Monitoring system for thermal barrier coatings with RF radar measurement

A monitoring principle able to identify damage on the thermal barrier coating of gas turbine blades and vanes is presented. The principle is applicable to in-service turbine monitoring. It is based on the resonant quarter wavelength effect of thin layers, which causes phase shift and attenuation of the reflected signal. The reflectivity is derived theoretically and evaluated by laboratory measurements. It was evident that especially the phase information is adequate to distinguish coated areas from those where coating has already been spalled off.