Effects of amorphous phase on hot corrosion behavior of plasma-sprayed LaMgAl11O19 coating

Plasma-sprayed LaMgAl11O19 (LaMA) coating as a novel thermal barrier coating candidate has been studied for more than a decade, but it tends to contain significant amounts of non-equilibrium amorphous phase due to rapid solidification. The microstructure of such as-sprayed LaMA coating was first studied in detail by a transmission electron microscope (TEM), which revealed that the coating consisted mostly of an amorphous phase but with very thin layers of crystalline (thickness similar to 100 nm) phase at the boundary of neighboring sprayed particles. A hot corrosion test was also conducted on an as-sprayed LaMA coating as well as on a fully-crystallized coating after heat treatment by a molten V2O5-Na2SO4 mixture at 1100 degrees C. The coatings' cross sectional observation showed that the as-sprayed coating was affected over a wider area by the corrosion reaction than the crystallized coating. To clarify the intrinsic reactivity of the amorphous and crystalline LaMA phases with the molten salt, a piece of as-sprayed coating was crushed into powder, mixed with a salt, and tested at temperatures below and above the crystallization temperature. The reacted powder was analyzed by an X-ray diffractometer (XRD) to quantify the amounts of various phases in the sample as a function of time. The results revealed that the amorphous LaMA has a remarkably higher reaction rate with the molten salt than the crystalline phase. In fact, at the testing temperature of 1100 degrees C, corrosion of the sprayed LaMA powder proceeded much faster than crystallization, as almost 90% of LaMA was corroded within 30 min.