Functionalized carbon nanostructures as lubricant additives - A review

In recent years, increased environmental concern and regulations have fueled a search for new lubricant addi-tives with reduced environmental impact. Owing to their attractive mechanical, thermal, and chemical prop-erties, carbon nanostructures have been proposed as potential candidates for a new generation of friction -modifying and antiwear additives. The aim of this literature review was to survey and systematize the growing body of literature on the topic of carbon nanostructures as lubricant additives and contribute to the establishment of better guidelines for future research and development on the subject. The various carbon nanostructures can exhibit a wide range of properties, and this literature review identified dimensionality, structural integrity and the nature of chemical interactions as particularly important properties governing the tribological behavior of carbon nanostructures. Critical evaluation of the theoretical basis for the proposed lubrication mechanisms revealed that several mechanisms are still quite poorly understood. However, despite certain ambiguities at the conceptual level, it can be concluded that certain carbon nanostructures (particularly, two-dimensional graphene-based nanostructures and carbon quantum dots) are promising candidates for a new generation of lubricant additives due to their excellent properties as carrier materials and the well-established chemistry of carbon. Together, these properties offer several routes for surface functionalization and endless opportunities for tailoring of additive properties. As a result, surface functionalization has the potential to not only affect dispersibility but also enhance the inherent lubrication mechanisms of the nanostructure and enable development of multifunctional additives.