Advancements in Eco-friendly Lubricants for Tribological Applications: Past, Present, and Future

Abstract

This chapter highlights the evolution of eco-friendly lubricants derived from natural oils and fats to green lamellar solid additives to a new class of “greener” functional fluids known as room temperature ionic liquids (RTILs). The attraction to these bio-based lubricants began with vegetable oils due to their low friction and wear properties. These superior tribological characteristics are a result of their chemical composition of triacylglycerol molecules made up of esters derived from glycerol and long chains of polar fatty acids. It is these fatty acids within the natural oils that establish monolayers that enable high lubricity in boundary-lubricated regimes. Despite these accolades, vegetable oils suffer from thermal-oxidative instability, high pour points, and inconsistent chemical compositions. To improve upon the tribological properties, vegetable oils were subjected to additives such as lamellar solid powders to establish more resilient transfer layers to mitigate wear and surface damage. Currently, RTIL lubricants derived from bio-based feedstock represent a promising potential solution to many of the problems associated with previous eco-friendly lubricants. An investigation into RTILs begins with a discussion on the history of ionic liquids and an assessment on their tribological properties. The chapter also includes a case study on the use of RTILs as additives in vegetable oils and as neat lubricants as well as exploring the effects of cation-anion moiety exchange within ionic liquids themselves. Ultimately, the RTILs are compared to more traditional bio-based lubricants for their tribological performance as a new class of eco-friendly lubricants and their potential as a future lubrication technology.