A high-frequency ultrasonic approach for testing and evaluating sprayed coating thickness is proposed in this paper. This technique is based on the maximum frequency interval method of the magnitude spectrum of the acoustic pressure reflection coefficient that adopts Welch spectrum estimation. The acoustic propagation model was set up at normal incidence, and the relationship between the maximum frequency interval by the Welch power spectrum and the coating thickness was established to provide the principle for determining coating thickness. According to this principle, the thickness of a series of stainless steel coatings and ZrO2–Y2O3 (yttria-stabilized zirconia (YSZ)) coatings were detected by scanning acoustic microscopy. The relative error was less than 4% with the microscope method, indicating that the proposed ultrasonic method provides a reliable nondestructive way to measure sprayed coating thickness. The uniformity of the sprayed coating thickness could be intuitively observed from C-scan images by programming.

With the rapid development of the aerospace industry, the quality inspection of complex curved components, such as aero-engine blades, is becoming increasingly strict. In contrast with other NDT methods, ultrasonic testing is easier to automate, while offering higher accuracy and efficiency in thickness measuring. To solve the challenge of the automated NDT inspection of aero-engine blades, in this study, an ultrasonic inspection system with a six degree of freedom (DOF) was proposed for industrial robots. Additionally, a defect detection model and a thickness detection method were proposed for the robotic ultrasonic inspection system, based on the thickness variation of the aero-engine blade. Through the quantitative analysis on engine blades with prefabricated defects and curved test blocks with stepped thicknesses, it can be concluded that our system is able to achieve high accuracy in defect detection and thickness measurement.

During the service or external loading of the surface coating, the damage accumulation may develop in the coating or at the interface between the substrate and the coating, but it is difficult to measure directly in the early stage, so the acoustic nonlinear parameters are used as the early damage index of the coating. In this paper, the nonlinear wave motion equation is solved by the perturbation method and the new relationship between the relative ratio of second-order parameter and third-order parameter was derived. The nonlinear ultrasonic testing system is used to detect received signals during tensile testing of for the specimen with Al2O3 coatings. It is found that when the stress is less than 260 MPa, the appearance of the coating has no obvious change, but the nonlinear coefficients measured by the experiment increase with the increase of the tensile stress. By comparing the curves of nonlinear coefficients and stress respectively, the fluctuation of curves the second-order nonlinear coefficient A2 and the relative nonlinear coefficient β′ to stress is relatively small, and close to the linear relationship with the tensile stress, which indicates that the two parameters of the specimen with Al2O3 coatings are more sensitive to the bonding conditions, and can be used as an evaluation method to track the coating damage.

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