Experimental Research On The Wetting And Penetrating Performance Characterization Of CMAS At High Temperature And Protection Of Thermal Barrier Coating

Thermal Barrier coatings(TBCs)have many excellent properties such as high heat insulation,high temperature and corrosion resistance,leading to the path for TBCs to become indispensable on the thermal applications of aeroengines,like turbine blade.However,when TBCs is served in the enviroment full of high speed gas containing impurity particles at high temperature,it may peel off due to interfacial oxidation,particle erosion or CMAS(the particle with the composition of calcium,magnesium,aluminum and silicon)corrosion.Among them,CMAS corrosion is considered as the most dangerous problem attributing to coatings peeling since the mixed oxides of calcium,magnesium,aluminum and silicon would melt at high temperature and then permeate into the coatings,causing the changes in structures,properties and composition of coatings rapidly.Obviously,the parameters like contact angle,wettability and the infiltrability of CMAS into coatings are the key parameters to evaluate its corrosion,which is also the basis to design the CMAS resist coatings.However,the latest research on wetting and infiltration behavior of molten CMAS is not in depth.There are many critical problems,such as the characteristic difficulty in high temperature contact angle and wettability,the lack of wettability characterization devices,the unclear of the infiltration rules and mechanisms,the unknow of the key factors of wettability and unclear optimization methods.Therefore,in this thesis,the contacting angle at high temperature was innovatively proposed as an important parameter to describe the wettability of CMAS.Starting from the independent development of the high temperature contact angle measuring device,combined with the characterization means such as scanning electron microscopy,energy dispersive spectrometer,as well as the analysis of phase diagrame and fractal theory,The wettability and infiltrality of CMAS on thermal barrier coatings and its key influencing factors were systematically analyzed.Based on this,a CMAS resisted coatings modified by Al2O3 was proposed.The main innovative achievements are as follows:(1)Based on the heating system for CMAS melting and dropping,the imaging system for CMAS wetting process recording,and the design of the special sample fixture at high temperature,a CMAS contact angle measuring device was developed independently,which Solved the problem of measuring quasi-static dropping of molten CMAS droplets with high viscosity and melting point,and realized the high-precision imaging of the wetting process of CMAS on coatings surface.Furthermore,a software system for calculating the contact angle was also developed.This device provides an important experimental platform for characterizing the wettability for thermal barrier coatings and other materials at high temperature.(2)According to the device mentioned above,the relationship between the contacting angles and wettability of CMAS and coatings process,roughness,viscosity,time and other parameters were investigated.The results show that the contacting angles of CMAS increase with the increasing of the surface roughness,moreover it's proportional to the square of the surface roughness,which indicates that increasing the roughness of the coatings surface could slow down the wetting process.In addition,the viscosity of molten CMAS has a great effect on the contacting angles near its melting point,and there is a positive correlation between them.Furthermore,when the surface morphology of the coatings is vertically arranged,the CMAS tends to infiltrate into the coatings,reducing the contacting angles.But when the pores on the coatings surface are distributed at random,the contacting angles of CMAS are relatively large.(3)Based on the fractal theory,a theoretical model of the penetration depth of CMAS into TBCs was established,combining with the experiments,the relationship between penetration depth and parameters of coatings and CMAS was built up.The results show that the infiltration process of molten CMAS conforms to the Gauss diffusion law,the temperature has the greatest influence on the penetration depth of CMAS,on the contrary,the surface roughness has the weak effect factor.Additionally,there is a critical saturation of CMAS during the infiltration process.When the CMAS in the coatings is less than the critical value,the infiltration depth is greatly affected.(4)Baesd on the change of the melting point and the penetration depth of CMAS,and in consist with the phase diagram calculation,one kind of Al2O3 modified YSZ coatings which show better thermodynamic properties was designed.The results show that adding 40 mol%Al2O3 into YSZ could increase the melting temperature of CMAS in the coatings,accelerating the formation of stable anorthite,thus increase the contacting angle of CMAS.Besides,the thermal conductivity of YSZ modified by 40 mol%Al2O3 increase and the thermal insulation effect of the coatings fall down,but the mechanical properties such as Young's modulus,hardness and fracture toughness increase as well,which improving the spalling resistance of the coatings.(5)Based on the composition designing of the modified YSZ coatings and EB-PVD method,a thermal barrier coatings with CMAS wetting and infiltrating resistance is obtained and the properties of the modified YSZ coatings are also characterized.The results show that the addition of Al2O3 is in favor of the formation of anorthite(CaAl2Si2Os),hence resist the continuing CMAS wetting and infiltrating.In summary,the preparation and performance characterization provide a significant guide for the practical application of the Al2O3 modified YSZ coatings.