Ultralow friction induced by tribochemical reactions: a novel mechanism of lubrication on steel surfaces.

The tribological properties of two steel surfaces rubbing against each other are measured while they are in contact with 1,3-diketones of varying structure. Such systems show after a short running-in period ultralow friction properties with a coefficient of friction of as low as μ = 0.005. It is suggested that the extremely favorable friction properties are caused by a tribochemical reaction between the 1,3-diketones and the steel surfaces, leading to formation of a chelated iron-diketo complex. The influence of temperature and the molecular structure of the 1,3 diketo-lubricants onto the friction properties of the system is elucidated under both static and dynamic conditions. With progression of the tribochemical reaction, the sliding surfaces become very conformal and smooth, so that the pressure is greatly reduced and further wear is strongly reduced. All iron particles potentially generated by wear during the initial running-in period are completely dissolved through complex formation. It is proposed that the tribochemical polishing reaction causes a transition from boundary lubrication to fluid lubrication.