SUMMARYThe topcoat thickness of thermal barrier coating (TBC) applied to a gas turbine blade was measured using terahertz waves. The refractive index of the topcoat was obtained by frequency analysis of the waves reflected from the topcoat surface and the interface between the topcoat and bondcoat. The surface roughness of the topcoat surface was considered for improving the accuracy of the refractive index calculation. The topcoat thickness was obtained from the refractive index and the time separation between the reflected waves. The validity of the method was confirmed using a TBC sample with variable topcoat thickness, and the obtained topcoat thickness was in agreement with measurement results using a contact thickness gauge. Error analysis showed that the measurement error of the topcoat thickness was about 3%. The method was applied to a gas turbine blade with TBC, and the measured topcoat thickness agreed with microscope observation results of the cross section to within 6%, which was within the range of measurement error. The effect of curvature of the gas turbine blade surface did not result in significant measurement error. The results showed that terahertz waves are effective for nondestructive measurement of the topcoat thickness of TBC applied to gas turbine blades. C⃝ 2014 Wiley Periodicals, Inc. Electr Eng Jpn, 189(1): 1-8, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com).

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.

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Conventional thermal power generation, as typified by gas turbines, has steadily increased power generation efficiency by elevating temperature of heat, but there is a limit to the maximum availability of electric energy. Exergy rate is a unified index indicating the quality of energy in deferent forms. We have no way in thermal conversion to extract all of the availability, while almost hydrocarbon fuels have exergy rate around 95%. 25% of exergy is inevitably lost through the combustion process from chemical to heat at maximum temperature of 2000°C. Hydrogen’s low exergy rate provides “exergy recuperation” in which degrading 12% from 95% to 83% can take low quality heat up to availability of 83% as a kind of chemical heat pump. Chemically Recuperated Gas Turbine (CRGT) is a specific example, and dimethyl ether (DME) is one of the most suitable fuels because steam reforming occurs around 300°C. Electrochemical partial oxidation (EPOx) is another way to convert mid-quality heat into electric energy as much as difference between change in Gibbs free energy and change in enthalpy. This paper reports concept and industrially-feasible applications of this unconventional and non-cascadic use of heat.