Titanium alloys offer many advantages such as high strength-to-weight ratio, corrosion resistance and biocompatibility, which make them suitable for various applications including automotive, aerospace, medical implants, etc. However, potential limitations in wear resistance caused by adhesive wear and relatively low hardness may hinder their application if a direct contact with other parts is needed. Appropriate surface treatments, lubrication strategies as well as design considerations can help to mitigate wear-related issues associated with titanium-based materials. Nitriding process, involving diffusion of nitrogen into the surface layer of the treated material and formation of nitride coating, has been proved as efficient method of increasing surface hardness and improving wear resistance. Recent reports showed that nitriding may be even more effective if surface plastic deformation, providing higher density of grain boundaries and/or generating a large amount of dislocations in the substrate subsurface area, is applied. Grain boundaries and dislocations provide pathways for nitrogen diffusion accelerating the formation of protective TiN coating. Following the phase transformations during nitriding process is possible only though advanced micro- and nanoscale studies.
Therefore, the effect of the surface plastic deformation of the Ti-6Al-4V ELI alloy on the formation of TiN coating during gas nitriding was investigated. The substrate material was subjected to turning, slide burnishing and/or shot peening in order to obtain sub-surface grain refinement. Hard TiN coating was produced with low-temperature gas nitriding realized at 540°C for 8, 16 and 24 h. The surface topography was investigated with contact profilometer. The microstructure analysis including microstructure observations as well as phase and chemical composition determination were carried out with scanning (SEM) and transmission (TEM) electron microscopes, both equipped with energy dispersive X-ray spectrometers (EDS). The microhardness of the sample was measured with a Vickers indenter.
The microstructure characterization with SEM and TEM microscopes confirmed that the surface of the nitrided alloy is uniformly covered with nitride coating. However, it should be noticed that the amorphous tribo-layer may act as an nitrogen diffusion barrier. In addition, the cross-sectional SEM studies showed that subsurface area may recrystallize during the mechanical surface treatment due to increased temperature resulting in the coarsening of -Ti grains simultaneously decreasing the growth rate of TiN layer. The latter is also hindered by the presence of aluminium oxide 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. For all investigated variants in the micro scale insignificant differences in wear resistance and dynamic friction after tribological tests with applied load F = 5 N were obtained. However, the surface layers resulting from the applied sequential treatments were characterized by different tribological properties in the range of loads F = 1 ÷ 3 N.