Research on Microarc Oxidation Film on Niobium Surface

       After 2010, the preparation of micro-arc oxidation ceramic coatings by Nb surface micro-arc oxidation technology has just started. Only a few data reports have proposed the feasibility of Nb surface micro-arc oxidation technology, mainly in the biocompatibility of Nb and optical coatings. Exploratory research.



Sowa et al. prepared MAO coating in phosphoric acid solution for the first time, and the loading voltage control range was 20~140V. The oxide film breakdown phenomenon occurred under the condition of 140V voltage, the MAO coating prepared was very thin, the surface was porous, and the main component of the film was Nb205. In addition, the polarization curve results show that the open circuit potential of the oxide film decreases after breakdown and the corrosion current increases. In the same year, Sowa used K2SiO3 electrolyte to prepare micro-arc oxidation coating by improving the electrolyte, and the applied voltage range was 100~400V. In addition to Nb205, the structure of the MAO coating contains a small amount of amorphous SiO2, which is the physical and chemical reaction of SiO3- ions in the electrolyte during the plasma discharge process and participates in the growth of the MAO coating. However, as the concentration of K2SiO3 in the electrolyte increases, the corrosion resistance of the MAO coating decreases. The preparation of the ceramic membrane is expected to be applied in biomedicine to improve the biocompatibility of metal Nb. Therefore, Sowa et al. changed the electrolyte composition again and added Ca(H2PO2)2 and Mg(CH3COO)2 to the electrolyte to obtain a MAO coating with a certain Ca/P ratio, and the applied voltage was 200~500V. The results showed that the coating contained Nb205, NbO, Mg3(PO4)2, Ca2P207 and Ca2Mg5(PO4)6. Electrochemical impedance spectroscopy tested in simulated body fluids shows that the impedance of MAO coatings prepared at low voltage (200V) is better, and high pressure is not conducive to the corrosion performance of MAO coatings. Pereira et al. used a two-step method to prepare MAO coatings containing Ca/P. The electrolyte used in the first step is phosphoric acid system electrolyte, and in the second step, calcium acetate system electrolyte is used. The prepared MAO coating has higher hydrophilicity, and the main component of the coating contains a large amount of calcium phosphate, which improves the coating The biocompatibility. The MAO coating prepared by a single phosphoric acid system or calcium acetate system electrolyte has large surface pores, poor scratch resistance, thinner coatings, and less electrolyte participates in the coating growth; phosphoric acid + calcium acetate system electrolyte can increase coating thickness , Scratch resistance and increase the amount of electrolyte doping in the coating.



Nb205 can be used as electrochromic coatings, optical coatings, and applied to solar cell devices, electrical sensors, and catalysis. Stoiadinovic et al. also studied the basic properties of the coating before and after the breakdown of the Nb surface micro-arc oxidation oxide film in phosphoric acid electrolyte. Before breakdown, the oxide film is mainly amorphous Nb205; after breakdown, the oxide film is mainly composed of hexagonal-Nb205. The photocatalytic properties of the MAO coating are controlled by the number of oxygen vacancies in the coating. The more oxygen vacancies, the better the photocatalytic performance of the coating. Adding Eu2O3 and Sm2O3 to the electrolyte can deposit trivalent Eu and Sm into the coating and improve the photoluminescence properties of the coating.



There are few researches on Nb surface micro-arc oxidation technology, and further exploration is needed to promote the development and application of Nb in more fields. Moreover, compared with other metals or alloys (Ti, Al, Mg, etc.), the research on the growth mechanism of the Nb surface micro-arc oxidation coating is still blank, requiring research and exploration.