Research On Terahertz Imaging Method For Debonding Defects Of Thermal Barrier Coatings

Modern aero engine engines usually spray a thermal barrier coating on the surface of turbine blades to improve the working conditions of the engine in all directions,and increase the operating temperature and thermal efficiency of the engine.In a high temperature environment,oxygen easily penetrates the porous ceramic layer on the top of the thermal barrier coating to form oxides at the interface between the ceramic layer and the metal bonding layer.As the thickness of the oxide layer increases,the tensile stress increases,which makes it easy to produce debonding defects at the interface of the bonding layer,and eventually cause the coating to fall off.Especially under the background of long-term work and long service period of aero-engines,it is very important to conduct debonding inspection of thermal barrier coatings on a regular basis.As a new type of non-destructive testing technology,terahertz imaging technology can detect the internal bonding state of the coating through subsurface imaging before the coating bulges or falls off,reducing the risk of coating failure.In this paper,focusing on the two goals of improving detection efficiency and enhancing imaging quality,the research on terahertz imaging detection methods for thermal barrier coating debonding defects has been carried out,and the following research results have been obtained:1.Aiming at the problem of long acquisition time of terahertz A-scan signal,the terahertz A-scan signal compression sampling and imaging of thermal barrier coating debonding defects were proposed.Considering that the sparse base and reconstruction algorithm greatly affected the reconstruction accuracy of compressed sensing,the performance of different compressed sensing methods was compared and analyzed,and the compressed sampling method of terahertz A-scan signal based on the DCT-OMP algorithm was determined,which improved the terahertz A Scan signal acquisition efficiency.2.Aiming at the difficulty of terahertz signal to characterize the coating’s micro debonding,a coating debonding imaging enhancement method based on stationary wavelet transform was proposed.The use of stationary wavelet transform detail components combined with threshold filtering enhanced the effective reflection peak of the terahertz signal and suppresses the echo signal.On this basis,the processed wavelet coefficients were substituted for the original signals to reconstruct the B-scan image of the coating,which improved the imaging contrast and image quality of the debonding defect.3.Aiming at the time-consuming and large amount of data for C-scan imaging of thermal barrier coatings,a method of terahertz image reconstruction based on compressed sensing was proposed.With the help of wavelet processing method,the imaging characteristics of coating debonding defects were determined,and the original C-scan image was constructed.In the next step,the performance of terahertz image reconstruction of thermal barrier coating debonding defects based on greedy OMP and TVAL3 algorithms was compared.The results showed that the debonded terahertz image of the coating reconstructed based on the TVAL3 algorithm had the advantages of high quality and fast speed.The paper has 47 pictures,8 tables,and 86 references.