Effect of Biodiesel Soot on Tribological Behavior of Liquid Paraffin

Biodiesel soot（BDS） was collected from the combustion of biodiesel using a self-made soot trap. The effect of BDS on the tribological behavior of liquid paraffin（LP） was investigated using a four-ball tribometer. A rotating viscometer was used to investigate the effect of BDS on the viscosity of LP. The morphology, composition, and tribological mechanism of BDS were studied by means of FETEM, XRD, XPS, SEM/EDS, and the 3D laser scanning microscopy. Test results showed that the BDS aggregates were chain-like, and the average diameter of BDS was 35 nm. The BDS existed in the form of graphitic layers and amorphous carbon. The oxygen-containing functional groups in BDS consisted of the（C-O-C） and（C-O-H）. With an increasing BDS content, the dynamic viscosity of LP increased and the maximum non-seizure load increased initially and became stable later. In addition, the average wear scar diameter（AWSD） of LP increased and the average friction coefficient of LP decreased at first and then increased later. The tribological mechanisms could be ascribed to the variation in content of BDS: BDS could act as a friction modifier for a lower friction coefficient in case of low BDS content. However, the BDS aggregates could lead to increase of abrasive wear to influence the lubricating oil film at higher content of BDS, which would reduce the friction reduction ability and wear resistance of LP.