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Characterization of Gas Turbine Blade Layers Using Eddy Current Testing, 15-9202

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Principal Investigators
Gary L. Burkhardt
Jay L. Fisher

Inclusive Dates: 07/03/00 - Current

Background - Advanced gas turbines are increasingly being used for electrical power generation. The turbine blades are protected from high-temperature oxidation and corrosion by metallic coatings made of CoCrAlY or NiCoCrAlY materials, often with a top layer of aluminum. As the coating ages, aluminum is depleted from the coating structure to support a protective aluminum oxide layer on the outer surface. The remaining service life of these blades depends on the state of degradation of the coating and, in particular, the remaining aluminum content. A nondestructive evaluation (NDE) technique is needed to measure the aluminum content or the thickness of the aluminum-rich layers in the coating to help determine remaining coating life.

 The Electric Power Research Institute (EPRI) has initiated a program to develop a field-deployable NDE system for life assessment of blade coatings. As a first step in this program, EPRI is performing a nonfunded round-robin testing program to identify the most suitable NDE technique using well-characterized test samples. EPRI has requested that SwRI be one of the participants in the test.

Approach - An existing SwRI model based on multifrequency eddy current testing (ECT) will be further developed and validated. The model will use ECT measurements to predict layer thicknesses associated with degradation of PWA-286 gas turbine blade coatings. This project will allow SwRI to participate in the EPRI round-robin test and provide test results to EPRI to show potential of SwRI's method for future EPRI funding and to allow SwRI to take advantage of the well-characterized EPRI test specimens to validate the model.

Accomplishments - The model was further developed to allow determination of the thickness of the aluminum-rich β-phase layer of the coating, as well as the thicknesses of the inner and outer depleted layers. The model was tested and validated on a set of initial EPRI specimens on which the coating thicknesses were known to SwRI. Good agreement was obtained between predicted and actual thicknesses, particularly the β-phase layer, which is of most importance. Blind round-robin tests were performed on additional specimens. Although the layer thicknesses of these specimens have not yet been made known to SwRI, the predicted thicknesses appear reasonable. The specimens will ultimately be examined metallurgically, and results will be made known for comparison with the predicted thicknesses.

Thickness of aluminum-rich β-phase layer of turbine blade coating as predicted by ECT measurements for EPRI specimens with known layer thicknesses

      Measurement & Nondestructive Evaluation of Materials & Structures Program
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