Evolution of high vacuum tribological performance of lead-doped hydrogenated diamond-like carbon coatings after atomic oxygen and ultraviolet irradiation

Pb/H-DLC coatings with varying Pb content were deposited using a unbalanced magnetron sputtering system, and their tribological properties were tested in vacuum. For the two coatings exhibiting superior performance, the surface damage caused by space irradiation was investigated in ground-simulated space environments, including atomic oxygen (AO) erosion and ultraviolet (UV) irradiation. The irradiation damage and friction mechanisms were analyzed using surface/interface characterization techniques. The results reveal that the hydrogen content and the ratio of I_D/I_G in the Pb/H-DLC coatings initially decrease and then increase with increasing the Pb content. The Pb/H-DLC coatings show improved mechanical and vacuum tribological properties when the Pb content is 0 at% and 2.4 at%. The lowest average friction coefficient is 0.014, and the lowest wear rate is 3.8 × 10⁻⁸ mm³N⁻¹m⁻¹. AO erosion induces graphitization and oxidation on the Pb/H-DLC coating surfaces, and the tribological properties of coating are deteriorated, while Pb doping effectively mitigates damage to the coatings' surface properties caused by AO erosion. The reduction in friction coefficient of the Pb/H-DLC coatings after UV irradiation can be mainly attributed to the formation of a soft layer consisting of HO-CO groups on the surface due to irradiation.