Induction effect of α-Al2O3 seeds on formation of alumina coatings prepared by double glow plasma technique

(FeAl/Al) + α-Al2O3 coatings were deposited directly onto the 316 L stainless steel substrate and then oxidized at a temperature as low as 580 °C using double glow plasma technique. The complex Al target containing 10 wt% α-Al2O3 seeds was used in the experiment. The microstructure, chemical composition and phase components of the sputter-deposited and oxidized coatings were characterized respectively by glancing-angle X-ray diffraction (GAXRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), field emission transmission electron microscopy (FETEM) and high-resolution transmission electron microscopy (HRTEM). The results indicated that the coatings were very dense and uniform. Cross-sectional FETEM observation indicated the thickness of the film was about 5.83 μm, by which the deposition rate was calculated as high as ~ 1.46 μm/h. The surface of the coatings was composed of pure α-Al2O3 while the inner layer of the coatings was mostly α-Al2O3 with little γ-Al2O3. These intriguing results were attributed to the dual effects of both the α-Al2O3 seeds and the high-energy ion bombardment, which provided more nuclei for the growth of α-Al2O3 and more energetic species at the surface of the coatings respectively. Furthermore, the coatings were well-bonded to the substrates because a SS/FeAl/(Al2O3+ Fe2O3)/Al2O3 structure was formed at the interface so as to improve the coefficients of thermal expansion mismatch between the substrate and the deposited coatings.