Tribological study of nitrogen plasma polymerized soybean oil with nitrogen heterocyclic structures

Two types of polymerized oils with high viscosities were synthesized by nitrogen plasma polymerization of soybean oil. The results of elemental analysis indicated that the N atoms were incorporated into the molecule of polymerized oil, which was pyrolyzed by pyrolysis gas chromatography with a mass selective detector to clarify the structure with nitrogen atoms. It was confirmed that in the molecule of polymerized oil, there were three nitrogen heterocyclic compounds, which played a key role in improving the tribological characteristics of the polymerized oils. The lubricating properties of polymerized oils were performed on the four ball friction and wear testers. The load-carrying capacities of polymerized oils reached 940.8 N and 1049 N, respectively, higher than that of soybean oil (646.8 N). Meanwhile, they showed better anti-wear properties under all tested loads and possessed preferable friction-reducing performances when the applied load surpassed 250 N. The friction surfaces lubricated by soybean oil and polymerized oil were observed by a scanning electron microscope (SEM), and the chemical states and compositions of the tribofilms generated during the rubbing process were analyzed by X-ray photoelectron spectroscopy (XPS). It was found that the nitrogen heterocyclic structure containing six N atoms possessed higher coordination capacity than the ester groups of soybean oil, and could form a durable organic nitrogen complex film on the metal surface. Simultaneously, the blended oils with different viscosity grades, which were prepared by diluting the polymerized oil with dioctyl sebacate, showed excellent receptivity on the anti-wear/extreme pressure additives of zinc dialkyl dithiophosphates and sulfurized isobutylene.