Tribological evaluation of a graphene-based lubricant in metal-on-metal and wet clutch interfaces

Abstract

Certain nanoparticles have been consistently demonstrated to exhibit remarkable lubricant improvements as used as additives. In particular, graphene nanosheets (GNS) added to lubricating oils have shown significant reduction of wear and coefficient of friction (CoF) in a wide range of metallic interfaces for different applications. Wet clutches from automatic transmissions are lubricated with high-performance automatic transmission fluid (ATF). Their efficiency, in terms of torque transmission by friction, depends on the tribological behavior of both the wet clutch sliding plates and ATF. However, commercial ATFs need continuous enhancement to comply new and more stringent OEM requirements. One of those requirements is to maintain the short- and long-term antishudder property of ATFs. This property, which is represented by a friction coefficient increasing with sliding speed, helps to keep reduced vibrations during clutch engagements and lock-up stages in the transmission, which is needed for driving comfort and increased durability of involved components in the transmission. Considering the tribological effectiveness of GNS in lubricants for reducing friction and wear, it can be a suitable additive for improving the antishudder property of ATFs. Although GNS have been largely studied as additives in lubricants for several metallic interfaces, their effects on wet clutches interfaces have been scarcely studied. Thus, this thesis contributes with an assessment of the tribological (friction and wear) behavior of graphene-based oils (blends of mineral base oil + GNS) at different concentrations in metal-on-metal contacts and the CoF behavior with speed (antishudder representation) in a wet clutch interphase using a pin-on-disc tester. The tests ran in metal-on-metal interfaces confirmed a significant wear rate reduction with unaltered CoF in aluminum and steel when a mineral oil with 1.0 wt.% GNS concentration was used in tests. In addition, the CoF of the wet clutch was reduced in general, while the antishudder property was enhanced by the addition of 1.0 wt.% GNS to the mineral oil.