Effect of Ionicity of Three Protic Ionic Liquids as Neat Lubricants and Lubricant Additives to a Biolubricant

Hong Guo,Angela Rina Adukure,Patricia Iglesias

doi:10.3390/coatings9110713

Abstract

Friction and wear of sliding surfaces are responsible for important energy losses and negative environmental effects. The use of environmentally friendly and cost-effective protic ionic liquids as neat lubricants and lubricant additives has the potential to increase the efficiency and durability of mechanical components without increasing the environmental damage. In this work, three halogen-free protic ionic liquids with increasing extent of ionicity, 2-hydroxyethylammonium 2-ethylhexanoate, 2-hydroxymethylammonium 2-ethylhexancate, and 2-hydroxydimethylammonium 2-ethylhexanoate, were synthesized and studied as neat lubricants and additives to a biodegradable oil in a steel–steel contact. The results show that the use of any protic ionic liquid as a neat lubricant or lubricant additive reduced friction and wear with respect to the biodegradable oil. The ionic liquid with the lowest ionicity reached the highest wear reduction. The one possessing the highest ionicity presented the poorest friction and wear behaviors as a neat lubricant, probably due to the more ionic nature of this liquid, which promoted tribocorrosion reactions on the steel surface. This ionic liquid performed better as an additive, showing that a small addition of this liquid in a biodegradable oil is enough to form protective layers on steel surfaces. However, it is not enough to accelerate the wear process with detrimental tribocorrosion reactions.

Highlights

Friction and wear of sliding surfaces are responsible for important energy losses and negative environmental effects
protic ionic liquids (PILs) tri-[bis (2-hydroxyethylammonium)] citrate is used as an additive in 2 wt.% to a mineral oil in aluminum–steel contact, it will form a tribolayer on the substrate surface, which may be responsible for a reduction of 16% in friction and 40% in wear compared with that just using mineral oil
The proton nuclear magnetic resonance (1 H NMR) spectra in Figure 1 confirmed the molecular structures of the three PILs [40]

Summary

Introduction

Friction and wear of sliding surfaces are responsible for important energy losses and negative environmental effects. Some researchers have quantified friction and wear losses to comprise as much as 23% of the world’s total energy consumption [1]. 20% of these energy losses are used to overcome friction and 3% to replace worn parts of machines. The use of advanced lubricants, new materials, and the application of novel surface technologies may reduce friction and wear losses of rubbing surfaces up to 40%, which corresponds to savings in the range of 1.00%–1.55% of a country’s gross domestic product [2,3]. The application of tribological advances has important environmental benefits. According to Holmberg and Erdemir [1], new advances in tribology can result in significant CO2 emission reductions

Methods

Results

Conclusion