A transmission electron microscopy study of the microstructures present in alumina coatings produced by plasma spraying

Alumina coatings deposited onto steel substrates by plasma spraying were studied using transmission electron microscopy. A number of allotropes of alumina distinct from the alpha-alumina of the starting powder were identified, The as-sprayed coatings consist primarily of metastable gamma-alumina and amorphous/nanocrystalline material. Areas of alpha-alumina were observed, generally in conjunction with the twinned metastable phases delta-and theta-alumina, and these were interpreted as resulting from solid-state transformation from the gamma phase, due to the annealing action of subsequent passes of the plasma torch, The absence of such phases from the first-and last-deposited layers is further evidence for such a mechanism.