Surface Texture Effect on Friction of a Microtextured Poly(dimethylsiloxane) (PDMS)

The effect of surface textures on the friction of a poly(dimethylsiloxane) (PDMS) elastomer has been investigated at both macro and microscales using a nanoindentation-scratching system. Friction tests were conducted by a stainless-steel bearing ball with a diameter of 1.6 mm (macroscale tests) and a Rockwell diamond tip with a radius of curvature of 25 μm (microscale tests) under normal loads of 5, 10, and 25 mN and with a sliding speed of 1 μm/s. Coefficient of friction (COF) on the pillar-textured surface was found to be much lower than that on the smooth surface of the same material, and it was reduced by about 59% at the macroscale tests and 38% at the microscale tests. The reduction of COF can be attributed to the reduced contact areas. The use of the JKR model revealed that the adhesion force has less effect on contacts under higher normal loads. COFs in different sliding directions on the groove-textured surfaces were compared, and a friction anisotropic behavior was identified and analyzed.     

Tribological Characteristics of Rubber-Based Friction Materials

This article deals with the rubber-based friction materials (RBFMs) which can be used in brake system. The physico-mechanical and tribological properties of a series of fiber filled RBFMs containing steel wool and aramid pulp at different concentrations along with a fiber-free reference material were characterized. Rubber–glass transition induced at higher sliding velocities was identified based on the friction fade behavior of the RBFMs. The rubber–glass transition which is inherently originated by viscoelastic response of polymeric binder was found to be influential on the tribological properties of the RBFMs. It was revealed that steel wool increased coefficient of friction (COF) and improved friction recovery behavior at low volume percent (7.5 vol.%) but it aggravated the COF at high concentration of steel wool (15 vol.%) and severe sliding conditions because of harsh abrasive mechanism. Aramid pulp improved the fade behavior at high sliding velocities and increased COF due to formation of sticky contact patches. It was revealed that steel wool increased the wear rate while aramid pulp did not affect the wear rate significantly, contrary to the increase in the friction coefficient of RBFM. SEM analysis was proved to be useful in correlating the wear rates of composites to the topographical changes on the worn surfaces.     

Wear characterization of electrodeposited Ni–WC composite coatings

The present contribution reports the tribological properties of Ni–WC composite coatings, electrodeposited on steel substrate. Commercial WC particles with an average size of 5 μm were codeposited with Ni on a mild steel substrate using a Watts bath at 50°C. The effect of plating variables on deposition behavior was studied. The amount of WC in the deposited layer decreased and plating efficiency increased with an increase in current density from 0.1 to 0.3 A/cm². The tribological properties of the coatings were studied using a small amplitude reciprocating friction wear tester. The addition of WC in Ni increases the microhardness of the electrodeposited coatings. An important result is that the presence of embedded WC particles in the electrodeposited coatings results in a much lower coefficient of friction (COF) of 0.34, when compared with pure Ni (COF 0.62) and mild steel (COF 0.54).     

Evaluation of the friction of WC/DLC solid lubricating films in vacuum

The accuracy of nanopositioning is to a large extent limited by the friction-caused errors, particularly in vacuum environments. An investigation of the friction behaviour of sp²-bonds dominating diamond like carbon (DLC) coatings and WC_1−x/DLC, WC(N)/DLC multilayer coatings, which are considered to be used in nanopositioning in vacuum, have been performed by a vacuum microtribometer. By using an atomically smooth Si sphere as a counterface, the reciprocating sliding friction was measured at a normal load <5 mN, and running speed at a 1–100 μm/s in ambient air and in ultra high vacuum (UHV) at 10⁻⁷ Pa, and correlated with microstructures and properties of the coatings. When tested in UHV, the coefficient of friction (COF) for pure DLC coatings (thickness: 700 nm) changes significantly between 0.2 and 0.4. Once the thickness of DLC layers is limited to 5 nm by formation of multilayer coatings, the COF in UHV decreases by nearly one order to 0.02–0.05. We suggest that the deformation of DLC films and the transfer films determines COF. Thick DLC coatings can induce more plastic deformation and consumes more energy in sliding resulting in a high COF. Thickening of the transfer film in running leads to a continuous decrease of COF since the deformation of the transfer films turns easier. The low COF of multilayer coatings is mainly due to their confinement of the thickness of DLC films. A consistent velocity-strengthening frictional behaviour of both WC_1−x/DLC and WC(N)/DLC coatings in UHV indicates that the transfer films acting as a thin layer of granular material. Further study of the friction behaviour with the presence of such granular materials might be interesting for the further development of tribological coatings for vacuum applications.     

Tribological Properties of Patterned NiFe-Covered Si Surfaces

The tribological properties of patterned surfaces were investigated under lubricated conditions. Micropatterns were fabricated on a Si surface using a combination of photolithography and plasma etching. NiFe film with a 150 nm thickness was then deposited on the patterned Si surface. We prepared four kinds of patterned surfaces: dimple, grating, bump, and mesh patterns. The dimensions of the patterns were: size 30–40 μm, pitch 120 μm, and depth 10–12 μm. Friction tests were carried out using a pin-on-plate tribometer. The pin specimen was made of cast iron and had a flat end. The normal load was varied from 9.8 to 98 mN, and the average sliding speed from 1.0 to 5.0 mm s⁻¹. Slideway lubricating oils or a gear oil were used as the lubricant, and the ISO viscosity grades of these oils were VG32, VG68, and VG320. The results showed that the friction coefficients of the two reverse patterns showed very similar tendencies and that circular patterns had a lower friction coefficient than did the rectangular patterns at a high bearing characteristic number. The surface geometry of the Si surface did not affect the friction coefficients at a low bearing characteristic number.     

Correlation Between Adsorption/Desorption of Surfactant and Change in Friction of Stainless Steel in Aqueous Solutions Under Different Electrode Potentials

Adsorption of sodium dodecyl sulfate (SDS) surfactant on the surface of gold or graphite in aqueous solutions has received extensive attention in the past. However, few studies have been done on the adsorption/desorption of SDS surfactant at surfaces of engineering materials as well as on their influence on friction behavior. In this article, quartz crystal microbalance (QCM), electrochemical spectroscopy, atomic force microscopy (AFM), lateral force microscopy (LFM), and ball-on-disc friction test have been jointly used to investigate the effects of electrode potential on adsorption and desorption of SDS surfactant, surfactant aggregate morphology on stainless steel surfaces, nanoscale and macroscale tribological behavior in dilute SDS aqueous solutions. Experiment results have shown that DS⁻ anions adsorb on the surface of the stainless steel electrode and form stripe-shaped aggregates at the open circuit potential (+0.03 V vs. SCE), which corresponds to a low friction coefficient. Under the negative potential of −0.4 V versus SCE, the adsorbed aggregates of DS⁻ anions are removed from the stainless steel surface, resulting in a high friction coefficient. By adjusting the electrode potential of stainless steel, both of the surfactant adsorption and tribological property can be controlled in a significant range.     

An estimation of friction from the indentation of metals by acute-angled conical indenters

The paper describes a method for estimating the coefficient of friction (COF) between sliding surfaces by an indentation of an acute-angled indenter. The COF estimated by the method is compared with the COF measured by a friction tester. The conical indenter, which is made of WC and has an apex angle of 45°, is used for the experiments. A micro-Vickers hardness tester is used for the indentation tests. The friction tester used is one of Bowden–Leben type. A copper and a 0.45% carbon steel are used for the indentation and sliding specimens. The estimation of the COF is based on the equilibrium equation of indentation, which takes the friction between the contacting surfaces of the indenter and the specimen into consideration. The results show that the estimated COF is approximately equal to the measured one. Based on the results, the validity of this method is discussed.     

Friction Study of a Ni Nanodot-patterned Surface

Nanoscale frictional behavior of a Ni nanodot-patterned surface (NDPS) was studied using a TriboIndenter by employing a diamond tip with a 1 μm nominal radius of curvature. The Ni NDPS was fabricated by thermal evaporation of Ni through a porous anodized aluminum oxide (AAO) template onto a Si substrate. Surface morphology and the deformation of the NDPS were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), before and after friction/scratch testing. SEM images after scratching clearly showed that, similar to what was assumed at the macroscale, the frictional force is proportional to the real area of contact at the nanoscale. It was found that adhesion played a major role in the frictional performance, when the normal load was less than 20 μN and plastic deformation was the dominant contributor to the frictional force, when the normal load was between 60 μN and 125 μN. Surprisingly, a continuum contact mechanics model was found to be applicable to the nanoscale contact between the tip and the inhomogeneous Ni NDPS at low loads. The coefficient of friction (COF) was also found to depend on the size of the tip and was four times the COF between a 100 μm tip and the Ni NDPS. Finally, the critical shear strength of the Ni nanodots/Si substrate interface was estimated to be about 1.24 GPa.     

Experimental Evaluation of Friction and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid Nitrogen

The friction and wear properties of the prevailing different solid lubricant coatings (Ion-plated Au, Ion-plated Ag and RF-sputtered PTFE on SUS440C stainless steel) used in the bearings of high-speed cryogenic-turbo-pumps of liquid rocket engines were experimentally evaluated in liquid nitrogen immersed conditions. Also the above experiments were carried out with two newly proposed solid lubricant coatings of sputter-ion-plated MoSTi and a new ion-plated Pb on SUS440C stainless steel. The friction coefficient and wear rates of the coatings of ion-plated Au, ion-plated Ag, RF-sputtered PTFE, the new ion-plated Pb and MoS₂Ti-SIP (with coating thickness of 0.7±0.1 μm) on SUS440C steel against SUS440C stainless steel ball in liquid nitrogen were compared. Worn surfaces were examined microscopically with a microscope and a profilometer for understanding the mechanisms of friction and wear and transfer film lubrication in liquid nitrogen. It is found that the newly proposed solid lubricant coatings are showing promising results for their use in liquid nitrogen immersed conditions. The sputter-ion-plated MoSTi coating on SUS 440C steel shows a minimum value of friction coefficient (μ=0.015) and wear rate (w_c=0.56 × 10⁻⁶ mm³/N m ) in liquid nitrogen.     

Ultralow Friction Behaviors of Hydrogenated Fullerene-Like Carbon Films: Effect of Normal Load and Surface Tribochemistry

Friction and wear behaviors of hydrogenated fullerene-like (H-FLC) carbon films sliding against Si₃N₄ ceramic balls were performed at different contact loads from 1 to 20 N on a reciprocating sliding tribometer in air. It was found that the films exhibited non-Amontonian friction behaviors, the coefficient of friction (COF) decreased with normal contact load increasing: the COF was ~0.112 at 1 N contact load, and deceased to ultralow value (~0.009) at 20 N load. The main mechanism responsible for low friction and wear under varying contact pressure is governed by hydrogenated carbon transfer film that formed and resided at the sliding interfaces. In addition, the unique fullerene-like structures induce well elastic property of the H-FLC films (elastic recovery 78%), which benefits the high load tolerance and induces the low wear rate in air condition. For the film with an ultralow COF of 0.009 tested under 20 N load in air, time of flight secondary ion mass spectrometry (ToF-SIMS) signals collected inside and outside the wear tracks indicated the presence of C₂H₃ ⁻ and C₂H₅ ⁻ fragments after tribological tests on the H-FLC films surface. We think that the tribochemistry and elastic property of the H-FLC films is responsible for the observed friction behaviors, the high load tolerance, and chemical inertness of hydrogenated carbon-containing transfer films instead of the graphitization of transfer films is responsible for the steady-state low coefficients of friction, wear, and interfacial shear stress.     

Friction and Wear Properties of Silicon Carbide in Water from Different Sources

Low friction and low wear of SiC sliding against itself in water at room temperature have been well reported in the past 20 years, and some practical applications have been developed. However, the properties of friction and wear in pure, deionized or distilled water have been mainly observed and not in water from sources in nature. In this article, the fundamental properties of friction and wear between SiC ball and disk are observed in water from ground, river, and sea, and the results are compared with those in deionized water in the viewpoints of modes of lubrication and wear and the resultant values of friction coefficient and wear rate. The smallest friction coefficient (μ_ℓ = 0.005) in steady state is observed in deionized water and the largest (μ_ℓ = 0.013) in sea water. The smallest wear rate (w _s = 2.2 × 10⁻⁷ mm³/Nm) is observed in sea water and the largest (w _s = 3.1 × 10⁻⁷ mm³/Nm) in deionized water. The intermediate values of μ_ℓ and w _s between the smallest and the largest ones are observed in ground and river water. The modes of lubrication and wear, which generated observed values of μ_ℓ and w _s, are considered as mixed lubrication and tribochemical wear. The chemical elements of Na, Cl, Mg, and K in sea water observed on wear particles and pits are thought effective to generate the largest value of μ_ℓ and the smallest value of w _s.     

Dry Sliding Friction and Wear Behaviour of an Electron Beam Melted Hypereutectic Al–Si Alloy

The economic and environmental benefits of using light-weighting technology in automotive applications continue to attract attention for feasible commercial solutions. This study investigates the use of pulsed electron beam melting of a hypereutectic Al–Si alloy as a possible modification procedure for cylinder crankcase bore facing surfaces. Machined surfaces of an A390 alloy were subjected to five pulsed electron doses with an applied cathode potential between 16.5 and 36 kV. It was found that increasing beam accelerating voltages led to an initial decrease (1.4 μm R _a) but subsequent increase (4.0 μm R _a) in average surface roughness values associated with surface crater formation due to sub-surface melting and eruption. Surfaces were tested under dry sliding tribological conditions against 52100 bearing steel in a reciprocating geometry. Average dynamic friction coefficients were higher (0.9) compared to the untreated alloy surface (0.6) as a result of a greater degree of adhesion to the counterface. However, FIB cross sections of worn surfaces indicated that this activated an oxidative type wear process which ultimately led to the formation of a beneficial surface tribo-film on the EBM-treated surfaces, improving the specific wear rates by up to 66%.     

The Tribological Behaviour of Fe–28Al–5Cr/TiC Under Liquid Paraffine Lubrication

The tribological behaviour of Fe–28Al–5Cr and its composites containing 15, 25 and 50 wt% TiC (corresponding to 19.3, 31.2 and 57.6 vol%), produced by hot-pressing process, was investigated under liquid paraffine lubrication against an AISI 52100 steel ball in ambient environment at varied applied loads and sliding speeds. It was found that the wear resistance increased and friction coefficient decreased with increasing of TiC content. The coefficients of friction are in the range of 0.09–0.14 at the given testing conditions. The wear rates of all the materials except the 50% composite are on the order of 10⁻⁶–10⁻⁵ mm³ m⁻¹, the wear rate for the 50% composite is too low to quantify under the two sliding conditions, (50 N, 0.04 m/s) and (100 N, 0.02 m/s). The wear rates of all the materials increase as applied load increases and the increasing extent diminishes with the increase of TiC content, but first increase slightly and then nearly remains steadiness with increasing sliding speed. The 50 wt% composite has wear resistance about 7–20 times better than pure Fe–28Al–5Cr at different sliding parameters. The enhanced wear resistance by TiC addition is attributed to the high hardness of the composites, as well as support of the oil lubrication film/layer by the hard TiC phase. The worn surfaces of all the materials are analyzed by a scanning electron microscope. The dominant wear mechanism of the Fe–28Al–5Cr and 15% composite is grooving and flaking-off, but those of the 25 and 50% composites are mainly shallow grooving.     

Speed and Atmosphere Influences on Nanotribological Properties of NbSe<Subscript>2</Subscript>

Nanotribological properties of NbSe₂ are studied using an atomic friction force microscope. The friction force is measured as a function of normal load and scan speeds ranging from 10 nm s⁻¹ to 40 μm s⁻¹ under two atmospheres (air and argon). At low speed, no effect of atmosphere is noticed and a linear relationship between the friction and normal forces is observed leading to a friction coefficient close to 0.02 for both atmospheres. At high speed, the tip/surface contact obeys the JKR theory and the tribological properties are atmosphere dependent: the shear stress measured in air environment is three times lower than the one measured under argon atmosphere. A special attention is paid to interpret these results through numerical data obtained from a simple athermal model based on Tomlinson approach.     

Frictional Properties of a Mesoscopic Contact with Engineered Surface Roughness

Friction between titanium spheres and an artificially structured silicon surface was measured with a friction force microscope. Two spheres with radii of 2.3 μm and 7.9 μm were firmly glued to the tip of the microscope cantilever. A periodic stripe pattern with a groove depth of 26 nm and systematically increasing groove width from 500 nm to 3500 nm was fabricated from a silicon wafer with a focused ion beam. The sphere substrate friction coefficient shows a strong enhancement at a certain groove periodicity, which is related to geometrical interlocking of the two surfaces. This shows that careful modification of the surface roughness can help to control the tribological behavior of mesoscale contacts.     

Anomalous Low Friction Under Boundary Lubrication of Steel Surfaces by Polyols

We present here anomalous low friction obtained with highly polished steel on steel hard contact lubricated by glycerol under severe mixed and boundary regimes (λ ratio below 1). We investigated the effects of contact pressure, sliding speed, and temperature on friction coefficient and electrical contact resistance. The mechanism of low friction (typically below 0.02) is thought to have two origins: first a contribution of an ultrathin EHL film of glycerol providing easy shear under pressure, second the chemical degradation of glycerol inside the contact when more severe conditions are attained, generating a nanometer-thick film containing shear-induced water molecules. This new mechanism, called “H-bond Network model”, is completely different from the well-accepted “Monolayer” model working with polar molecules containing long aliphatic chains. Moreover, we show outstanding superlubricity (friction coefficient below 0.01) of steel surfaces directly lubricated by a solution of myo-inositol (also called vitamin Bh) in glycerol at ambient temperature (25 °C) and high contact pressure (0.8 GPa) in the absence of any long chain polar molecules. Mechanism is still unknown but could be associated with friction-induced dissociation of inositol and H-bond interactions network of water-like species with steel surface.     

Influence of Surface Roughness, Material and Climate Conditions on the Friction of Human Skin

The gliding comfort and performance of personal care and wellness products is strongly influenced by the sliding friction behaviour of human skin. In the open literature, most of the results on skin friction are related to the performance of cosmetic products or to the slip and grip properties of surfaces. Experiments were usually carried out on the forearm or the fingertips. The influence of the surface roughness and the material of engineering surfaces have received little attention so far, especially not in sliding contact with the skin of the cheek, or under different climate conditions. A custom-built rotating ring device was used to study the influence of the probe surface roughness (R _a = 0.1–10 μm), the probe material (metals, plastics), the climate conditions (21–29 °C, 37–92% RH) and skin hydration on the frictional behaviour of the skin on the cheek and the forearm. The amplitude of the surface roughness has a dominant influence on the friction behaviour: the smoother the surface, the higher the friction. Differences can be as large as a factor 5–10, especially in the range R _a < 1 μm. The probe material itself has no significant influence; except for PFTE which reduces the friction by approximately 25% compared to the other materials. In a humid climate, the skin becomes hydrated and the friction is twice as high as in a dry climate. The effect of skin hydration is smaller on the cheek than on the forearm, probably due to the presence of beard stubbles. A simple friction model for human skin is presented, based on adhesion friction, contact mechanics of rough surfaces and the interfacial shear stress of thin organic films. The model explains the effects of the probe surface roughness and skin compliance. Quantitative application of the model indicates that the biomechanical indentation and shearing behaviour of the stratum corneum is influenced by the same physical process, i.e. the intercellular bonding strength of the corneocytes.     

Temperature-Dependent Atomic Scale Friction and Wear on PbS(100)

Friction and wear on PbS(100) surfaces have been investigated on the atomic scale as a function of temperature with atomic force microscopy. At room temperature and above, the PbS(100) surface exhibited low friction (μ < 0.05) in contact with a silicon nitride probe tip, provided that interfacial wear was not encountered. In the absence of wear, friction increased exponentially with decreasing temperature, transitioning to an athermal behavior near 200 K. An Arrhenius analysis of the temperature dependence of friction yielded an activation energy ∆E = 0.32 ± 0.02 eV for the sliding contact of a silicon nitride tip on PbS(100).     

Comparative study of micro- and nano-ZnO reinforced UHMWPE composites under dry sliding wear

This is a comparative study between ultra-high molecular weight polyethylene (UHMWPE) reinforced with micro-zinc oxide (ZnO) and nano-ZnO under different filler loads. These composites were subjected to dry sliding wear test under abrasive conditions. The micro- and nano-ZnO/UHMWPE composites were prepared by using a hot compression mould. The wear and friction behaviours were monitored using a pin-on-disc (POD) test rig. The pin-shaped samples were slid against 400 grit SiC abrasive papers, which were pasted, on the stainless steel disc under dry sliding conditions. The worn surfaces and transfer film formed were observed under the scanning electron microscope (SEM). Experimental results showed that UHMWPE reinforced with micro- and nano-ZnO would improve the wear behaviour. The average coefficient of friction (COF) for both micro- and nano-ZnO/UHMWPE composites were comparable to pure UHMWPE. The weight loss due to wear for nano-ZnO/UHMWPE composites are lower compared to micro-ZnO/UHMWPE and pure UHMWPE. The optimum filler loading of nano-ZnO/UHMWPE composites is found to be at 10 wt%. The worn surface of ZnO/UHMWPE composites shows the wear mechanisms of abrasive and adhesive wear. Upon reinforcement with micro- and nano-ZnO, the abrasive and adhesive wear of worn surfaces transited from rough to smooth.     

Ultralow Friction Between Steel Surfaces Achieved by Lubricating with Liquid Crystal After a Running-in Process with Acetylacetone

A suitable running-in process is advantageous for reducing friction. The aim of the present work was to study the influence of the running-in with acetylacetone on tribological performance of 4-Cyano-4’-pentylbiphenyl (5CB) liquid crystal. Friction tests were performed between steel surfaces in a ball-on-disk sliding system. After a running-in period of 240 s, the COF of 5CB was measured to be 0.013, which is about a quarter of the value (0.055) without running-in. The reduced contact pressure, caused in running-in process, does not directly lead to a drop in COF. The generation of tris(acetylacetonato) iron(III) induced by the tribochemical reactions between acetylacetone and steel surfaces, and the unique physical properties of liquid crystal are assumed to be reasons for the ultralow COF. Surface analysis was performed to correlate COF with the topography of wear surfaces. An evenly distributed specific grooved structure observed on wear area of the ball may have a beneficial effect on COF as well. We believe our findings can provide an effective and simple solution to reduce COF of liquid crystal between steel surfaces. A better understanding of the tribological behavior is needed for the development of this tribological system and for the possible future applications.