Detailed surface analyses and improved mechanical and tribological properties of niobium treated by high temperature nitrogen plasma based ion implantation

Transition metal nitrides, as is the case of NbN, exhibit an attractive mixture of physical, chemical and mechanical properties and can be used to overcome a severe constraint of pure niobium, that is the high oxidation rate for temperature above 400 °C. In this work nitrogen ions were implanted into niobium by means of plasma based ion implantation (PBII) and high temperature PBII (HTPBII) in order to produce NbN on the surface of the material. In the case of HTPBII the treatment was performed in the temperature of 1000 °C and 1250 °C. In the process, negative pulses of 10 kV/20 μs/500 Hz were applied to Nb samples for 1 h. The depth of the modified layer reached up to 4.5 μm due to the diffusion of nitrogen atoms implanted into the material. X-ray diffraction spectra showed the presence of ɛ-NbN, β-Nb2N and γ-Nb4N3 phases. Wear rate was reduced from 1.5 × 10−2 mm3/Nm up to 2.6 × 10−6 mm3/Nm for treated samples at high temperature in comparison with pristine samples, while friction coefficient was reduced from 0.8 to 0.25. Hardness was significantly increased. Surface topography was measured by optical profilometry. Surface roughness increases with the sample temperature but it remains lower than the one obtained by conventional PBII, very probably due to the heating method, which was performed by electron bombardment.