Tribological properties of Ti-doped diamond-like carbon coatings under dry friction and PAO oil lubrication

Titanium-doped diamond-like carbon (Ti-DLC) coatings with Ti concentration of 4 at.% (Ti_4at.%-DLC) and 27 at.% (Ti_27at.%-DLC) were prepared by a hybrid ion beam deposition system for comparison. The tribological behaviors of Ti-DLC coatings under dry friction and boundary lubrication conditions were systematically investigated. Results showed that, under dry friction, the Ti_4at.%-DLC coating displayed lower friction coefficient (0.07) and wear rate due to the continuous transfer film formed in the sliding interface, while Ti_27at.%-DLC coating was worn out at initial stage due to severe abrasive wear. And under boundary lubrication, both the Ti_4at.%-DLC and Ti_27at.%-DLC coatings showed excellent tribological properties attributing to the formation of oil film between sliding interface. In particular, Ti_27at.%-DLC performed the lowest wear rate of 1.12 × 10⁻¹⁶ m³ N⁻¹ m⁻¹ in this friction case. In conclusion, compared with Ti_27at.%-DLC coating, Ti_4at.%-DLC coating exhibited better tribological performances both under dry friction and boundary lubrication. The present result provides guidance for the selection of DLC coatings according to the realistic environment of starved-oil and rich-oil conditions.