Surface modification of pure niobium by plasma nitriding

Plasma nitriding of pure niobium was investigated using a dc square waveform pulsed power supply of 5.0 kW, aiming to change the material surface characteristics due to heating and physical-chemical reactions caused by the ions and fast neutral bombardment during the treatment. Nitriding was performed for temperatures of 500, 750, 915 and 1080 °C, at 90% N2 + 10% H2 gas mixture, pressure of 1200 MPa (9 Torr), at 300 sccm gas flow rate and nitriding time of 7.2 ks. Characterization of the samples was carried out by means of SEM, XRD, nanoindentation technique, scratch testing and roughness analysis. Strong increase of the surface hardness was verified. Nanoindentation results indicate values on the order of 2.5 GPa for pure niobium in the start condition (as non-nitrided) and up to 21 GPa in the as-nitrided condition. The XRD patterns show the presence of ε-NbN and β-Nb2N hexagonal phases in addition to the original Nb peaks. Significant changes on surface morphology and roughness of the treated samples were observed for the studied conditions and are discussed considering the sputtering and plasma species bombardment effect.