Direct Correlation of the Crack Distribution in APS Thermal Barrier Coatings with the Measured Synchrotron XRD Residual Stress Distribution

Abstract

Residual stress distributions as a function of depth in APS TBCs after different heat treatment times have been measured by synchrotron XRD. Three types of residual stress distribution were observed. The first type was a low stress state from the as-received sample; the second type was a compressive stress state increasing from the surface to the interface with a discontinuous "jump feature" in the trend near the interface; the third type of residual stress distribution has two "jump" features in the trend, one was observed near the sample surface and the other near the interface. To understand the double "jump" features in the third kind of trend, the 3D microstructures of the measured samples were systematically measured X-ray μ-CT. Vertical cracks and edge cracks were observed. We have shown by image based modelling that the vertical cracks do not have a large effect on the residual stress distribution and that edge cracks are related to the "jump features" near the sample surface. The edge crack can be observed in all the measured samples with the third kind of the residual stress trend; and the measured samples with the second kind of the residual stress trend are crack free. The relationship between the middle and the through edge cracks and the residual stress distribution was also investigated by image based modelling. It was found that the middle crack can also cause a "jump feature" near the sample surface while the through edge crack seems to not have a large effect on the residual stress distribution. A semi-destructive method was developed to determine the existence and the position of horizontal cracks in the APS TBC samples based on the above findings.