Evidence of scuffing initiation by adiabatic shear instability

A mechanism for the initiation of scuffing based on adiabatic shear instability was assessed experimentally through the measurement of dislocation densities by X-ray diffraction. Scuffing was predicted to occur when the rate of local thermal softening exceeded that of work hardening in the tribological contact. Since these rates were dependent on microstructure, the theory was tested by performing scuffing tests on SAE 4340 steel subjected to five different heat treatments. The scuffing resistances were consistent with the theory, and thermal events during the tests and the resulting microstructural morphologies were consistent with a locally adiabatic environment. The dislocation densities measured in samples from tests stopped before, during, and after scuffing were consistent with the theory. Use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the U.S. Department of Energy under Contract W-31-109-ENG-38.     

Study of plastic shear deformation in surface layer at friction. Simulation results. Part II. Effect of friction parameters

Deformation and temperature in the surface layer of plastic material and thickness of the plasticized layer are calculated using the one-dimensional macroscopic friction model developed in the first part of the paper [1]. We have studied the effect of sliding velocity, contact spot area, hardening ability, and thermal conductivity of material on the frictional heating and plastic deformation of the surface layer during a single touch between the contact spot and counterbody. Repeated contacts of the specimen and counterbody are simulated and the accumulation of plastic shear is considered for different contact spot sizes.     

Scuffing Performance of Amorphous Carbon During Dry-Sliding Contact

Scuffing is a major problem that limits the life and reliability of sliding tribo-components. When scuffing occurs, friction force rises sharply and is accompanied by an increase in noise and vibration; severe wear and plastic deformation also occur on the damaged surface. Attempts have been made over the years to combat scuffing by enhancing the surface properties of the machine elements, and by methods involving lubricant formulation and coating application.

In this study, the authors evaluated the scuffing performance of an amorphous, near-frictionless carbon (NFC) coating that provides super-low friction under dry sliding conditions. The test configuration used a ball-on-flat contact in reciprocating sliding. The coating was deposited on HI3 steel. An uncoated 52100 steel ball was tested against various coated flats in room air. Compared to uncoated surfaces, the carbon coating increased the scuffing resistance of the sliding surfaces by two orders of magnitude. Microscopic analysis shows that scuffing occurred on coaled surfaces only if the coating had been completely removed. It appears that depending on coating type, the authors observed that coating failure occurs before scuffing failure by one of two distinct mechanisms: the coating failed in a brittle manner and by spoiling, or by gradual wear.     

Tribological Behavior of Polytetrafluoroethylene under Unidirectional Rotation,Reciprocating Sliding,and Torsion Motion

The experimental friction coefficient of polytetrafluoroethylene (PTFE) increased in the order of unidirectional rotating motion (URM), reciprocating sliding motion (RSM), and face-on-face torsion motion (FTM), whereas the wear mass loss decreased. The contact zone of PTFE composites exhibited different wear mechanisms with various motions. Under URM, RSM, and FTM, the wear particles were presented as banding, long strips, and irregular floccus clumps. The higher shear force produced by more coupled nodes would result in a higher friction coefficient. The accumulated elastic hysteresis and plastic deformation in corresponding trajectories potentially enabled a better understanding of the particle morphology under URM, RSM, and FTM, respectively.     

Temperature Analysis in Lubricated Simple Sliding Rough Contacts

A temperature analysis of dry sliding fully plastic contact is extended to calculate the asperity temperatures between a sliding lubricated rigid smooth plane and a stationary elastic rough surface. First, surface roughness is generated numerically to have a Gaussian height distribution and a bilinear autocorrelation function. Lai and Cheng's elastic rough contact computer program is then used to determine the asperity contact loads and geometries of real contact areas. Assuming different frictional coefficients for shearing the lubricant film at the noncontact areas, shearing the surface film at the asperity contacts and shearing the oxide film as the asperity temperature exceeds a critical temperature, asperity temperature distributions can be calculated. Eight cases in Durkee and Cheng's scuffing tests of lubricated simple sliding rough contacts are simulated by using 20 computer-generated rough surfaces. The results show that scuffing is correlated to high-temperature asperities which are above the material-softening temperature.     

Thermal and thermomechanical effects in dry sliding

Whenever friction occurs in dry sliding of mechanical components, mechanical energy is transformed into heat through surface and volumetric processes in and around the real area of contact. This frictional heating, and the thermal and thermomechanical phenomena associated with it, can have a very important influence on the tribological behavior of the sliding components, especially at high sliding velocities. Significant developments in the study of these phenomena are reviewed in this paper. Among the topics reviewed are mechanisms of frictional heating and the distribution of heat during sliding friction, the measurement and analysis of surface and nearsurface temperatures resulting from frictional heating, thermal deformation around sliding contacts and the changes in contact geometry caused by thermal deformation and thermoelastic instability, and the thermomechanical stress distribution around the frictionally heated and thermally deformed contact spots. The paper concludes with a discussion of the influence of the thermal and thermomechanical contact phenomena on wear, thermocracking and other modes of failure of sliding mechanical components.     

脂肪醇对Ti6Al4V/钢摩擦副摩擦磨损性能的影响

采用SRV型摩擦磨损试验机分别考察了Ti6Al4V/钢摩擦副在多种脂肪醇润滑下的摩擦磨损性能。结果表明,与液体石蜡相比,碳链长度小于碳8的脂肪醇作为Ti6Al4V/钢摩擦副的润滑剂表现出良好的润滑性能,其润滑机制是在Ti6Al4V磨损表面形成吸附膜。载荷和频率明显影响Ti6Al4V/钢摩擦副在脂肪醇润滑下的摩擦磨损行为和摩擦磨损机制:当载荷较小时,Ti6Al4V磨损表面主要发生轻微的擦伤;随着载荷增加,Ti6Al4V磨损表面擦伤严重并在更高载荷下发生较为严重犁沟和塑性变形。     

An experimental study of wear of ferrous metals

An experimental study of tracks generated by point contact sliding surfaces under different loads has been carried out on a Bowden-Laben machine. The damage to mild steel, cast iron and carburized steel under repeated rubbing was studied by microhardness testing and microscope examination.The microhardness value at various depths below the track can be an indication of severe wear by sliding action. A critical value for a combination of materials was determined. The mechanism of wear and its gradual change at the contact surface with increased load and cycles of reciprocating sliding motion are discussed.     

Role of Frictional Heating in Rubber Friction

The energy dissipation in the contact regions between solids in sliding contact can result in high local temperatures which may strongly affect the friction. This is the case for rubber sliding on road surfaces at speeds above 1 mm/s. I derive equations which describe the frictional heating for arbitrary (non-uniform) motion, taking into account that some of the frictional energy is produced inside the rubber due to the internal friction in rubber. Numerical results are presented for one limiting case for steady sliding.     

Prediction of Wear in Reciprocating Dry Sliding via Dissipated Energy and Temperature Rise

An alternative technique aimed at facilitating the calculation of frictional power dissipation in reciprocating dry sliding is presented. The proposed technique can be employed for the prediction of wear in circumstances where the direct measurement of power dissipation is encumbered by practical limitations. Experimental tests are carried out to investigate the relationship between the system’s wear rate, power dissipation, and thermal response. A convenient technique is also proposed to estimate the average contact temperature in a reciprocating sliding contact. The predicted temperatures agree with the experimental measurements. It is also shown how the predicted temperatures can be used for the estimation of wear under reciprocating dry sliding configuration.     

The effect of frictional force amplitude on the degradation of SiC abrasive papers

The degradation of 600 and 100 grade SiC abrasive papers by unidirectional and reciprocating frictional contact was studied. The mechanism of deterioration is basically different for each grit size. Detachment of grits from the resin base and the volume of metal removed depend on the type of friction. The rate of detachment of blunt grits is higher for reciprocating frictional contact than for unidirectional frictional contact. This can be explained in terms of the fatigue strength of the cementing resin base under completely reversed stress and pulsating stress caused by the rubbing action.     

Kinetic friction and dynamic elastic contact behaviour of polymers

Friction and contact between a glass hemisphere and plates of various polymers were studied under elastic contact. The contact at various sliding speeds was observed by means of Newton's rings. Although the frictional force was proportional to the area of contact observed, it was necessary to consider the increase in the shearing strength of the adhesive junctions with increasing load W. The area of dynamic contact of low density polyethylene (LDPE) was proportional to W^0.78 in a wide speed range, while the area of static contact was proportional to W^2/3 as expected from Hertz' equation. The shape of the dynamic contact was affected by the retarded elasticity of the polymer and frictional heating. The area of dynamic contact decreased and the shear strength increased in proportion to log v (where v is the speed) provided that frictional heating was negligible. The time dependence of elastic compliance 1/E of LDPE could be obtained by applying Hertz' equation to the dynamic contact observed at various speeds. However, the value of 1/E decreased with decreasing load in sliding. It was shown that the time dependence of elastic compliance plays an important role in determining the area of dynamic contact of polymers.     

A system for wear prediction in lubricated sliding contacts

A system of analysis is developed to predict the rate of wear in sliding contacts. The essence of the approach is the proposal that the rate of wear can be predicted only in probabilistic terms. Therefore, the estimation of the probability of wear, which can be regarded as synonymous with the probability of surface asperity contacts, precedes the calculation of the wear rate. Further, recognising the fact that wear takes place within the actual area of contact, it is argued that this area consists of plastic and elastic contacts between asperities which, in turn, have different shear strengths and contribute differently to the wear process. In the case of lubricated contact, a frictional film defect represents the influence of a lubricant on the wear process. Moreover, as in this type of contact the load is supported by both lubricating film and contacting asperities, a special procedure is provided to estimate the load supported by the asperities, because it is only that part of the load which contributes to the wear. The catastrophic form of wear in lubricated contacts, that is termed ‘scuffing’, is also considered, and the probability of scuffing, under a given set of operating conditions, is estimated. The predictive system has been tested and its predictions are compared with available experimental results.     

Microstructural evolution during scuffing of hardened 4340 steel—implication for scuffing mechanism

Microstructural changes occurring during scuffing failure in hardened 4340 steel were studied with a block-on-ring test rig using a step-loading test protocol. Tests were interrupted before, during, and after scuffing. Both surface and sub-surface changes in the original tempered microstructure were characterized by scanning electron microscopy. Results of our study showed that scuffing occurred by severe and sudden plastic deformation of near-surface material in a sub-second time frame. Based on this observation and other previously observed microstructural changes, i.e. formation of a large fraction of retained austenite during scuffing, a scuffing mechanism based on adiabatic shear instability is proposed. The proposed scuffing mechanism can effectively explain most of the phenomenological observations associated with scuffing failure.     

热力耦合下微接触点塑性应变沿深度变化分析

在考虑粗糙实体弹塑性变形、热力耦合、微凸体间相互作用和摩擦热流耦合等影响下,运用有限元法数值模拟具有三维分形特性的粗糙面与刚性平面间滑动摩擦过程,分析了粗糙实体接触凸点塑性变形随深度变化情况。发现：在速度的突变和闪点温度形成时,摩擦接触表层等效塑性应变增大明显;在这一摩擦表层,过不同接触点的纵向剖面塑性应变沿深度分布不同：有的是接触表面塑性变形最大,有的是在接触微凸体表面下某一深度塑性变形最严重,而接触凸点表面的塑性应变稍小些。这与相关文献用SEM研究干摩擦后金属摩擦表层变形照片后发现的结果一致。滑动摩擦过程中,金属粗糙摩擦接触表层塑性变形的不断累积,将会导致材料表层中的夹杂或微观缺陷周围萌生微孔和裂纹源。     

Investigation on the Molecular Shear-Induced Organization in a Molecularly Thin Film of N-hexadecane

The frictional properties of a thin hexadecane film confined between two atomically smooth surfaces of mica were studied using the surface forces apparatus equipped with a 3D actuator–sensor attachment specially designed to investigate static and dynamic forces in three orthogonal directions simultaneously. The use of this attachment allows the relative alignment of the reciprocal sliding motion to be changed by an angle of 90° while maintaining the film under the same confinement conditions. The effects of the commensurability of the confining mica surfaces as well as the relative sliding direction on the frictional behavior of the hexadecane film were determined for different temperatures (18–29 °C) and sliding velocities (4 nm/s to 4 μm/s). The confined hexadecane film exhibited smooth sliding friction whose amplitude increased with the commensuration of the surfaces. A progressive evolution in the kinetic friction force toward a steady-state value was observed over reciprocal sliding motion for given experimental conditions of applied load, sliding velocity and environmental temperature. This friction evolution shows to be dependent on the sliding history of the film and could result from a partial molecular ordering, occurring during shear.     

Correlation between the frictional behaviour and the physical properties of metals

The surface topography of mating surfaces is characterized by plastically deformed asperities which form real areas of contact. As sliding leads to the shearing of junctions, it is feasible that physical properties will affect adhesion and shear strength and influence frictional behaviour.The sliding friction of similar metals and of metals sliding against a steel specimen was investigated for most commonly used metals in terms of their fundamental physical properties including atomic volume, surface energy and thermal properties.     

The influence of specimen geometry and sliding mode on the friction and wear of iron-chromium alloys in controlled environments

The friction and wear behaviour of iron-chromium alloys containing 5–20% Cr has been studied in unidirectional and reciprocating motions under conditions where frictional heating should be minimal. These experiments were performed in an ultrahigh vacuum chamber in which the oxygen partial pressure could be maintained from 10^?6 Pa to atmospheric pressure. At 10^?6 Pa the friction behaviour and the pattern of wear were essentially similar for both types of motion, although slightly lower wear was observed under reciprocating motion. At higher oxygen partial pressures the containment of debris within the track led to the formation of compacted oxide “islands”. Hemispherical upper specimens seemed more effective than the conical ones in trapping this debris.     

基于最小能量法的高速切削锯齿状切屑变形分析

以高速切削典型变形特征为研究对象,建立了锯齿状切屑的几何模型。根据高速切削绝热剪切理论,通过对剪切面相对滑移失稳瞬间的切屑块受力平衡分析建立力学模型和运动学模型,获得切屑摩擦力和剪切力及剪切速度和切屑流动速度,并确定了切削能量方程。在考虑应变、应变率、切削温度和变形硬化因素条件下按最小能量原理求变形方程,结合变形曲线的分析和切削理论确定了高速切削变形方程及影响变形的因素。分析结果表明,切屑锯齿化是塑性变形超过临界失稳条件的结果。     

Shear deformation of voids with contact modelled by internal pressure

The behaviour of voids in a ductile material subject to simple shear or to a shear-dominated stress state is analyzed numerically. Here the stress triaxiality is so low that instead of void volume growth to coalescence there is void closure leading to micro-cracks that rotate in the shear field. At some stage of the deformation, the void surfaces will come in contact so that sliding with or without friction will start to occur. To avoid problems with strong mesh distortion in the large strain field around the deforming void and with mesh resolution at the tip of the crack, an internal pressure is applied as an approximate representation of void surfaces pressed together in frictionless sliding, and also remeshing is applied. This micromechanical model for a strain hardening elastic–plastic material shows that a maximum overall shear stress is reached, at which localization of plastic flow occurs, leading to final failure in the material.