Tribology Letters

In practice, the sliding speed is an important parameter for materials applied in sliding condition. We have conducted an experimental study to explore the effect of sliding speed on friction and wear performance of a copper–graphite composite. The sliding tests were carried out over a wide range of speeds with a pin-on-disc configuration. The results show that there is a critical speed at which there is a transition of the friction and wear regimes of the composite. In addition, the formation of a lubricant layer on the contact surface (surface modification) determines the actual tribological performance of the composite. The wear mechanisms in different wear regimes are also discussed.

As soft aqueous hydrogels have moved from new materials to the basis for real engineered devices in the last 20 years, their surface friction and lubrication are emerging as critical aspects of their function. The flexibility to alter and augment their mechanical and surface properties through control of the crosslinked 3D polymer networks has produced materials with diverse surface behaviors, even with the relatively simple composition of a single monomer and crosslink chemistry. Correspondingly with new understandings of the bulk behavior of hydrogels has been the identification of the mechanisms that govern the lubricity and frictional response under dynamic sliding conditions. Here we review these efforts, closely examining and identifying the internal and external influences that drive tribological response in high water content crosslinked hydrogels. The roles of surface structure, elasticity, contact response, charge, water interaction and water flow are addressed here as well as current synthesis and testing methods. We also collect open questions as well as the future needs to fully understand and exploit the surface properties of hydrogels for sliding performance.

A polycrystal model has been developed to study the tribological behavior of an aggregate BCC crystal under extreme contact loading conditions. The polycrystal behavior is defined from the local behavior of a single crystal within the framework of the Hill’s self-consistent scheme by considering the temperature effect. The tribological problem simulated in this work is a contact problem where a pin (tool) moves on a workpiece with a fairly high pressure and large sliding velocity. The results show the importance of the physical parameters as the hardening effect, the restoration of the dislocation and the crystallographic orientations of grains on the contact. The effect of the frictional heat partition and the friction coefficient have been particularly discussed.

Atomic force microscopy and molecular dynamics simulation are used to study the nanoscale wear of a silicon dioxide tip sliding on a copper substrate. Wear is characterized in terms of structural and chemical evolution of the system where the latter is possible experimentally using atom probe tomography of the slid tips. Comparison of the experimentally observed and simulation-predicted wear reveals that adhesive wear is dominant in the short sliding distances of the simulation at any applied load, while the sliding distances in the experiments are long enough to observe load-induced transitions between adhesive-dominated and abrasive-dominated wear.

The effect of a rare earth (RE) surface treatment on the mechanical and tribological properties of carbon fiber (CF) reinforced polytetrafluoroethylene (PTFE) composites was experimentally investigated. The tensile properties of the CF reinforced PTFE (CF/PTFE) composites treated with air oxidation and RE modifier were superior to those of untreated CF/PTFE composites, while RE treatment was most effective in promoting the tensile strength and strain at break of the CF/PTFE composite. The bending strength of the RE treated CF/PTFE composite was improved by about 16% compared with that of untreated composites, while 2% improvement was achieved by air oxidation. Under oil-lubricated conditions, RE treatment was more effective than air oxidation to reduce the friction coefficient and wear of PTFE composite. RE treatment effectively improved the interfacial adhesion between CF and PTFE. The strong interfacial coupling of the composite made CF not easy to detach from the PTFE matrix, and prevented the rubbing-off of PTFE, accordingly improved the friction and wear properties of the composite.

In most practical applications, surface roughness is characterized by just one or two parameters (numbers). I show that the standard maximum surface height parameters fluctuate strongly between different surface realizations (or measurements), and should not be used in the design of engineering components. I show how some roughness parameters depend on the size of the roll-off region in the surface roughness power spectra, and introduce a new height parameter which is very reproducible. The numerical results presented agree well with experimental observations.

Stimulating the human hand with a tactile device in order to simulate pile fabric touch is a challenge. The stimulation has to be designed from the friction characteristics of the investigated pile surfaces, i.e. velvet fabrics. The tactile illusion of pile is given when touching the smooth plate of the tactile stimulator STIMTAC by modulating the coefficient of friction between the plate and the finger during an active movement. In a preliminary study, five tribological features as velvet fabric characteristics were identified, used for the design of the stimulator’s control signal, and validated via psychophysical studies where real and simulated fabrics were compared. But a specific tribological feature described and expected by individuals was missing. Then, a tribological investigation has been done in order to obtain this tribological feature, with the five previous ones, by changing experimental conditions: slider shape, texture, and joint stiffness. The obtained results show that a rounded shape of the slider has an influence only on the friction force level, but a texture of the slider close to fingerprints and a joint stiffness is crucial to obtain the missing characteristic and therefore for the pile surface tribological characterization. The role of the fingerprints in touching grooved surfaces has been published before but not for pile surfaces.

Friction is an important limitation of energy efficiency performances of MEMS/NEMS but is, in the same time, a great opportunity for harvesting energy by designing optimized tribo-electric nano-Generators (TENG). Thus, frictional behaviour can be accurately controlled in real time by using thermally sensitive periodic patterned self-assembled monolayers of n-octadecyltrichlorosilane (OTS) grafted on MEMS surfaces. Nanopatterns are currently used in order to limit the wear rate without modifying the frictional behaviour. In this work, patterns have been created by micro-contact printing ($$\upmu \hbox {CP}$$) using a polydimethylsiloxane (PDMS) stamp displaying a trapezoidal profile. Hence, pattern periodicity can be continuously changed—and then optimized from discontinuous to pseudo-continuous—by applying a controlled normal load on the soft PDMS stamp. A multiscale tribological study has been carried out on these nanopatterns by using both single-asperity and multi-asperity nanotribometers. Lateral force microscopy (LFM) provides the individual frictional behaviour of each pattern’s component, whereas the multi-asperity nanotribometer rather gives the emerging frictional behaviour induced by the patterning according to temperature. As a macroscopic crucial parameter while designing TENG’s devices, this macroscopic behaviour has to be carefully optimized for each practical applications at the molecular scale. Thus, the microscale frictional behaviour can be precisely optimized by the pattern’s periodicity, whereas the macroscopic one can be accurately controlled with values of friction coefficient ranging from 0.12 to 0.04 by varying the contact temperature. In addition, any inertial effects observed in the thermal-controlled frictional behaviour of nanopatterns can be drastically reduced using infra-red emission as thermal source.

The wear behaviour of two railway wheel steels, ER8 and SUPERLOS®, was studied through pin-on-disc tests, and the results were correlated with those previously obtained with twin-disc tests. The work-hardening of the steels was investigated with Vickers hardness measurements, and the wear mechanisms were studied using scanning electron microscopy. ER8 discs showed higher wear resistance, lower work-hardening ability and less wear damage than SUPERLOS® ones, confirming the results of the twin-disc tests. Therefore, sliding pin-on-disc experiments are recommended as a simple laboratory technique that can be used as a screening method for wheel steel performance prior to more complex and more expensive tests. The damage in both steels was due to the concomitance of oxidative wear, abrasive wear and fatigue wear. Iron oxide formation protects the steels from severe wear, whereas its detachment causes abrasive wear; furthermore, surface fatigue cracks initiate and propagate leading to the detachment of material flakes.

We develop an analytical model of adhesive wear between two unlubricated rough surfaces, forming micro-contacts under normal load. The model is based on an energy balance and a crack initiation criteria. We apply the model to the problem of self-affine rough surfaces under normal load, which we solve using the boundary element method. We discuss how self-affinity of the surface roughness, and the complex morphology of the micro-contacts that emerge for a given contact pressure, challenge the definition of contact junctions. Indeed, in the context of adhesive wear, we show that elastic interactions between nearby micro-contacts can lead to wear particles whose volumes enclose the convex hull of these micro-contacts. We thereby obtain a wear map describing the instantaneous produced wear volume as a function of material properties, roughness parameters and loading conditions. Three distinct wear regimes can be identified in the wear map. In particular, the model predicts the emergence of a severe wear regime above a critical contact pressure, when interactions between micro-contacts are favored.