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Friction between two surfaces is due to nano- and micro-asperities at the interface that establish true contact and are responsible for the energy dissipation. To understand the friction mechanism, often single-asperity model experiments are conducted in atomic-force microscopes. Here, we show that the conventional interpretation of the typical results of such experiments, based on a simple mass-spring model, hides a fundamental contradiction. Via an estimate of the order of magnitude of the dissipative forces required to produce atomic-scale patterns in the stick-slip motion of a frictional nano-contact, we find that the energy dissipation must be dominated by a very small, highly dynamic mass at the very end of the asperity. Our conclusion casts new light on the behavior of sliding surfaces and invites us to speculate about new ways to control friction by manipulation of the contact geometry.

Most of the previous studies on fretting fatigue have been accomplished under constant contact load condition, and a less attention has been paid to cyclic contact load. That is the case in many engineering applications such as bearing cap bolted joints of the V-type engines and dovetail joint of turbines. In this study, the effect of contact geometry on fretting fatigue behavior of Al7075-T6 under cyclic normal contact loading is investigated by experiment, numerical simulation and analysis. Two contact types including flat-on-flat and cylinder-on-flat are considered in this study. The crack initiated at a lower cycle for cylinder-on-flat contacts. It was found that in comparison with the constant contact loading condition, the cyclic contact loadings have more damaging effect on fretting fatigue life, particularly for cylinder-on-flat contact. With the increase in the pad width, fretting fatigue life increased for flat and cylindrical pads, particularly for high cycle fatigue (HCF) conditions. Examination of the fretting scars was performed using optical microscopy. Crack propagation life was determined by numerical simulation using the commercial FE codes ABAQUS® and FRANC2D/L®. Crack initiation life was calculated by a combination of numerical simulation and experiment.

Graphene was grown by low-pressure chemical vapour deposition on polished steel samples using ethanol as a precursor at different pressures: 200 and 400 mTorr. The samples were characterized by Raman spectroscopy, and the Raman maps clearly show the formation of an inhomogeneous graphene coating with surface regions covered with single-layer, bilayer, and multilayer graphene, the last one covering most of the surface area of the sample when the sample was prepared at 400 mTorr. Scanning electron microscopy revealed the partial coverage of the sample surface when the graphene coating was prepared at a pressure of 200 mTorr. Friction measurements were taken using a micro-tribometer in a ball-on-flat contact geometry with a stainless steel ball as the counterbody. An important reduction in the coefficient of friction was verified even with partial coverage of the steel surface, and it was attributed to the formation of nanographitic structures in the wear track region and partially transferred to the counterbody.

During sliding low elastic modulus, materials can form ridges and distinct zones of contact vice the commonly observed Schallamach wave phenomena. It was shown that at low velocities (0.001 and 0.01 m/s), the contact geometry retained a single region of contact and at higher velocities (0.1 and 1 m/s) the contact separated into two distinct zones. It was also shown that this phenomenon was independent of the applied normal load.

The present work studies the different effects in the contact surfaces subjected to fretting movements and electric current flow on an austenitic stainless steel. The main purpose is to establish a relationship between the energy dissipation by friction and the energy dissipation by the Joule effect as well as the synergetic effects. As this type of contact is often used in a large number of small connectors, especially on the interface of printed circuit boards, there is a significant interest in studying the entire problem. Austenitic stainless steels have some properties, mainly the low oxidation rate, that, allied with relatively low cost and availability, qualify this type of material for this specific task. The fretting tests consisted in reciprocating movements on a sphere-on-flat-surface contact geometry. Both specimens were made of AISI 304 stainless steel. The electric current was kept constant during the entire test at various set values, ranging from 0 mA (no current), corresponding to no energy dissipation by the Joule effect, to 120 mA. The addition of energy in the contact by the Joule effect increases the oxidation of the contact surfaces, inducing wear acceleration in both specimens by synergetic effect. A synergetic model was introduced in order to quantify individual contributions to total wear volume, which is composed by mechanical wear, a Joule effect parcel of wear and an incremental factor of degradation due to the combined effect.

An instrument has been developed that allows in situ optical analysis and tribological measurements for contacts between solid bodies; an interferometric optical analysis can be used to measure and observe contact size, contact geometry, near contact topography, tribofilm formation, tribofilm motion, tribofilm thickness, wear debris formation, and wear debris morphology. The optical arrangement is in such a way that a 0th order interference fringe highlights the real contact area of contact, while near contact regions are height-mapped with higher order Newton’s rings interference fringes. Images synchronized with force and position measurements allow for the potential to test and validate models for contact mechanics, adhesion, and sliding. The contact and friction measurement between a rough rubber sphere and a polished glass counterface were studied over a range of loads from 1 to 50 mN.

Nano-scale adhesive contact mediated by intermolecular van der Waals forces has become a typical fundamental problem in many areas. Interpretation and control of the strength and efficiency of the nano-scale adhesive contacts require a proper modeling considering the actual interfacial forces, the varying contact area, and clearance. In this article, the finite-element (FE) method is developed to model the nano-scale adhesive contact of elastic bodies with an adhesive pressure derived from the interatomic interaction Lennard-Jones potential, which permits numerical solutions for a variety of interface geometries. Compared with the analytical results from conventional Hertz, JKR, and DMT models, the validity of the FE model is verified. For nano-scale contact, the assumption of equivalent radius adopted in the Hertz model is initially investigated and proved to be improper for nano-scale adhesive contact due to the distribution variations of interfacial force caused by local contact geometry. Then adhesive contact behaviors of four typical nano-scale contact geometries inspired by tip shapes of bio-adhesive pads are investigated in detail, which are flat punch tip, sphere tip, mushroom tip, and empty cup tip. The simulation results indicate that the nano-scale tip geometry plays a dominant role on the pull-off strength. Within the investigated geometries, cup tip results in a highest adhesion efficiency followed by flat punch tip, sphere tip, and mushroom tip, respectively, which are highly geometry dependent and verified by former experimental results. The dominant effect is found coming from the contact area ratio of the adhesive area to the sticking area or the whole contact area. The FE modeling can serve a useful purpose in revealing the nano-scale geometry-based adhesion contact for surface topography design in MEMS to avoid stiction failure and for the artificial sticky feet in bionics to increase adhesion strength.

Frictional force behavior during fretting fatigue and its interdependence on other fretting variables are investigated. Both coefficient of static friction and the normalized frictional force (i.e., the ratio of frictional force and normal contact load) increase during the earlier part of a fretting fatigue test and then both reach to a stabilized value. The variation of temperature in the contact region and normalized frictional force with increasing cycle numbers and bulk stress show similar trend implying that normalized frictional force represents the average friction in the contact region during a fretting fatigue. An increase in bulk stress, relative slip, and hardness of pad material results in an increase of the normalized frictional force, while an increase in contact load, frequency and temperature decreases the normalized frictional force. The normalized frictional force is also affected by the contact geometry. On the other hand, coefficient of static friction increases with an increase in the hardness of mating material, temperature and roughness from shot-peening treatment, but is not affected by contact geometry and displacement rate. Further, the normalized frictional force is not affected by the contact geometry, roughness and applied bulk stress level when fretting fatigue test is conducted under slip controlled mode, however it increases with increasing applied relative slip and decreasing contact load in this case.

Tribological properties of a material depend on a wide range of parameters/conditions. While some of them are obvious, some are quite subtle. One of these subtle parameters has been identified and reported here. Under sliding conditions, whenever surface layers are formed due to the interaction at the sliding interface (e.g., tribochemical interaction), the resultant tribological response has been conjectured to depend upon the relative length of the pin, parallel to the sliding direction, with respect to the corresponding length of the disc track. The results obtained from the experiments reported in this paper as well as those reported by different researchers in the past, furnish evidence to support this conjecture. It can be considered as a general parameter because of this wide spectrum of applications. It is shown in this paper that this parameter helps in selecting the optimum contact geometry of the pin under various experimental conditions. This might reduce the observed scatter in tribological test results. Such a study leads to the conclusion that, for tribological testing, a square/rectangular contact configuration of pin is preferable to a circular one.