Optical measurement of the thermal diffusivity of intact thermal barrier coatings

Abstract

The thermal diffusivity of vapor deposited thermal barrier coatings is measured using an all-optical approach. The method combines CO2 laser heating of the coating surface with detection of the temperature variation in the laser induced Cr3+ luminescence signal from the thermally grown oxide (TGO) formed under the coating during service. The thermal diffusivity is determined from the delay between a surface heating laser pulse and the heating of the TGO. Data analysis is performed using a three layer heat equation model. Various locations on several coatings were investigated with different average heating laser powers and modulation frequencies. The normalized standard deviation was found to be of the order of 20%, which is thought to be largely due to localized variations in coating thickness and density. The advantage of the method described here is that it does not require knowledge of either the thermal properties of the alloy, its temperature, or thickness, and is therefore well suited for measurement on coated parts of complex shape, such as a hot section gas turbine blade. © 2008 American Institute of Physics.