Study On Thickness Measurement Of Thermal Barrier Coatings Using Terahertz Technology

Thermal barrier coating is an advanced high-temperature protective coating,which is a technology of spraying thermal insulation ceramic material on the surface of the turbine blade base to improve the thermal efficiency,thrust and working temperature of the engine.The structural properties such as the thickness and uniformity of the thermal barrier coating are related to the parameters of the preparation process,which directly affect the thermal insulation effect of the thermal barrier coating.With the service time increasing,the thermal barrier coating works under high temperature environment,the ceramic coating is thinned and the performance of the thermal barrier coating is poor.Therefore,the non-destructive testing and evaluation of the thickness of the thermal barrier coating is a key issue that needs to be solved urgently.Terahertz non-destructive testing technology is a new type of non-destructive testing technology.Because of its coherence,transient,and ability to penetrate ceramic layers,it is suitable for non-destructive testing of thermal barrier coatings.Therefore,a new method for non-destructive measurement of thermal barrier coating thickness based on terahertz technology is proposed,which improves the accuracy,efficiency and reliability of thermal barrier coating thickness detection.The terahertz theoretical model was deduced and established,and then combined with the time-of-flight method to detect the thickness of the thermal barrier coating.The main achievements and innovations are as follows:1.Based on the theory of the interaction between terahertz waves and multilayer samples and Rouard's method,a terahertz theoretical model for multilayer coatings is derived.Considering that the coating surface and interface are not smooth,and will cause scatter,Kirchhoff approximation was used to modify the model.The results show that the theoreticalmodel established is correct,and the detection accuracy of the model considering the roughness is higher than without considering the roughness,the relative error of measurement is within 1%.2.A thickness detection method of thermal barrier coating based on time of flight wasstudied.The effects of the measured material's refractive index,extinction coefficient,thickness and roughness on the terahertz time-domain signal were studied by simulation.According to the simulation results,a method for measuring the thickness of the thermal barrier coating by time of flight was proposed,then thermal barrier coating specimens were produced,optical parameters were extracted according to the model of refractive index extraction.Finally,the thickness and the uniformity of ceramic coating of the thermal barrier coating was measured and analyzed.The range of measurement relative error varies greatly,ranging from 0.2% to 45%.3.Furthermore,a thermal barrier coating thickness detection method based on a theoretical model was proposed,and a global optimization algorithm is used for parameter optimization.The measurement error of 1~3# thermal barrier coating specimens is 0.3%~5.3%,so the accuracy of this method is higher then the method based on time of flight.4.The method of combining terahertz theoretical model and time-of-flight was used to investigate the thickness of thermal barrier coating.First,the refractive index of the sample was predicted based on the model-based inversion algorithm,and then the thickness of the ceramic coating in the thermal barrier coating is calculated by time-of-flight,and the unevenness of the ceramic coating was evaluated.The measurement error of 1~3# thermal barrier coating specimens is 0.3%~6.2%,The results show that the combined method has higher detection accuracy than the time-of-flight method and higher detection efficiency than the terahertz theoretical model.