Experiments on the Testing Sensitivity for Microstructure Evolutions of Coatings by Electro-Mechanical Impedance Method in kHz and MHz Frequencies

Two kinds of plasma sprayed Cr2O3 coatings are characterized by electro-mechanical impedance (EMI) method with PZT patch in kHz and MHz frequencies. Comparing frequency shift of impedance peaks f and root mean square deviation (RMSD) of PZT electric impedance signatures, the influence of frequency on the testing sensitivity is investigated in some frequency bands among 10-238 kHz and 5.45-6.33 MHz. The results show that in the kHz frequency range, the frequency shifts f increase with testing frequency and the RMSD values first increase and then decrease. The latter may be attributed to the decreasing of peak number and the decline of impedance magnitude. In MHz frequency range, the frequency shifts f become more significant, and a prominent frequency shift of 206.1 kHz is observed near 6 MHz. The RMSD values are also much larger than that in kHz frequency range.