Ti-6Al-4V ELI alloy subjected to mechanical surface modification and gas nitriding

Abstract text (incl. figure legends and references)

Titanium alloys, like most widely used Ti-6Al-4V, are well known for their high specific strength, biocompatibility and corrosion resistance. However, relatively low wear resistance hinders its eventual application when a direct contact with other ones is required. Thus, a number of methods have been developed with the aim to increase the wear resistance and extend the life-time of the parts produced of such materials. Nitriding has been confirmed as one of the most promising methods of producing a hard and well adhering TiN coating on the top of Ti-based material. Recent reports indicate that nitriding may be even more effective in temperature-time scale if fast diffusion channels are formed by increasing the density of grain boundaries and/or generating a large amount of dislocations in the substrate subsurface area by its plastic deformation. However, understanding the phase transformations taking place at the surface is possible only though the micro- and nanoscale study carried out with the help of the cutting-edge electron microscopes.

Therefore, in this work, the influence of the surface plastic deformation of the Ti-6Al-4V ELI alloy on the formation of TiN coating during gas nitriding was studied. The substrate material was subjected to turning and subsequent low-temperature gas nitriding realized at 540°C for 8, 16 and 24 h. The surface topography was analyzed with Hommel Tester T1000. The microstructure characterization, phase and chemical analysis were carried out with ThermoFisher"s scanning (SEM) and transmission (TEM) electron microscopes i.e. Scios 2 Dual Beam and Themis G2 200 FEG, respectively, both equipped with energy dispersive X-ray spectrometers (EDS). The surface analysis was also done with the scanning Microprobe PHI 5000 Versa Probe II X-ray photoemission spectrometer (XPS). The microhardness of the sample was determined with an FM 7 stand equipped with a Vickers indenter.

The SEM, TEM and XPS characterization of the subsurface area allowed to reveal that the whole surface of the as-turned alloy is coated with un-evenly distributed amorphous tribo-layer acting as an effective nitrogen diffusion barrier preventing formation of titanium nitrides. The growth of TiN layer is also hindered by the presence of continuous a-Al2O3 at the tribo-material/substrate. The thickness of TiN layer after 24h of gas nitriding was measured to be ~100 nm. The results of indentation experiments showed an increase in hardness by ~10% measured for as-machined material and after 24 h of gas nitriding treatment.

Acknowledgment: This work was financially supported by National Science Centre of Poland, grant number UMO-2020/39/D/ST8/02610