Titanium doped MoSe₂ coatings – Synthesis, structure, mechanical and tribological properties investigation

In this work, we doped sputtered MoSe₂ coatings with Ti to improve their mechanical and tribological properties. Mo-Se-Ti coatings were prepared onto steel discs by High Target Utilisation Sputtering (HiTUS). The Ti was in the range 7–31 at.%, which led to Se/Mo ratios of 1.7–2.7. The Mo-Se-Ti coatings possessed a polycrystalline structure for the lower Ti contents and became amorphous when Ti content was above approx. 18 at.%. The hardness of the initially soft (0.3 GPa) pure MoSe₂ coatings was dramatically improved with Ti additions (up to 7.5 GPa). Sliding experiments confirmed excellent tribological properties: coefficient of friction values from 0.02 to 0.08 and very low wear rates (8×10^-8mm³N^-1m⁻¹) of the Ti-containing coatings tested in the humid air at the loads in the range of 2–45 N. The outstanding self-lubricant properties of Mo-Se-Ti coatings were attributed to the transformation of the topmost amorphous part of the coating into crystalline MoSe₂ tribofilm while preserving inner parts of the coating in as-deposited state ensuring good load-bearing capacity. There is strong experimental evidence that Ti has a gettering effect in the tribological contact, which protects MoSe₂ tribofilm against oxidation.