Mechanical and unlubricated tribological properties of titanium-containing diamond-like carbon coatings

Titanium-containing diamond-like carbon (Ti-DLC) coatings were deposited on steel with a close-field unbalanced magnetron sputtering in a mixed argon/acetylene atmosphere. The morphology and structure of Ti-DLC coatings were investigated by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Nanoindentation, nanoscratch and unlubricated wear tests were carried out to evaluate the hardness, adhesive and tribological properties of Ti-DLC coatings. Electron microscopic observations demonstrated the presence of titanium-rich nanoscale regions surrounded by amorphous carbon structures in Ti-DLC coating. The Ti-DLC coatings exhibit friction coefficients of 0.12-0.25 and wear rates of 1.82 × 10−9 to 4.29 × 10−8 mm3/Nm, depending on the counterfaces, sliding speed and temperature. The Ti-DLC/alumina tribo-pair shows a lower friction coefficient than the Ti-DLC/steel tribo-pair under the identical wear conditions. Increasing the test temperature from room temperature to 200 °C reduces the coefficient of friction and, however, clearly increases the wear rate of Ti-DLC coatings. Different wear mechanisms, such as surface polishing, delamination and tribo-chemical reactions, were found in the tribo-contact areas, depending on different wear conditions.