Oliver Bruchwald*, Wojciech Frackowiak, Wilfried Reimche and Hans Jürgen Maier
Institut für Werkstoffkunde, Leibniz Universität Hannover, 30823 Garbsen, Germany
Thin metallic coatings are widely used for corrosion protection or to improve the functional and wear properties of the base material. Because these coatings can wear out and lose their functionality during lifetime, a non-destructive characterization of the coating condition is needed. The eddy current technology allows for a non-destructive characterization of metallic coatings as well as measuring the coating thickness of both metallic or non-metallic, non-conductive coatings. However, very thin coatings with a low electrical conductivity, e.g. the PtAl- or MCrAlY-coatings typically deposited on turbine blades, cannot be characterized separately from the metallic base material due to the high penetration depth when using conventional eddy current technology with testing frequencies below 5 MHz. Experimental investigations were carried out using a new high frequency eddy current testing (HF-EC) system with a frequency range between 5 and 100 MHz and test samples with zinc coatings between 10 and 45 μm as well as high pressure turbine blades made of nickel-base alloys with a PtAl coating and a non-conductive thermal barrier coating TBC. It will be shown that the HF-EC technology allows for a quick estimation of the thin coating’s condition and its thickness by analyzing the local electric properties and the lift-off effect.
High frequency eddy current testing, material characterization, coating condition, thin coatings, turbine blades.
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