Topcoat Thickness Measurement of Thermal Barrier Coating of Gas Turbine Blade using Terahertz Wave

Fuse

Mizuno

et al. 2016

Elect Comm in Japan

SUMMARYA method to measure the refractive index of the ceramic layer of a thermal barrier coating (TBC) using the intensity ratio between the reflection of terahertz waves from the ceramic surface and a reference metal plate is proposed. This method uses a variable aperture to change the relative proportions of specular refraction and scattering from the ceramic surface. The refractive index is calculated based on the specular reflectivity, which is obtained from the intensity ratio when the field of view is extrapolated to zero. Measurement of a TBC specimen resulted in a refractive index of 4.7, which was in good agreement with the refractive index of the ceramic material obtained by terahertz time‐domain reflectometry. In addition, the surface roughness of the ceramic surface was obtained based on the frequency characteristics of the reflections from the ceramic surface and a reference metal plate. A surface roughness of 14 μm was obtained, which matched the results of microscopic observation of a TBC specimen whose coating was applied using the same lot as the specimen used in the measurement.

Section: Results Of Measuring the Refractive Indexsupporting

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Measurement of Refractive Index of Thermal Barrier Coating Using Reflection of Terahertz Waves and Variable Aperture

Fuse

Mizuno

et al. 2016

Elect Comm in Japan

“…After THz measurement, the blade was sliced at 30%, 70%, and 90% height, and the topcoat thickness at each position was measured by microscopic observation. The results are compared in Table . The topcoat thickness measured using THz waves was in agreement with the microscopic observation results to within 11 μ m, or 6%.…”

Section: Topcoat Thickness Measurementsupporting

confidence: 76%

“…. Positions a , c are at 90% and 70% height on the suction side; positions b , d are at 30% height on the suction side; position e is at 70% height on the pressure side .…”

Section: Topcoat Thickness Measurementmentioning

confidence: 99%

Nondestructive inspection of thermal barrier coating of gas turbine high temperature components

IEEJ Transactions Elec Engng

Ozeki

Okada

et al. 2016

Self Cite

A thermal barrier coating (TBC) is applied to high-temperature components in gas turbines, and consists of a ceramic topcoat and a metallic bondcoat. Various kinds of TBC degradation and damage occur in high-temperature components during service, such as topcoat thinning, topcoat delamination, and formation of a thermally grown oxide (TGO) layer below the topcoat, each of which can be examined using a suitable nondestructive inspection technique. Topcoat thinning can be detected by topcoat thickness measurement using terahertz waves, which are electromagnetic waves in the frequency region between optical and radio waves. The measurement resolution is about 10 μm, which is comparable to microscopic observation of the cross section in destructive inspection. Topcoat delamination can be detected by active thermography, in which the topcoat surface is scanned by a heating laser and the surface temperature distribution is measured by a thermal infrared camera. The combination of temperature peak and residual thermal image detection is effective in eliminating false detection. The TGO layer can be detected using photoluminescence, in which the Cr 3+ ions included as an impurity in Al 2 O 3 are detected. Since delamination tends to occur at locations at which the TGO layer has grown, TGO layer detection provides an effective method to select regions where delamination has occurred or is likely to occur. An inspection flow based on these techniques is proposed, which is expected to aid the establishment of condition-based maintenance strategies of high-temperature components.

Topcoat Transmission Measurement and Sensitivity Evaluation for Detection of Thermally Grown Oxide Layer of Thermal Barrier Coating

Eto

Okada

et al. 2016

Elect Comm in Japan

Self Cite

SUMMARY The thermally grown oxide (TGO) layer, which forms below the topcoat of thermal barrier coating (TBC), is an important factor that causes topcoat delamination, and its detection is important for health monitoring of TBC. Photoluminescence detects the emission from Cr3+ in the TGO layer upon laser excitation, and is an effective, nondestructive method to detect the TGO layer. In this method, the laser light and Cr3+ photoluminescence must transmit through the topcoat, so the topcoat transmittance is a limiting factor of the measurement sensitivity. In this report, the topcoat transmittance was measured by comparing the photoluminescence intensities from the TGO layer in the presence and absence of the topcoat. The round‐trip transmittance of the topcoat of thickness 300 μm was 1.26%, corresponding to an attenuation coefficient of 7.3 mm‐1. The sensitivity of the luminescence measurement was evaluated by placing ND filters of known attenuation in front of the TGO layer, to simulate the attenuation for different topcoat thickness. The results showed that luminescence measurement is possible for a maximum topcoat thickness of about 700 μm. A method to adjust the photoluminescence intensity for different topcoat thickness is presented.