Formation of plasma electrolytic oxidation coatings on pure niobium in different electrolytes

Abstract

The development of PEO surface treatments for Nb (niobium) provides a possibility to produce functionalized coatings with a combination of new compositions and phases. In the frame of this work, PEO coatings on pure Nb are produced in three electrolytes containing aluminate, phosphate and silicate. Furthermore, the influence of the electrolyte composition on the PEO process and the microstructure, composition and properties of the coatings were studied. It was observed that most of the coating forming species did not participate in the plasma reactions directly, while the chemical-, electrochemical- and thermal stimulated reactions are dominating the coating formation. The results show that all the coatings are mainly composed of the Nb2O5 phase. An additional mixture of oxides (Al2O3 and AlNbO4) is found in the coating formed in aluminate-containing electrolyte, and the coating formed in phosphate-containing electrolyte contains a small amount of Nb2(PO4)3 and amorphous phosphate. No additional crystalline phase other than the amorphous silica phase is detected in the coating formed in silicate-containing electrolyte. Additionally, different morphology of the coatings is revealed and the most uniform coating is produced in silicate-containing electrolyte, which further improves its corrosion resistance. This coating also shows the highest photocatalytic activity due to the combination of Nb2O5 and silica. Since all PEO coatings on pure Nb show superior corrosion resistance and photocatalytic activity, they might be interesting for a wide range of applications ranging from transportation, biomedical implants to environmental protection.