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.     

Experimental investigations of porous bearings under vertical sinusoidally fluctuating loads

There are many papers on the experimental investigations of porous bearings under static loads but there is no paper on the experimental investigations under dynamic loads. In the present paper, the results of experimental investigation of porous bearings under vertical sinusoidally fluctuating loads are presented. The friction force was measured under various conditions of fluctuating load/steady load ratio, journal frequency and load frequency. The investigations were carried out in the hydrodynamic lubrication regime in a specially designed and fabricated test rig. It was found that at any given rpm, as the fluctuating specific load/steady specific load ratio, P_f/P_s, increases, the mean coefficient of friction μ_m increases. It was also found that the mean coefficient of friction is not affected by the load frequency even when the load frequency is half of the journal frequency.     

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.     

Evaluation of Friction Behavior and Its Contact-Area Dependence at the Micro- and Nano-Scales

The friction behavior of two different materials, mica and ultra-high molecular weight polyethylene (UHMWPE), was evaluated at the nanoscale with an atomic force microscope and with a custom-built ball-on-flat microtribometer at the microscale. The same counterface (Si₃N₄ probe), environmental conditions (25 °C, RH < 10%), and similar load ranges were maintained for all experiments. The friction-force data obtained were analyzed for contact-area dependence. Friction force between silicon nitride and mica at the nanoscale showed initial non-linearity with normal load up to a certain load, beyond which surface damage was observed resulting in a linear dependence of friction force on normal load. At the microscale, the friction force of the mica–silicon nitride interface exhibited linear dependence on normal load. Friction force between silicon nitride and UHMWPE exhibited non-linearity with normal load at both the length scales, for the applied load ranges of our experiment. An appropriate contact mechanics theory was applied to calculate an interfacial shear strength value for the material pair at both the scales. The values at both the scales were similar, when the conditions were carefully maintained to be the same across scales.     

Load Induced Microstructure Evolution and Friction in an Organic Monolayer Self-assembled on a Silicon Substrate

In sliding of organic self-assembled monolayer against a probe the friction force is generally found to vary linearly with normal load. Here, lateral force microscopy is used to track the physical changes at the interface brought about when an octadecyltrichlorosilane monolayer, self-assembled on a silicon wafer, is slid against a Si₃N₄ tip in the 0–30 nN load range. Regarding a morphologically heterogeneous monolayer domain to be made up of tiles of characteristic friction forces, each tile is in a unique physical state; the variation of area fraction (in a scan area) of each tile is tracked as a function of normal load. The area averaged friction force at a load is obtained by summing the fractional forces of constituent friction tiles. The friction force obtained thus, is found to vary linearly with normal tip load. It is observed that this force is dominated by the low-friction crystalline tiles at low loads and by the high friction more amorphous tiles at high loads. This suggests that for a self- assembled monolayer the load governance of friction as implied by the Amontons Law may be attributed to the physical changes that are brought about at the interface by changing the normal load.     

Ni nanodot-patterned surfaces for adhesion and friction reduction

Surface nano-patterning with Ni nanodot arrays was investigated for adhesion and friction reduction of contacting interfaces. Self-assembled anodized aluminum oxide (AAO) templates in conjunction with thermal evaporation was used to fabricate nano-patterned surfaces with ordered Ni nanodot arrays on Si substrates. Surface morphology of the Ni nanodot-patterned surfaces (NDPSs) was characterized by scanning electron microscopy (SEM). Adhesion and friction studies on a Ni NDPS and a baseline smooth Si(100) surface were conducted using a TriboIndenter employing a diamond tip with 100 μm nominal radius of curvature. The results show that the ordered Ni nanodot-patterning reduced the adhesion forces and coefficients of friction up to 92 and 83%, respectively, compared to those of the smooth silicon surface. Surprisingly, the nanoscale multi-asperity contact between the diamond tip and inhomogeneous Ni NDPSs under low loads follows a continuum contact mechanics model.     

Tribophysical phenomena on sliding surfaces of polyester composites evaluated by spectroscopic and thermal analysis

Polyethylene terephthalate/polytetrafluoroethylene (PET/PTFE) composites were slid under different temperatures or contact pressures and surfaces are scanned with Raman spectroscopy to examine orientation and crystallisation effects induced by frictional stresses and flash temperatures. Characteristic absorption bands at 996 cm⁻¹ and 857 cm⁻¹ or 2,908 cm⁻¹ and 2,960 cm⁻¹ are used for quantification of amorphous, crystalline and rigid-amorphous phases after sliding. With almost an identical amount of total trans conformation either obtained after sliding at high temperatures or normal loads, the orientation of the molecular backbone is mainly concentrated in the formation of a crystalline phase during sliding at high temperatures. When sliding under high normal loads, orientation is concentrated in the rigid-amorphous phase with a stress-induced transition from rigid-amorphous phase into crystalline phase above 25 MPa, yielding a reduced slope in a wear rate versus normal load plot. Visual and thermal analysis of the wear debris shows that degradation under low normal loads is attributed to wear of the amorphous phase with long resident times of debris in the sliding interface. The formation of a load-carrying transfer film is attributed to a post-polymerisation reaction of the wear debris at low contact pressures.     

Contrasting static-to-kinetic friction transitions on layers of an autophobically dewetted polymer film using Fourier-analyzed shear modulation force microscopy

Fourier analysis of oscillating forces at a laterally modulated tip provides new insight into static-to-kinetic friction transitions on ultrathin polyvinyl alcohol (PVA) films. In addition to contrast in sliding friction, layers of autophobically dewetted PVA films exhibit remarkable contrast in the transition from static to kinetic friction as derived from spatially resolved Fourier analysis. These differences relate to strong adsorption of first layer to mica substrate and concomitant conformational arrest, as compared to bulk-like behavior in the second layer. The third Fourier harmonic is found to be a sensitive gauge to variable degrees of sliding as a function of both lateral drive amplitude (0.25–25 nm) and normal load (tensile to compressive). For a 2.5-nm drive on PVA, it is discovered that a largely static contact at compressive loads becomes a largely sliding contact at tensile loads. This finding has implications for the analysis of shear modulation force microscopy of polymers in the context of contact mechanics models, and for studies under variable sample compliance as a function of temperature or plasticizer absorption.     

Dynamic thermoanalytical test method for qualifying brake lining materials

A thermoanalytical test method is described for qualifying brake lining materials, based upon the recording of coefficient of friction f versus temperature T diagrams. Tests are performed at a low sliding speed, thus avoiding substantial increase in temperature due to dissipation of frictional energy. During testing, the temperature in the friction interface of a brake lining-metal ring friction couple is increased linearly with time by means of time-controlled inductive heating of the metal ring. It is shown that the method yields a ‘fingerprint’ of the brake lining material, which is characteristic for its friction-temperature behaviour as related to composition and structure and which can thus be used for identification purposes and in acceptance testing. Repeated runs in the range T_o–T_max, performed at , yield information on the f−T behaviour of lining materials which, in previous runs, have been exposed to a maximum contact temperature T_max. The method is illustrated with results obtained with a resin/asbestos and a cermet/graphite material.     

Effect of MoS2 and WS2 Nanotubes on Nanofriction and Wear Reduction in Dry and Liquid Environments

Nano-objects in dry and liquid conditions have shown reductions in friction and wear on the macroscale. Studies in low viscosity liquids with nanoparticles and nanotubes made of lubricating materials such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) are limited. In this research, MoS₂ and WS₂ nanotubes with spherical gold (Au) nano-objects as a control are studied on the nanoscale under dry and low viscosity liquid environments for their effect on friction and wear reduction. Atomic forces microscopy (AFM) experiments on the nanoscale are performed in single-nano-object contact with an AFM tip, where nano-objects are laterally manipulated and multiple nano-object contact with a tip attached to a glass sphere sliding over several nano-objects. Wear tests were performed on the nanoscale by means of AFM as well as on the macroscale using a ball-on-flat tribometer to relate friction and wear reduction on both scales. Results indicate that nano-objects such as MoS₂ and WS₂ nanotubes contribute to friction and wear reduction due to the reduced contact area and the possible rolling and sliding on the nanoscale. On the macroscale, reductions in friction and wear occur due to possible exfoliation of outer layers in addition to other mechanisms just mentioned.     

Friction and wear of carbon steel near T1-transition under dry sliding

The wear phenomenon of metals under dry sliding is, generally, divided into two modes of severe and mild wear. A discontinuous transition between the wear modes often takes place in a certain load range. The T₁-transition is usually observed at lower levels of load or sliding velocity. There is a great difference in wear rate between severe and mild wear. This indicates that the occurrence of severe wear should be avoided, especially in the field of machine design to prevent energy loss, occurrence of noise and vibration, and life reduction of machines and their components. Therefore, it is important for machine designers to know the relationship between friction and wear and the difference in properties of the wear surfaces in the two wear modes. In this study, wear tests of 0.35% C steel in contact with itself under constant load were conducted in moist air at various contact loads under dry sliding. The friction and wear were measured continuously throughout each test. After the tests, the relationship between friction and wear and the difference in properties of the wear surfaces were investigated in each wear mode. From the results, the upper and lower critical loads (P_acr and P_Acr) appeared between severe and mild wear. The phenomenon of zero wear has been newly found in the early period at very low loads. The zero wear continued for a long sliding distance and then changes to mild wear. The critical load between zero wear and mild wear is defined as P_zerowear. The load was changed once in a step-wise manner from low to high levels in process of test. Since the rubbing history under mild wear condition at the low load in the first stage affected the properties of wear surface, the wear mode at the high load in the second stage changed from ‘mild wear’ to ‘quasi-mild wear’ having a low rate. From the relationship between sliding distance necessary for the appearance of quasi-mild wear and contact load in the first stage, the boundary curve between severe wear and quasi-mild wear in the second stage is hyperbolic. This curve gradually approaches P_zerowear with decreasing contact load. Thus, P_zerowear is one of the important critical loads for elucidating the test results under varying load.     

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

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

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.     

The effect of ethoxylated esters on the lubricating properties of their aqueous solutions

In many applications strict ecological criteria are crucial. Water is often used in these cases as a lubricant base. Its disadvantageous properties, in particular lubricity, can be modified by introduction of additives. Ethoxylated methyl esters of fatty acids from rapeseed oil were chosen for that purpose. In order to verify tribological properties of aqueous solutions of these compounds, anti-seizure properties under linearly increasing load, friction and wear under 2 kN (four-ball machine) were assessed. Not only radical change of tribological properties as compared to water was observed, but also the results are comparable to the ones obtained for mineral oils, which contained typical, commercial additives. Significant changes in scuffing load (P _t), seizure load (P _oz), limiting pressure of seizure (p _oz) were recorded even at 0.1% wt concentration of the additive. At optimal concentration P _t, P _oz, p _oz, increased even almost 6-, 2- and 3-fold (respectively). The quantities measured increased with ethoxylation degree. Also at constant load, friction (6-fold) and wear (almost 2-fold) were reduced. In this case, however, no influence of concentration and ethoxylation degree on the quantities measured, was observed.The tribological properties can be discussed in terms of results of physical–chemical tests. Ethoxylates are surface active compounds. Within the range of concentrations analyzed, they form micelles in a bulk phase and liquid-crystalline structures in a surface phase. High surface activity of these additives and creation of specific structures in solutions were proved by measuring surface tension, wetting angle and mesophases observation in polarized light. On the basis of the results of the physical–chemical and tribological tests one can conjecture that profitable tribological properties of the lubricating compositions are the result of strong affinity to the surface and their ability to form ordered structures. One can expect that over the cooperating surfaces a deposit is formed. Presence of the deposit increases real contact area and prevents seizure. This hypothesis gives a simple explanation of the unexpected improvement of anti-seizure properties, which accompanies the increase of ethoxylation degree. Ethoxylates with higher number of ethylene oxide attached, reveal smaller surface activity and better solubility in water. On the other hand, they can be characterized by higher dehydratation temperature. This means that at higher temperatures they reveal surface activity. That is why, during seizure tests, at high temperature, under high loads the esters of higher ethoxylation degree protect friction couples against seizure much more efficiently.     

Friction and Wear Behavior of Single-Phase Fe2B Bulk under Dry Sliding Condition

The effects of normal load and velocity on the friction and wear behavior of single-phase Fe₂B bulk have been investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Results indicate that the friction coefficient and wear rate both decrease at first and then increase with increasing load and velocity, respectively. Attributed to the formation of a lamellar film on the Fe₂B surface, the lowest friction coefficient and wear rate are obtained at a velocity of 0.2 m/s under a load of 12 N. The dynamic friction coefficients under loads of 4 and 12 N are around 0.8 in the initial steady stage and then decrease to about 0.6, whereas the friction coefficient at 20 N shows no obvious change and remains around 0.82. The lubricating film consisting of Fe₂O₃, B₂O₃, SiO₂, and H₃BO₃ reduces the friction coefficient at 0.2 m/s under a load of 12 N.     

Friction characteristics of nanoscale sliding contacts between multi-asperity tips and textured surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.     

Theory of Friction with Applied Load

We present a molecular dynamics simulation for the static friction under the effect of load. We chose a system formed by slabs of molecules of N ₂ deposited on a (111) surface of Pb. In contrary to many calculations, we assume that the Pb atoms are not kept fixed in the lattice positions, but can vibrate in their own phonon’s field. This has the important consequence that the upper and lower block can exchange energy and momentum. During the molecular dynamics simulation, the two systems can reach a thermodynamical equilibrium. When in our molecular dynamics simulation the equilibrium is reached the N ₂ plane at the interface reconstructs. The unit cell is still hexagonal but it contains 16 molecules in disordered positions. These positions of the N ₂ molecules are strongly modified by the presence of load. For small load there is a small increase of the disorder that produces a small reduction in the static friction. For larger loads the formation of clusters begins, and for high loads the size of the clusters increases and there is a tendency to the formation of vacancies. These effects are producing a large increase in the force friction. We can then distinguish different regions that characterize the behaviour of the static friction as a function of the load.     

Advanced Tribometer for In Situ Studies of Friction, Wear, and Contact Condition—Advanced Tribometer for Friction and Wear Studies

An advanced ball-on-disk tribometer was developed for in situ studies of friction, wear, and contact condition during sliding. Kinetic friction force, contact resistance (R _c), acoustic emission (AE), ball position perpendicular () to the plane of the disk (ball and disk wear), and disk surface reflectance (disk wear) were all measured simultaneously during sliding experiments. Metal (440C steel) balls were slid against ceramic (n-doped polysilicon) wafers at light load (10g) and short test duration (2.5min). Significant changes in measured parameters were observed as sliding progressed. These changes are discussed, and when considered together provide new insights into friction and wear mechanisms not readily obtainable from more standard tribometers. The effects of disk run-out (effective surface waviness) on and R _c were also investigated. Friction and R _c were periodic with a period equal to the period of disk rotation. The behavior was complex, but generally going up a hill increased and decreased R _c, with the opposite behavior going down a hill. We established a critical link between low-frequency friction oscillations (LFFO) and the nature of the contact between sliding surfaces (R _c measurement). The geometric ratchet mechanism was ruled out as a cause of LFFO, as the surface slope was too small to explain the large friction oscillations. Coating the balls and wafers with lower friction materials resulted in negligible LFFO, which makes it unlikely that LFFO were simply a result of an oscillating normal force created by dynamic effects. LFFO likely have their origins in the complex nature of the contact between rubbing surfaces.     

Nanoscale to Macroscale Investigation of the Frictional Properties of Physisorbed Layers of Self-Organized Phthalocyanine Derivatives

The frictional properties of molecules physisorbed on a graphite surface were investigated on the macroscale using a pin-on-plate tribometer and on the nanoscale using atomic force microscopy (AFM). Hydrodynamic lubrication was observed even at a contact pressure of about 1 GPa under the AFM tip. Selection of the Stribeck parameter of η _eff V/(W/R), which accounts for the change in effective viscosity, is proved effective for comparing the frictional properties of the physisorbed layers of self-organized molecules on nanoscale with the properties on macroscale.     

载荷和相对湿度对微摩擦力的影响

利用自制的微摩擦和粘附力测试装置，研究了在微牛量级载荷下，载荷和相对湿度对Si（100）材料与Si3N4材料组成的摩擦副的最大静摩擦和滑动摩擦的影响。实验发现，最大静摩擦力和滑动摩擦力均随着载荷的增大而增大，而最大静摩擦因数和滑动摩擦因数则随载荷增大而降低。相对湿度对摩擦的影响则随不同载荷的范围而有所不同。载荷较小时，相对湿度对摩擦因数的影响较大；载荷较大时，则相反。利用单峰接触的模型，建立了相对湿度与摩擦力关系的公式。