G. Antonelli et al., THICKNESS MEASUREMENT OF MCRALY HIGH-TEMPERATURE COATINGS BY FREQUENCY SCANNING EDDY-CURRENT TECHNIQUE, Journal of engineering for gas turbines and power, 120(3), 1998, pp. 537-542
Nondestructive characterization of nonserviced, high-temperature coati
ngs can be considered one of the important factors to achieve a higher
level of structural integrity of advanced gas turbines. The present p
aper describes art innovative eddy current technique especially develo
ped for measuring the thickness of metallic (MCrAlY) coatings applied
by vacuum plasma spray on Ni-base superalloys. Conventional eddy curre
nt techniques, well established for quality control of coating thickne
ss, are not applicable in this case because of the low difference of e
lectrical conductivities of coating and base materials, which is a con
sequence of their quite similar physical and chemical properties. The
new technique employs fast frequency scanning of the electromagnetic f
ield 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 measur
ed data (i.e., probe impedance versus frequency), in order to estimate
the relevant diagnostic parameters (coating thickness, coating, and b
ase 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 meet
s the requirements for in-shop quality control.