Characterization of Friction and Heat Partition Coefficients during Machining of a TiAl6V4 Titanium Alloy and a Cemented Carbide

This article aims at characterizing the frictional behavior of a TiAl6V4 alloy and a carbide tool under extreme conditions corresponding to those occurring at the cutting tool–work material interface. A specially designed open tribometer was used to characterize the macroscopic friction coefficient, heat partition coefficient, and adhesion in the contact versus sliding velocity and contact pressure. It has been shown that titanium leads to intense adhesion, which seems to be even more intensive with high contact pressure and high sliding velocity, which limits the local sliding movement at the interface (stuck layer). However, the tribometer provides the evolution of an apparent friction coefficient and a macroscopic heat partition coefficient related to the shearing of titanium between the adhesive layer and the bulk material. An increase in sliding velocity or contact pressure induces a small decrease in the apparent friction coefficient as well as the heat partition coefficient. It has been shown that adhesion is thermally activated by a combination of contact pressure and sliding velocity, which leads to a threshold effect. Furthermore, the application of an emulsion showed a small decrease in the apparent friction coefficient associated to a decrease in adhesion. Finally, this work provides quantitative data on the apparent friction and heat partition coefficients versus sliding velocity and contact pressure that can support the development of macroscopic cutting models for titanium alloys.     

The effect of sliding time and speed on the wear of composite materials

The friction and wear properties of an Al 201 alloy and a unidirectionally oriented graphite fiber-aluminum matrix composite (T50-Al 201) were investigated. The experiments were conducted on a pin-on-disc type friction machine. The diameter of the pin was 0.22 cm and the load 4.46 N. The sliding velocity varied between 0.17 and 0.43 m s^?1. The disc counterface was of commercially pure iron. It has been found that the friction coefficient μ and the wear rate W_L of the composite material decrease as the sliding time is increased until a steady state value is reached. The steady state wear rate is proportional to the reciprocal of the sliding speed in accord with a recently proposed model. Scanning electron microscopy and Auger electron spectroscopy observations indicate that the high initial values of μ and W_L are due to a high degree of matrix adhesion to the counterface accompanied by fiber breaking and transfer. The low steady state values of μ and W_L are due to the formation of a film that impedes adhesion and confers some degree of self-lubrication. It is suggested that the observed variation of W_L with sliding speed is related to changes in the degree of subsurface damage as the velocity is varied.     

双粗糙面滑动过程摩擦因数的变化分析

采用W-M函数建立具有分形特征的三维双粗糙面接触模型,考虑了接触界面间的黏着效应,在滑动速度、法向载荷及界面剪切强度等参数变化下,运用有限元方法探讨了粗糙体在滑动过程中摩擦因数的变化情况。结果显示,滑动速度、法向载荷及界面剪切强度等参数对摩擦因数的变化有一定的影响,边界润滑工况下平均摩擦因数为0.28,无润滑工况下平均摩擦因数为0.713,最大界面剪切强度时的平均摩擦因数为0.73;随着界面剪切强度的减小、法向载荷的增大、滑动速度的增加,滑动摩擦因数有所减小。与相关文献结论或实验结果进行比较,证明了上述结果的正确性。分析结果可为摩擦学设计和摩擦材料的制备提供理论参考。     

Deformation by scratches and frictional properties of MgO crystals

The subsurface structure of MgO crystals frictionally damaged on their (001) faces by scratching with a diamond stylus in the 〈100〉 direction was investigated using a scanning electron microscope in both the cathodoluminescence and the secondary electron modes. The friction is independent of the load below a critical value, and above this value it depends on the load W according to μ ∝ Wⁿ where μ is the coefficient of friction.The subsurface beneath the sliding contact consists of four characteristic material zones: zone I is severely deformed with a high defect density which causes quenching of the luminescence; zone II is a plastically deformed intermediate zone showing enhanced luminescence; zone III is a slipped zone which on etching shows a dislocation pit pattern; zone IV consists of non-deformed crystalline material. The normalized depth $\text{d}\text{a}$, where d is the depth of each deformed zone and a is the track half-width, is independent of the load.The frictional behaviour was examined by using empirical relations for the damage to estimate the plastic work done during the scratching process.     

Sliding wear behavior of Fe-Al and Fe-Al/WC coatings prepared by high velocity arc spraying

A comparative study was carried out to investigate the microstructure and tribological behavior of Fe-Al and Fe-Al/WC iron aluminide based coatings against Si₃N₄ under dry sliding at room temperature using a pin-on-disc tribotester. The coatings were prepared by high velocity arc spraying (HVAS) and cored wires. The effect of normal load on friction coefficient and wear rate of the coatings was studied. The microstructure and the worn surfaces of the coatings were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscope (EDS). The results showed that, the main phases in both coatings were iron aluminide (Fe₃Al and FeAl) and α. WC/W₂C particles were embedded in the matrix of the composite coating. With adding WC hard particles, the Fe-Al/WC composite coating exhibited higher wear-resistance than Fe-Al coating. But the friction coefficient of both coatings showed little difference. As the load increased, the friction coefficient decreases slightly due to a rise of friction contact temperature and larger areas of oxide film formation on the worn surface, which act as a solid lubricant. Increasing load causes the maximum shear stress occurring at the deeper position below the surface, thereby aggravating the wear. The coating surface is subjected to alternately tensile stress and compression stress during sliding, and the predominant wear mechanism of the coatings appears to be delamination.     

Estimation of rubber sliding friction from asperity interaction modeling

Interaction between a soft rubber asperity and its hard counterpart is traced with the help of a finite element computation. The analysis is aimed to estimate the influence of adhesion between rubber and rigid surfaces and the energy losses arising from the deformation of rubber bulk to the sliding resistance. At the contact zone, interfacial bonds are formed due to adhesion and their resistance to sliding is represented by the shear strength of the contact interface. In the rubber bulk, the hysteresis loss is calculated using an appropriate model of the viscoelastic mechanical behavior of rubber for large strains. Dependence of friction on sliding speeds and temperature is hence detected. Influence of surface roughness and contact pressure on friction is also examined.     

The Relationship Between Contact Mechanics and Adhesion in Nanoscale Contacts Between Non-Polar Molecular Monolayers

Atomic and friction force microscopy were employed to examine adhesion and friction between dodecanethiol self-assembled monolayers in pure media as well as in two-component heptane/acetone mixtures. In media that did not contain hydrogen bond donors, the pull-off forces were found to be in very good agreement with theoretic predictions based on the Lifshitz theory. As the hydrogen bond donor ability of the medium increased, the adhesion energy was found to be increasingly underestimated by the model, illustrating the importance of the medium–medium interactions outside the contact area in determining the adhesive properties of the contact at the nanoscale. Exceptionally, in n-octanol, the pull-off forces were considerably lower than predicted and a dual slope linear friction–load relation was observed. These observations were rationalized by the formation of physisorbed layers of octanol on the surfaces. The friction–load relationship in the other media was found to be dependent on the magnitude of adhesion. For weakly adhering systems, the friction–load relationship was linear, but as adhesion increased, a sublinear relationship was observed. The data were rationalized by treating the friction as the sum of an adhesion-dependent shear term characterized by a surface shear strength τ and a molecular plowing term characterized by a coefficient of friction μ. Thus, Amontons’ law appears to describe the limiting case of very weak adhesion where viscoelastic plowing is primarily responsible for energy dissipation, while a sublinear friction–load relationship emerges in other situations due to the dissipation of energy in shearing adhesive contacts.     

Fundamental aspects and technological implications of the solubility concept for the prediction of running properties

The first Goodzeit rule, which relates resistance to cold welding and seizure with mutual solubility of metal sliding couples, has been investigated. The sliding properties of Au-Ag and Pd-Ag alloys against SAE 1045 carbon steel were observed in ultrahigh vacuum and in argon. The abrupt change in friction and wear coinciding with the composition of the alloys at which the solubility of iron into the alloy vanishes found by De Gee was not confirmed. Instead a continuous decrease in friction coefficient and wear was observed with increasing silver content. Pure silver showed a very limited tendency to metal transfer, although in some cases the transferred layer was removed spontaneously after 200–300 cycles. Electrical conduction measurements made during sliding revealed a correlation between the momentary friction force and the electrical conductivity which indicated that variations in the friction force are synchronous with variations in the contact area.A model is proposed in which the conditions of friction and metal transfer are described in terms of one system and two material properties, i.e. the interfacial shear strength τ_i, the shear strength τ¹ and the strain hardening exponent n of the junction materials. The friction behaviour is described by a modified adhesion theory: instead of a contaminating layer it is the specific atomic interaction of the contacting metals acting over small regions of atomic contact which controls τ_i. The junction growth mechanism is correlated with strain hardening.It is suggested that the first Goodzeit rule must be interpreted as a result of atomic interaction in the interface between the contacting metals. In this way prediction of the running behaviour of both metals and alloys would become possible.     

In Situ Analysis of Third Body Contributions to Sliding Friction of a Pb–Mo–S Coating in Dry and Humid Air

Reciprocating sliding tests of ion-beam deposited (IBD) Pb–Mo–S coatings were performed with an in situ tribometer that allows real-time visualization and Raman analysis of the sliding contact through a transparent hemisphere. Experiments were performed in dry air, ambient air (∼50% RH) and mixtures of dry and humid air cycled between low and high humidity. Third bodies formed in the sliding contact were monitored through an optical microscope and analyzed by Raman Spectroscopy. Third body velocity accommodation modes were identified and correlated with friction behavior in dry and ambient air. The dominant velocity accommodation mode in both dry and humid air was interfacial sliding between the outer surface of the transfer film and the wear track; this interface, based on present and earlier studies, is crystalline MoS₂. Therefore, the friction coefficient was controlled by the interfacial shear strength of MoS₂ sliding against MoS₂. Humid air sliding was accompanied by a rise in the friction coefficient and a small but observable second velocity accommodation mode: shear/extrusion of the transfer film. It is concluded that the friction rise in humid air was due to an increase in the interfacial shear strength, and that the rise in friction caused the third body to deform rather than the deformation causing the friction to rise.     

Tribological Behavior of Rice Bran Ceramics in a Vacuum Environment

ABSTRACT

In this study, we investigated the friction and wear of rice bran (RB) ceramics—hard porous carbon materials made from rice bran—in a vacuum environment. Sliding friction tests for RB ceramic pin–RB ceramic disk contact were performed using a pin-on-disk-type friction tester installed in a vacuum chamber. The ambient pressure was controlled at 0.02, 0.6, 30, and 10⁵ Pa (i.e., atmospheric pressure). The normal load was 0.49 or 2.94 N, the sliding velocity was 0.01 or 0.1 m/s, and the number of friction cycles was 50,000. The friction coefficient tended to decrease with decreasing ambient pressure for all combinations of normal load and sliding velocity; by contrast, the specific wear rate of the RB ceramic pin and disk specimens tended to increase with decreasing ambient pressure. The friction coefficient exhibited a low value of 0.05 or less at 0.02 Pa. The results suggested that the reduced surface roughness and graphitization of the sliding surface of the RB ceramic pin and disk due to induced friction, as well as the increased ratio between the partial pressure of water vapor and the ambient pressure, are related to the reduction in the friction of RB ceramic–RB ceramic dry sliding contact under vacuum conditions.     

Frictional characteristics of ceramics under water‐lubricated conditions

Friction experiments were conducted on four kinds of ceramics (SiC, Si₃N₄, Al₂O₃ and ZrO₂) against themselves in water under different contact pressures and sliding velocities. The variations of friction coefficients as a function of sliding distance, and the effects of mean contact pressure and sliding velocity on friction coefficients were shown. Friction coefficients lower than 0.03 were observed under a certain combination of mean contact pressure and sliding velocity for each material. The friction coefficient suddenly increased when the mean contact pressure was above a threshold value, which depended on both sliding velocity and the material of sliding pairs. SiC had a higher threshold value than the other three ceramics at every sliding velocity investigated. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tribological Behavior of Composites Based on ZA-27 Alloy Reinforced with Graphite Particles

The objective of this investigation is to assess the influence of graphite reinforcement on tribological behavior of ZA-27 alloy. The composite with 2 wt% of graphite particles was produced by the compocasting procedure. Tribological properties of unreinforced alloy and composite were studied, using block-on-disk tribometer, under dry and lubricated sliding conditions at different specific loads and sliding speeds. The worn surfaces of the samples were examined by the scanning electron microscopy (SEM). The obtained results revealed that ZA-27/graphite composite specimens exhibited significantly lower wear rate and coefficient of friction than the matrix alloy specimens in all the combinations of applied loads (F _n ) and sliding speeds (v) in dry and lubricated tests. The positive tribological effects of graphite reinforcement of ZA-27 in dry sliding tests were provided by the tribo-induced graphite film on the contact surface of composite. In test conditions, characterized by the small graphite content and modest sliding speeds and applied loads, nonuniform tribo-induced graphite films were formed leading to the increase of the friction coefficient and wear rate, with increase of the sliding speed and applied load. In conditions of lubricated sliding, the very fine graphite particles formed in the contact interface mix with the lubricating oil forming the emulsion with improved tribological characteristics. Smeared graphite decreased the negative influence of F _n on tribological response of composites, what is manifested by the mild regime of the boundary lubrication, as well as by realization of the mixed lubrication at lower values of the v/F _n ratio, with respect to the matrix alloy.     

Evaluation of friction coefficient using indentation model of Brinell hardness test for sheet metal forming

This work presents an indentation model of the Brinell hardness test, which is a rigid ball-deformable plane contact model (RB-DP model), to elucidate the sliding friction mechanism of sheet metal forming. In the proposed model, the friction force can be defined as a combination of shear (shearing effect) and plough (ploughing effect) forces. The real contact area ratio α is determined from the RBDP model under sliding condition. Moreover, the lateral contact area ratio A _c /A _r can be specified as a function of the real contact area ratio α. Based on Meyer’s law and Hertz contact problem, the maximum contact area ratio α _u , a limiting condition of the real contact area ratio α, can be described as a function of the strain hardening exponent n. Additionally, a limiting condition applies: the strain hardening exponent n must be less than 0.64 in the present model. The present friction model reveals that the friction coefficient μ _d is a function of strain hardening exponent n, the real contact area ratio α and the maximum contact area ratio α _u . The calculated friction coefficient μ _d agrees with the published experimental results.     

Effect of variable friction coefficient on contact tractions

Friction coefficient plays an important role in determining the contact tractions and consequently the stresses that drive damage mechanisms in components subjected to fretting fatigue. Friction coefficient evolves to different magnitudes at different points in the contact interface. Effect of this spatial variation in friction coefficient on the shear distribution for contacts involving similar isotropic materials has been presented in this paper. Full sliding tests have been conducted to obtain friction coefficient as a function of sliding distance. Well characterized set of experiments to determine the average friction coefficient in fretted specimens is briefly discussed. Existing analysis for obtaining the shear traction for arbitrary smooth profiles has been modified to take the variation of friction coefficient from one point to another into account. Using this analysis, spatial variation of friction coefficient in fretted specimens has been obtained. It has been found that using a constant friction coefficient (equal to the slip zone friction coefficient) throughout the contact zone is a good enough approximation in most of the cases. However, in some cases, when the slip zones extend into regions where the friction coefficient has not reached the asymptotic value, actual distribution of friction coefficient gives a different shear stress distribution as compared to a constant slip zone friction coefficient.     

Effects of entraining velocity of lubricant and sliding velocity on friction behavior in stainless steel sheet rolling

A series of experiments was carried out using a rolling-type tribometer to investigate the effects on friction behavior of the entraining velocity of the lubricant at the inlet to the contact zone (V) and sliding velocity during deformation (ΔV). Experiments with stainless steel sheets of two different surface roughnesses showed that the variations in the friction coefficient with entraining velocity V and sliding velocity ΔV are largely dependent on the initial surface texture of the workpiece. For a smooth workpiece, the friction coefficient decreases with increasing sliding velocity ΔV but keeps almost constant with increasing entraining velocity V. However, for a rough workpiece, the friction coefficient initially decreases slowly and increases largely with increasing sliding velocity ΔV or decreasing entraining velocity V. Observation of the rolled surface for a smooth workpiece shows that, with increasing entraining velocity V, the slip band becomes more marked, and with increasing sliding velocity ΔV, the rubbed portions become more conspicuous. For a rough workpiece, galling occurs at high sliding velocity ΔV. The critical condition for galling outbreak is shown on the V-ΔV graph. The galling outbreak process is observed by interrupting the rolling process.     

An Interfacial Friction Law for a Circular EHL Contact Under Free Sliding Oscillating Motion

The friction response of a lubricated interface under free sliding oscillating motion is investigated as a function of the contact pressure and the rheology of the lubricant in terms of viscosity and piezoviscosity. For loaded contacts, both velocity dependent friction, referred to as viscous damping, and friction independent of the instantaneous sliding velocity contribute to the energy dissipation. Viscous damping mainly corresponds to the dissipation in the lubricant meniscus surrounding the contact, while dissipation within the confined lubricated interface is mainly independent of the instantaneous sliding velocity. The friction coefficient independent of the instantaneous sliding velocity falls on a master curve for the wide range of tested operating conditions and lubricant rheological properties. The master curve is a logarithmic function of a dimensionless parameter corresponding to the ratio of the viscosity of the confined lubricant to the product of the pressure and a characteristic time. The physical meaning of this latter and the friction law are discussed considering the confined interface as a viscoelastic fluid or a non-Newtonian Eyring fluid.     

Friction and Shear Strength of Polymers

The authors have measured the shear strength and friction of a number of polymers. The shear strength measurements were made on thin films of the polymers deposited on hard, smooth substrates. The values of the coefficients of friction obtained from these shear strength measurements have been compared with the values obtained when bulk polymer slides either on itself or on a hard, smooth surface. The results support the view that in the sliding of bulk polymer energy is dissipated within a thin region close to the sliding interface and it is the pressure in the contact which largely governs the magnitude of the interfacial shear strength.     

On dry sliding friction and wear behaviour of PEEK and PEEK/SiC-composite coatings

In this work, polyetheretherketone (PEEK) and PEEK/SiC-composite coatings were deposited on Al substrates using a printing technique to improve their surfaces performance. The objective of this work was to investigate coatings friction and wear behaviour. Especially, the effect of sliding velocity and applied load on coatings friction coefficient and wear rate was evaluated in range of 0.2-1.4 m/s and 1-9 N, respectively. Compared to Al substrate, the coated samples exhibit excellent friction coefficient and wear rate. For PEEK coating, under an applied load of 1 N, the increase in sliding velocity can result in decreasing of friction coefficient at a cost of wear resistance. Under a load of 9 N, however, PEEK coating exhibits the highest friction coefficient and wear rate at an intermediate velocity. These influences appear to be mainly ascribed to the influence of contact temperature of the two relative sliding parts. In most test conditions, the composite coating exhibits better wear resistance and a little higher friction coefficient. SiC reinforcement in composite coating plays a combined role. First of all, it might lead to energy dissipation for activation of fracture occurred on the interface of PEEK and the powders. Moreover, it can reduce coating ploughs and the adhesion between the two relative sliding parts.     

Contributions of adhesion and hysteresis to coefficient of friction between shoe and floor surfaces: effects of floor roughness and sliding speed

Abstract

Improving shoe–floor friction in order to reduce slip and fall accidents requires thorough understanding of the factors that contribute to friction. The friction between a sliding viscoelastic material (shoe) and a hard surface (floor) has two major components: adhesion and hysteresis. This study aimed to quantify the effects of floor roughness and sliding speed on adhesion and hysteresis to determine how each component contributes to the coefficient of friction. Experiments were conducted on a pin on disc tribometer using ceramic tiles with three levels of roughness, six sliding speeds, two common shoe materials and four liquid lubricants. Hysteresis was measured using a lubricant that minimised adhesion. Dry and lubricated adhesion was measured by subtracting hysteresis from the coefficient of friction. Analysis of variance regression models were used to determine the contributions of hysteresis, dry adhesion, sliding speed and fluid to lubricated coefficient of friction. Increased floor roughness led to increased hysteresis, while increased sliding speed reduced both adhesion and hysteresis. These findings are consistent with theory that states that larger asperities increase hysteretic deformation and that sliding speed affects deformation and real area of contact between a viscoelastic material and a hard surface. The model correctly predicted 83% of variation in coefficient of friction based on dry adhesion, hysteresis and fluid dependent constants. The sensitivity of hysteresis friction to shoe material and floor roughness indicates that optimising these parameters may be effective at reducing slip accidents on oily floor surfaces.