Principle And Experimental Investigation Of Ultrasonic Characterization For Thermal Barrier Coating

The investigation has been carried out on the ultrasonic charactedzaton for the thermal barrier coatings(TBC),which are prepared by the methods of plasma-sprayed and Electron Beam Physical Vapor Deposition(EB-PVD).Two major problems limit the application of ultrasonic method in characterizing TBC.One is the coating is so thin that the echoes from the front and back of the coating overlap,bringing out ultrasonic velocity and attenuation coefficient unavailable.The other is the influence of coating inhomogeneity on the ultrasonic propagation,and the existing homogeneous layered medium model can not describe the ultrasonic dispersion.The model and theory on propagating in inhomogeneous layered medium is investigated in this paper.The expressions of coating sound pressure reflection coefficient amplitude and phase are obtained,and the theoretical relation of phase extreme and coating density is established.Moreover,a new parameterγnamed dispersion factor is introduced for describing the dispersion extent in the coating quantificationally.The coating thickness, density and inhomogeneity are characterized in this paper,and the main results are as following:(1).The thickness of ZrO₂-8%Y₂O₃ coating prepared by the plasma-sprayed method is measured by the sound pressure reflection coefficient phase spectrum,and the thickness range is 318-337μm which is according to the metallographic analysis.(2).Based on the relation of phase extreme and coating density,qualitative analysis on the densities at different location in the coating is fulfilled.The result shows the dense extent of pore and microcrack are different and the coating is inhomogeneous.(3) The density of ZrO₂-7%^Y₂O₃ coating prepared by EB-PVD method is measured,and the coating density is 4.79 g/cm³.The error is about 5.8%,relative to the value obtained by Archimedeam method.(4) Surface modification on the Cr₂O₃ and ZrO₂-7%Y₂O₃ coating is accomplished by High-Intensity Pulsed Ion Beams(HIPIB) in order to obtain the coatings containing the pore, microcrack and density of different levels.The dispersion in the Cr₂O₃ and ZrO₂-7%Y₂O₃ coatings is characterized by sound pressure reflection coefficient amplitude and phase spectrum,and the attenuation coefficients of the coatings are determined by numerical fitting technique.For the as-sprayed and irradiated Cr₂O₃ coming with 1 and 5 shots,the dispersion factor and attenuation coefficient reduce,which shows the inhomogeneous level of the coating has a decrease.The SEM observations are in agreement with the ultrasonic characterization.The similar result has been found for the as-deposited and irradiated ZrO₂-7%Y₂O₃ coating with 1 and 5 shots.