In a general perspective, lubricants facilitate motion between two contacting surfaces by minimizing friction. This results in minimal wear and tear which translates to lower maintenance costs, for the case of machines. Consequently, they have found application in every aspect of our lives that involve motion; our bodies included. Inopportunely, selection of the appropriate materials as the boundary lubricant has poised a great challenge. The material in question should possess both load-bearing properties, that is it should be solid like, and self-healing attributes, that is it should be liquid like, at the same time. These two prerequisite attributes should be present and act simultaneously. Unfortunately, this is not the case in real life applications. Generally, it has been seen that good load-carrying results in poor self-healing and vice versa. In quest to resolve this shortcoming, researchers have stumbled upon ionic liquids have been seen to be fascinating materials with unique combination of solid and liquid properties, nonetheless, they have not been investigated exhaustively.
In a recent publication in Advance Engineering Materials authored by Dr. Xiao Gong at Wuhan University of Technology and Dr. Lei Li at University of Pittsburgh carryout a thorough and critical review concerning the applicability of ionic liquids as boundary lubricants. Their main motivation was the high thermal stability and promising tribological properties of ionic liquids which trigger the need for better comprehension of the relationship between molecular arrangement of such ionic liquids confined to a solid and the tribological properties.
In their report, the authors performed an in-depth review of the fundamentals on boundary lubrication and the state-of-the-art lubricants. Secondly, they assessed the progress on the molecular structure of ionic liquids confined to a solid surface. Lastly, they engaged in a detailed discussion concerning the experimental and computational efforts on the ionic liquids as boundary lubricants where they prioritized emphasis on the effect of ionic liquid structure, solid substrates, and ionic liquid-solid interaction.
Xiao Gong and Lei Li observed that ionic liquids presented a better load-carry capability than many conventional boundary lubricants owing to their high tendency to form extended solid-like layers near a solid surface. Additionally, they noted that the assessed ionic liquids had very good thermal stability; an attribute highly desired specifically under tribological contact.
In a nutshell, Xiao Gong- Lei Li presented a thorough and expert review of the recent progress on ionic liquids potential applicability as boundary layers. In general, ionic liquids were seen to be promising material candidate for boundary lubrication. It was also seen that the tribological performance of ionic liquids thin film was determined by a wide range of factors, including: the chemical structure of anion and cation, solid substrate, ionic liquid-solid interaction, and possibly the experimental conditions such as humidity. Altogether, as with any other lubricant, the long-term stability, cost, as well as toxicity will always be the concern for real life applications therefore more studies should be undertaken. Dr. Lei Li indicated that “nanometer-thick ionic liquids are promoting as lubricants in hard disk drives and MEMS.“
Xiao Gong, Lei Li. Nanometer-Thick Ionic Liquids as Boundary Lubricants. Advanced Engineering Materials 2018, volume 20, 1700617.Go To Advanced Engineering Materials 2018