Novel insight into tribology of carbon black soot particles in engine oil

An investigation into the interaction between soot and steel surfaces can improve the development of soot wear-resistant lubricants. In this study, we have tested the wear response of carbon black (CB; a surrogate for engine soot) on different steel surfaces using the high-frequency reciprocating rig and scanning electron microscopy - energy-dispersive X-ray spectroscopy. Results show that CB in 0W20 reacts to the preconditioned steel surface differently than the nonpreconditioned steel surface. Chemisorption of zinc dialkyldithiophosphates (ZDDP) on the steel surface prior to the wear test resulted in severe wear, but the shear-induced chemisorption of ZDDP on the steel surface showed the least wear. Changes in wear were related to the concentration of zinc and phosphorus, which represent antiwear phosphate film on the surface. Chemisorption of ZDDP prior to the test triggered an aggressive response by CB, as there was a complete removal of antiwear phosphate film. Thus, unprotected steel was exposed to sulfur, a corrosion enhancer in the lubricant that resulted into micropitting and grooves on the disk surface, a representation of the corrosion-abrasion wear mechanism. Shear-induced chemisorption of ZDDP on steel surface allowed retention of antiwear phosphate films. As a result, there were no grooves or severe micropitting on the steel surface, indicating a protection against CB. This study shows that preconditioning the steel can have a profound effect on the soot wear mechanism.