Unidirectional Friction Research Articles

Tribological effects of different scaled chevron-shaped microstructures on the Stribeck curve of parallel contacts under unidirectional friction

Laser surface texturing is a widely used technology to improve the tribological behavior of articulating surfaces. Especially for lubricated contacts working in unidirectional motion, laser-generated chevron shapes are one of the most promising surface designs to reduce friction and wear. But, investigations regarding the effect of different chevron dimensions on the tribological performance of lubricated contacts are still lacking in literature. Therefore, we used a 300-femtosecond laser to apply chevron microstructures with different scaled leg widths (5–400 µm). Friction analyses were performed to determine the Stribeck curve of lubricated ring-on-disk samples at sliding velocities from 2.3 to 0.006 m/s and a constant load of 50 N. Results show that small-scale chevrons (5–40 µm) improve the overall tribological behavior of the contact partners. With small-scale chevrons, friction could be reduced by 29% in the boundary, 85% in the mixed, and up to 56% at the transition point from mixed to the lubrication regime compared to an unstructured reference. With a chevron leg width of 5 µm, a significant shift of the critical velocity for the lubrication regime transition to lower values was observed (reduction by 36% compared to the reference). In contrast, no substantial change of the coefficient of friction was observed with large-scale (leg width 200–400 µm) chevrons.

Influence of different aligning surfaces on the morphology of dichroic squaraine films

Dichroic organic films have found a broad application in various optoelectronic devices. One efficient method for the production of thin dichroic films is the deposition of the film onto aligning polymer surface. Scientists propose different explanations concerning the mechanisms of the molecules aligning. Here, we report the aligning of squaraine (Sq) molecules on the different aligning polymer surfaces, independent from the morphology of the Sq deposits. Aligned polytetrafluoroethylene (PTFE) layers were produced by either unidirectional hot friction transfer or by PTFE decomposition in vacuum with the activation of the gas by accelerated electrons and the consequent unidirectional rubbing with a cloth. Sq films grown on the aligned PTFE surfaces and on the nanostructured polycarbonate (PC) surfaces were found to exhibit dichroic optical absorption spectra. Despite different shape and orientation of the Sq crystals on the PTFE and the PC surfaces, the absorption of polarized light was shown to exhibit one pattern. Variations in the morphology of Sq films on different surfaces were explained by the competitive participation of different growth mechanisms, which depended on the properties of the deposited compound, the aligning layer and the growth conditions. The PTFE aligning layers can be useful in organic electronics due to low-temperature processing.

Dry wear characteristics of machined ZL109 aluminum-silicon alloy surface under unidirectional and reciprocating rolling-contact friction

Dry wear characteristics of machined ZL109 piston aluminum-silicon alloy surface were investigated in this paper. The wear characteristics of unidirectional friction is also analyzed to reveal its reciprocating wear characteristics better. The wear rates, wear debris and surface morphology versus wear time, and the causes of friction squeal under unidirectional and reciprocating rolling-contact were studied in detail. The surface topography, surface phase, surface roughness and residual stress of ZL109 aluminum-silicon alloy machined surface before and after wear were measured. The results indicate that the wear rates fluctuates with the wear time increasing under unidirectional rolling-contact, however, the wear rates first decreases and then stabilizes at about 1.5 mg min−1 with reciprocating wear. The wear rate under reciprocating wear is obviously higher than that of unidirectional wear, which is mainly affected by the reciprocating stress, new phase production and its content in the wear surface. There is no friction noise during reciprocating rolling-contact, however, and the fluctuation of high-hardness corundum content could lead to the friction squeal in unidirectional wear. There are flake and granular wear debris mainly, and the granular debris is mainly produced by extruding the flake debris repeatedly. There is high-degree surface plastic deformation during reciprocating wear versus that of unidirectional wear. The maximum surface residual compressive stress of reciprocating wear is higher than that of unidirectional wear, which is accounts for 62.9% of ZL109 aluminum-silicon alloy’s yield limit. Fatigue wear is main mechanism of machined ZL109 aluminum-silicon alloy surface. Fatigue wear and delamination wear are all essentially the surface material failure phenomena caused by the expansion of micro-cracks and micro-holes in the sub-surface layer.

Effect of reciprocating and unidirectional sliding motion on the friction and wear of phenolic resin based composite

Purpose The purpose of this paper is to investigate the effect of reciprocating and unidirectional sliding motions on friction and wear of phenolic resin-based composite. Design/methodology/approach The phenolic resin-based composite was fabricated by hot press molding, and then the tribological properties were tested on a CSM tribometer with two types of friction motion modes – reciprocating friction and unidirectional friction. Findings The results showed that the composite exhibited low friction coefficient in unidirectional test. However, the wear factor recorded under unidirectional sliding condition was 12-16 times higher than the reciprocating friction results. The SEM and optical microscopy test results showed that changing the relative motion mode resulted in different topography of transfer film, which is responsible for the different friction and wear characteristics of the composite under reciprocating and unidirectional friction conditions. Originality/value Effect of different friction modes, reciprocating friction and unidirectional friction, on friction characteristics of the composite is sought. Different topography of transfer film formed under reciprocating and unidirectional friction conditions contributed to the different friction characteristics.

Fractal Characterization of Relative Movement Modes in Simulated Friction and Wear Tests

The selection of the relative movement mode between the friction pairs in simulated friction and wear tests is a crucial factor for the characterization and evaluation of metallic material properties. In this study, two kinds of commonly used metal materials (stainless steel 0Cr18Ni9 and aluminum alloy 2A12) were applied for the simulated friction and wear tests with four different modes of relative motion, including unidirectional, reciprocating, circular and multi-directional cross-shear. The corresponding surface topography and statistical parameters such as surface roughness Ra were obtained via OLYMPUS ultra-depth microscope. Based on the fractal theory, fractal dimension D was calculated to characterize the surface topography and the results indicate that, the fractal dimension D of the unidirectional and multi-directional cross-shear simulated friction and wear tests is the minimum and maximum respectively, which reflects that the fractal dimension D is able to characterize the complexity of the surface morphology. The fractal dimension D may be related to the intrinsic physical properties of metal materials, such as hardness and toughness, which needs to be further studied.

Dry sliding friction and wear behaviour of hybrid AA7075/Si3N4/Gr self lubricating composites

The present investigation aims to study the effect of graphite on the friction and wear behaviour of AA7075/Si3N4 composites. Four different sample compositions have been fabricated by stir casting technique. The percentage of silicon nitride has been fixed as 8 wt% and the percentage of graphite has been varied from 0–6 wt%. The cast composites have been tested for microhardness and density prior to the tribological testing. Unidirectional friction and wear testing has been carried out for all compositions under five different loading conditions. Moreover, in order to elucidate the effect of speed on the tribological behaviour testing at 4 m s−1 and 7 m s−1 has also been carried out. SEM analysis has also been carried out for morphological analysis of the worn surfaces. A decrease in microhardness and density of the composites is observed with an increase in graphite wt% in the base alloy. The addition of graphite led to a linear decrease in the COF. However, the wear loss exhibited a decreasing trend till 4 wt% graphite and thereafter it increased. The increase in the normal load led to an increase in COF as well as wear loss. The increase in the sliding speed led to an increase in the wear loss, however the COF exhibited a decreasing trend with an increase in speed. The SEM images revealed that the addition of graphite resulted in a decrease in the abrasive wear. Mild delamination and smoother surfaces were observed for higher content of graphite. The fabricated self lubricating composite could serve various anti-wear and antifriction applications particularly in automotive sector.

Interparticle Bonding of Cu Powder under Repetitive Unidirectional Friction

Interparticle Bonding of Cu Powder under Repetitive Unidirectional Friction

Dry Sliding Friction and Wear Behavior of AA7075-Si3N4 Composite

The present investigation is aimed at identifying the influence of Si3N4 reinforcement on the mechanical and tribological behavior of AA7075-Si3N4 composite. Five different composites of AA7075 aluminum alloy reinforced by silicon nitride particles have been fabricated by the stir casting route. The percentage of silicon nitride was varied from 0-8 wt%. The cast composites were tested for hardness, density and compression strength. Unidirectional friction and wear testing was carried out for all compositions under five different loading conditions (10 N, 20 N, 30 N, 40 N and 50 N) at a constant sliding speed of 1 m/s. SEM and EDS analysis was also carried out for worn surface analysis and elemental analysis of the composites. The hardness and compression strength of the composites exhibited an increasing trend with an increase in wt% of reinforcement in the base alloy, showing 20% improvement in hardness and around 50% improvement in compression strength for 8 wt% Si3N4 addition. The addition of Si3N4 particles led to an improvement in the wear resistance by 37% at low loads (10 N) and 61% at higher loads (50 N). The COF for all varied compositions at low load (10 N) and high load (50 N) ranges from 0.10 to 0.20 and 0.25 to 0.30 respectively. Moreover, the COF is observed to increase until 4 wt% and beyond it decreases. Microscopic studies of worn surfaces revealed a dominance of delamination wear at lower concentrations (0 wt% and 2 wt%) and ploughing at higher concentrations (6 wt% and 8 wt%). The developed composites exhibited better mechanical and anti-wear properties and could serve as potential candidates in sliding applications such as bearings, brake drums, gears, sprockets and brake rotors.

In Situ Measurements of Contact Dynamics in Speed-dependent Hydrogel Friction

The friction behavior of soft, aqueous, biotribological contacts depends on contact geometry, speed, and pressure. Previous efforts to measure the surface profile of soft sliding contacts have been stymied by the matching index of refraction of aqueous materials submerged in water. Here, hydrogel surface deformations were imaged using confocal microscopy to experimentally investigate the contact geometry as a function of sliding speed. In situ fluorescence confocal microscopy measurements of the contact deformation during unidirectional friction experiments revealed the existence of a front/back asymmetry that increased with increasing sliding speed. A polyacrylamide hydrogel disk (96.5% water) was polymerized with fluorescent dyes and used as a rotating countersample below a polished glass hemispherical pin (1mm radius of curvature). All experiments were performed submerged in a dilute (0.7wt%) suspension of 1μm red fluorescent microspheres in ultrapure water. Imaging of the contact was performed using a confocal microscope in situ with unidirectional sliding at 0.1, 1, 10, and 100mm/s. The friction coefficient increased monotonically with increasing speed from μ~0.04 at 0.1mm/s to μ~0.20 at 100mm/s. All imaging was performed relative to the stationary glass probe under steady-state conditions. The contact line measured by connecting the leading and exiting contact points was nearly perpendicular to the loading direction at 0.1mm/s sliding speed, but distorted as sliding speed increased. Consistent with the theories of viscoelastic contributions to polymer friction, the tangent of the contact-line angle correlated with friction.

A Novel Slack-Enabling Tendon Drive That Improves Efficiency, Size, and Safety in Soft Wearable Robots

Tendon drives are widely used in robotics. The compliance of the tendon in such drives suits them for soft robots, including soft wearable robots, but several issues impede their use. Generally, the tendon should always maintain tension to prevent derailment from the spool. However, in soft robots, tendon tension induces high friction forces owing to the absence of ball bearings. Because the kinematics of the soft robot is basically nonlinear and changed by the deformation of the structure, the kinematic difference between the soft structure and the spool causes derailment of the tendon. Moreover, continuously maintained tension in soft wearable robots causes safety issues. The linear actuator can be an option. However, the need to increase the length of the linear actuator to accommodate the excursion length of its tendon is a barrier to its use in small-scale applications. To preclude this issue, a slack-enabling actuator that employs a spool is proposed. The space efficiency of the spool enables the mechanism to be small, and a one-way clutch applies unidirectional friction force to the tendon to tighten the tendon around the spool. This paper describes the design concept for the slack-enabling mechanism, its design optimization, and system identification for force control.

Mechanism of interparticle bonding of metal powder by repetitive unidirectional friction process

Mechanism of interparticle bonding of metal powder by repetitive unidirectional friction process

Analysis of Strain Rates and Microstructural Evaluation during Metal Forming: Role of Surface Texture and Friction

The surface texture of a die plays an important role in friction during metal forming. In the present study, unidirectional and random surface finishes were produced on hardened steel plate surfaces. To understand the influence of surface texture on friction, experiments were conducted using Al-Mg alloy pins that slid against steel plates of different surface textures. In the sliding experiments, a high coefficient of friction was observed when the pins slid perpendicular to the unidirectional grinding marks and low friction occurred when the pins slid on the random surfaces. Finite element simulations were performed using the measured friction values to understand the stress and strain evolutions in the deforming material using dies with various friction. The numerical results showed that the states of stress and strain rates are strongly influenced by the friction at the interface and hence would influence the final material microstructure. To substantiate the numerical results, laboratory compression tests were conducted. Different surface textures were obtained in order to experience different friction values at different locations. A large variation in the microstructure at these locations was observed during experiments, verifying that surface texture and die friction significantly influence fundamental material formation behavior.

Discussion on “A bidirectional and homogeneous tuned mass damper: A new device for passive control of vibrations” by Jose L. Almazan, Juan C. De la Llera, Jose A. Inaudi, Diego Lopez-Garcia, Luis E. Izquierdo [Eng Struct 29 (2007) 1548–1560

The aforementioned paper proposes a new type of pendulum tuned mass damper with unidirectional friction damper. The proposed tuned mass damper is novel. However, the authors use the inappropriate examples to illustrate their device. In their example, the length of pendulum is too small, and then the angle of pendulum is very large. The aim of this discussion is to point out that the examples are impractical.

Spring-block model for a single-lane highway traffic

Abstract A simple one-dimensional spring-block chain with asymmetric interactions is considered to model an idealized single-lane highway traffic. The main elements of the system are blocks (modeling cars), springs with unidirectional interactions (modeling distance-keeping interactions between neighbors), static and kinetic friction (modeling inertia of drivers and cars) and spatiotemporal disorder in the values of these friction forces (modeling differences in the driving attitudes). The traveling chain of cars correspond to the dragged spring-block system. Contrary to most of the studies in the field of highway traffic here we focus on a measure characteristic for one car in the row. Our statistical analysis for the spring-block chain predicts a non-trivial and rich complex behavior. As a function of the disorder level in the system a dynamic phase-transition is observed. For low disorder levels uncorrelated slidings of blocks are revealed while for high disorder levels correlated avalanches dominates.

Tribological Behavior of Polyimide/Zinc Oxide Hybrid Films

A series of polyimide/ZnO films were successfully prepared via a solution process. The friction and wear behavior of the hybrid polyimide films in dry sliding against Si3N4 ball was examined on a DF-PM ball-on-block unidirectional friction and wear test rig. The worn surface morphologies of the films were observed with a scanning electron microscope. It was found that the introduction of zinc oxide led to improving the tribological characteristics and thermal stability of the polyimide films. The polyimide/zinc oxide hybrid films had much better friction-reducing ability and wear-resistance than the neat PI film.

Characterization and tribological investigation of self-assembled MPTS-MPTES/RE composite films

γ-mercapto-propyl trimethoxysilane (MPTS) and γ-methacryloxy propyltrimethoxysilane (MPTES) were self-assembled on a hydroxylated glass substrate to form a two-dimensional organic monolayer (MPTS-MPTES SAM). The terminal thiol groups (–SH) in the MPTS-MPTES SAM were in-situ oxidized into sulfonic acid groups (–SO 3H) to endow the film with good chemisorption ability. Then rare earth (RE) (lanthanum-based) composite thin films were prepared by self-assembly technique based on the as-prepared SAM, taking advantage of the chemisorption ability of the sulfonic acid groups. Automatic force microscope (AFM), X-ray photoelectron spectrometry (XPS), contact angle measurement and ellipsometer were used to characterize MPTS-MPTES/RE composite films. The macrofriction and wear behaviors of the films sliding against an AISI-52100 steel ball were examined on a unidirectional friction and wear tester, and the worn surface morphologies were observed on an AFM. The results showed that MPTS-MPTES/RE films had a low friction coefficient (0.09) and a long wear life (5980 sliding pass) at a light load (50 mN). It indicated that the superior tribological properties of the MPTS-MPTES/RE composite films were attributed to the special characteristic of RE elements, the mobility of the films and good bonding strength.

Tribological investigation of MPTS‐MPTES SAMs

PurposeThis paper's aim is to study the tribological properties of hyrid monolayer composited by γ‐mercapto‐propyl trimethoxysilane (MPTS) and γ‐methacryloxy propyltrimethoxysilane (MPTES), and estimated the action mechanism.Design/methodology/approachMPTS‐MPTES were self‐assembled on a hydroxylated silicon substrate to form a two‐dimensional hybrid monolayer. Atomic force microscope (AFM), X‐ray photoelectron spectrometry and contact angle measurement were used to characterize the MPTS‐MPTES hybrid self‐assembled monolayer (SAM). The macrofriction and wear behaviors of the film sliding against an AISI‐52100 steel ball were examined on a unidirectional friction and wear tester, and the worn surface morphologies were observed on an AFM.FindingsThe tribological results show that the friction coefficient of silicon substrate reduces from 0.86 to 0.18 after the formation of the MPTS‐MPTES hybrid SAM on its surface, and the thin film has a long wear life (2,620 sliding pass). It is demonstrated that the MPTS‐MPTES SAM exhibited good wear resistant property with a low friction coefficient, and the superior friction reduction and wear life were attributed to the low surface energy and the characteristics of the hybrid SAM.Research limitations/implicationsThe film's tribological performance under dynamic load is not estimated.Practical implicationsA hybrid monolayer with superior tribological property was synthesized on a silicon substrate by self‐assembly process, and maybe it is the potential solution for micro‐electromechanical‐system lubrication.Originality/valueThis paper provides a study way of hybrid SAM on a silicon substrate as lubricating coating.

Different responses of several kinds of copolymerized polyimide films to ultraviolet irradiation

The surfaces of three kinds of copolymerized polyimide films were irradiated by ultraviolet (UV) light. The chemical composition changes of the films after irradiation were investigated by X-ray photoelectron spectroscopy (XPS), while their topographical changes were studied by atomic force microscopy (AFM). Besides, the surface topographical variation induced changes in friction behavior were recorded on a DF·PM unidirectional friction and wear test rig in a ball on block contact configuration. It was found that photodegradation took place, which induced the chain scission of the polyimide molecules and formation of low molecular weight oxidized material (LMWOM). Due to different photostability of the three kinds of polyimide, different chemical composition changes and forms of “LMWOM” were observed. Also, different friction behavior variations of the three kinds of polyimide films were recorded. It is thought that the polyimide film of PI (PPA + PMDA) with rod-like structures might have stronger photo-resistance, whose form of “LMWOM” was “nodules” and whose friction coefficient increased at 2 h of irradiation possibly due to cross-linking. In the case of PI (OPA + PMDA) and PI (MPA + PMDA) films, they mainly underwent photodegradation, which might soften the surfaces and increased the friction coefficients with increasing irradiation time.

A bidirectional and homogeneous tuned mass damper: A new device for passive control of vibrations

Passive tuned-mass dampers (TMDs) are a very efficient solution for the control of vibrations in structures subjected to long-duration, narrow-band excitations. In this study, a Bidirectional and Homogeneous Tuned Mass Damper (BH-TMD) is proposed. The pendular mass is supported by cables and linked to a unidirectional friction damper with its axis perpendicular to the direction of motion. Some advantages of the proposed BH-TMD are: (1) its bidirectional nature that allows control of vibrations in both principal directions; (2) the capacity to tune the device in each principal direction independently; (3) its energy dissipation capacity that is proportional to the square of the displacement amplitude, (4) its low maintenance cost. Numerical results show that, under either unidirectional or bidirectional seismic excitations, the level of response reduction achieved by the proposed BH-TMD is similar to that obtained from an “ideal” linear viscous device. Moreover, experimental shaking table tests performed using a scaled BH-TMD model confirm that the proposed device is homogeneous, and, hence, its equivalent oscillation period and damping ratio are independent of the motion amplitude.

Mechanical properties and tribological behavior of ZrO2 thin films deposited on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

A silane coupling reagent (3-mercaptopropyl)trimethoxysilane (abridged as MPTS) was self-assembled on a single-crystal Si substrate to form a two-dimensional organic monolayer (MPTS-SAM). The terminal –SH group in the MPTS-SAM film was in-situ oxidized to –SO3H group to endow the film with good chemisorption ability. Then ZrO2 thin films were deposited on the oxidized MPTS-SAM by way of the enhanced hydrolysis of aqueous zirconium sulfate (Zr(SO4)2·4H2O) in the presence of aqueous HCl at 50 °C, making use of the chemisorption ability of the –SO3H group. The thickness of the ZrO2 films was determined with an ellipsometer, while their morphologies and corresponding friction forces were analyzed by means of atomic force microscopy. The hardness and elastic modulus of the ZrO2 thin films were determined on a Nanoindentation II (MET) instrument. The macro-friction and wear behaviors of the ZrO2 films sliding against an AISI-52100 steel ball were examined on a unidirectional friction and wear tester and the worn surface morphologies observed on a scanning electron microscope (SEM). As the results, the as-deposited ZrO2 thin film at a deposition duration of 100 h is about 100 nm thick, it decreases to 48 nm after annealing at 500 °C and further decreases to 45 nm after heating at 800 °C. The as-deposited ZrO2 film is relatively rougher, with the rms to be about 1.0 nm, while the ZrO2 thin films heated at 500 and 800 °C have surface roughness rms of 0.76 nm and 0.68 nm, respectively. The ZrO2 film annealed at 800 °C has a high hardness to elastic modulus (H/E) ratio (0.062) as compared to the as-deposited ZrO2 film and the film annealed at 500 °C. Both the two annealed ZrO2 films show excellent wear-resistance as they slide against AISI-52100 steel at a normal load below 2.0 N, while the one annealed at 800 °C has better wear-resistance. The differences in the friction and wear behaviors of the as-deposited ZrO2 film, the ZrO2 film annealed at 500 °C and that annealed at 800 °C are attributed to their different micro structures and compositions. Since the ZrO2 films was well adhered to the underlying MPTS-SAM, it might find promising application in the surface-protection of single crystal Si and SiC subject to sliding at small normal load in microelectromechanical systems (MEMS).