Effects of amorphous phase on hot corrosion behavior of plasma-sprayed LaMgAl ₁₁ O ₁₉ coating

Plasma-sprayed LaMgAl ₁₁ O ₁₉ (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 ~ 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 V ₂ O ₅ -Na ₂ SO ₄ mixture at 1100 °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 °C, corrosion of the sprayed LaMA powder proceeded much faster than crystallization, as almost 90% of LaMA was corroded within 30 min.