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 non-destructive 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.
Highlights
Ceramic-based insulation such as Thermal barrier coating (TBC) has steadily grown to become the material of choice for thermal insulation and corrosion resistance industrial applications, such as gas turbine and aero-engine parts
A thin layer of TBCs is applied onto the metal substrate of gas turbine components to allow increased engine temperatures without sacrificing durability
The complex reflection coefficient is measured at discrete frequency points on the TBC surface using an Open-ended rectangular waveguides (OERWs)
Summary
Ceramic-based insulation such as Thermal barrier coating (TBC) has steadily grown to become the material of choice for thermal insulation and corrosion resistance industrial applications, such as gas turbine and aero-engine parts. The study found that the operational frequency of microwave OERW is robust to the changes of the thickness and dielectric constant of the TBC layer which can be effectively used for detecting dielectric thickness and delamination between the dielectric and metal substrate based on measuring the phase of the reflection coefficient. A defect evaluation technique is proposed in [29] to measure the delamination depth of GFRP pipe based on microwave open-end waveguide. A simple and efficient method is proposed to determine the delamination thickness in TBCs to an acceptable degree of accuracy In this method, the complex reflection coefficient is measured at discrete frequency points on the TBC surface using an OERW.
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