Superlubricity of TiN coating using glycerol with the addition of Cu nanoparticles

Superlubricity was achieved by the addition of copper nanoparticles (CuNPs) as an additive to glycerol, a sustainable lubricant, which was applied to TiN-coated steel that underwent ball-on-flat reciprocating testing at an average contact pressure of 54.6 MPa. A coefficient of friction of ∼ 0.007 and a wear rate of 2.0 × 10–8 mm3/N m were achieved on the TiN coating when lubricated with glycerol + 0.1 wt% CuNPs at a sliding speed of 120 mm/s. XPS and Raman analysis suggested that a surface tribofilm containing CuNPs and a carbon-based film derived from the degradation of glycerol were deposited onto the contact area of the tribopairs, which effectively prevented the direct asperity contact between the steel ball and TiN-coated steel, enabling ultra-low friction.