Tribological behavior of reciprocating friction drive system under lubricated contact

The dynamic friction and wear behaviors are investigated in reciprocating friction drive system using a 0.45% carbon steel pair. The effects of various operating parameters on the traction force, stick and slip time, and friction modes are examined under the lubricated contacts. Moreover, the critical operating conditions in classifying three friction modes are also established. Results show that the fluid friction induced by the shearing of lubricant dominates the variation of traction force and produces the positive slope γ at the first period of slip in the traction force–relative sliding velocity curve. The γ value decreases at higher driver speed during stick-slip motion due to the thicker fluid film and shear thinning effect. The γ value increases due to the asperity interactions as the friction region is transferred from stick-slip to sticking with normal load from 196 to 980 N. Furthermore, it is also found that the static friction force is independent of stick time for the tangential loading rate ranged from 1.12 to 16.8 s⁻¹. The transition region produces the severest wear under the different driver speeds, but the wear is insensitive to the friction regions and the severe wear only occurs at higher normal load due to the action of Hertzian contact.     

Study on slip displacement and stick-slip characteristics in reciprocating friction drive system

The effects of follower mass, time ratio, driver speed, and normal load on the slip displacement and stick-slip characteristics are experimentally and theoretically investigated in the reciprocating friction drive system under dry contact using a 0.45% carbon steel pair. Results show that the accumulative slip displacement is linearly proportional to the rotating cycle under various operating conditions. The slope for this linear relationship is defined as the slip rate. The slip rate increases with increasing driver speed, the mass of the follower, and |δ−1|, but decreasing normal load. There are three modes of relative motion between the driver ring and the follower in the present study, namely, unstable stick-slip (USS), stable stick-slip (SSS), and sticking (ST) regimes. They are significantly influenced by the driver speed, normal load, time ratio and mass of the follower. The critical operating conditions among unstable stick-slip, stable stick-slip, and sticking regimes were also established. The critical frequency can be theoretically calculated and agrees well with experimental results.     

A procedure for evaluating the positioning accuracy of reciprocating friction drive systems

The dynamic characteristics of stick-slip motion in reciprocating friction drive systems are investigated under dry contact using a 0.45% carbon steel pair. Based on this dynamic analysis, the stick-slip motion can be eliminated under certain experimental conditions depending upon driver speed, normal load and spring constant. The effects of normal load, driver speed, and spring constant on the positioning accuracy of the reciprocating friction drive system are examined under harmonic oscillation without stick-slip motion. Results show that at very low spring constants, the slip increases with increasing driver speed such that higher normal load has better positioning accuracy or smaller slip than does lower normal load. However, at high spring constants and high normal loads, there exhibits severe wear or peak at two limit positions on the sliding region. As a result, smaller normal loads have a better positioning accuracy than larger normal loads due to severe wear at high normal loads.     

A new tribometer for friction drives

Friction drives have great potentials in future small-sized machines for their unique features, such as no-noise operation and high position accuracy coupled with simpler design. One of the main challenges of a friction drive is to find suitable material pair for the driver and follower components such that there is no slip and/or wear at the frictional contact regions. In this paper, we present a new tribometer that can be used to study the performance of a material for friction drive applications. The design and fabrication of the apparatus are presented with initial test results conducted to investigate the power efficiency and wear performances of several polymer and metal samples. Our results show that for a fixed power transmission torque, there exists a critical pre-load between the drive components at which the wear and slip are maximum and the value of critical pre-load is independent of the material. Both wear and slip can be minimized to near-zero if an optimum pre-load, which is higher than the critical pre-load, is applied between the two components.     

Third body effects on friction and wear during fretting of steel contacts

Fretting wear proceeds through particle detachment from the contacting surfaces which, while trapped in the contact zone, can affect the frictional and wear response. Ball-on-flat fretting experiments were carried out between steel specimens under gross slip regime. A transition in the coefficient of friction was linked to a critical contact pressure. The microstructure and chemical composition of the third body evolve with the applied pressure. The evolution of the friction coefficient is strongly dependent on the third body properties. The wear is controlled by the applied load and thus the real contact area within the wear track.     

Normal displacement and dynamic friction characteristics in a stick-slip process

The dynamic friction-velocity relationship in the slip stage of stick-slip vibration was investigated with a pin-on-flat apparatus. A reasonable relationship was derived from the measured displacement and acceleration of the sliding element, using a computing technique which provided smooth, reliable curves. It was found, from an examination of the effect of data smoothing on computed friction characteristics, that the moving average method can be recommended. Variation of normal displacement of the sliding body during the slip period was examined together with tangential displacement and electrical contact resistance. The friction-velocity relationship was well explained by an assumption that the instantaneous friction variation results from the change in real area of contact resulting from normal displacement occuring in an elastic contact.     

Oil-Lubricated Fretting Behaviors of 304 Stainless Steels under Different Fretting Strokes and Normal Loads

The influence of oil lubrication on the fretting wear behaviors of 304 stainless steel flat specimens under different fretting strokes and normal loads has been investigated. The results proved that fretting regimes and fretting wear behaviors of 304 stainless steels were closely related to the fretting conditions. In general, the increase in normal load could increase wear damage during sliding wear. However, according to the results, a significant reduction in wear volume and increase in friction coefficient was observed when the normal load was increased to critical values of 40 and 50 N at a fretting stroke of 50 μm due to the transformation of the fretting regime from a gross slip regime to partial slip regime. Only when the fretting stroke further increased to a higher value of 70 μm at 50 N, fretting could enter the gross slip regime. There was low wear volume and a high friction coefficient when fretting was in the partial slip regime, because oil penetration was poor. The wear mechanisms were fatigue damage and plastic deformation. There was high wear volume and low friction coefficient when fretting was in the gross slip regime, because the oil could penetrate into the contact surfaces. Unlike the wear mechanisms in the partial slip regime, fretting damage of 304 stainless steels was mainly caused by abrasive wear in the gross slip regime.     

Stability criterion for stick-slip motion using a discontinuous dynamic friction model

An analysis of stick-slip stability is presented using a discontinuous friction model which consists of a negative damping action for the slip acceleration phase and a constant frictional resistance for the slip deceleration phase. Limiting conditions for stability due to a positive viscous damping action are defined, where transition from self-generated vibrations to smooth sliding occurs. The implications of the theory are discussed for the design and analysis of equipment subject to such vibrations.     

曲线半径及轴重对轮轨摩擦功影响的研究

基于非Hertz滚动接触理论利用数值计算方法详细分析了静态接触情况下,轴重和曲线半径对轮轨接触质点间等效应力、接触斑粘滑区的分布、总滑动量和摩擦功的影响。分析计算表明,轴重增加引起轮轨接触质点间等效应力,接触质点间粘滑区的面积以及总滑动量的变化,同时对轮轨接触质点阍的摩擦功的变化有重要影响;小曲线半径处轮轨接触质点间的总滑动量,接触斑滑移区的面积以及摩擦功都明显增大,导致曲线上钢轨磨损加剧。因此曲线半径和轴重是影响轮轨滚动接触磨损的重要因素。     

Stress drop and contact stiffness measured from stick-slip experiments on PMMA-PMMA friction

We present results from an extensive stick-slip study on PMMA-PMMA dry friction, where we studied the influence of a wide range of normal stresses, loading velocities and roughnesses of the sliding surfaces. In this paper we focus (a) on the analysis of a residual coefficient of friction, i.e., shear stress measured at the end of the slip phase divided by the corresponding normal stress, and (b) on the contact stiffness measured by plotting the relative displacement between sample against the shear stress during the stick phase. It is shown that the residual coefficient of friction (i) decreases as normal stress increases, (ii) shows a slight increase when the roughness of the sliding surfaces increases and (iii) does not vary according to the loading velocity. The contact stiffness proved independent of loading conditions and of the roughness of the sliding surfaces. These results are interpreted in terms of asperity interlocking. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fretting Wear Behavior of UHMWPE—Influence of Load and Stroke

In this study, the tribological behavior of ultra-high-molecular-weight polyethylene (UHMWPE) against a GCr 15 steel ball during fretting wear conditions was investigated using an oscillating reciprocating tribometer. The aim of this study was to characterize the critical value of normal load and stroke corresponding to this transition in UHMWPE worn surface at room temperature. Results showed that there existed a critical value of load or stroke at fixed condition. The friction coefficient and wear volume loss of UHMWPE at or near the critical values of load and stroke exhibited extreme changes. Based on observation of the worn surface by scanning electron microscopy (SEM) and 3D surface profiler measurements, it can be found that damage to the worn surface can be linked to the contact load and stroke. In addition, results showed that during the process of fretting wear under different load or stroke conditions, the gross slip regime dominated throughout the whole test period.     

Autowave friction and nonequilibrium dynamic tribosystem self-regulation

The transition to the condition of autowave motion is illustrated on the example of a dynamic system consisting of weights linked to a pulley moving with friction in the pulley axis under the force of gravity. The coherent interaction between the external modes of nonstationary directed motion and the internal wave modes under the studied conditions induces the effects of negative dynamic friction. The tangential intensification of the counterbody friction in the intervals of positive feedback actuates both the tangential and normal component of the full contact response that has no counterparts in the equilibrium tribodynamics. This effect is explained by the resonance automodulation of contact friction by elastic waves of stresses. The qualitative parameters controlling the changes in external friction conditions are introduced, similar to the Reynolds number in hydroaeromechanics. The general problems of contact tribodynamics and hydroaerodymics of the boundary layer are analyzed in respect to the coherent interaction in the friction system and the wave impulse transfer.     

Development of a friction energy capacity approach to predict the surface coating endurance under complex oscillating sliding conditions

In the case of surface coatings application it is crucial to establish when the substrate is reached to prevent catastrophic consequences. In this study, a model based on local dissipated energy is developed and related to the friction process. Indeed, the friction dissipated energy is a unique parameter that takes into account the major loading variables which are the pressure, sliding distance and the friction coefficient. To illustrate the approach a sphere/plane (Alumina/TiC) contact is studied under gross slip fretting regime. Considering the contact area extension, the wear depth evolution can be predicted from the cumulated dissipated energy density. Nevertheless, some difference is observed between the predicted and detected surface coating endurance. This has been explained by a coating spalling phenomenon observed below a critical residual coating thickness. Introducing an effective wear coating parameter, the coating endurance is better quantified and finally an effective energy density threshold, associated to a friction energy capacity approach, is introduced to rationalize the coating endurance prediction. The surface treatment lifetime is then simply deduced from an energy ratio between this specific energy capacity and a mean energy density dissipated per fretting cycle. The stability of this approach has been validated under constant and variable sliding conditions and illustrated through an Energy Density–Coating Endurance chart.     

Static friction coefficient of some plastics against steel and aluminum under different contact conditions

The static friction behavior was investigated in the case of POM-based polymers containing additives, designated DE20076, DE100KM and DE12017 as well as Nylon-66 designated ZY103HSL, when in contact with AISI 1045 steel and 6061 aluminum alloy. A designed apparatus that detects the exact moment corresponding to the transition from rest to slip was used to measure the coefficient of static friction. The impact of normal load, dwell time before slip, apparent area of contact and lubrication by water and paraffin oil on the static friction characteristics was studied. A decrease of the static coefficient of friction with low normal load followed by a levelling off governed by its effect on the real area of contact and stiffness of the counterface was observed. This also yielded a linear decrease of slip acceleration as well as stick time. A polynomial equation, which increases monotonically, was found to express well the effect of stick time on static friction, independently of the materials in contact. Adhesive effects were generated by the presence of both lubricants with the result of increase of the static coefficient of friction.     

Analysis of interface temperature,forward slip and lubricant influence on friction and wear in cold rolling

To improve cold rolling processes, it is necessary to understand and to optimise contact at roll–strip interface. Thus a simulation test has been developed in our laboratory. The upsetting rolling test (URT) enables to study friction and iron fines pollution by the reproduction of the main industrial contact conditions such as plastic strain, normal and tangential stresses and forward slip. On the basis of the URT, a new experimental protocol has been developed to reproduce industrial lubrication regime. Moreover, a new heating system has been designed to simulate interface temperature which has a decisive effect on lubricant behaviour. These optimisations permit to analyse contact temperature, forward slip and lubricant influence on friction, iron fine pollution and surface aspects. A great influence of temperature and lubricant on friction and wear has been put forward. Actually an increase of the Coulomb friction coefficient associated with a decrease of the iron fines quantity have been shown with an increase of temperature. These results seem to indicate more adhesive wear when temperature increases.     

The effect of an alumina counterface on friction reduction of CuO/3Y-TZP composite at room temperature

The friction behavior of CuO/yttria-stabilized tetragonal zirconia (3Y-TZP) composite in dry sliding against alumina at room temperature has been investigated. The results show that an alumina counterface has a crucial role on the frictional behavior when sliding against CuO/3Y-TZP composite in comparison with other counter materials. Pure 3Y-TZP shows high friction and wear under the same conditions. It is found that the friction reduction behavior is dependent on the sliding test conditions such as load and humidity. A thin aluminum-rich layer less than 200 nm thick on the contact surface during the low friction situation has been found by various analyzing techniques including interference microscopy, micro-Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microcopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The induced change of contact conditions and interfacial chemical reaction between CuO and alumina to form the phase CuAlO₂ increase the wear of alumina and accelerates the formation of an aluminum-rich surface layer. The presence of such a layer in the contact is beneficial for reducing friction. After a certain sliding distance, the coefficient of friction shifts from a low value to a high value due to a change in the dominating wear mechanism. This transition is shown to be caused by a different composition and thickness of the interfacial layer.     

Numerical analysis of stick-slip instability by a rate-dependent elastoplastic formulation for friction

In various fields of engineering, it is important to clarify friction-induced vibration, such as stick-slip motion, for a wide range of scales from microscopic elements to continental plates. In the present study, we apply a rate- and state-dependent friction model [30] (Hashiguchi and Ozaki, 2008), which can rationally describe the reciprocal transition between the static friction and the kinetic friction by a unified formulation, to the simulation of stick-slip instability for a one-degree-of-freedom spring-mass system under various conditions. It is verified that the various basic experimental findings on stick-slip motion can be pertinently described by the present approach. Moreover, the effect of the dynamic characteristics of the system, such as the mass, stiffness and driving velocity, is discussed, and parameters prescribing the rate of reciprocal transition of static-kinetic frictions and the preliminary microscopic sliding on the instability of the stick-slip motion are also discussed.     

Effect of friction cycles on wear particle generation of carbon nitride coating against a spherical diamond

The in-situ observations of wear particle generation of carbon nitride coating on silicon repeatedly sliding against a spherical diamond have been studied in terms of the critical friction cycles and normal loads. An environmental scanning electron microscope (E-SEM), in which a pin-on-disk tribotester was installed, has in-situ provided direct evidence of when and how the wear particle generation do occur during the repeated sliding of carbon nitride coating against a spherical diamond. The in-situ observations of non-conductive carbon nitride coating are therefore available free from surface charging with controllable relative humidity. The repeated sliding tests at a sliding speed of 50 μm/s have been carried out with the purpose of observing the ‘No wear particle generation’ region when varying normal load from 10 to 250 mN. It appears that until 20 friction cycles, the maximum Hertzian contact pressure P_max for ‘No wear particle generation’ can be improved from 1.39 Y to 1.53 Y if silicon is coated by carbon nitride with a thickness of 10 nm, where Y is defined as the yield strength of silicon. The applicable enlargement of the ‘No wear particle generation’ region of carbon nitride coating has therefore been comparatively discussed with the silicon substrate from the view points of the friction coefficient and the specific wear rate. The mode transition maps have also been summarized for the repeated sliding of carbon nitride coating in terms of ‘No wear particle generation’, ‘Wear particle generation by microcutting’ and ‘Wear particle generation by microcutting and microfracturing’ three typical modes.     

Velocity dependent friction laws in contact mode atomic force microscopy

Friction forces in the tip–sample contact govern the dynamics of contact mode atomic force microscopy. In ambient conditions typical contact radii between tip and sample are in the order of a few nanometers. In order to account for the large interaction area the dynamics of contact mode atomic force microscope (AFM) is investigated under the assumption of a multi-asperity contact interface between tip and sample. Thus, the kinetic friction force between tip and sample is the product of the real contact area between both solids and the interfacial shear strength. The velocity strengthening of the lateral force is modeled assuming a logarithmic relationship between shear-strength and velocity. Numerical simulations of the system dynamics with this empirical model show the existence of two different regimes in contact mode AFM: steady sliding and stick–slip where the tip undergoes periodically stiction and kinetic friction. The state of the system depends on the scan velocity as well as on the velocity dependence of the interfacial friction force between tip and sample. Already small viscous damping contributions in the tip–sample contact are sufficient to suppress stick–slip oscillations.