Effect Of Microstructure On Radiation Properties Of Thermal Barrier Coatings

Thermal barrier coatings(TBCs)are widely used in advanced turbine blades owing to low thermal conductivity,excellent thermal shock resistance and favorable erosion resistant properties.However,the YSZ material consisting of TBCs is translucent over a broad spectral region where thermal radiation tends to be concentrated;therefore,the investigations on radiative properties of TBCs receive extensive attenation.In particular,as the temperature increases,thermal radiation energy and its contribution to total heat flux will significantly increase.Hence,carrying on a research about radiative performance of TBCs can provide an important theoretical guidance for the coating design as well as its practical applications.A systematic study that involves experimental measurements and theoretical analysis is carried out to investigate the radiative properties of thermal barrier coatings(TBC).For the experiment,two types of typical TBC are prepared.The microstructures of the APS and the EB-PVD TBC are observed using scanning electron microscopy.The morphologies of TBC with different microstructures are acquired and recorded.Moreover,a parameterization procedure is developed.An experimental system is set up to investigate the effects of the TBC microstructure on its radiative properties.The spectral radiative properties of TBC(reflectance,transmittance,and absorption)are obtained using UV/Vis/NIR and FTIR Mid-IR spectrometers,and the correlation between the microstructure and the radiative properties of TBC are further explored.The results indicate that distinctive micro nanostructures of TBC have a significant effect on its radiative properties.In particular,the large grain boundary and pore architecture(i.e.,size,morphology,and distribution)strongly affect spectral reflectance and transmittance.In theoretical research,based on the experimental results,the key radiative parameters(i.e.,scattering coefficient and absorption coefficient)are obtained using the radiative identification model.Based on the precious spectral scattering and absorption coefficient,the radiative heat flux as well as the temperature distribution across the TBC is computed by adopting an improved coupling thermal conduction and multispectral radiation two flux model,and the proportion of radiative flux accounted for the total flux is evaluated.The theoretical analysis indicates that the effects of the microstructure of TBC on its radiative properties mainly include scattering behavior caused by these defects.The anisotropic scattering effects caused by the grain boundary and the pores strongly influence the radiative transfer across the coatings.In order to explore the radiation transfer mechanisms inside the coatings,a new model based on spheroidal pore scattering is established through careful analysis of the microstructures.This model can predict the effects of pore size,morphology,and arrangement on the radiative properties of TBC.The study indicates that the radiative properties of APS TBC are strongly affected by pores and cracks parallel to the substrate.Compared with spherical pores of the same volume,parallel cracks can significantly increase the scattering section,which reduces the cross section of the radiative heat flux.Further,by optimizing pore morphology,the radiative flux and heat insulation performance can be improved.Based on the abovementioned investigation results,a new type of APS TBC with a gradient porosity is designed by optimizing the microstructure of traditional coatings to provide guidance for practical coating applications.The results showed that TBC with a gradient porosity improve spectral reflectance and reduce transmittance.Model analysis indicated that TBC with gradient porosity increase the scattering coefficient in the short wavelength,thereby reducing the radiative heat flux across the coatings.Experimental results show the feasibility of microstructural optimization.In order to investigate the effects of microstructures on the radiative properties of TBC comprehensively,the radiative properties of EB-PVD TBC are studied by combining experimental measurement and theoretical analysis,and the correlation between the scattering coefficient and the columnar microstructure is determined.In addition,a method of finite-difference-time-domain is employed to investigate radiation transfer mechanisms.This study indicated that the radiative properties of EB-PVD TBC are closely related to the unique feathery microstructure as well as nanoscale closed pores inside the column.Increasing the inclination angle of the feathery microstructures as well as the closed porosity in the main column can dramatically improve the scattering coefficient,and thus reduce the radiative flux across the coatings.Comprehensive experimental and theoretical studies found that the micro/nanostructure of TBC have a significant influence on its radiative properties,and by optimizing the microstructure of coatings to enhance its backscattering effects,the scattering coefficient can be effectively increased and the radiative heat flow and substrate temperature can be reduced,thereby providing a better thermal protection for the turbine blade.