Nondestructive Detection And Quantitative Characterization Of Interface Defects In Thermal Barrier Coatings By Infrared Thermography

The application of thermal barrier coatings(TBCs)can effectively improve the temperature-bearing capacity of high temperature components in aircraft engines.However,TBCs have complex multi-layer structures and its service environment is extremely harsh,which make the coating extremely prone to interface defects such as debonding and buckling.The further expansion of these defects will lead to spalling failure of TBCs,which seriously threaten the safe application of TBCs on turbine blades.Therefore,the researches on quantitative characterization of the size and location of internal defects in the TBCs are essential to reduce accident losses of aero engines.Long pulsed thermography(LPT)is a safe,reliable,economic and contactless nondestructive testing method,which can effectively characterize the state of internal defects in materials.The topic was based on LPT to detect the samples of atmospheric plasma spraying(APS)TBCs containing prefabricated flat bottom holes defects and interface defects.Image processing techniques such as principal component analysis(PCA)and fitting background reduction were used to reduce the noise,and the influence of uneven heating in the thermal infrared images was removed.A onedimensional heat conduction model of the TBCs with interfacial defects was established.By comparing the analytical solutions of the temperature distribution at different size defects and the temperature distribution measured by experiments,the quantitative relationship between the size parameters of model and the actual sizes of the defects were obtained.Quantitative characterization of the diameters of the interface defects were achieved by the temperature distribution of the defects in the infrared thermal image.The main researches include the following:(1)The coating was sprayed on the surface of the substrate by atmospheric plasma spraying(APS),and flat bottom holes defects with different diameters were prefabricated on the bottom surface of the substrate by electrical discharge machining;Infrared nondestructive testing of flat bottom holes defects in TBCs were carried out,and a background reduction method for image noise reduction was proposed;Then the Gaussian function was fitted to the temperature data of the defects on the surface of the coating,and the quantitative relationship between the fitting parameters and the diameters of the defects were obtained,the quantitative characterization of the flat bottom hole defect of TBCs with a diameter as low as 4 mm was achieved.(2)Using electrical discharge machining to prefabricate cylindrical pits with different sizes on the surface of the substrate,filled the pits with brass to complete the prefabrication of interface defects,and the coating was sprayed on the surface by APS.Infrared nondestructive testing of interface defects in TBCs was carried out,an image which contained most of the temperature information was extracted from infrared thermal images by PCA.Furthermore,the extracted image was processed by the image algorithm of fitting background subtraction,which could eliminate the influence of uneven heating of the excitation source on defect identification in the infrared thermal images.The studies found that when the defect size was greater than 6 mm,the temperature curve at the defect had a platform characteristic,and the platform width decreases as the defect size decreases.When the defect size was less than 6 mm,the platform disappears and the temperature curve at the defect was arched.(3)A one-dimensional heat conduction model of TBCs with interfacial defects was constructed.By comparing the analytical solution of the temperature distribution of TBCs with defects of different sizes and the results of experimentally measured temperature distribution,the quantitative relationship between the size parameters of the model and the size of defects was obtained.Then,by comparing the analytical solution of the model with the temperature data of a defect of unknown size,the fitting parameter of this defect was determined;Combined with the quantitative relationship between the model size parameters and the defect size,the quantitative characterization of the interface defect of TBCs with a diameter as low as 2 mm was achieved.