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Abstract:
Four-hour oxidation processes of niobium in an air atmosphere at temperatures of 400 °C, 425 °C, 450 °C and 500 °C were carried out. In order to characterise the layers produced, the cross-sectional microstructure, chemical and phase composition as well as surface roughness were examined. The mechanical properties of the surface were determined by performing Vickers microhardness tests. In order to verify the properties from a biological point of view, contact angle analysis and corrosion tests in Ringer's solution were carried out. The results revealed the formation of layers composed of a solid solution of oxygen in niobium Nb(O) at oxidation temperatures of 400 °C, a solution of Nb(O) and niobium pentoxide Nb2O5 at 425 °C, and Nb2O5 at 450 °C and 500 °C. Increased oxidation temperature resulted in an increase in hardness and surface roughness, and each process contributed to improved corrosion resistance. Oxidation at too high temperature (≥450 °C) caused degradation of the material's surface due to niobium's low heat resistance. At 450 °C the first cracks in the material were visible, and at 500 °C the layer was inhomogeneous, brittle and underwent significant chipping. The highest hardness, roughness and hydrophobic properties were shown by niobium oxidised at 500 °C, which underwent surface degradation at this temperature. In turn, niobium oxidised at 400 °C and 425 °C showed outstanding properties in the biological aspect, achieving both high hydrophilicity and the highest corrosion resistance.

Keywords:
Niobium, Oxidation, Microstructure, Corrosion, Contact angle, Surface engineering