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Abstract:
Niobium nitride (δ-NbN) coatings were deposited on AISI 316L austenitic steel using reactive DC magnetron sputtering. This study investigates the effects of air oxidation on the surface morphology, topography, roughness, nanohardness, adhesion, and wear resistance of NbN coatings. Their microstructure and thickness were analyzed by scanning electron microscopy (SEM), while surface morphology and roughness were assessed using atomic force microscopy (AFM), and surface topography was assessed by an optical profilometer. Nanohardness was measured using a Berkovich indenter. Adhesion was evaluated via progressive-load scratch testing and Rockwell indentation (VDI 3198 standard). Wear resistance was assessed using the “ball-on-disk” method. Both as-deposited and oxidized NbN coatings improved the mechanical performance of the substrate surface. Air oxidation led to the formation of an orthorhombic Nb2O5 surface layer, which increased surface roughness and reduced hardness. However, the brittle oxide also contributed to a lower coefficient of friction. Despite reduced adhesion and increased surface development, the oxidized coating exhibited a significantly lower wear rate than the uncoated steel, though several times higher than that of the non-oxidized NbN. Considering its good wear and corrosion performance, along with the bioactivity confirmed in earlier research, the oxidized NbN coating can be considered a promising candidate for biomedical applications.

Keywords:
Nb2O5, NbN, magnetron sputtering, oxidation, adhesion, wear, surface engineering

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