Experimental Measurement Of Elastic Modulus And Residual Stress Of Thermal Barrier Coatings

Thermal barrier coatings(TBCs)have been widely used as high temperature thermal protection materials in high temperature parts of turbine engines.The elastic modulus of the thermal barrier coating is an important parameter to characterize its mechanical properties,and residual stress of thermal barrier coatings has a negligible effect on its serviceability.Therefore,study the elastic modulus and residual stress of the thermal barrier coating has an important meaning.Thermal barrier coating is porous brittle film material,measurement of its elastic modulus and residual stress will face great challenges.Based on the three-point bending and four-point bending experiments,combined with the non-contact optical deformation measurement method: digital image correlation method,the elastic modulus and residual stress test characterization of plasma-sprayed thermal barrier coatings were studied.The main research contents and conclusions are as follows:(1)Based on the three-point bending experiment,the theoretical calculation formulas of the elastic modulus and residual stress of the coating are established respectively.Based on the four-point bending experiment,the theoretical calculation formula of the elastic modulus and residual stress of the coating is established,combined with the digital image correlation method,the initial elastic modulus and residual stress value of the coating after preparation,and the change trend of the elastic modulus and the residual stress after the high temperature heat treatment are obtained.(2)In the process of measuring the elastic modulus of the coating by three-point bending,if the initial curvature is not considered,a large measurement error will be generated;the full-field displacement measurement method based on the digital image correlation method has higher accuracy and reliability than the midpoint displacement measurement method;the four-point bending test can simultaneously determine the elastic modulus and residual stress of the coating,which is more convenient and faster than the three-point bending.(3)The experimental results show that the elastic modulus of the coating under pressure is greater than that at the time of tension,and the initial elastic modulus is between 44 GPa and 54 GPa,and the elastic modulus increases rapidly after high temperature heat treatment.It gradually becomes gentle;the initial residual stress of the coating is compressive stress,which quickly becomes tensile stress after heat treatment,and tends to be gentle with the heat treatment time;the experimental results of threepoint bending and four-point bending are basically the same.(4)Based on the scanning electron micrograph of the coating section,a finite element calculation model containing pores and microcracks was established,and the influence of microstructure on the elastic modulus of the coating was simulated.The results show that the porosity and microcrack density of the coating decrease rapidly with the increase of heat treatment time,and then gradually become gentle.The elastic modulus of the coating increases rapidly with the heat treatment time,and then gradually becomes gentle,which is consistent with the experimental results.Therefore,the pores and microcracks inside the coating are the main factors whose elastic modulus changes with the heat treatment time.