Accurate thickness evaluation of thermal barrier coatings using microwave resonator sensor

Abstract

Thermal barrier coatings (TBC) are frequently employed in aircraft engines and turbine blades for protection against high temperatures. Constant exposure to in-service stresses, however, leads to a variety of anomalies in TBC that could prove to be potentially catastrophic for the assemblies they protect. This raises the need for inspection using non-destructive techniques (NDT) to ensure the reliability and structural integrity of the coated assemblies. Thickness evaluation of the topcoat of the TBC is paramount to determining structural integrity of the TBC. Microwaves are particularly suited for such applications since they readily penetrate low loss TBC. However, the performance of previous microwave-based TBC thickness evaluation techniques was limited in accuracy to 15 μm. In this paper, accurate thickness evaluation of thermal barrier coating (TBC) is performed using a microwave resonator sensor operating at a relatively low frequency (<1 GHz). TBC thicknesses as low as 100 μm are evaluated based on shift in resonant frequency of the probe. Overall, practically relevant thicknesses ranging from 100 μm to 600 μm are evaluated herein and multiple trials are performed to ascertain repeatability and assess uncertainty in measurements. For a 250 μm thick TBC sheet, a mean error of 0.9 μm was accomplished which translates to 0.36 % error with a standard deviation of 1.5 μm. Compared to other microwave sensors reported in the literature, a better estimation accuracy is demonstrated in this work, in particular for thicknesses less than <250 μm.