Nondestructive Evaluation of Coatings Delamination Using Microwave Time Domain Reflectometry Technique

Abstract

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.

Microwave Nondestructive Testing for Defect Detection in Composites Based on K-Means Clustering Algorithm

“…, and the open-ended rectangular waveguide (OERW) inspection method is popular among them [13]- [17]. The working principle microwave NDT using OERWs is based on recording and analyzing the reflected, scattered electric field from an OERW probe scanned over a surface of the insulated structure using a calibrated Vector Network Analyzer (VNA).…”

“…The reference plan of OERW is used to scan a metal-backed sample that consists of a dielectric layer which placed near the waveguide. Traditional TDR uses the time of receiving the reflection from a certain discontinuity to estimate the distance between it and the reference plane [17]. This implies knowledge of the speed at which the waves travel through the media under consideration, which is usually the speed of light.…”

IFFT-Based Microwave Non-Destructive Testing for Delamination Detection and Thickness Estimation

“…Therefore, this NDT technique mandates a costly apparatus to obtain an accurate estimation of delamination thicknesses. Because of the above, alternative microwave-based NDT techniques have emerged [18]- [23] . The main advantage of these techniques is the ability of electromagnetic signals in the microwave frequency range to penetrate dielectric coatings.…”

“…Recently, an alternative microwave NDT approach has been proposed to reveal small changes in various type of dielectric coating by converting the recorded reflections to the time domain. By doing so, the reflections from various discontinuities inside the dielectric were detected by tracking the reflected pulses to the OERW probe [21], [23]. Such methods are similar to the well-established Time Domain Reflectometry (TDR) method used to estimate the defects' locations in faulty cables [26], and also the previously mentioned THz technology [16], [17].…”

“…Despite the ability of this method to detect various defects, estimating the delamination thickness proved to be challenging due to the nonlinear relation between the reflection peak amplitude and the actual time shift. Another method proposed in [23] used zero-padding on the measured frequency domain reflection coefficient in order to artificially increase the bandwidth of the signal. This process has the effect of oversampling the time domain signal, which causes the reflection pulses to be more pronounced and hence, provides the possibility of tracking the shift in the peak reflection in the time domain.…”