Influence of roughness parameters and surface texture on friction during sliding of pure lead over 080 M40 steel

In the present investigation, tests were conducted on a tribological couple made of cylindrical lead pin with spherical tip against 080 M40 steel plates of different textures with varying roughness under both dry and lubricated conditions using an inclined pin-on-plate sliding tester. Surface roughness parameters of the steel plates were measured using optical profilometer. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the surface texture of hard surfaces. A newly formulated non-dimensional hybrid roughness parameter called ‘ξ’ (a product of number of peaks and maximum profile peak height) of the tool surface plays an important role in determining the frictional behaviour of the surfaces studied. The effect of surfaces texture on coefficient of friction was attributed to the variation of plowing component of friction, which in turn depends on the roughness parameter ‘ξ’.     

The genesis of friction

The genesis of friction is explained in terms of a new theory. Contrary to the postulates of the adhesion theory of friction, this theory postulates that the frictional force (and thus the friction coefficient μ) is affected by the sliding distance and the environment because of the changing contributions of three components of friction, i.e. that due to the deformation of surface asperities (denoted μ_d), that due to plowing by wear particles and hard asperities (denoted μ_p) and that due to the adhesion of the flat portions of the sliding surface (denoted μ_a). Therefore the coefficient of friction is not a simple material property. There are four or six stages of the friction regime depending on the sliding conditions. The initial friction coefficient μ_i can range from 0.1 to 0.2 for most machined surfaces. μ_i is largely independent of environmental conditions (including lubricants), materials and surface topography. μ_p varies from 0 to 1.0 and μ_a from 0 to 0.4. Only μ_a depends on the quality of surface adhesion. The history-dependent frictional behavior of materials is represented in the “friction space” diagram. According to this theory, the compatibility of sliding surfaces is dictated more by the mechanical properties of materials such as hardness than by their relative solubility at low temperatures.     

Observations of Stick-Slip Friction in Velcro®

Friction, and in particular stick-slip friction, occurs on every length scale, from the movement of atomic force microscope tips at the nanoscale to the movement of tectonic plates of the Earth’s crust. Even with this ubiquity, there still appears to be outstanding fundamental questions, especially on the way that frictional motion varies generally with the mechanical parameters of a system. In this study, the frictional dynamics of the hook-and-loop system of Velcro^® in shear is explored by varying the typical parameters of driving velocity, applied load, and apparent contact area. It is demonstrated that in Velcro^® both the maximum static frictional force and the average kinetic frictional force vary linearly with apparent contact area (hook number), and moreover, in the kinetic regime, stick-slip dynamics are evident. Surprisingly, the average kinetic friction force is independent of velocity over nearly two-and-a-half orders of magnitude (~2 × 10^?4 to ~6 × 10^?2 m/s). The frictional force varies as a power law on the applied load with an exponent of 0.28 and 0.24 for the maximum static and kinetic frictional forces, respectively. Furthermore, the evolution of stick-slip friction to more smooth sliding, as controlled by contact area, is demonstrated by both a decrease in the spread of the kinetic friction and the spread of the fluctuations of the average kinetic friction when normalized to the average kinetic friction; these decreases follow power-law behaviors with respect to the increasing contact area with exponents of approximately ?0.3 and ?0.8, respectively. Lastly, we note that the coefficients of friction μ _s and μ _k are not constant with applied load but rather decrease monotonically with power-law behavior with an exponent of nearly ?0.8. Phenomenologically, this system exhibits interesting physics whereby in some instances it follows classical Amontons–Coulomb (AC) behavior and in others lies in stark contrast and hopefully will assist in the understanding of the friction behavior in dry surfaces.     

A study on the effect of surface topography on rough friction in roller contact

The friction behaviour of gear teeth in the context of tribology can have a strong effect on housing vibration, noise and efficiency. One of the parameters that greatly influences the friction under certain running conditions is surface roughness. In this work, rough friction was studied in lubricated sliding of roller surfaces, which were manufactured to simulate the real gear surfaces. By examining 3D surface topography of two mating bodies, both surface roughness and its effect on friction behaviour can be studied. In a previous study, a rough-friction test rig has been designed, constructed and initially verified. The types of surfaces involved in this study are ground, shot-peened, phosphated and electrochemically deburred. These rollers were subjected to the same friction testing procedures. Roller surfaces were then examined, and correlation between the topography and the frictional behaviour was analysed. Friction behaviour was interpreted in terms of Stribeck curves (friction coefficient as the function of Hersey parameter (ην/p)). The results showed that electrochemically deburred and certain phosphated surfaces provide lower friction coefficient values which are competitive to fine-ground surfaces in lubricated rolling/sliding contact.     

Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate

Surface texture of harder mating surfaces plays an important role during sliding against softer materials and hence the importance of characterizing the surfaces in terms of roughness parameters. In the present investigation, basic studies were conducted using inclined pin-on-plate sliding tester to understand the surface texture effect of hard surfaces on coefficient of friction and transfer layer formation. A tribological couple made of a super purity aluminium pin against steel plate was used in the tests. Two surface parameters of steel plates, namely roughness and texture, were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture and are independent of surface roughness (R_a). Among the various surface roughness parameters, the average or the mean slope of the profile was found to explain the variations best. Under lubricated conditions, stick–slip phenomena was observed, the amplitude of which depends on the plowing component of friction. The presence of stick–slip motion under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities.     

Experimental investigation of tribological performance of laser textured stainless steel rings

A series of experiments is conducted to examine the frictional characteristics of laser surface-textured, heat-treated 17-4 PH stainless steel specimens. Two dimple shapes are tested: circular and elliptical. The circular dimples are designed with different sizes, dimple densities and dimple depth-to-diameter ratios. It is found that the cavitation pressure (P_cav) in a circular dimple is strongly affected by the operational speed. The surface textured specimens provide low coefficient of friction compared with plain (dimple-free) surfaces. However, for the material used in the current experiments, the surface texture decreases the surface's resistance to wear.     

Correlation between standard roughness parameters skewness and kurtosis and tribological behaviour of contact surfaces

The aim of the present research was to investigate the correlation between surface roughness parameters and friction. For this purpose 100Cr6 steel plate samples were prepared using different grades and combinations of grinding and polishing in terms of similar S_a but different S_sk and S_ku values. Dry and lubricated pin-on-disc tests, using different contact conditions, were carried out, using Al₂O₃ ball as counter-body. Test results show that surfaces with higher S_ku and negative S_sk values tend to reduce friction.     

The effect of temperature and environment on the friction of some well characterized refractory surfaces

This paper describes a study of the frictional behaviour of a hard slider traversing the clean surface of molybdenum, tungsten and vanadium carbide crystals in ultra high vacuum. The effect of adding very low pressures of oxygen or hydrogen sulphide was investigated. The surfaces involved were characterized by means of a high energy electron gun incorporated in the system to allow glancing incidence diffraction studies throughout the friction experiments.At all temperatures the coefficient of friction (μ) of molybdenum and tungsten surfaces is isotropic. At high temperatures molybdenum surfaces show frictional behaviour which is heavily dependent on the nature of the slider. Thus with polycrystalline molybdenum sliders the friction rises with temperature but with sapphire sliders it falls. This effect is also seen on tungsten surfaces but is less pronounced.In the presence of an active gas a number of unexpected phenomena are observed. The (110) surface of tungsten gives low values of μ in high pressures of oxygen and at room temperature but the (100) surface does not. At higher temperatures both surfaces of tungsten are protected by oxygen. On molybdenum surfaces hysteresis effects are seen. Thus cooling a (100) molybdenum surface in oxygen from 1000°C leads to a higher friction than that shown while heating. The same effect is seen on both tungsten and molybdenum surfaces in the presence of traces of H₂S. This leads to the singular observation that over a range of conditions adding traces of oxygen or hydrogen sulphide causes an increase in μ.On vanadium carbide surfaces the frictional behaviour is typical of a very brittle solid. It is shown that the fall in friction on adding oxygen is due to the chemisorbed oxygen rather than any crystalline oxide formed.     

Investigation of the overall friction coefficient in single-pass scratch test

Single-pass scratch test on bilinear elastic–plastic materials with a conical indenter was simulated using a three-dimensional finite element model. The influence of the interfacial friction coefficient μ_s and the apical angle α of the indenter on the induced maximum tangential force F_T, normal force F_N, and the overall friction coefficient μ=F_T/F_N, were systematically studied. It was found that the induced tangential force is greater than the normal force when the apex is small and vice versa when the apex is large. The tangential force increases with μ_s, but the normal force decreases with μ_s. The overall friction coefficient μ was found to increase linearly with μ_s and tangent of the attack angle of the indenter. The relationship between the adhesion frictional component (μ_a), the plowing frictional component (μ_p), and the interfacial friction coefficient μ_s was analyzed. An analytical model for the overall friction coefficient μ was also developed based on the interaction between the indenter and the specimen and compared to the numerical results. The model was found to yield a good agreement with the finite element simulation results.     

Compositional dependence of the microscopic frictional properties of single crystal metal carbides

The frictional properties of TiC(100), Ti_0.3V_0.6C(100), and VC(100) surfaces in contact with a silicon nitride probe tip have been investigated by atomic force microscopy (AFM) under ambient pressures of dry nitrogen as well as environments of different relative humidities. Calibration of normal and lateral force has permitted the determination of the quantitative frictional properties of the three carbide samples on a nanometer length scale. In these studies, TiC(100) exhibits the lowest friction coefficient, ranging from ∼0.044 to ∼0.082 under the different environments. VC(100) and Ti_0.3V_0.6C(100) have similar friction coefficients (∼0.07) under dry nitrogen conditions, yet VC exhibits a larger friction coefficient (∼0.158) than Ti_0.3V_0.6C (∼0.129) under conditions of higher relative humidity (∼55%). Condensation of water vapor with increasing relative humidity results in an increase in the frictional response for all the three samples. The experimental results demonstrate that the frictional properties of the three carbide samples are correlated to their surface composition and surface free energy.     

Kinetic frictional behaviour of alumino-silicate ceramics

The frictional behaviour was experimentally investigated of alumino-silicate ceramics (3Al₂O₃, 2SiO₂) rubbing against a hard steel surface under static and kinetic friction conditions. Tests were carried out on a pin-on-disc machine under both dry and wet contact conditions. Results showed that the frictional behaviour under either static or kinetic conditions was highly dependent on the ceramic body phase transformation which in turn was controlled by the firing temperature during ceramic processing and treatment. Lower friction values were evident when using specimens of ceramic bodies containing a high mullite crystalline phase, which are attained at high firing temperatures. Both the running speed and applied loads had insignificant effects at high loads.During kinetic friction tests lower frictional values were displayed than for static friction tests under wet contact conditions, and under dry conditions when using high mullite ceramic bodies. For specimens of ceramics fired at relatively low temperatures, kinetic friction tests produced higher frictional values than static friction tests.     

Impact of Wire-EDM on Tribological Characteristics of ZrO<Subscript>2</Subscript>-based Composites in Dry Sliding Contact with WC–Co-Cemented Carbide

In the present investigation, experiments were conducted by unidirectional sliding of pins made of FCC metals (Pb, Al, and Cu) with significantly different hardness values against the steel plates of various surface textures and roughness using an inclined pin-on-plate sliding apparatus in ambient conditions under both the dry and lubricated conditions. For a given material pair, it was observed that transfer layer formation and the coefficient of friction along with its two components, namely adhesion and plowing, are controlled by the surface texture of the harder mating surfaces and are less dependent of surface roughness (R _a) of the harder mating surfaces. The effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces. It was also observed that the variation of plowing friction as a function of hardness depends on surface textures. More specifically, the plowing friction varies with hardness of the soft materials for a given type of surface texture and it is independent of hardness of soft materials for other type of surface texture. These variations could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding. It was also observed that among the surface roughness parameters, the mean slope of the profile, Δ _a, correlated best with the friction. Furthermore, dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing taking place at the asperity level.     

The role of alloying elements in the friction and wear of copper alloys

The effect of Sn and Al on the wear and frictional properties of Cu-Sn and Cu-Al alloy systems was investigated by electron probe microanalysis and Auger electron spectroscopy. The coefficient of friction and the wear loss in the Cu-Sn alloy increased with increasing Sn content and reached a maximum at about 40% Sn, which corresponds to ?-Cu₃Sn. With the Cu-Al alloy system the coefficient of friction initially decreased with increasing Al content and reached a minimum at about 20% Al corresponding to the composition Cu₉Al₄. The wear loss increased initially, reached a maximum at about the solubility limit of Al and then decreased rapidly.The coefficient of friction of a Cu-Sn solid solution showed a maximum at the initial stage in the friction coefficient-sliding time curve, which was not found with Cu-Al and Cu-Zn. This phenomenon is discussed on the basis of surface compositional changes during the friction process.     

Sliding velocity dependency of the friction coefficient of Si-containing diamond-like carbon film under oil lubricated condition

In this study we investigated the sliding velocity dependency of the coefficient of friction for a Si-containing diamond-like carbon (DLC-Si) film in an automatic transmission fluid (ATF) under a wide range of contact pressures. The DLC-Si film and a nitrided steel with a surface roughness, Rz_JIS, of around 3.0 μm were used as disk specimens. A high-carbon chromium steel (JIS-SUJ2) bearing ball was used as a ball specimen. Friction tests were conducted using a ball-on-disk friction apparatus under a wide range of sliding velocites (0.1-2.0 m/s) and contact pressures (P_max: 0.42-3.61 GPa) in ATF. The friction coefficients for the nitrided steel had a tendency to decrease with an increase in sliding veloicity under all the contact pressure conditions; however, the friction coefficients for the DLC-Si film were stable with respect to sliding velocities under all the contatct pressures. These results indicate that the DLC-Si film suppresses the stick-slip motion during sliding againt steel in ATF, which is a desired frictional characteristic for the electromagnetic clutch disks used under lubrication. Furthermore, the DLC-Si film showed a higher wear resistance and lower aggression on the steel ball specimen than the nitrided steel. There were less hydrodynamic effects on the friction coefficient for the DLC-Si film possibly due to maintenance of the initial surface roughness and its poorer wettability with the fluid. X-ray photoelectron spectroscopy (XPS) analysis of the sliding surfaces revealed that the adsorption film derived from the succinimide on the sliding surfaces of the DLC-Si film and the mating steel ball also contributed to the sufficient and less sliding-velocity-dependant friction coefficients.     

Tribological Characteristics and Tribochemical Reactions of Various Ceramics Lubricated with HFC-134a Gas

The role of tribochemical products in the friction and wear reduction of ceramics with different fractional ionic character in CF₃CH₂F (HFC-134a) gas was investigated using a ball-on-disk type tribometer. Without exposure to air, the wear tracks on the disks were characterized with the aid of a micro-spot X-ray Photoelectron Spectroscope (XPS) whose analytical chamber was connected to the friction chamber of the tribometer. Further, the adsorption and desorption behaviors of HFC-134a molecules on the nascent surfaces of the ceramics were studied using an adsorption test apparatus in high vacuum. It was found that the lubricating effect of HFC-134a gas was closely related to the fractional ionic or covalent characters of the ceramics. HFC-134a gas was more effective in lubricating ionic ceramics than the covalent ceramics. XPS analysis revealed that metal fluorides were mainly formed on the frictional surfaces of the ionic ceramics, whereas the composition of the tribochemical products on the frictional surfaces of the other ceramics was complicated. The adsorption tests proved that HFC-134a was decomposed to an olefin CF₂=CHF on the nascent surfaces of the ionic ceramic Al₂O₃ and the covalent ceramics. However, the formation of organic fluorine-containing compounds was not detected on the frictional surfaces of the ionic ceramics by XPS. This result implies that the mechanism of tribochemical reactions is strongly dependent on the bond type of ceramics. It is concluded that the low friction and wear of the ionic ceramics in HFC-134a gas result from the metal fluorides formed with high surface concentration on the sliding surfaces.     

Investigation of adhesive and frictional behavior of GeSbTe films with AFM/FFM

Frictional force microscope (FFM) was used to investigate the nanoscale frictional behavior of GeSbTe films deposited by magnetron sputtering. The effects of relative humidity, scanning velocity and surface roughness on friction were taken into account. Besides, the frictional behavior of GeSbTe films with different compositions was analyzed. Experimental results show that the coefficient of friction of GeSbTe films is almost independent of scanning velocity, while the frictional force decreases with increasing velocity. Both the relationship of friction vs. normal load and that of friction vs. RMS keep relatively linear, and the coefficient of friction increases with the increase in RMS. The influence of humidity on adhesion between the tip and the GeSb₂Te₄ film is more significant than that between the tip and the Ge₂Sb₂Te₅ film.     

Tribochemistry of dry-sliding wear of structural TiAl(Nb,Cr,Zr)B,La intermetallics family against the chromium steel

Effects of microstructure and composition on dry friction, wear and tribochemistry of worn surfaces of new intermetallics Ti–44Al–5Nb–3Cr–1.5Zr, Ti–44Al–5Nb–2Cr–1.5Zr–0.4B–0.07La and Ti–44Al–5Nb–1Cr–1.5Zr–1B–0.17La (at%) sliding against the steel 40Cr have been investigated in air. XRD, SEM, EDX and on-depth ion sputtering techniques were used to investigate the alloys, their worn surfaces and debris particles. For all three alloys the friction coefficient (f) increased vs. sliding way within the range of 0.16–0.39, afterwards followed by the steady state. Neither grain refinement degree, nor content of (Ti,Nb)B, La₂O₃ abrasives in intermetallics affected the stationary f value and wear rate. This tribological behavior reflects the kinetics of FeO-based cover formation on the worn surface, resulting from local oxidation of counterbody steel under the frictional heating-up.     

Wettability versus roughness: Multi-scales approach

These authors experimentally investigate the influence of multi-scale roughness on contact angle measurements, and they propose a model combining the Wenzel and Cassie–Baxter equations with the 3D roughness parameters defined by ISO25178. To do this, a generic methodology based on a statistical method (bootstrap and correlation coefficient) is developed and applied to a polypropylene textured femtosecond laser (55 surfaces, ablation depths=5–45 μm and depth/diameter=0.07–0.53). A mixed model is proposed according to the correlations obtained between the contact angle of a plane surface, θ₀, and a textured surface, θ, with the roughness parameters as the developed surface, S_dr, closed hills area, S_ha and the closed dales area, S_da, with regard to the models of Wenzel and Cassie–Baxter.     

Modeling of surface roughness effect on dry contact friction in metal forming

Interfacial conditions such as friction and roughness substantially affect the process characteristics of metal forming. This study developed a dry friction model that accounted for the adhesion and interference effects of surface roughness. A sliding friction coefficient was suggested to provide fundamental information about the interfacial conditions of the contact surface. The proposed model was easily verified by published experiments and predicted values agreed with experimental results. Accordingly, friction coefficient ?? clearly increased as relative roughness R _m (=?roughness of tool $ R_a^T $ /roughness of workpiece $ R_a^M $ , measured as interference effect) increased. Simulations confirmed that the friction coefficient ?? decreased as dimensionless stress S _m (=?contact pressure p _m /tensile strength $ \sigma_u^0 $ ) increased at small strain hardening exponent n-values. Under the conditions of large n and small R _m values, the friction coefficient ?? initially decreased and then increased. It then slightly decreased as dimensionless stress S _m increased. However, this trend became less apparent as relative roughness R _m increased since friction coefficient ?? simply decreased.