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Ceramic based coatings such as Thermal Barrier Coatings (TBCs) are widely deposited onto gas turbine engine components to protect their metal substrate against high temperature. However, improper adhesive application, thermal stress and moisture penetration through the insulation materials can cause defects such as delamination to grow between the insulation and the metal surface. Such defects could lead to catastrophic failure if not immediately detected and repaired. Hence, it is imperative to non-destructively inspect these interfaces for delamination to avoid such an event. In this paper, a novel microwave nondestructive testing technique for ceramic-based coatings is proposed. This technique is based on scanning the surface of the coating with an open-ended rectangular waveguide and analyzing the reflections in the time domain. The proposed time-domain technique appears to reveal more about the depth of any existing delamination than any other microwave non-destructive testing (NDT) technique. Here, a ceramic sample with machined delamination is scanned using rectangular waveguides operating from 26.5 to 40 GHz. After applying Fourier analysis to the frequency domain waveguide reflections, time-domain results showed the delamination with significantly better depth resolution compared to frequency domain analysis. The results reported in this paper prove the advantages of the time-domain technique to forecast the relative delamination depth, which is important for many industrial applications. The depth information of delamination can be used in a condition-based predictive maintenance schedule to plan the inspection intervals efficiently, minimize the unnecessary replacements and provide practical guidance for future design.

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Recent years have witnessed an increase in the use of composite coatings for numerous applications, including aerospace, aircraft, and maritime vessels. These materials owe this popularity surge to the superior strength, weight, stiffness, and electrical insulation they exhibit over conventional substances, such as metals. The growing demand for such materials is accompanied by the inevitable need for fast, accurate, and affordable nondestructive testing techniques to reveal any possible defects within the coatings or any defects under coating. However, typical nondestructive testing (NDT) techniques such as ultrasonic testing (UT), infrared thermography (IRT), eddy current testing (ECT), and laser shearography (LS) have failed to provide successful results when inspecting composite coatings. Consequently, microwave NDT techniques have emerged to compensate for the shortcomings of traditional NDT approaches. Numerous microwave NDT methods have been reported for composite coatings inspection. Although existing microwave NDT methods have shown successful inspection of composite coatings, they often face several challenges, such as low spatial image quality and extensive data interpretation. Nevertheless, many of these limitations can be addressed by utilizing microwave NDT techniques with modern technologies such as soft computing. Artificially intelligent techniques have greatly enhanced the reliability and accuracy of microwave NDT techniques. This paper reviews various traditional NDT techniques and their limitations in inspecting composite coatings. In addition, the article includes a detailed review of several microwave NDT techniques and their benefits in evaluating composite coatings. The paper also highlights the advantages of using the recently reported microwave NDT approaches employing artificial intelligence approaches. This review demonstrates that microwave NDT techniques in conjunction with artificial intelligence approaches have excellent prospects for further enhancing composite coatings inspection and assessment efficiency. The review aimed to provide the reader with a comprehensive overview of most NDT techniques used for composite materials alongside their most salient features.

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Microwave Non-Destructive Testing (NDT) techniques for dielectric coatings are vital processes in many industrial applications due to their superior defect detection capabilities over traditional NDT methods. However, the limited bandwidth and post-processing complexities of these techniques cause them to fall short of accurately estimating the delamination thickness in such structures. In this paper, a novel low-complexity signal processing method is proposed to estimate the delamination thickness in metalbacked dielectric coatings using bandwidth-limited Open-Ended Rectangular Waveguide (OERW) probes. Here, Inverse Fast Fourier Transform (IFFT) process is used to convert the measured complex reflection coefficients at the surface of the coating to the time domain. Next, the amplitude of a specific time step is used to accurately estimate the delamination thickness without extra time-consuming processing. Using a 3 mm-thick macor samples with machined defects, the proposed technique is validated by correctly estimating delamination thickness down to 0.4 mm using an OERW probe with 13.5 GHz bandwidth. Hence, this technique overcomes the challenge of the OERW's limited bandwidth, which highly affects the possibility of tracking shifting in the peak reflection in the time domain. Moreover, the proposed method does not add to the complexity of the NDT process, which makes it suitable for in-situ real time applications. Hence, following this approach would be of great importance to numerous industries, where accurate thickness estimation of minute delamination in coatings is essential to avoid system failure.

Insulation has been widely used to protect metal substrates in various industries such as turbine blades and oil and gas pipelines. Due to aging and cyclic processes, delamination between the metal component and insulation could grow. Such defects represent a critical problem that may result in catastrophic failure of the asset. Therefore, detecting and evaluating delaminated areas is mandatory for providing urgent maintenance solutions before the failure occurrence. In this paper, microwave nondestructive technique (NDT) is proposed for delamination detection and thickness estimation under dielectric insulation. In microwave NDT, the electromagnetic waves are induced into the dielectric material's surface using a ridge waveguide. The complex reflection coefficients are acquired using a vector network analyzer for further analysis in the time domain. Next, a time-domain reflectometry (TDR) technique-based inverse discrete Fourier transform is used to evaluate the complex reflection coefficients in terms of delamination's size and thickness estimation. The delamination evaluation is performed by measuring the maximum peak's time-step variations instead of the magnitude reduction in conventional TDR techniques in terms of the defect and defect-free reflections. Next, a comparison is made between implementing the proposed technique using a ridge waveguide and a regular rectangular waveguide. The results prove the superiority of using the ridge waveguide for delamination evaluation in terms of sizing and thickness with a minimum error rate of 5% and 7.87%, respectively. These novel results of ridge waveguide due to its small aperture and relatively wide bandwidth. The microwave NDT technique reported here could provide a predictive maintenance tool for many industries to minimize the maintenance effort and cost.