Triboelectrification of Metallic Surfaces Lubricated by Used Oils

ABSTRACTThe present work investigates the tribological properties of used oils such as friction coefficient and wear. Besides, the voltage generated from the triboelectrification of the sliding surfaces lubricated by the used oils was measured. It was found that, friction coefficient displayed by used oil I significantly decreased with increasing the load, while friction slightly increased with increasing the running distance. For used oil II, friction coefficient increased up to maximum at 6000 km then decreased with increasing running distance. Blending the used oil III by fresh one slightly increased friction coefficient up to maximum then decreased with increasing running distance. The sliding velocity showed insignificant effect on friction coefficient displayed by used oil I. Wear displayed by the used oil I increased as the running distance increased. As the load increased wear significantly increased indicating the loss of the lubricating properties of the used oil. In contradiction to the friction wear showed significant decrease with increasing running distance for used oil II. At lower load, wear slightly increased with increasing fresh oil content blending used oil, while at higher load wear decreased with increasing fresh oil content. The relatively higher velocity displayed wear higher than that shown for the lower velocity. As the sliding velocity increased the voltage increased. Voltage generated by the triboelectrification of the sliding materials, lubricated by used oil I, decreased with increasing running distance. The voltage generated showed drastic decrease with increasing running distance for used oil II. Voltage significantly increased with increasing the fresh oil content blending the used oil. It seems that generation of carbon soots was responsible for the voltage decrease, where the oil conductivity increased. This behaviour can be used to test the validity of the used oil and to determine its life time.