Field Of Nanotribology Research Articles

Effect of temperature-induced contact quality evolution on nanoscale friction

Multiple subcontacts formation/rupture and puckering the evolution of contact quality play crucial roles in modulating friction on two-dimensional lamellar materials but have up to now been studied separately in the field of nanotribology. In this work, by using variable temperature friction force microscopy in ultrahigh-vacuum conditions, we investigate friction on graphite surfaces with different numbers of free edges as a function of temperature from 295.0 to 100.0 K. We introduce subcontacts formation/rupture simply by changing temperature and tune the out-of-plane deformation compliance by changing the number of free edges. It is found that friction on a plane with large deformation compliance is constantly larger than that with small deformation compliance at cryogenic temperatures. While appearing to be influenced secondly by deformation compliance, the coefficients of friction increase, however, linearly with temperature. By performing systematic stick-slip measurements, the aforementioned separate friction mechanisms have been linked, where the temperature-induced increase of subcontacts rupture force can give rise to the frictional contact quality which additionally becomes more prominent on a plane with large deformation compliance at cryogenic temperatures. These results emphasize a way to vary the contact quality via temperature and demonstrate that the subcontacts formation/rupture and the contact quality must be correlatively accounted for.

Atomistic observation on diffusion-mediated friction between single-asperity contacts.

The field of nanotribology has long suffered from the inability to directly observe what takes place at a sliding interface. Although techniques based on atomic force microscopy have identified many friction phenomena at the nanoscale, many interpretative pitfalls still result from the indirect or ex situ characterization of contacting surfaces. Here we combined in situ high-resolution transmission electron microscopy and atomic force microscopy measurements to provide direct real-time observations of atomic-scale interfacial structure during frictional processes and discovered the formation of a loosely packed interfacial layer between two metallic asperities that enabled a low friction under tensile stress. This finding is corroborated by molecular dynamic simulations. The loosely packed interfacial layer became an ordered layer at equilibrium distances under compressive stress, which led to a transition from a low-friction to a dissipative high-friction motion. This work directly unveils a unique role of atomic diffusion in the friction of metallic contacts.

In-Situ Nanotribological Properties of Ultrananocrystalline Diamond Films Investigated with Ambient Pressure Atomic Force Microscopy

The relationship between nanoscale friction and the surrounding environment has long been a critical issue in the field of nanotribology. Here, we utilized ambient pressure–atomic force microscopy to investigate the effect of environmental gas on nanoscale friction of ultrananocrystalline diamond (UNCD) films. The frictional forces were measured in an atomic force microscopy (AFM) chamber in the environmental range from an ultrahigh vacuum to near ambient pressure in the presence of oxygen, nitrogen, and water. We observed that friction increased with the pressure of the oxygen responsible for the oxidation of the surface of the UNCD, while that in nitrogen gas remained unchanged. Interestingly, friction decreased in water, due to the tribochemical reaction caused by surface passivation. When two diamond materials come into contact under water conditions, the water molecules are dissociated because of normal pressure between the AFM tip and diamond surface, and the dissociative water molecule adsorption passivates the surfaces of the diamond-coated tip and UNCD, resulting in a reduction of friction force. The chemical state of the UNCD surface in various environmental conditions was confirmed using near ambient pressure X-ray photoelectron spectroscopy. This result elucidates the role of vapor-phase oxygen and water in the tribological properties of carbon-based materials.

Squeezing and stick–slip friction behaviors of lubricants in boundary lubrication

The fundamental questions of how lubricant molecules organize into a layered structure under nanometers confinement and what is the interplay between layering and friction are still not well answered in the field of nanotribology. While the phase transition of lubricants during a squeeze-out process under compression is a long-standing controversial debate (i.e., liquid-like to solid-like phase transition versus amorphous glass-like transition), recent different interpretations to the stick-slip friction of lubricants in boundary lubrication present new challenges in this field. We carry out molecular dynamics simulations of a model lubricant film (cyclohexane) confined between molecularly smooth surfaces (mica)--a prototypical model system studied in surface force apparatus or surface force balance experiments. Through fully atomistic simulations, we find that repulsive force between two solid surfaces starts at about seven lubricant layers (n = 7) and the lubricant film undergoes a sudden liquid-like to solid-like phase transition at n < 6 monolayers thickness. Shear of solidified lubricant films at three- or four-monolayer thickness results in stick-slip friction. The sliding friction simulation shows that instead of shear melting of the film during the slip of the surface, boundary slips at solid-lubricant interfaces happen, while the solidified structure of the lubricant film is well maintained during repeated stick-slip friction cycles. Moreover, no dilation of the lubricant film during the slip is observed, which is surprisingly consistent with recent surface force balance experimental measurements.

Scaling Effects on Materials Tribology: From Macro to Micro Scale.

The tribological study of materials inherently involves the interaction of surface asperities at the micro to nanoscopic length scales. This is the case for large scale engineering applications with sliding contacts, where the real area of contact is made up of small contacting asperities that make up only a fraction of the apparent area of contact. This is why researchers have sought to create idealized experiments of single asperity contacts in the field of nanotribology. At the same time, small scale engineering structures known as micro- and nano-electromechanical systems (MEMS and NEMS) have been developed, where the apparent area of contact approaches the length scale of the asperities, meaning the real area of contact for these devices may be only a few asperities. This is essentially the field of microtribology, where the contact size and/or forces involved have pushed the nature of the interaction between two surfaces towards the regime where the scale of the interaction approaches that of the natural length scale of the features on the surface. This paper provides a review of microtribology with the purpose to understand how tribological processes are different at the smaller length scales compared to macrotribology. Studies of the interfacial phenomena at the macroscopic length scales (e.g., using in situ tribometry) will be discussed and correlated with new findings and methodologies at the micro-length scale.

Replenishment Speed of Depleted Scar in Submonolayer Lubricant

The evaluation of submonolayer lubricant mobility is becoming important in the field of nanotribology, in particular, in hard disk drives for realizing the near-contact or surfing–recording. This paper experimentally and theoretically investigates the replenishment speed of a depleted scar in a submonolayer lubricant caused by the head–disk contact. The theoretical analysis is based on continuum mechanics. The replenishment process of a submonolayer lubricant height profiles in a depleted scar caused by the head touchdown operation was experimentally measured for the Z-tetraol lubricant with a 0.24 nm mobile lubricant thickness and compared with the numerical simulation. It was found that the analytical replenishment process can fairly agree with the experimental one if the ratio of Hamaker constant to effective lubricant viscosity is properly determined. By using the validated basic equation, a simple but useful generalized formula is proposed to evaluate the replenishment speed in relation to the depleted scar width of the mobile lubricant, the lubricant thickness, and the ratio of Hamaker constant to effective lubricant viscosity.

Nanotribology and nanomechanics

The recent emergence and proliferation of proximal probes, in particular scanning probe microscopies (the scanning tunneling microscope and the atomic force microscope), the surface force apparatus, and of computational techniques for simulating tip-surface interactions and interfacial properties, have led to the appearance of the new field of nanotribology, which pertains to experimental and theoretical investigations of interfacial processes on scales ranging from the atomic- and molecular- to the microscope, occurring during adhesion, friction, scratching, wear, indentation, and thin-film lubrication at sliding surfaces. Nanotribological studies are needed to develop fundamental understanding of interfacial phenomena on a small scale and to study interfacial phenomena in micro/nanostructures used in magnetic storage systems, micro/nanoelectromechanical systems (MEMS/NEMS), and other applications. Proximal probes have also been used for nanomechanics and materials characterization studies. In this paper, we present a review of significant aspects of nanotribology, nanomechanics, and materials characterization studies conducted using an atomic force microscope.

Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy.

A few years after the invention of the scanning tunneling microscope (STM), the atomic force microscope (AFM) was developed. Instead of measuring tunneling current, a new physical quantity could be investigated with atomic-scale resolution: the force between a small tip and a chosen sample surface. This paper reviews progress and recent results obtained with AFM and other closely related techniques in the field of nanotribology, and attempts to point out many of the unresolved questions that remain. The goal of this paper is to demonstrate that AFM is capable of producing atomic-scale knowledge. As such, the authors will focus upon some of the contributions of the AFM to nanotribology. They will almost exclusively discuss results that shed light on the actual atomic and molecular processes taking place, as opposed to the more applied investigations of microscale properties which are also carried out with AFM. They will accompany this discussion by mentioning related theoretical efforts and simulations, although their main emphasis will be upon experimental results and the techniques used to obtain them, as well as suggested future directions. In many ways, AFM techniques for quantitative, fundamental nanotribology are only in a nascent stage; certain key issues such as force calibration,more » tip characterization, and the effects of the experimental environment, are not fully resolved or standardized. The authors thus begin with a critical discussion of the relevant technical aspects with using AFM for nanotribology. 289 refs.« less