Amorphous coatings on tantalum formed by plasma electrolytic oxidation in aluminate electrolyte and high temperature crystallization treatment

In this paper, amorphous coatings on tantalum obtained by plasma electrolytic oxidation (PEO) in alkaline sodium aluminate electrolyte (2–10 g/l NaAlO2 + 2 g/l KOH) are investigated. It was found that the amorphous component of the oxide coatings increased with NaAlO2 concentration in the electrolyte. The coatings obtained in the electrolyte of 10 g/l NaAlO2 + 2 g/l KOH are completely amorphous. Subsequently, the amorphous coatings were subjected to vacuum heat treatment at 600, 800, 900 and 1300 °C to study their crystallization behavior. The results show that crystallization does not occur at 600 °C. However, crystallization occurred partially at 800 °C and completed at 1300 °C, with orthorhombic AlTaO4 as the main phase structure. Nanoindentation tests show that hardness of the coating at dense regions increases after crystallization, but defects and cracks in the coating are also increased after heat treatment. The amorphous coatings have excellent corrosion resistance, but the defects generated in the crystallized coatings are detrimental to the corrosion performance. The reason for the formation of amorphous coatings may be derived from rapid cooling and the glass-forming ability of the binary system of Ta2O5 and Al2O3.