Friction control by load-induced structure modification of overbased detergent in fully formulated lubricant

The increasing application of diamond-like carbon (DLC) coatings in engine mechanical parts requires the investigation on interaction mechanism between DLC and fully formulated engine oil. This study shows that the friction can be controlled by monitoring the load for a steel/DLC contact, lubricated by a fully formulated 5W30 engine oil in boundary/mixed lubrication regime. A calcium carbonate-rich patchy tribofilm, mainly formed on steel ring, was characterized by multiple surface analyses. X-ray photoelectron spectroscopy (XPS) indicates an increase quantity of calcium carbonate (CaCO3) with increasing contact load. Transmission Electron Microscope (TEM) showed that the original calcium carbonate in the overbased detergent additive within the lubricant itself was amorphous. However, the amorphous calcium carbonate crystallizes in vaterite (low load), aragonite (medium load) and calcite (high load) during the tribological tests. The intrinsic lubricity of each calcium carbonate polymorph was estimated by density functional theory calculations, while the tribological properties of the sliding interface composed by H-passivated diamond and CaCO3 polymorphs were investigated by ab initio molecular dynamic simulations. The in-depth investigation presented here sheds light into the main factors leading to the unexpected pressure-induced friction reduction.