Low Kinetic Friction Research Articles

Using lubricant composition to control friction-induced-vibration in an elastomer-steel contact representing a hydraulic seal

Hydraulic seals are key industrial components that can suffer from unwanted friction induced vibration (FIV). The types of FIV mechanism that occur in these components, and how they may be controlled, are not well known. We conducted sliding friction tests on contacts between seal materials, lubricated by hydraulic fluids, under speeds and contact pressures typical of hydraulic machines. FIV that occurred under certain conditions was captured and analysed. Results suggest 1) two FIV mechanisms are occurring: classic stick-slip and speed dependant friction instability, the 2) occurrence and severity of these mechanisms are correlated with the ratio of static to dynamic friction and the gradient of the friction vs. speed (Stribeck) curve, respectively, 3) these parameters can be controlled by including an appropriate additive package in the lubricant and this significantly reduces FIV, 4) most FIV occurs at low temperature and high load since these both lead to low kinetic friction (due to thicker hydrodynamic films and smoother surfaces) and thus promote stick-slip. This may explain instances of noise known in practice as the “Monday-morning effect” and suggests how this too may be alleviated through lubricant formulation.

Dynamics of Droplets Impacting on Aerogel, Liquid Infused, and Liquid-Like Solid Surfaces.

Droplets impacting superhydrophobic surfaces have been extensively studied due to their compelling scientific insights and important industrial applications. In these cases, the commonly reported impact regime was that of complete rebound. This impact regime strongly depends on the nature of the superhydrophobic surface. Here, we report the dynamics of droplets impacting three hydrophobic slippery surfaces, which have fundamental differences in normal liquid adhesion and lateral static and kinetic liquid friction. For an air cushion-like (super)hydrophobic solid surface (Aerogel) with low adhesion and low static and low kinetic friction, complete rebound can start at a very low Weber (We) number (∼1). For slippery liquid-infused porous (SLIP) surfaces with high adhesion and low static and low kinetic friction, complete rebound only occurs at a much higher We number (>5). For a slippery omniphobic covalently attached liquid-like (SOCAL) solid surface, with high adhesion and low static friction similar to SLIPS but higher kinetic friction, complete rebound was not observed, even for a We as high as 200. Furthermore, the droplet ejection volume after impacting the Aerogel surface is 100% across the whole range of We numbers tested compared to other surfaces. In contrast, droplet ejection for SLIPs was only observed consistently when the We was above 5-10. For SOCAL, 100% (or near 100%) ejection volume was not observed even at the highest We number tested here (∼200). This suggests that droplets impacting our (super)hydrophobic Aerogel and SLIPS lose less kinetic energy. These insights into the differences between normal adhesion and lateral friction properties can be used to inform the selection of surface properties to achieve the most desirable droplet impact characteristics to fulfill a wide range of applications, such as deicing, inkjet printing, and microelectronics.

Directed transport of a particle on a horizontal surface under asymmetric vibrations

Asymmetric unbiased fluctuations coupled with the effect of nonlinear dry (Coulombic) friction are observed to generate a directed transport even in the absence of any net external force or gradient. This phenomenon, called the ratchet effect, occurs when the driving force does not overcome the static friction threshold symmetrically. An elementary model is used to generate directed and unidirectional motion of a particle placed on top of a horizontal rigid surface which is given a zero mean spatially asymmetric motion through a multi-frequency in-plane excitation. We obtain steady-state periodic solutions of the nondimensional transport equations analytically to assess the distinct single/multiple sticking and sliding phases of particle motion and various parametric conditions for phase transitions. These analytical results are then validated by solutions obtained semi-analytically using Galerkin projection and numerically using the fourth-order Runge–Kutta method (RK4) and Newton–Raphson method with a fixed arc-length based continuation scheme. Also, a multibody dynamics model of this particle-on-plate system is built in MSC Adams to help us visualize the actual particle behavior under real dimensional system parameters and compare it with the foregoing analyses. A highly counter-intuitive phenomenon is encountered as the particle drift velocity increases non-monotonically with the increase in friction whereas a very high or a very low amount of friction impedes motion. Maximum transport velocity is found to be independent of the amplitude ratio of the vibratory input for low values of kinetic friction and is observed to manifest at a critical static friction threshold beyond which the particle motion becomes unidirectional. The effect of viscous drag of the ambient fluid on transport is assessed to be quite insignificant. Optimal vibratory input parameters to achieve the best transport current under varying friction conditions are determined. At low kinetic friction, a fixed set of system parameters is observed to generate distinct solution branches that coexisted just beyond the static friction threshold corresponding to a motion transition from “slip” to “stick–slip”.

Vibroarthrographic analysis of patellofemoral joint arthrokinematics during squats with increasing external loads

BackgroundThe patellofemoral joint (PFJ) provides extremely low kinetic friction, which results in optimal arthrokinematic motion quality. Previous research showed that these friction-reducing properties may be diminished due to the increase in articular contact forces. However, this phenomenon has not been analyzed in vivo during functional daily-living activities. The aim of this study was the vibroarthrographic assessment of changes in PFJ arthrokinematics during squats with variated loads.Methods114 knees from 57 asymptomatic subjects (23 females and 34 males) whose ages ranged from 19 to 26 years were enrolled in this study. Participants were asked to perform 3 trials: 4 repetitions of bodyweight squats (L0), 4 repetitions of 10 kg barbell back loaded squats (L10), 4 repetitions of 20 kg barbell back loaded squats (L20). During the unloaded and loaded (L10, L20) squats, vibroarthrographic signals were collected using an accelerometer placed on the patella and were described by the following parameters: variation of mean square (VMS), mean range (R4), and power spectral density for frequency of 50–250 Hz (P1) and 250–450 Hz (P2).ResultsObtained results showed that the lowest values were noted in the unloaded condition and that the increased applied loads had a significant concomitant increase in all the aforementioned parameters bilaterally (p < 0.05).ConclusionThis phenomenon indicates that the application of increasing knee loads during squats corresponds to higher intensity of vibroacoustic emission, which might be related to higher contact stress and kinetic friction as well as diminished arthrokinematic motion quality.

Tube-Based DBD Plasma Treatment for Improving the Performance of the Slippery Coating Layers on Medical Catheters

This work develops a tube-based dielectric barrier discharge (DBD) plasma treatment for improving the performance of the slippery coating layers on medical catheters. The raw materials of the polymer catheters usually exhibit hydrophobic surface property and need a hydrophilic slippery coating layer for reducing the friction during catheter insertion. However, the high friction force may cause an uncomfortable feeling or even the infective or thrombosis injuries on the epithelium. This study develops an atmospheric tube-based DBD plasma for treating the surface of the plastic catheter for enhancing the coating performance of the slippery layer coating on medical catheters. The tube-based DBD plasma system is a semiclose and co-axial concentric circles system composed of a glass tube as the DBD layer and the inner supporting hook for catheter production as the center electrode for DBD discharges. The treatment for the inner and outer catheter tube can be achieved simultaneously with a small carrier flow rate of 30–100 sccm. Results show that the contact angles of these catheter tubes are significantly reduced after plasma treatment and the hydrophilicity of the treated surface maintains 57% for more than 30 min after the treatments. The catheters with different plasma treatments are coated with the commercial slippery layer and tested the kinetic friction under the ASTM D1894 standard. The kinetic friction test indicates that the catheters with plasma treatment exhibit lower kinetic friction force, indicating that lower friction during catheter medical insertion operations. The experimental results have shown that the medical catheter treated by the developed tube-based DBD plasma can be effectively modified and facilitated in the subsequent coating process.

Interactions between Friction Modifiers and Dispersants in Lubricants: The Case of Glycerol Monooleate and Polyisobutylsuccinimide-Polyamine.

The structural and frictional properties of 10 wt % solutions of the amphiphilic molecules glycerol monooleate (GMO) and polyisobutylsuccinimide-polyamine (PIBSA-PAM) in squalane are studied using molecular dynamics simulations in bulk and under confinement between iron oxide surfaces. GMO is a friction modifier, PIBSA-PAM is a dispersant, and squalane is a good model for typical base oils. A range of liquid compositions and applied pressures is explored, and the formation and stability of reverse micelles are determined under quiescent and shear conditions. Micellization is observed mainly in systems with a high GMO content, but PIBSA-PAM may also form small aggregates on its own. In the confined systems under both static and shear conditions, some surfactant molecules adsorb onto the surfaces, with the rest of the molecules forming micelles or aggregates. Shearing the liquid layer under high pressure causes almost all of the micelles and aggregates to break, except in systems with around 7.5 wt % GMO and 2.5 wt % PIBSA-PAM. The presence of micelles and adsorbed surfactants is found to be correlated with a low kinetic friction coefficient, and hence, there is an optimum composition range for friction reduction. This work highlights the importance of cooperative interactions between lubricant additives.

The mechanics of snow friction as revealed by micro-scale interface observations

ABSTRACTThe mechanics of snow friction are central to competitive skiing, safe winter driving and efficient polar sleds. For nearly 80 years, prevailing theory has postulated that self-lubrication accounts for low kinetic friction on snow: dry-contact sliding warms snow grains to the melting point, and further sliding produces meltwater layers that lubricate the interface. We sought to verify that self-lubrication occurs at the grain scale and to quantify the evolution of real contact area to aid modeling. We used high-resolution (15 µm) infrared thermography to observe the warming of stationary snow under a rotating polyethylene slider. Surprisingly, we did not observe melting at contacting snow grains despite low friction values. In some cases, slider shear failed inter-granular bonds and produced widespread snow movement with no persistent contacts to melt (μ &lt; 0.03). When the snow grains did not move and persistent contacts evolved, the slider abraded rather than melted the grains at low resistance (μ &lt; 0.05). Optical microscopy revealed that the abraded particles deposited in air pockets between grains and thereby carried heat away from the interface, a process not included in current models. Overall, our results challenge whether self-lubrication is indeed the dominant mechanism underlying low snow kinetic friction.

Friction and noise of rubber belt in low temperature condition: The influence of interfacial ice film

Abstract The friction of rubber belt on pulley was examined under cold conditions. Special emphasis was given to the investigation of the influence of interfacial ice film on friction and noise. A test rig was constructed to study the friction and noise behavior of rubber belt with interfacial ice film under low temperatures. Belt friction and noise were measured using the test rig in the field as well as in a cold environmental chamber. The temperatures used for room testing and cold testing were respectively 23 °C and − 20 °C. It is found that the rubber belt friction is strongly dependent on temperature and the presence or absence of interfacial ice film. The belt friction is found to increase under dry-cold conditions when compared to room temperature conditions. The wet belt under cold conditions could form a thin ice film leading to high static friction and low kinetic friction. The belt sound associated with dry-cold friction is a continuous squeal with a distinct frequency of 9 kHz and associated harmonics, whereas the sound associated with wet-cold friction is impulsive with wideband spectrum stretching up to 20 kHz.

Frictional properties of new developed cold work tool steel for high tensile strength steel forming die

Currently developed tool steel SLD-MAGIC™ was examined and compared with other SKD11 (corresponding to AISI D2) type tool steels from the view point of machinability, friction property and anti-galling property. The experimental results indicated the SLD-MAGIC shows very low kinetic friction coefficient and shows excellent machinability. The anti-galling property of the alloy was studied and compared with typical SKD11 and model alloys eliminating characteristic elements from the newly developed steel. The anti-galling property of the alloy was discussed with a role of elements included.

Sliding friction of polyethylene on ice: tribometer measurements

It is well known that the low kinetic friction experienced when sliding on snow and ice is due to water films generated through frictional heating. There is, however, uncertainty concerning the thickness and the distribution of these water films. Since direct observation of the water films is difficult, tribometer studies coupled with temperature measurements have been carried out on a large-scale, pin-on-disc tribometer (diameter 1.80 m). IR sensors were used to measure the temperature of the ice track in front of and behind the contact region. In addition, thermocouples integrated into the polyethylene slider measured the temperature close to the interface. The kinetic friction between polyethylene and ice has been measured as a function of temperature, velocity, load, and apparent contact area. The friction coefficient, as well as the temperature increase of the slider and the ice track, depends on all of these parameters. Interpretation of the results is given on the basis of hydrodynamic lubrication, taking into account the generation and shearing of thin water films in the contact regions.

Drift Turning of Lateral Guided Vehicle with Sensor Steering Mechanism. Application of a Variable Kinetic Friction Model.

In the previous paper, the simulated loci of high speed cornering are not correspond with the experimental results, because of the tire model is only used the sticking in Coulomb friction model. If the centrifugal force becomes larger than the cornering force of tires, the vehicle does not maintain the desired course. This demeanor includes the sticking and slipping condition of the tire. In order to simulate the trajectory at the high speed corner, this paper is assumed three stages of friction condition; the low kinetic friction force at the beginning of slipping, the period of kinetic friction force, and returning to the static friction at the small slip angle. In addition of the RR (Rear motor Rear drive) type vehicle, FF (Front motor Front drive) type with SSM (Sensor Steering Mechanism) and four-wheel steering mechanism is developed, and both robots run on the oval course at high speed. Numerical simulation and experimental results show the effectiveness of the presented friction model.

Morphology control in polymeric multiphase systems with block copolymers and structured latexes

AbstractSome application possibilities of block copolymers for the morphology control in polymeric multiphase systems are reviewed. The microdomain formation of block copolymers in the solid state is illustrated in the case of functionalized block copolymers and for interpolymer complexes. The preparation of “hairy latex” by emulsion polymerization in the presence of hydrophilic‐hydrophobic block copolymers is shown in connection with their applications in the controlled agglomeration process of latexes and for the preparation of polymer particles having microvoids. The surface activity of block copolymers in polymeric oil‐in‐oil systems is illustrated for silicone oil filled polymers. These materials have a low kinetic friction coefficient and act as reservoir systems with the lubricant incorporated in the polymer matrix.

Frictional heat and strength loss in some rapid landslides

Frictional heat generated within fluid-saturated landslide slip zones can create an increase in pore fluid pressure. This rise in fluid pressure is enhanced by large values of friction coefficient, initial porosity and slide displacement, and by small values of slip zone thickness and compressibility. If maintained under conditions of fast slip, and small wall rock permeability and shear dilatancy, fluid pressure rise can induce rapid frictional strength loss during sliding. Moderate sliding can thereby be converted under certain conditions into catastrophic descent. The rate of temperature rise within a slip zone diminishes as pore fluid pressures rise. Vaporization seems possible only under restricted conditions, and results suggest that large gas pockets are not generally produced. However, rock melting or dissociation may be relatively common under portions of large slides. Frictional heat-induced fluid pressure enhancement is proposed as a possible explanation for the problem of low kinetic friction posed by the Vaiont slide. La chaleur de friction engendrée à l'intérieur des zones de glissement de terrain saturées de fluide peut créer une augmentation de la pression du fluide interstitielle. Cette augmentation de la pression du fluide est rehaussée par des valeurs élevées du coefficient de frottement, de la porosité initiale et du déplacement de glissement, et aussi par des valeurs basses de l'épaisseur et de la compressibilité de la zone de glissement. Si elle est maintenue sous des conditions de glissement rapide, de perméabilité réduite du mur de roche et de dilatance de cisaillement, l'augmentation de la pression du fluide peut causer une perte rapide de la résistance par frottement pendant le glissement. Comme résultat, sous de certaines conditions du glissement modéré peut se convertir en affaissement catastrophique. La rapidité de l'augmentation de temptéature à l'intéieur d'une zone de glissement diminuit à mesure que les pressions du fluide interstitielles s'accroissent. Il semble que la vaporisation soit possible seulement sous des conditions restreintes, et les résu1tats donnent l'impression que de grandes poches de gaz ne se produisent pas généralement. Cependant, la dissolution ou la disassociation de la roche peut se produire relativement souvent sous des parties de grands glissements de terrain. L'augmentation de la pression du fluide causée par la chaleur de frottement est proposée comme explication du problème de la friction cinétique réduite posé par le glissement de terrain qui eut lieu à Vaiont.

Influence of Glycol Molecular Configuration on Friction

Boundary or mixed friction studies on a wide range of synthetic lubricants and mineral oils demonstrated that ethylene glycol gave higher static friction and much lower kinetic friction than did other lubricants tested. To investigate this unusual properly of ethylene glycol, poly-hydroxy alcohols of various molecular configurations were evaluated. The results indicate, that kinetic friction increases and static friction decreases as either the number of atoms between primary hydroxyl groups increases or primary hydroxyl groups are replaced with secondary hydroxyl groups. Surface coatings containing oxidation products of the glycols are believed to be responsible for the observed friction characteristics. Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Pittsburgh, Pennsylvania, October 16–18, 1962.