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In modern aircraft engines,  a high number of complex high-performance components are employed, which are partly subjected to extreme loading. For instance, the high-pressure turbine blades of the first stage after the combustor are both thermally-mechanically loaded and experience severe corrosive attack. Therefore they are furnished with several protective systems in order to improve the fatigue life and reliability of the high-temperature material blades employed. A ceramic thermal barrier coating insulates the internally and film-cooled blades against the hot gas stream whilst the underlying layer (PtAl, Al, MCrAlY) protects the substrate material against corrosive attack and oxidation. Owing to the low electrical conductivity of the materials employed in the multilayered system of the high-pressure turbine blades with layer thicknesses of 20–150 μm, conventional eddy-current and thermographic technologies are only suitable to a limited extent to non-destructively detect the condition of the individual layers separately from the substrate material. In contrast to this, with the aid of multi-parameter, high-frequency eddy-current technologies and high-frequency induction thermography using pulsed excitation, the eddy-currents with test frequencies in the mega-Hertz range (up to 100 MHz) can be limited to the near subsurface and produce standard penetration depths of &lt;50 μm; which enables the layer and substrate materials to be differentially studied. The objective of the present study was to non-destructively detect the condition of the coatings, to characterise and determine the thickness of the coating as well as to detect the condition of the substrate material and sensitively analyse local damage and defects.

htm journal of heat treatment and materials

Non-destructive determination of local damage and material condition in high-performance components: High-frequency eddy current and induction thermography techniques