Dominant Friction Mechanisms Research Articles

Investigating the Wettability, Rheological, and Tribological Properties of Ammonium-Based Protic Ionic Liquids as Neat Lubricants for Steel–Steel and Steel–Aluminium Contacts

This study aims to evaluate the tribological properties of two protic ionic liquids (PILs) under different tribological conditions as a sustainable alternative for mineral oil-based neat lubricants. The synthesis of PILs in this study uses a relatively simple and less expensive method. The Fourier transform infrared (FTIR) spectroscopy results help validate the synthesised PILs’ formation. Further, their physicochemical and tribological properties were investigated. The PILs as neat lubricants were tested on a ball-on-plate reciprocating tribometer using bearing steel–bearing steel and bearing steel–aluminium alloy friction pairs at 30 °C and 80 °C. The results show that the investigated PILs significantly reduced the coefficient of friction and wear. The dodecylamine-based PILs performed better in friction and wear reduction than the other investigated lubricants. The formation of the adsorption layer on the friction pairs was assumed to be the dominant friction and wear reduction mechanism.

Nanofriction Properties of Mono- and Double-Layer Ti3C2Tx MXenes.

Unique structure and ability to control the surface termination groups of MXenes make these materials extremely promising for solid lubrication applications. Due to the challenging delamination process, the tribological properties of two-dimensional MXenes particles have been mostly investigated as additive components in the solvents working in the macrosystem, while the understanding of the nanotribological properties of mono- and few-layer MXenes is still limited. Here, we investigate the nanotribological properties of mono- and double-layer Ti3C2Tx MXenes deposited by the Langmuir-Schaefer technique on SiO2/Si substrates. The friction of all of the samples demonstrated superior lubrication properties with respect to SiO2 substrate, while the friction force of the monolayers was found to be slightly higher compared to double- and three-layer flakes, which demonstrated similar friction. The coefficient of friction was estimated to be 0.087 ± 0.002 and 0.082 ± 0.003 for mono- and double-layer flakes, respectively. The viscous regime was suggested as the dominant friction mechanism at high scanning velocities, while the meniscus forces affected by contamination of the MXenes surface were proposed to control the friction at low sliding velocities.

Fluctuation-induced quantum friction in nanoscale water flows.

The flow of water in carbon nanochannels has defied understanding thus far1, with accumulating experimental evidence for ultra-low friction, exceptionally high water flow rates and curvature-dependent hydrodynamic slippage2-5. In particular, the mechanism of water-carbon friction remains unknown6, with neither current theories7 nor classical8,9 or ab initio molecular dynamics simulations10 providing satisfactory rationalization for its singular behaviour. Here we develop a quantum theory of the solid-liquid interface, which reveals a new contribution to friction, due to the coupling of charge fluctuations in the liquid to electronic excitations in the solid. We expect that this quantum friction, which is absent in Born-Oppenheimer molecular dynamics, is the dominant friction mechanism for water on carbon-based materials. As a key result, we demonstrate a marked difference in quantum friction between the water-graphene and water-graphite interface, due to the coupling of water Debye collective modes with a thermally excited plasmon specific to graphite. This suggests an explanation for the radius-dependent slippage of water in carbon nanotubes4, in terms of the electronic excitations of the nanotubes. Our findings open the way for quantum engineering of hydrodynamic flows through the electronic properties of the confining wall.

Nanoscale dry friction: Dependence on load and sliding velocity

Numerous experimental and theoretical studies show different relationships between the kinetic dry friction force and the sliding velocity depending on the interacting materials, load, velocity range, etc. In our work, we developed a methodology for data analysis and interpretation to understand such complex and often unclear relationships. We measured the dependence of the friction force on the sliding velocity and load for SiOx–trichloro(3,3,3-trifluoropropyl)silane friction pair using the atomic force microscope. The results were transformed into a dependence on the velocity-to-load parameter, which allowed for a new insight into the nature of friction. All the transformed curves exhibited a similar minimum and their subsequent ranges were assigned to dominant friction mechanisms such as “stick-slip”, smooth sliding, adhesive friction and rebinding.

Effect of temperature on friction and galling behavior of 7075 aluminum alloy sheet based on ball-on-plate sliding test

Abstract In this work, the friction and galling properties of 7075 aluminum alloy sheet at different temperatures from ambient to 400 °C have been investigated by performing linear unlubricated reciprocating sliding tests on a ball-on-plate tribometer. It was found that a compacted layer could be formed on the worn surface and its effect on the friction behavior of the alloy was determined. Thereafter, coupled with dissolution-precipitation of the hard phase, a friction evolution model of the alloy was established and the friction mechanism at different temperatures was revealed. At 25 °C and 150 °C, the friction mechanism of 7075 aluminum alloy sheet is mainly ploughing friction. When higher than 300 °C, the dominant friction mechanism becomes the adhesive peeling-off.

Tribological behavior of self-lubricating carbon nanoparticle reinforced metal matrix composites

The present study focuses on investigating the dominant friction and wear mechanisms in case of dry sliding of carbon nanoparticle reinforced nickel matrix composites under elastic and elasto-plastic contact conditions. For this purpose, multi-wall carbon nanotubes (CNT), onion-like carbon (OLC) and nanodiamonds (nD) were chosen to represent a large variety of carbon nanoparticles as they can be systematically distinguished regarding their carbon hybridization state (sp 2 vs. sp3) as well as their morphology and size (“0D” vs. “1D”). Contact simulations based on the Greenwood-Williamson model are conducted in order to calculate the required contact loads. Friction and wear analysis is supported by complementary characterization techniques, including scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, Raman spectroscopy, light microscopy as well as laser scanning microscopy. It is found, that only CNT provide efficient lubrication as reinforcement phase in composites, presenting different lubrication mechanisms for the tested contact conditions. The high aspect ratio of CNT is found to be essential for the lubrication mechanisms, allowing the particles to be dragged into the direct tribological contact. The lubrication effect increases with increasing volume content of CNT, reaching a maximum steady state frictional reduction of 50% compared to the unreinforced nickel reference.

Friction mechanism in the running-in stage of copper: From plastic deformation to delamination and oxidation

The friction mechanisms during the running-in stage and its effect on the subsequent steady-state were investigated by examining the evolution of surface topography and subsurface microstructures in pure copper sliding against a WC-Co ball. Scanning electron microscopy observations indicated that plastic deformation induced grain refinement occurs after only several sliding passes. A transition in the dominant friction mechanism from plastic deformation to delamination and oxidation was found in the running-in stage, corresponding to a peak coefficient of friction and onset of detectable wear.

Tribological behavior of copper-molybdenum disulfide composites

The dry sliding friction and wear behavior of Cu-MoS2 composite pins against bearing steel (AISI 52100) discs were investigated in order to determine the optimal additive content. The MoS2 content ranged from 0–40vol%. The microstructure and composition of the worn surface, wear debris and transfer layer were observed and analyzed in detail to reveal the dominant friction and wear mechanisms. It is confirmed that MoS2 is an effective lubricant for copper matrix composites against steel because the friction coefficient decreased from 0.67 to 0.18 when adding 20vol% of the MoS2. The wear rates of the composites tended first to increase then decrease as the MoS2 content increased. The high wear rates of Cu-5vol% MoS2 and Cu-10vol% MoS2 composites are mainly attributed to the cracks associated with the MoS2 particles, and the decrease of wear rate for composites with MoS2 above 20vol% is derived from the formation of a nearly continuous lubricating film on the worn surface. The formation mechanism for MoS2 lubricating film has been clarified by observing its microstructure and characterizing its distribution in the worn surface.

Study of the temporal characteristics of friction and contact behavior encountered during braille reading

Beyond the sense of sight, the sense of touch is one of the primary ways that individuals experience their surrounding environment. Fundamentally understanding the relationship of skin-surface tribology and its elicited tactile attributes could provide a breakthrough in improving the ability to efficiently transmit tactile information to those who rely on the sense of touch to interact with their surroundings, such as the blind and visually impaired (BVI) community. The tactile language of braille has been adopted by the BVI community, employing configurations of raised dome-shape dots to convey what is ordinarily presented in text and image form. The coefficient of friction caused by skin sliding across a these dot features is hypothesized to affect the reader's tactile sensitivity, and skin-on-braille coefficient of friction has been investigated in previous work, where macro-scale deformation of the human fingerpad sliding over the dot contour was identified as the dominant friction mechanisms. This investigation succeeds that study by examining a simplified large-scale, two-dimensional representation of skin-on-braille sliding to characterize the underlying contact mechanisms in the loading behaviors that dictate the resulting coefficient of friction. This was accomplished by using a multi-axis tribometer to sliding a 25.4mm radius cylindrical polyurethane (representing a human fingerpad) rod over a lubricated 3.17 mm aluminum half rod (representing a braille dot) under displacement-displacement-controlled conditions. The results from the tribometer study indicate that the presence of the dot feature drastically affects the vertical and lateral loading behavior by vertically displacing the body's elastic bulk, generating rubber-like Poisson effect contributions. Most importantly, the Poisson effect rapidly increases the lateral load when the body contacts the dot's leading edge, and rapidly decreases when the body rests largely in contact with the dot's trailing edge. This rapid decrease is caused by a “propulsion” effect, where vertical compression expands the material laterally, and when situated on the trailing edge of the dot, propels it into the direction of sliding, virtually negating adhesive surface friction. Computational modeling of this system discovered that while normal contact pressures dominated the fluctuations seen in the vertical loading, effects due to both normal contact pressures and frictional shears nearly equally drove the lateral loading behavior.

Observation of a Dislocation-Related Interfacial Friction Mechanism in Mobile Solid $$^{4}$$ 4 He

We report a study of the temperature and stress dependence of the friction associated with a relative motion of crystallites of solid $$^{4}$$ He in contact with each other. A situation where such motion exists emerges spontaneously during a disordering of a single crystal contained inside an annular sample space of a torsional oscillator (TO). Under the torque applied by the oscillating walls of the TO these crystallites move relative to each other, generating measurable dissipation at their interface. We studied this friction between 0.5 and 1.8 K in solid samples grown from commercially pure $$^{4}$$ He and from a 100 ppm $$^{3}$$ He– $$^{4}$$ He mixture. The data were analyzed by modeling the TO as a driven harmonic oscillator. In this model, an analysis of the resonant frequency and amplitude of the TO yields the temperature dependence of the friction coefficient. By fitting the data to specific forms, we found that over our temperature range, the dominant friction mechanism associated with the interfacial motion of the crystallites results from climb of individual dislocations. The characteristic energy scale associated with this friction can be 3 or 6 K, depending on the sample. The motion of the solid in the presence of such friction can perhaps be described as the low limit of “slip–stick” motion.

Friction and surface fracture of a silicon carbide ceramic brake disc tested against a steel pad

Friction coefficients of a SiC ceramic disc were measured on a laboratory-scale dynamometer by testing against a mild steel pad under different initial braking speeds, and its friction surface was investigated with microscopy techniques. At bedding, averaged friction coefficient for a braking stop varied significantly with the initial braking speed; after bedding, it converged to ∼0.6, regardless of braking speed. Surface fracture on the SiC disc was responsible for the transformation from a flat surface into a rough one, making ploughing a dominant friction mechanism at bedded stage. It was found that fracture surface and non-contact regions directly contributed friction coefficient variation at bedding stage. Friction layer composed of iron oxides and plastic deformation with partial dislocations activated appeared on SiC surface, but were unsustainable owing to surface fracture. A quantitative analysis is provided to understand friction coefficient variation and SiC surface fracture during braking.

Contact mechanics analysis of oscillatory sliding of a rigid fractal surface against an elastic–plastic half-space

Oscillatory sliding contact between a rigid rough surface and an elastic–plastic half-space is examined in the context of numerical simulations. Stick-slip at asperity contacts is included in the analysis in the form of a modified Mindlin theory. Two friction force components are considered – adhesion (depending on the real area of contact, shear strength and interfacial adhesive strength) and plowing (accounting for the deformation resistance of the plastically deformed half-space). Multi-scale surface roughness is described by fractal geometry, whereas the interfacial adhesive strength is represented by a floating parameter that varies between zero (adhesionless surfaces) and one (perfectly adhered surfaces). The effects of surface roughness, apparent contact pressure, oscillation amplitude, elastic–plastic properties of the half-space and interfacial adhesion on contact deformation are interpreted in the light of numerical results of the energy dissipation, maximum tangential (friction) force and slip index. A non-monotonic trend of the energy dissipation and maximum tangential force is observed with increasing surface roughness, which is explained in terms of the evolution of the elastic and plastic fractions of truncated asperity contact areas. The decrease of energy dissipation with increasing apparent contact pressure is attributed to the increase of the elastic contact area fraction and the decrease of the slip index. For a half-space with fixed yield strength, a lower elastic modulus produces a higher tangential force, whereas a higher elastic modulus yields a higher slip index. These two competing effects lead to a non-monotonic dependence of the energy dissipation on the elastic modulus-to-yield strength ratio of the half-space. The effect of interfacial adhesion on the oscillatory contact behaviour is more pronounced for smoother surfaces because the majority of asperity contacts deform elastically and adhesion is the dominant friction mechanism. For rough surfaces, higher interfacial adhesion yields less energy dissipation because more asperity contacts exhibit partial slip.

Abrasive Wear Performance of CNT/PP Composites

In this work, abrasive wear performance of multi-walled carbon nanotube (CNT) reinforced polypropylene (PP) has been studied. Various weight percentage of CNT/PP nanocomposites were prepared through melt intercalation with the aid of twin screw extruder. Prepared composites were subjected to sliding wear against silicon carbide abrasive paper with the aid of pin-on-disc configuration. Worn out surface morphology were observed under microscope and non-contact profilometer to understand the wear mechanism. Since the dominant friction mechanism observed to be is ploughing, no lubrication effect of CNT was observed. CNT/PP composite exhibits inferior wear resistance than unfilled PP due to its brittleness. With increased CNT, wear resistance found to decreases.

High-vacuum adhesion and friction properties of sliding contact-mode micromachines

The evolution of adhesion and friction in contact-mode micromachines operated in high vacuum was studied by tracking changes in the adhesive pressure, interfacial shear strength, and static coefficient of friction with accumulating sliding cycles. Low adhesion and high static friction observed during the initial stage of sliding were followed by monotonically intensifying adhesion and decreasing friction until reaching an equilibrium stage at steady-state sliding. This trend revealed the existence of two friction regimes in which asperity deformation and adhesion were the dominant friction mechanisms. Scanning electron microscopy and atomic force microscopy observations indicated that sliding resulted in physical and chemical surface changes. The evolution of the adhesion and friction properties with sliding cycles is attributed to the increase of both the real contact area and the work of adhesion due to nanoscale surface smoothening and the removal of contaminant adsorbents, respectively.

Sidewall Adhesion and Sliding Contact Behavior of Polycrystalline Silicon Microdevices Operated in High Vacuum

The reliability and performance of contact-mode microelectromechanical systems (MEMS) depend strongly on the tribological properties of contact interfaces. Knowledge of the dominant friction and wear mechanisms at submicrometer length scales is therefore of paramount importance to the design of MEMS devices with contact interfaces. The objective of this study was to examine changes in the adhesion behavior and morphology of sliding sidewall surfaces of polycrystalline silicon MEMS devices operated in high vacuum (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> torr) and under low apparent contact pressures (0.1-18 kPa) and correlate these changes to the operation lifetime. Sidewall adhesion increased with applied contact pressure. Typically, a twofold to fourfold increase in side wall adhesion was measured upon cessation of the device operation (typically, ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> sliding cycles) due to the increase of the static friction force above the restoring force available by the device. Scanning electron microscopy (SEM) revealed very small amounts of ultrafine wear debris (10-140 nm) on the sidewall surfaces of about half of the tested devices, without discernible changes in the surface topography. Cross-sectional transmission electron microscopy (TEM) showed that sliding did not cause the removal of the silicon oxide film (5-13 nm in average thickness) from the sidewall surfaces. Atomic force microscopy (AFM) indicated that sliding contact was confined at the top of a few elevated ridges on the sidewall surfaces, resulting in nanoscale wear that smoothened locally the surfaces. SEM, TEM, and AFM results of this study show that the tribological properties of contact-mode MEMS devices operating in high vacuum are controlled by only a few nanoscopic contacts, which depend on the local nanotopography of the interacting surfaces.

Friction, wear and structure of Cu samples in the lubricated steady friction state

Friction and wear of copper rubbed with lubrication in wide range of loads and sliding velocities were studied. The results of friction and wear experiments are presented as the Stribeck curve where the boundary lubrication (BL), mixed (ML) and elasto-hydrodynamic lubrication (EHL) regions are considered. The structural state of subsurface layers in different lubricant regions is studied by X-ray photoelectron spectroscopy, optical, transmission and scanning microscopy analysis. Dislocation density of dislocations in EHL and BL lubricant regimes was determined. Nanohardness at thin surface layers rubbed under different lubricant regimes is compared. The dominant friction and wear mechanisms in different lubrications regions are discussed.

Structural evolution in copper layers during sliding under different lubricant conditions

Friction and wear of copper rubbed in a wide range of loads and sliding velocities were studied. The results of friction and wear experiments in PAO-4 lubricant are presented as the Stribeck curve where the boundary, mixed and elasto-hydrodynamic lubrication regimes are considered. The structural state of surface layers in different lubricant regimes is studied by optical and scanning electron microscopy and X-ray photoelectron spectroscopy analyses. The dominant friction and wear mechanisms in different lubrication regimes are discussed. Severe plastic deformation of subsurface layers under friction is correlated with the nanocrystalline structure obtained by different methods of grain refinement.

Comparative surface and nano-tribological characteristics of nanocomposite diamond-like carbon thin films doped by silver

In this study we have deposited silver-containing hydrogenated and hydrogen-free diamond-like carbon (DLC) nanocomposite thin films by plasma immersion ion implantation-deposition methods. The surface and nano-tribological characteristics were studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and nano-scratching experiments. The silver doping was found to have no measurable effect on sp 2–sp 3 hybridization of the hydrogenated DLC matrix and only a slight effect on the hydrogen-free DLC matrix. The surface topography was analyzed by surface imaging. High- and low-order roughness determined by AFM characterization was correlated to the DLC growth mechanism and revealed the smoothing effect of silver. The nano-tribological characteristics were explained in terms of friction mechanisms and mechanical properties in correlation to the surface characteristics. It was discovered that the adhesion friction was the dominant friction mechanism; the adhesion force between the scratching tip and DLC surface was decreased by hydrogenation and increased by silver doping.

Microscale friction phenomena in oscillatory sliding contacts

Microscale friction phenomena encountered in oscillatory sliding contacts were examined with a special reciprocating surface micromachine. Variations in static and dynamic friction forces were tracked in situ throughout testing under controlled loading and environmental conditions. Stick-slip surface interactions emerged at high numbers of sliding cycles. An unexpected binary friction behavior occurred as sliding transitioned between two-body and three-body conditions due to the formation of fine wear particles. The dominant friction mechanisms arising at the asperity scale are interpreted in the context of temporal evolutions of the static and dynamic friction forces and the decrease of the static and dynamic operational safety factors with accumulating sliding cycles. An important finding is that oscillating microdevices tend to fail in static friction mode rather than in dynamic friction mode. The results of this study illustrate the important role of microscale stick-slip phenomena in high-speed oscillatory microcontacts and the need for developing dynamic friction theories applicable at the microscale.

Dependence of nanomechanical modification of polymers on plasma-induced cross-linking

The nanomechanical properties of low-density polyethylene (LDPE) modified by inductively coupled, radio-frequency Ar plasma were investigated by surface force microscopy. The polymer surface was modified under plasma conditions of different ion energy fluences and radiation intensities obtained by varying the sample distance from the plasma power source. Nanoindentation results of the surface stiffness versus maximum penetration depth did not reveal discernible differences between untreated and plasma-treated LDPE, presumably due to the small thickness of the modified surface layer that resulted in a substrate effect. On the contrary, nanoscratching experiments demonstrated a significant increase in the surface shear resistance of plasma-modified LDPE due to chain cross-linking. These experiments revealed an enhancement of cross-linking with increasing ion energy fluence and radiation intensity, and a tip size effect on the friction force and dominant friction mechanisms (adhesion, plowing, and microcutting). In addition, LDPE samples with a LiF crystal shield were exposed to identical plasma conditions to determine the role of vacuum ultraviolet (VUV) and ultraviolet (UV) radiation in the cross-linking process. The cross-linked layer of plasma-treated LDPE exhibited much higher shear strength than that of VUV/UV-treated LDPE. Plasma-induced surface modification of the nanomechanical properties of LDPE is interpreted in the context of molecular models of the untreated and cross-linked polymer surfaces derived from experimental findings.