Characterization for Young's modulus of TBCs using soft lithography gratings and moiré interferometry

Thermal Barrier Coatings (TBCs) are widely used for thermal protection in turbine engines, and the mechanical characterization for TBCs is of great importance. In this study, a new method to measure Young's modulus of TBCs is proposed combining soft lithography gratings with moiré interferometry. Neutral plane locating method and strain-bending moment method are developed to determine Young's modulus of TBCs subjected to four-point bending load. In the measurement, a soft lithography grating with the frequency of 1200 lines/mm was transferred on the specimen surface. After loading, the strain field was measured using a moiré interferometer. Then the data were used to calculate the Young's modulus. In addition, the influence factors of measurement errors on the determination of Young's modulus of TBCs are analyzed. The analysis results indicate that the thickness of the substrate as well as high quality null field plays an important role in improving measurement accuracy. Besides, in order to obtain the strain field from the fringe patterns, the linear fitting strain extraction method is proposed, which can achieve satisfying measurement accuracy and is easy for operation. Using the proposed method, Young's modulus of TBCs was measured to be 34 GPa. Finally, validation test was conducted with a bi-material sample made by Ni-based alloy and aluminum. Once the Young's modulus of Ni-based alloy is given, that of aluminum can be obtained. The results indicate that the relative error of the measured Young's modulus is less than 6%.