Mariana DONATO1,2, Jonas DEUERMEIER3, Rogério COLAÇO4, Luís C. BRANCO1, Benilde SARAMAGO2
1LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Lisbon, Portugal
2Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
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
Nano and microelectromechanical devices (NEMs and MEMs) are miniaturized devices usually with silicon in their composition, which are significantly changing our day-to-day lives. In 2018, the global total value in the market for these devices was 10 billion euros and that value is expected to reach 50 billion until 2024. This set of new devices lacks efficient lubrication and the scientific community has been trying to find greener alternatives to the commercial lubricants.
Ionic Liquids (ILs) are low-melting organic salts with interesting physico-chemical properties such as 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 in several research fields.
One of the potential applications of ILs is in the lubricants field, in which they have shown very interesting properties, namely for NEMs/MEMs lubrication. ILs are conductive fluids, so they lead to the minimization of the contact resistance between sliding surfaces, which is an important parameter for various electrical applications. However, since the manufacturing costs of ILs are usually high, their application as neat lubricants is impracticable. A viable solution to overcome this drawback is the use of ILs as additives to commonly used base oils.
Protic Ionic liquids (PILs) are a subclass of ILs that have recently been considered as potentially good lubricants. The PILs composed by protonated ammonium cations possess the advantage of being easily prepared with low cost and toxicity. Besides that, they exhibit low viscosity when compared to the conventional aprotic ionic liquids, making them good candidates as lubricants.
In this work, we report the use of protic ILs based on different organic cations combined with two anions (hydrogen sulfate and methyl sulfate) as additives to the commonly used base oil polyethylene glycol MW 200 (PEG 200). All the prepared lubricants were characterized in terms of their water content, viscosity, wettability and tribological properties. Two tribological pairs were tested: steel and silicon spheres against Si surfaces. Friction and wear were determined and the worn sliding surfaces were imaged and chemically analyzed in order to understand the wear mechanisms. The most promissory PILs containing the protonated cation 4-picolinium showed a good tribological performance, both in terms of friction and wear reduction comparing to commercial lubricant PEG 200 making them very good candidates for future applications in electronic devices.
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.