry powder Y2SiO5,the optimum pulling speed of dipping and pulling process was 6000 ?m/s,the optimum dipping time was 60s,and the optimum curing method was freeze-curing.The optimum sintering process is 1550? holding for 3 hours(heating rate is 5?/min).