The nondestructive evaluation of thermal barrier coatings: Measurements of thermal properties and associated defects

Phase of thermal emission spectroscopy is developed to nondestructively determine the thermal properties of thermal barrier coatings (TBC). Harmonic heating of the coating is established using a CO2 laser. The phase of thermal emission is measured from the front surface as a function of laser frequency. A mathematical model is developed to describe the heat transport in the measurement and establish all important dependencies of the emission phase. Least square fitting of the experimental measurement using the model permits determination of the unknown properties of the TBC. Besides the thermal diffusivity of the coating, both the thermal conductivity and volumetric heat capacity of the coating can be determined simultaneously. A UCSB TBC deposited by electron-beam physical vapor deposition (EB-PVD) is measured and used to illustrate the measurement and analysis.; Various TBC samples deposited by different deposition techniques with varying process parameters and materials are measured with phase of thermal emission spectroscopy. A set of TBC specimens deposited by EB-PVD with varying process parameters are measured. The influences of coating thickness, rotation rate, and deposition tilt on the thermal properties of coatings are studied. Another set of TBC samples deposited by directed vapor deposition (EB-DVD) with various processing parameters and different coating materials are also measured. The effects of rotation rate of the substrate and the substrate temperature for deposition are investigated. The thermal properties of two kinds of coating materials, 7wt% yittria, partially stabilized zirconia (7YSZ) and samarium zirconate (Sm2Zr2O7) are compared.; Based on the phase of thermal emission spectroscopy, a surface area mapping method is developed for inspecting defects in TBCs non-destructively, which is desirable for monitoring the performance and predicting the failures of TBCs in service. Mapping the phase shift over a region of interest can be used to characterize changes in coating properties and the presence of defects. By varying the modulation frequency, this method is able to provide depth information concerning the defects. The unknown properties of the delaminated TBC and the thermal contact resistance introduced by a local delamination are determined quantitatively by phase of thermal emission spectroscopy.