High-temperature cyclic oxidation and microstructural behavior of CoMoCrSi-based composite coatings with Al2O3 and YSZ on T91 steel

This investigation focuses on the cyclic oxidation behaviour and microstructural evolution of Cobalt-Molybdenum-Chromium-Silicon coatings reinforced with alumina and yttria stabilised zirconia (YSZ) deposited on T91 steel by the atmospheric plasma spraying method. Characterization of the as-sprayed coatings was done to provide a consistent base line for comparison of the intercoatings. Cyclic oxidation experiments at 800 o C for 50 oxidation cycles was carried out with surface and cross section analyses using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) for the evaluation of oxidation scale formation, element distribution and phase stability. After cyclic oxidation, XRD showed an appearance of stable oxide constituents, which demonstrates the formation of protective scales on the coating surfaces. In comparison with unreinforced and alumina reinforced coatings, the YSZ reinforced coating displayed comparatively superior oxidation behavior as reflected in the more stable oxidation and less degradation during cyclic exposure. SEM examination of oxidized surfaces showed that the ceramic additions helped to increase coating integrity, improve scale adherence and reduce oxygen penetration. These effects were amplified for the YSZ-containing coating, showing a good response for high temperature oxidation. Overall, the addition of ceramic reinforcements, especially of YSZ, improved the phase stability and oxidation resistance of the CoMoCrSi-based coatings, which supports the suitability of the coatings for demanding boiler environments.