Ultrasonic Characterization For Surface Coatings Using Pressure Reflection Coefficient Spectrum Of Bulk Wave

The investigation on the ultrasonic testing and characterization of surface coatings is significant to the development and application of the coatings technique. At present, there are two major problems on the immersion ultrasonic pulse echo method used to test and characterize the properties of the coatings. (1) The coating is so thin (10-8-10-7m) that the ultrasonic echoes reflected from the front and back surfaces of the coating overlap. (2) For the inhomogeneous coating, ultrasonic wave occurs complex scattering and dispersion when it propagates in such kinds of coating containing the random pores and microcracks.To overcome the problem mentioned above, the systematical work is carried out from the views of the theoretical model, signal processing, numerical simulation and experimental tests. For the problem (1), a water/coating/substrate model is established for the homogeneous coating. According to the interference caused by the reflection waves from the interfaces of water/coating and coating/substrate, the thickness and density of the coatings are determined by the ultrasonic reflection coefficient amplitude and phase spectrum (URCAS & URCPS). Then, the velocity and thickness are determined simultaneously by introducing the inversion technique. Meanwhile, the variation of URCAS extreme is very sensitive to the changes of acoustic impedance match between the coating and its adjacent medium, which can be used to test flaws of the interface, such as disbonding and the existence of TGO (thermally grown oxide).For the problem (2), the influence of inhomogeneous microstructure on propagation of ultrasonic wave is investigated from the view of experiment and numerical simulation. It is found that dispersion phenomenon occurs when the ultrasonic signals transmits in the inhomogeneous coating. Accordingly, the constant forms of longitudinal velocity and attenuation coefficient in the expressions of URCAS and URCPS of homogeneous coating are modified as the frequency dependent form, so the expressions of URCAS and URCPS of inhomogeneous coating are obtained. Then the resonant frequency deviation coefficient (RFDC) is introduced to quantify the degree of dispersion, which provides the basis on the ultrasonic characterization of the inhomogeneous coating.Based on the model and the principle above, ultrasonic testing and characterization of the homogeneous aluminum coating, epoxy coating and inhomogeneous TBC are carried out in this work. The major results are as following:1. The thickness of aluminum coating on PVC substrate (404-425μm) is measured by URCAS, and range of absolute error between ultrasonic method and metallographic observations is±7μm, the range of relative error±2%. The density of aluminum coating is determined by the relation of URCPS extreme and coating density, and the relative error maximum of measurement results between ultrasonic and Archimedes method is 5.8%.2. The ultrasonic longitudinal velocity, density and thickness of homogeneous epoxy coating (277μm) on the aluminum substrate are characterized by the combination of URCAS and inverse method. The relative errors between the velocity and thickness obtained by two parameters inversion based on Gauss-Newton method and "reference value" are 2.67% and 2.35%, respectively.3. The specimens of the aluminum layer/epoxy bonding layer/glass substrate system are tested according to the influence of acoustic impedance match relation between the coating and its adjacent medium on URCAS. Both of the experimental URCAS of disbonding and perfect specimen are obtained, and it is found that the regularity of the experimental URCAS is consistent with the theoretical prediction.4. Ultrasonic testing is carried out on the TBC system (YSZ coating/NiCoCrAlY bonding layer/GH33 substrate) before and after aging under the conditions of 1050℃×1 h after 15 cycles. TGO (main component:Al2O3) with thickness of about 3μm generates between the YSZ coating and NiCoCrAlY bond layer due to the oxidation, which changes acoustic impedance match relations between the coating and its adjacent medium. This makes the law of URCAS extreme occurring of the coating before and after aging reverse. The experimental results agree well with the theoretical prediction, which shows the present method can be used to test TGO in the TBC system.5. The inhomogeneous coating physical simulation model (ICPSM) is proposed to describe the inhomogeneous coating prepared by plasma sprayed and electron beam physical vapor deposition (EB-PVD) based on the random medium theory and statics method. Then the simulation on the ultrasonic propagation in the ICPSM is fulfilled by the FDTD (Finite-difference time-domain) method. It is found that RFDC increase with the increase of coating porosity, which shows that the dispersion degree increases with the increase of inhomogeneous degree of the coating. The constant forms of velocity and attenuation coefficient in the expressions of URCAS and URCPS of homogeneous coating are modified as the frequency dependent form. Then the expressions of URCAS and URCPS of inhomogeneous coating are obtained first, which are particularly suitable to ultrasonic characterization of inhomogeneous coating.6. The thickness of YSZ coating (256-330μm) on 1Cr18Ni9Ti substrate is measured by URCAS, and the range of absolute error between ultrasonic method and metallographic observations is±10μm, the range of relative error±3%. 7. The density of Cr2O3 coating is determined by the relation of URCPS extreme and coating density. It is shown the microstructure of the Cr2O3 coating is inhomogeneous, and the ultrasonic measurement results agree well with SEM observation. Similarly, the density of YSZ coating the prepared by EB-PVD method is determined by the relation of URCPS extreme and coating density, and the relative error maximum of measurement results between ultrasonic and Archimedes method is 4.59%.8. The ranges of RFDC of as-deposited TBC and irradiated TBC by high-intensity pulsed ion beam (HIPIB) with beam densities 100 and 300 A/cm2 are determined by URCPS, and they are 0.80-0.85,0.85-0.88 and 0.91-0.93, respectively. Further, the attenuation coefficients of three kinds of coating are obtained by numerical fitting technique. The attenuation coefficient decreases with increase of beam density. These results indicate that the coating density and uniformity have increased after irradiation and those of coating after 300 A/cm2 irradiation increases most significantly. The ultrasonic results are in agreement with the SEM observation.