Ionic Liquids as Extreme Pressure Additives for Bearing Steel Applications

Mariana DONATO^1,5, Pranjal NAUTIYAL², Jonas DEUERMEIER³, Benilde SARAMAGO⁵, Luís C. BRANCO¹, Rogério COLAÇO⁴, Robert CARPICK^2
1LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Lisbon, Portugal
²Department of Mechanical Engineering & Applied Mechanics, University of Pennsylvania, Philadelphia, United States
³CENIMAT|i3N and CEMOP/UNINOVA, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Lisbon, Portugal
⁴IDMEC-Instituto de Engenharia Mecânica, Departamento de Engenharia Mecânica, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
⁵Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal

The automotive industry is one of the main responsible for energy consumption, primarily due to energy losses in tribological contacts. Efficient lubrication using low viscosity oils is an important contribution to reduce those energy losses.
Ionic Liquids (ILs) are low-melting organic salts that have shown very interesting properties as neat lubricants or lubricant additives. They exhibit high chemical and thermal stability, almost negligible vapor pressure, high ionic conductivity, non-flammability and ease in dissolving organic, inorganic and polymeric materials. One of the most attractive characteristics of ILs is related with the possibility to design the cation-anion combinations according to the desired properties as well as the final application. By choosing the adequate cation/anion combination, low viscosity fluids with adequate lubricant performance for specific metallic contacts can be obtained.
This work focuses on the synthesis and application of ILs for lubrication of bearing steel, as it is widely used in vehicle parts manufacturing, due to its hardness and high mechanical resistance.
Several ionic liquids were studied and two of them stand out as promising additives: 1-hexyl-methyl imidazolium triflate [C6mim][TfO] and 1-hexyl-4-picolinium triflate [C6-4-pic][TfO]. These ILs were studied as 2 wt.% additives to base oil polyethylene glycol MW 200 (PEG 200) in extreme pressure conditions (1.12 GPa) using a Mini-Traction Machine (MTM). MTM tests were conducted under mixed rolling-sliding conditions, which mimics several real-world tribological contacts. Boundary lubrication conditions were chosen in order to assess the IL’s contribution in terms of friction and wear reduction. The additives showed very interesting properties, particularly in terms of surface protection: [C6mim][TfO] 2%PEG allowed for a decrease in wear up to ~76% and [C6-4-pic][TfO] 2%PEG up to ~46% when comparing to neat PEG 200. We hypothesize that these ILs are surface active and readily adsorb to the steel surface, forming protective tribofilms that reduce wear between the contacting surfaces under boundary regime.
The relevance of these results derives from the possibility of using these IL mixtures as competitive greener alternatives to commercial lubricants as they cause less wear of bearing steel moving parts, allowing for great savings in replacing damaged parts, as they will last longer.
 
 
Acknowledgements
The work was financed by the Portuguese Foundation for Science and Technology (FCT) through the projects UIDB/00100/2020, UIDP/00100/2020 and IMS-LA/P/0056/2020 and through the PhD grant SFRH/BD/140079/2018. Additionally, this work was financed by national funds from FCT in the scope of the projects UIDB/50022/2020 (IDMEC/LAETA) and LA/P/0037/2020, UIDP/50025/2020 and UIDB/50025/2020 of the Associate Laboratory Institute of Nanostructures, Nanomodelling and Nanofabrication – i3N.