Laminated structure in internally oxidized Ru–Ta coatings

During the development of refractory alloy coatings for protective purposes at high temperature under oxygen-containing atmospheres, previous studies noted and examined the internal oxidation phenomenon for Mo–Ru and Ru–Ta coatings. The internally oxidized zone shows a laminated structure, consisting of alternating oxygen-rich and deficient layers stacked with a general orientation. Previous studies proposed a forming mechanism. To investigate in detail, Ru–Ta coatings were prepared with various rotating speeds of a substrate-holder. The coatings were annealed at 600 °C in an atmosphere continuously purged with 1% O2–99% Ar mixed gas for 30 min. Transmission electron microscopy was used to examine the laminated-layer periods. Auger electron spectroscopy depth profiles certified the periodical variation of the related constituents. X-ray photoelectron spectroscopy proved the valence variation of Ta in the near surface, accompanied by the introduction of oxygen ions. The inward diffusion of oxygen was dominated by lattice diffusion. - Highlights: ► Laminated Ru–Ta coatings consisted of a cyclical gradient concentration. ► The as-deposited coatings showed a laminated structure with a period of 4–34 nm. ► Internal oxidation of Ru–Ta coatings executed after annealing in 1% O2–Ar atmosphere. ► Oxygen inward diffusion was dominated by lattice diffusion.