Unveiling residual stresses in air plasma spray coatings by digital image correlation

We report a new three-dimensional digital image correlation (3D-DIC) based method for in-situ probing of the residual stresses in air plasma spray (APS) coatings. The inherent surface texture and coloration of APS coatings enabled extraction of surface profiles with accuracies of better than ±8.8μm. The residual stress history of a three-layer thermal barrier coating system (TBC) was obtained experimentally by 3D-DIC curvature measurements from 22 to 800 °C coupled with a newly developed residual stress model for multilayered TBC systems. The residual stress in TBC systems was released in an observed linear trend through 800 °C. Anomalous increases in curvature and residual stress upon heating between temperatures of 800 °C and 900 °C were partially caused by expansive tetragonal to monoclinic (t→m) phase transformation within the partially stabilized zirconia top coat. A coupled stress-temperature dependent mechanism was used to uncover the phase transformation induced stresses. The technique opens up new opportunities for monitoring the residual stress development in manufacturing and operation of coatings, thin films, and structures of dissimilar materials.