SURFACE DYNAMIC FRICTION OF POLYMER GELS

The sliding friction of various kinds of hydrogels has been studied and it was found that the frictional behaviors ofthe hydrogels do not conform to Amonton's law F=μW which well describes the friction of solids. The frictional force andits dependence on the load are quite different depending on the chemical structures of the gels, surface properties of theopposing substrates, and the measurement condition. The gel friction is explained in terms of interracial interaction, eitherattractive or repulsive, between the polymer chain and the solid surface. According to this model, the friction is ascribed tothe viscous flow of solvent at the interface in the repulsive case. In the attractive case, the force to detach the adsorbing chainfrom the substrate appears as friction. The surface adhesion between glass particles and gels measured by AFM showed agood correlation with the friction, which supported the repulsion-adsorption model proposed by the authors.

Static/dynamic friction and wear of some selected polymeric materials for conformal tribo-pairs under boundary lubrication conditions

This work is aimed at investigating the friction and wear performance of different polymeric materials having potential for hydraulic system components under lubricated sliding conditions against a steel counter face.A pin-on-disc test configuration was used for the experimental study.The different polymeric materials selected for these studies were commercial polyimides(PI),polyether ether ketone(PEEK),and flouropolymers.Some of these materials were bulk materials whereas others were used as coatings applied on to the cast iron substrate.The tribological characteristics of the polymers were compared with a reference grey cast iron.The frictional characteristics were evaluated in both static and dynamic conditions.The results have shown that by using polymeric materials it is possible to reduce breakaway friction by an order of magnitude compared to grey cast iron.However,the breakaway friction increased significantly after the wear tests.The polymeric materials having lowest breakaway friction have shown the highest wear with the exception of the PEEK-PTFE coating which showed low wear.PI with graphite fillers also showed low wear but it resulted in relatively high friction.The carbon fibre reinforced materials resulted in unstable friction as well as higher wear compared to the PI materials with graphite fillers.

Dynamic friction energy dissipation and enhanced contrast in high frequency bimodal atomic force microscopy

Dynamic friction occurs not only between two contact objects sliding against each other,but also between two relative sliding surfaces several nanometres apart.Many emerging micro-and nano-mechanical systems that promise new applications in sensors or information technology may suffer or benefit from noncontact friction.Herein we demonstrate the distance-dependent friction energy dissipation between the tip and the heterogeneous polymers by the bimodal atomic force microscopy(AFM)method driving the second order flexural and the first order torsional vibration simultaneously.The pull-in problem caused by the attractive force is avoided,and the friction dissipation can be imaged near the surface.The friction dissipation coefficient concept is proposed and three different contact states are determined from phase and energy dissipation curves.Image contrast is enhanced in the intermediate setpoint region.The work offers an effective method for directly detecting the friction dissipation and high resolution images,which overcomes the disadvantages of existing methods such as contact mode AFM or other contact friction and wear measuring instruments.

Effects of hydraulic oil and lubricant additives on dynamic friction properties under various reciprocating sliding conditions

The friction characteristics of a shock absorber are very complex because the reciprocating motion is not always identical.In this study,a device was developed and used to analyze the dynamic friction characteristics under various reciprocating sliding conditions to determine the sliding materials and hydraulic oils that improve the shock absorber performance.This study describes the influence of hydraulic oil additive on the fine reciprocating friction characteristics of steel and copper alloy.Hydraulic oils were prepared by blending a paraffinic mineral oil with zinc dithiophosphate(ZnDTP)and polyhydric alcohol ester as additives.The results show that the dynamic frictional characteristics vary mainly depending on the additive concentration.A specific additive formulation induces a unique amplitude-dependent friction behavior.In addition,the influence of different additives on the lubrication mechanism is investigated based on the instrumental analysis of the friction surface.

Anomalous friction of graphene nanoribbons on waved graphenes

Friction plays a critical role in the function and maintenance of small-scale structures, where the conventional Coulomb friction law often fails. To probe the friction at small scales, here we present a molecular dynamics study on the process of dragging graphene nanoribbons on waved graphene substrates. The simulation shows that the induced friction on graphene with zero waviness is ultra-low and closely related to the surface energy barrier. On waved graphenes, the friction generally increases with the amplitude of the wave at a fixed period, but anomalously increases and then decreases with the period at a fixed amplitude. These findings provide the insights into the ultraqow friction at small scales, as well as some guidelines into the fabrication ofgraphene-based nano-composites with high performance.

Effect of the time-weakening friction law during the nucleation process

Based on dynamic rupture simulations on a planar fault in a homogeneous half-space, we investigated the nucleation processes using the time-weakening friction law. Both the characteristic time and the rupture speed in the nucleation asperity play an important role in determining rupture behaviors on a fault plane following the time-weakening friction law, with which rupture starts from a single point in the nucleation asperity and propagates at a given speed toward the boundary of the nucleation area. Rupture with a small characteristic time or a large rupture speed in the nucleation asperity propagates earlier from the hypocenter. Rupture following the slipweakening friction law requires a smaller radius of nucleation patch to have similar rupture front contours of the time-weakening friction law. Even if the rupture velocity in the nucleation patch of the time-weakening friction law increases to infinity, the peak slip rate in the nucleation asperity is smaller than that of the slip-weakening law. The peak ground velocity distributions of ruptures following the two friction laws are also compared.

Evolution of aggregate surface texture due to tyre-polishing

Three kinds of aggregates were polished by genuine pneumatic rubber tyres.The initial states of surface texture and dynamic friction coefficient were measured and their developments in polishing process were monitored.The characterizations of height distribution and power spectral density of aggregate surface texture were estimated.The changes of micro-texture were also investigated based on a fractal filtering method with sound theoretical backgrounds of rubber friction on rough surfaces.Global height reduction and differential removal of mineral component are observed in polishing process.It is concluded that the tyre-polishing action plays the critical roles in the micro-scale texture,and the evolution of friction of aggregate is governed by the micro-texture changes due to the differential removal of mineral component.

Effect of surface roughness on the surface lubrication performance of galvanized steel sheets with a self-lubricated coating

Four kinds of galvanized steel sheets having different surface roughness values were used to prepare the steel sheets with a self-lubricated coating. The effects of surface roughness on the surface lubrication performance of the steel sheets were examined using a friction coefficient tester. Results revealed large dynamic friction coefficients for the galvanized steel sheets, which increased remarkably with surface roughness. Once the self-lubricated coating was applied, significant drops in the dynamic friction coefficients were measured. After the first stage of the friction test,the coefficients were almost unchanged, which reflected a weak dependence on the surface roughness of the self-lubricated steel sheets. However, the dynamic friction coefficients gradually increased as the test progressed, where these increase clearly correlated with the surface roughness of the self-lubricated steel sheets.

干摩擦对临界阻尼状态缓冲特性的影响

根据单自由度、临界阻尼状态的动力学模型．考虑了干摩擦的存在对缓冲特性的影响，导出了包装件受冲击作用时产品的最大冲击加速度和缓冲材料的最大形变的表达式．并与无阻尼系统的相应情况进行了比较。

Analysis of blasting damage in adjacent mining excavations

Following a small-scale wedge failure at Yukon Zinc＇s Wolverine Mine in Yukon, Canada, a vibration monitoring program was added to the existing rockbolt pull testing regime. The failure in the 1150 drift occurred after numerous successive blasts in an adjacent tunnel had loosened friction bolts passing through an unmapped fault. Analysis of blasting vibration revealed that support integrity is not compromised unless there is a geological structure to act as a failure plane. The peak particle velocity（PPV） rarely exceeded 250 mm/s with a frequency larger than 50 Hz. As expected, blasting more competent rock resulted in higher PPVs. In such cases, reducing the round length from 3.5 m to 2.0 m was an effective means of limiting potential rock mass and support damage.

Laser Surface Textured PTFE Inhibitation for Stick-Slip Phenomenon Under Boundary Lubrication

When the machine tool is in the start-and stop-stages,the stick-slip phenomenon will be observed and highprecision positioning,machining accuracy and fretting feed will not be guaranteed. The most critical reason is that there is the difference between the dynamic and the static friction coefficients. Textures with different area ratios are fabricated on the surfaces of the upper PTFE-based composite and the friction tests are carried out on a reciprocating tribotester under the boundary lubrication and flat-on-flat contact conditions. The results show that there exists an optimal textured area ratio of 19.6% that can minimize the difference between the dynamic and the static friction coefficients.

MECHANICAL EFFECTS OF THE SELF-ORGANIZATION OF DISLOCATIONS AND RELATED CRITICAL CHARACTERISTICS

The effects of the dislocation pattern formed due to the self-organization of the dislocations in crystals on the macroscopic hardening and dynamic internal friction (DIF) during deformation are studied. The classic dislocation models for the hardening and DIF corresponding to the homogeneous dislocation configuration are extended to the case for the non-homogeneous one. In addition, using the result of dislocation patterning deduced from the non-linear dlislocation dynamics model for single slip, the correlation between the dislocation pattern and hardening as well as DIF is obtained. It is shown that in the case of the tension with a constant strain rate, the bifurcation point of dislocation patterning corresponds to the turning point in the stress versus strain and DIF versus strain curves. This result along with the critical characteristics of the macroscopic behavior near the bifurcation point is microscopically and macroscopically in agreement with the experimental findings on mono-crystalline pure aluminum at temperatures around 0.5T(m). The present study suggests that measuring the DIF would be a sensitive and useful mechanical means in order to study the critical phenomenon of materials during deformation.

Development and validation of an integrated mechatronic apparatus for measurement of friction coefficients of agricultural products

An integrated mechatronic apparatus was developed based on tilting plate method in order to precisely measure static friction coefficient(SFC)and dynamic friction coefficient(DFC)of agricultural products.The apparatus consisted of two main parts(mechanical and electrical parts).The main element of mechanical part was rotary container.Meanwhile,the electrical part included control,display,goniometer,level controller,rotational power supply,and infrared unit.The apparatus was initially simulated in simulation environment and practically calibrated to achieve high precision measurements.To appraise performance of the apparatus,the SFC and DFC of three grains were measured on five contact surfaces.Experiments were also conducted by means of a typical apparatus operating based on puling force method.Some statistical descriptor parameters such as mean absolute percentage error(MAPE),average of absolute values of measurement errors(AAVME),maximum of absolute values of measurement errors(MAVME),and correlation coefficient were used to compare accuracy of the apparatus with the typical one.The acceptable AAVME(<10%),MAVME(<10%),MAPE(<5%),and correlation coefficient(>0.9)indicated high accuracy,stability,and efficiency of the apparatus for automatic measurements of the SFC and DFC.From practical point of view,the mechatronic apparatus would be a beneficial tool for experimental,educational,demonstrational,and research works.

Friction moment analysis of space gyroscope bearing with ribbon cage under ultra-low oscillatory motion

This paper presents the model of calculating the total friction moment of space gyroscope ball bearings which usually work under ultra-low oscillatory motion and are very sensitive to the friction moment. The aim is to know the proportion of the friction moment caused by each frictional source in the bearing＇s total friction moment, which is helpful to optimize the bearing design to deduce the friction moment. In the model, the cage dynamic equations considering six degree-of-freedom and the balls dynamic equations considering two degree-of-freedom were solved.The good trends with different loads between the measured friction moments and computational results prove that the model under constant rate was validated. The computational results show that when the speed was set at 5 r/min, the bearing＇s maximum total friction moment when oscillation occurred was obviously larger than that occurred at a constant rate. At the onset of each oscillatory motion, the proportion of the friction moment caused by cage in the bearing＇s total friction moment was very high, and it increased with the increasing speed. The analyses of different cage thicknesses and different clearances between cage pocket and ball show that smaller thickness and clearance were preferred.

Numerical analysis of frictional behavior of dense gas-solid systems

Dense gas-solid flows show significantly higher stresses compared with dilute flows, mainly attributable to particle-particle friction in dense particle flows. Several models developed have considered particle-particle friction; however, they generally underestimate its effect in dense regions of the gas-solid system, leading to unrealistic predictions in their flow patterns. Recently, several attempts have been made to formulate such flows and the impact of particle-particle friction on predicting flow patterns based on modified frictional viscosity models by including effects of bulk density changes on frictional pressure of the solid phase. The solid-wall boundary is also expected to have considerable effect on friction because particulate phases generally slip over the solid surface that directly affects particle-particle frictional forces. Polydispersity of the solid phase also leads to higher friction between particles as more particles have sustained contact in polydispersed systems. Their effects were investi- gated by performing CFD simulations of particle settlement to calculate the slope angle of resting material of non-cohesive particles as they settle on a solid surface. This slope angle is directly affected by frictional forces and may be a reasonably good measure of frictional forces between particles. The calculated slope angle, as a measure of frictional forces inside the system are compared with experimental values of this slope angle as well as simulation results from the literature.

Interlayer shear of nanomaterials:Graphene–graphene,boron nitride–boron nitride and graphene–boron nitride

In this paper, the interlayer sliding between graphene and boron nitride （h-BN） is studied by molecular dynamics simulations. The interlayer shear force between h-BN/h-BN is found to be six times higher than that of graphene/graphene, while the interlayer shear between graphene/h-BN is approximate to that of graphene/graphene. The graphene/h- BN heterostructure shows several anomalous interlayer shear characteristics compared to its bilayer counterparts. For graphene/graphene and h-BN/h-BN, interlayer shears only exit along the sliding direction while interlayer shear for graphene/h-BN is observed along both the translocation and perpendicular directions. Our results provide significant insight into the interlayer shear characteristics of 2D nanomaterials.