Nanotribology is a young and dynamic field of research which aims to investigate friction,
wear and adhesion phenomena down to the nanometer scale. Since these phenomena occur
in all natural, artificial or conceptual situations involving two surfaces (at least)
in contact or in close proximity to each other, it is not surprising that, knowingly
or not, many physicists, materials scientists, mechanical engineers or chemists have
to contend with these topics sooner or later in their careers.
This Thematic Series is intended as an “invitation to nanotribology” addressed to
the attentive readership of the Beilstein Journal of Nanotechnology. The first goal
of this Thematic Series is simply to make more colleagues and students aware of the
existence of this fascinating subject, which, in spite of its universality, is often
considered as a niche topic even at prestigious scientific conferences. The second
goal, which is closely related to the first one, is to provide fresh examples of the
most recent advancements in the field worldwide. This is substantiated in a series
of ten original contributions whose authors are quite well distributed around the
world (from India, China, Argentina, Cameroon, Russia and USA back to many countries
in Old Europe).
The covered topics include lubrication, surface preparation and theoretical models
of friction at the nanoscale. Regarding the first topic, this Thematic Series gives
examples of cutting-edge aqueous solutions including nanodiamonds [1] and novel materials
such as nitrogen-doped graphene oxide [2] and imidazolium-based ionic liquids [3]
used as additives to mineral oils. Standard large-scale applications to steel surfaces,
but also to a material of key importance in micro- and nanoelecromechanical systems,
i.e., silicon oxide, are recognized. The quality of the surface condition is addressed
experimentally by the example of cryogenically treated martensitic stainless steel
[4] and theoretically by an analysis of the influence of micro-dimple textures on
hydrodynamic lubrication [5]. On a more fundamental level, different authors have
modeled the influence of electrical double layers on hydrodynamic lubrication [6],
the occurrence of a second-order phase transition in ultrathin lubricant films [7]
and the velocity dependence of dry friction on crystal surfaces at the atomic scale
[8].
While many experimental techniques for materials characterization are those typical
of surface science (e.g., X-ray diffraction, SEM, TEM, XPS and Raman spectroscopy),
more specific to nanotribology are nanoindenters, nanotribometers, quartz force microbalance
and especially atomic force microscopy (AFM), which, without a doubt, has triggered
a true revolution in our understanding of friction at the atomic scale. This is exemplified
by the lattice resolved friction force images on oxidized silicon surfaces in the
tribochemistry study presented by the Bennewitz group [9]. On a larger scale, alternative
surface scan methods (e.g., circular mode AFM) for investigation of abrasive wear
are proposed by Noel et al. [10].
Considering the interdisciplinary nature of the subject, and the variety of materials,
lubricants, and possible applications, the previous examples, in spite of their high
quality, are still not enough to achieve our goals. For this reason, the research
articles in this Thematic Series are complemented by a rather comprehensive but concise
review paper [11], which was born out of the collaboration of nine researchers (experimentalists
and theoreticians) within the European COST Action MP 1303 “Understanding and Controlling
Nano and Mesoscale Friction”, which ran from 2013 to 2017. Here the covered topics
include but are not limited to controlled manipulation of nanoparticles, optically
trapped colloidal and ionic systems, superlubricity of graphene, sliding friction
of organic molecules, charge density waves, and perspectives of tuning friction using
photo-assisted reactions.
Hopefully, new ideas and further research work will be stimulated from this Thematic
Series, which could not have come to light without the contributions of all authors
and the constant support from the Beilstein-Institut and specifically, Dr. Wendy Patterson
and Dr. Uli Fechner.
Enrico Gnecco, Susan Perkin, Andrea Vanossi, and Ernst Meyer
Jena, Oxford, Trieste and Basel, August 2018