Comparative investigation on the thermal aging behavior of the plasma sprayed coatings of La0.25Nd0.25Sm0.25Gd0.25MgAl11O19 and LaMgAl11O19

High-entropy LnMgAl₁₁O₁₉ (Ln = La, Nd, Sm, Gd; LnMA) and LaMgAl₁₁O₁₉ (LaMA) powders were successfully synthesized via solid-state reactions. This study primarily investigates the changes in recrystallization behavior during thermal aging of plasma-sprayed LnMA coatings compared to LaMA coatings. The results demonstrate that thermal aging at temperatures ranging from 800°C to 1200°C leads to the formation of various nanostructures in the LnMA coatings, including nano-equiaxed grains, nano-layered crystals, and high-aspect-ratio subgrains. This behavior contrasts sharply with the recrystallization observed in LaMA coatings. The differences in grain size during recrystallization are attributed to the high-entropy diffusion retardation effect in the LnMA coatings, which significantly influences the overall performance of the coatings. Notably, after aging for 100 h at 1400°C, the thermal expansion coefficient of the LnMA coatings closely matches that of sintered LnMA bulk material and exceeds that of LaMA bulk material. This indicates superior thermophysical and mechanical properties for the LnMA coatings. To further explore the performance differences, phase stability between the LnMA and LaMA coatings was compared using simultaneous thermal analysis and X-ray diffraction. Additionally, the microstructural and mechanical property changes of both coatings during thermal aging were investigated through scanning electron microscopy (SEM) and nanoindentation measurements, with a particular focus on the recrystallization mechanisms in the amorphous coatings. This study demonstrates that LnMA coatings offer superior structural integrity and mechanical performance in high-temperature service environments.