Nanotribology
Nanotribology is the branch of tribology that studies friction, wear, adhesion and lubrication phenomena at the nanoscale, where atomic interactions and quantum effects are not negligible.
[1][2][3] Microscopy techniques, including Scanning Tunneling Microscope (STM), Atomic-Force Microscope (AFM) and Surface Forces Apparatus, (SFA) have been used to analyze surfaces with extremely high resolution, while indirect methods such as computational methods[4] and Quartz crystal microbalance (QCM) have also been extensively employed.
[5][6] Changing the topology of surfaces at the nanoscale, friction can be either reduced or enhanced more intensively than macroscopic lubrication and adhesion; in this way, superlubrication and superadhesion can be achieved.
In micro- and nano-mechanical devices problems of friction and wear, that are critical due to the extremely high surface volume ratio, can be solved covering moving parts with super lubricant coatings.
On the one hand, the scientific approach of the last centuries towards the comprehension of the underlying mechanisms was focused on macroscopic aspects of tribology.
In 1969 the very first method to study the behavior of a molecularly thin liquid film sandwiched between two smooth surfaces through the SFA was developed.
[7] From this starting point, in 1980s researchers would employ other techniques to investigate solid state surfaces at the atomic scale.
Direct observation of friction and wear at the nanoscale started with the first Scanning Tunneling Microscope (STM), which can obtain three-dimensional images of surfaces with atomic resolution; this instrument was developed by Gerd Binnig and Henrich Rohrer in 1981.
Thanks to these techniques, the nature of bonds and interactions in materials can be understood with a high spatial and time resolution.
SFA 2000, which has fewer components and is easier to use and clean than previous versions of the apparatus, is one of the currently most advanced equipment utilized for nanotribological purposes on thin films, polymers, nanoparticles and polysaccharides.
SFA 2000 has one single cantilever which is able to generate mechanically coarse and electrically fine movements in seven orders of magnitude, respectively with coils and with piezoelectric materials.
And so, the adhesion force is measured with the following formula: Using the DMT model, the interaction energy per unit area can be calculated: where
[14][15][2] The Scanning Tunneling Microscope is used mostly for morphological topological investigation of a clean conductive sample, because it is able to give an image of its surface with atomic resolution.
The Atomic Force Microscope is a powerful tool in order to study tribology at a fundamental level.
It provides an ultra-fine surface-tip contact with a high refined control over motion and atomic-level precision of measure.
Contact mode is commonly applied on hard sample, on which the tip cannot leave any sign of wear, such as scars and debris.
[21] The atomic force microscope can be used as a nanoindenter in order to measure hardness and Young's modulus of the sample.
For this application, the tip is made of diamond and it is pressed against the surface for about two seconds, then the procedure is repeated with different loads.
[13] In an atomistic simulation, every single atom's motion and trajectory can be tracked with a very high precision and so this information can be related to experimental results, in order to interpret them, to confirm a theory or to have access to phenomena, that are invisible to a direct study.
[13] It has been demonstrated with an atomistic simulation that the attraction force between the tip and sample's surface in a SPM measurement produces a jump-to-contact effect.
[25] Simulating an AFM scansion in contact mode, It has been found that a vacancy or an adatom can be detected only by an atomically sharp tip.
Whether in non-contact mode vacancies and adatoms can be distinguished with the so-called frequency modulation technique with a non-atomically sharp tip.
For instance, Amonton's second law states that friction coefficient is independent from the area of contact.
Two thin films made of perfluorinated lubricants (PFPE) with different chemical composition were found to have opposite behaviors in humid environment: hydrophobicity increases the adhesive force and decreases lubrication of films with nonpolar end groups; instead, hydrophilicity has the opposite effects with polar end groups.
[33] Even if at the macroscopic scale friction involves multiple microcontacts with different size and orientation, basing on these experiments one can speculate that a large fraction of contacts will be in superlubric regime.
This leads to a great reduction in average friction force, explaining why such solids have a lubricant effect.
Other experiments carried out with the LFM shows that the stick-slip regime is not visible if the applied normal load is negative: the sliding of the tip is smooth and the average friction force seems to be zero.
[36] With the introduction of AFM and FFM, thermal effects on lubricity at the atomic scale could not be considered negligible any more.
[40] On the other hand, adhesion was also investigated for its biomimetic applications: several creatures including insects, spiders, lizards and geckos have developed a unique climbing ability that are trying to be replicated in synthetic materials .
At the nanoscale, however, measuring such volume can be difficult and therefore, it is possible to use evaluate wear by analyzing modifications in surface topology, generally by means of AFM scanning.
