Crack healing mechanisms in atmospheric plasma sprayed Yb-silicate coatings during post-process heat treatment

The microstructural evolution of partially amorphous, atmospheric plasma-sprayed Yb-silicate coatings was investigated after heat treatment (HT, 1300 degrees C). Open porosity as well as crack area and widths in the coatings were characterized. A correlation was found between the increasing amorphous content of the as-sprayed coatings and the reduction in crack area due to crack healing after HT. Characterization results also suggested larger crack widths in the coatings after HT. The first crack-healing mechanism proposed was the capillaritydriven viscous flow of the amorphous phase in the coatings. It was experimentally shown that viscous flowdriven crack spheroidization (crack width enlargement), and crack healing started in the coatings at 1050 degrees C. Secondly, metastable to stable phase transformation (> 1100 degrees C) induced expansion in the constrained coatings was discussed as a crack healing mechanism. Finally, a two-step heat treatment was designed to prevent crack width enlargement during HT resulting in reduced porosity.