Thickness Measurement of MCrAlY High-Temperature Coatings by Frequency Scanning Eddy Current Technique

Nondestructive characterization of nonserviced, high-temperature coatings can be considered one of the important factors to achieve a higher level of structural integrity of advanced gas turbines. The present paper describes an innovative eddy current technique especially developed for measuring the thickness of metallic (MCrAlY) coatings applied by vacuum plasma spray on Ni-base superalloys. Conventional eddy current techniques, well established for quality control of coating thickness, are not applicable in this case because of the low differences of electrical conductivities of coating and base materials, which is a consequence of their quite similar physical and chemical properties. The new technique employs fast frequency scanning of the electromagnetic field in the range 100 kHz–10 MHz, corresponding to probing depths from 1 mm to 0.1 mm. Dedicated hardware has been developed featuring high sensitivity, stability, and harmonic rejection. Analysis of the measured data (i.e., probe impedance versus frequency), in order to estimate the relevant diagnostic parameters (coating thickness, coating, and base metal electrical conductivities), is carried out on the basis of a theoretical model of interaction between a plane electromagnetic wave and test piece. The results of tests performed on a nonserviced first stage blade are reported and compared with reference destructive data. Reliability, accuracy and practical applicability of the method meets the requirements for in-shop quality control.