Showing papers in "Tribology Letters in 2005"

Ultralow-friction and wear properties of IF-WS2 under boundary lubrication

Abstract: Tested in boundary lubrication, inorganic fullerene-like WS2 nanoparticles used as additives in oil present interesting friction reducing and anti-wear properties. A dispersion with only 1 wt% of particles leads, from a contact pressure of 0.83 GPa, to a drastic decrease of the friction coefficient below 0.04 and to very low wear. High resolution transmission electron microscopy (HRTEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Raman Spectroscopy and video imaging were used to explain the lubrication mechanisms. A structural modification of fullerene-like nanoparticles into sheets during the friction test was evidenced to be the main effect at the origin of these properties.

Ultralow friction of DLC in presence of glycerol mono-oleate (GNO)

Abstract: This paper presents a unique tribological system that is able to produce no measurable wear of material combination and that reduces friction markedly in the ultralow regime under boundary lubrication. Ultralow friction (0.03) was obtained by sliding hydrogen-free Diamond-Like-Carbon ta-C against ta-C lubricated with Poly-alpha Olefin base oil containing Glycerol Mono-Oleate (GMO) additive. The origin of ultralow friction in these conditions has been investigated by surface analysis techniques. Results are in agreement with the formation of a OH-terminated carbon surface. This new surface chemistry might be formed by the tribochemical reaction of alcohol function groups with the friction-activated ta-C atoms. The origin of low friction could be due to the very low-energy interaction between OH-terminated surfaces.

Fundamental studies of Au contacts in MEMS RF switches

Abstract: Microelectromechanical systems (MEMS) radio frequency (RF) switches hold great promise in a myriad of commercial, aerospace, and military applications. However, there is little understanding of the factors determining the performance and reliability of these devices. Fundamental studies of hot-switched gold (Au) contacts were conducted using a micro/nanoadhesion apparatus as a switch simulator. Experiments were conducted in a well defined air environment under precisely controlled operating conditions. Fundamental properties were connected to performance with an emphasis on the effects of contact force and electric current on contact resistance (R), microadhesion, and reliability/durability. Electric current had the most profound effect on switch performance. Observations at low current (1–10 μA) include: (1) slightly higher R; (2) asperity creep; (3) high adhesion after rapid switching; (4) switch bouncing; and (5) reasonable durability. Conversely, observations at high current (1–10 mA) include: (1) slightly lower R; (2) melting; (3) no measurable adhesion; (4) less propensity for switch bouncing; (5) necking of contacts; and (6) poor reliability and durability due to switch shorting. Low current behavior was dominated by the propensity to form smooth surface contacts by hammering, which led to high van der Waals force. High current behavior was dominated by the formation of Au nanowires that bridge the contact during separation. Data suggest the presence of an adventitious film containing carbon and oxygen. Aging of the contacts in air was found to reduce adhesion.

The Frictional Properties of Newtonian Fluids in Rolling–Sliding soft-EHL Contact

Abstract: A combined experimental and numerical study has been carried out to explore friction in rolling-sliding, soft-EHL contact. Experimental work has employed corn syrup solutions of different concentrations in water to provide a range of lubricant viscosities and has measured Couette friction in mixed rolling-sliding conditions over a wide range of entrainment speeds. A Stribeck curve has been generated, ranging from the boundary to full film, isoviscous-elastic lubrication regime. In the latter regime, friction coefficient is approximately proportional to the product of (entrainment speed x viscosity) raised to the power 0.55. Numerical solution of the isoviscous-elastic lubrication regime has been used to derive predictive equations for both Couette and Poiseuille friction in circular, soft-EHL contacts. This shows that in soft-EHL the Poiseuille or "rolling" friction can have magnitude comparable to the Couette friction. The calculated Poiseuille friction coefficient can be predicted from non-dimensional load and speed using a simple power law expression similar to that used for film thickness. However accurate prediction of calculated Couette friction coefficient requires a two-term power law expression. Comparison of experimental and numerical Couette friction coefficients shows quite good agreement between the two, with a similar non-dimensional speed dependence, but slightly lower predicted than measured values.

Friction coefficient of soft contact lenses: measurements and modeling

Abstract: Tribological conditions for contact lenses have very low contact pressures in the range 3–5 kPa and sliding speeds around 12 cm/s. Using a microtribometer a series of experiments was run on commercially available contact lenses made from Etafilcon-A. These tests were run using 10–50 mN of normal load at speeds from 63 to 6280 μm/s using a 1-mm radius glass sphere as a pin. The resulting contact pressures are believed to be nearly an order of magnitude larger than the targeted 3–5 kPa. It is hypothesized that the viscoelastic nature of the hydrogel, viscous shearing of the packaging solution, and interfacial shear between the glass sphere and the contact lens all contribute to the friction forces. A model that includes all three of these contributors is developed and compared to the experimental data. The experimental friction coefficients vary from μ = 0.025 to 0.075. The calculated fluid film␣thicknesses were between 1 and 30 nm. The average surface roughness of the lens and the glass sphere are Ra=15 nm and Ra=8 nm, respectively, suggesting that the contact is not in full elastohydrodynamic lubrication. Finally, the largest contributors to the friction force in these experiments were found to be viscous dissipation within the hydrogel and interfacial shear within the contact zone.

Frictional anisotropy of oriented carbon nanotube surfaces

Abstract: This report examines highly anisotropic tribological behavior of multi-walled nanotube films oriented in mutually orthogonal directions. The average values of coefficient of friction varied from extremely high values (μ=0.795) for vertically aligned nanotubes grown on rigid substrates to very low values (μ=0.090) for nanotubes dispersed flat on the same substrates. The results were insensitive to humidity, in contrast to graphite materials, and indicate that nanotubes could be utilized as both low and high frictional surfaces.

The Role of Transfer Layers on Friction Characteristics in the Sliding Interface between Friction Materials against Gray Iron Brake Disks

Abstract: The effect of transfer layer formation on friction performance was studied using a brake friction material containing 15 ingredients. Based on a base formulation, 13 friction material specimens containing different relative amounts of ingredients were produced and they were tested on gray iron disks using a small-scale friction tester. A non-destructive four-point probe technique to measure electrical resistance of the thin film was used to estimate the transfer layer thickness. Results showed that the transfer layer formation was highly dependent on the relative amount of ingredients in the friction material and temperature. Among various ingredients, solid lubricants and iron powders increased the transfer layer thickness but no apparent correlation between transfer layer thickness and the coefficient of friction was found. Strong influence from individual ingredients was observed, dominating the friction characteristics during sliding. On the other hand, the thick transfer layers on the disk surface tended to reduce the friction material wear and the amplitude of the friction coefficient oscillation during sliding.

Multiscale roughness and modeling of MEMS interfaces

Abstract: Investigation of contact and friction at multiple length scales is necessary for the design of surfaces in sliding microelectromechanical system (MEMS). A method is developed to investigate the geometry of summits at different length scales. Analysis of density, height, and curvature of summits on atomic force microscopy (AFM) images of actual silicon MEMS surfaces shows that these properties have a power law relationship with the sampling size used to define a summit, and no welldefined value for any is found, even at the smallest experimentally accessible length scale. This behavior and its similarity to results for fractal Weierstrass-Mandelbrot (W-M) function approximations indicate that a multiscale model is required to properly describe these surfaces. A multiscale contact model is developed to describe the behavior of asperities at different discrete length scales using an elastic single asperity contact description. The contact behavior is shown to be independent of the scaling constant when asperity heights and radii are scaled correctly in the model.

Tribofilms generated from ZDDP and DDP on steel surfaces: Part 1, growth, wear and morphology

Abstract: The growth and morphology of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and an ashless dialkyldithiophosphate (DDP) over a wide range of rubbing times (10 s to 10 h) and concentrations (0.1–5 wt% ZDDP), have been examined using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy at the O, P and S K-edges and the P, S, and Fe L-edges. The physical aspects of the growth and morphology of the tribofilms will be presented in Part I and the chemistry of the films will be discussed in Part II. The major components of all films on 52100 steel are Zn and Fe phosphates and polyphosphates. The average thickness of these phosphate films has been measured using P K-edge XANES and XPS profiling. For ZDDP, a very significant phosphate film (about 100 A thick) forms after 10 s, while film development for DDP is substantially slower. However, for both additives, the average film thickness increases to 600–800 A after 30 min of rubbing, before leveling off or decreasing. The antiwear properties of pure ZDDP and in combination with DDP at different rubbing times and concentrations have also been examined. It was found that under all conditions, the performance of ZDDP as an antiwear agent is superior to that of DDP. However, DDP has no adverse effect on the performance of ZDDP when the two are mixed. The AFM results show that ZDDP forms larger and better developed “pads” than DDP at short rubbing times. At longer rubbing times, both films become more uniform. For the 1 h ZDDP films, the film thickness is surprisingly independent of the ZDDP concentration from 0.1 to 5 wt% ZDDP. The film thickness is also independent of the ratio of ZDDP/DDP concentrations.

Study on Tribological Properties of Multi-walled CarbonNanotubes/Epoxy Resin Nanocomposites

Abstract: Multi-walled carbon nanotubes/epoxy resin (MWNTs/EP) nanocomposites with different MWNTs contents have been prepared successfully. The influence of MWNTs on the friction and wear behaviors of the nanocomposites was investigated by a friction and wear tester under dry-sliding contact conditions. The relative humidity of the air was about 50±10%. Contrast to pure EP, MWNTs/EP nanocomposites showed not only higher wear resistance but also smaller friction coefficient. MWNTs could dramatically reduce the friction and improve the wear resistance behaviors of the nanocomposites. The mechanisms of the significant improvements on the tribological properties of the MWNTs/EP nanocomposites were also discussed.

Frontiers of fundamental tribological research

Abstract: This review summarizes recent advances in the area of tribology and the challenges to achieve a molecular level understanding of friction and wear and makes specific recommendations towards attaining such a fundamental understanding. This document represents the results of a two-day workshop, sponsored by the U.S. National Science Foundation, at which participants were charged with defining the outstanding challenges in obtaining a fundamental understanding of friction and wear, thus assisting the National Science Foundation in the effective allocation of resources to address these challenges.

Tribological duality of Ti3SiC2

Abstract: We report here on the friction behavior of fine- and coarse-grained Ti3SiC2 against steel and Si3N4 balls. Two successive friction regimes have been identified for both grain sizes and both counterparts. First, Type I regime is characterized by a relatively low (0.1–0.15) friction coefficient, and very little wear. Sliding occurs between a tribofilm on the ball and the Ti3SiC2 plane when against steel. Then, a Type II regime often follows, with increased friction coefficients (0.4–0.5) and significant wear. Compacted wear debris seems to act as a third body resulting in abrasion of the ball, even in the case of Si3N4. The transition between the two regimes occurs at different times, depending on various factors such as grain size, type of pin, and normal load applied. Some experiments under vacuum showed that the atmosphere plays also a major role. The reason for this evolution is not fully clear at that time, but its understanding is of major technological importance given the unusual good properties of this material.

Solid lubrication by multiwalled carbon nanotubes in air and in vacuum

Abstract: Aligned and dispersed multiwalled carbon nanotubes (MWNTs) were evaluated for solid lubrication applications. Results obtained from unidirectional sliding friction experiments indicate that MWNTs have superior friction properties and sustainability in air and vacuum. Based on analyses of wear surfaces, transfer films, wear debris, and microstructures, the lubrication mechanism of the MWNTs is discussed.

The Tribological Properties of Low-friction Hydrogenated Diamond-like Carbon Measured in Ultrahigh Vacuum

Abstract: In this paper, we investigate the sliding friction and wear behavior of a hydrogenated diamond-like carbon (DLC) film in ultrahigh vacuum (UHV) and under partial pressures of water vapor, oxygen, nitrogen and hydrogen. The initial friction coefficient of the film in UHV was ~0.15, but decreased steadily to values as low as 0.03 after about 30 sliding passes. During longer duration tests, the friction coefficient increased again to values as high as ~0.15 and such an increase in friction coincided with hydrogen desorption from the contacting surfaces (as detected by a mass spectrometer). Heating DLC to temperatures higher than 360 K also caused desorption of hydrogen and a resulting marked increase in friction. The presence of molecular nitrogen, oxygen and hydrogen in the test chamber did not have any noticeable effect on friction, but the presence of thermally dissociated or ionized hydrogen within the close proximity of sliding surfaces had a beneficial effect by restoring the low friction behavior of the DLC films. The introduction of water vapor into test chamber had an adverse effect on friction. The results of this study confirm that hydrogen is key to low friction behavior of hydrogenated DLC films and that the presence of water molecules has an adverse effect on their friction behavior.

Frictional dynamics of perfluorinated self-assembled monolayers on amorphous SiO2.

Abstract: Silicon micromachines in microelectromechanical systems (MEMS) are coated with self-assembled monolayers (SAMs) in order to reduce the wear and stiction that are commonplace during operation Recently, perfluorinated SAMs have been the focus of attention because they have better processing properties than hydrocarbon SAMs In this study, we perform molecular dynamics simulations that model adhesive contact and friction for perfluorinated alkylsilane (Si(OH)3(CF2)10CF3) self-assembled monolayers (SAMs), which are commonly used in MEMS devices Amorphous silica is used as the substrate for the SAMs in the simulations The frictional behavior is investigated as a function of applied pressure (50 MPa–1 GPa) for a shear velocity of 2 m/s and compared to recent simulation results of hydrocarbon alkylsilane SAMs The microscopic friction coefficient for the perfluorinated SAMs is the same as was measured for the hydrocarbon SAMs, but the shear stress is slightly larger than in the case of the hydrocarbon SAMs on amorphous silica

Wear resistant solid lubricant coating made from PTFE and epoxy

Abstract: A composite coating of polytetrafluoroethylene and epoxy shows 100 × improvements in wear resistance as compared to either of its constituents alone and reduced friction coefficient under testing on a pin-on-disk tribometer. This coating is made by impregnating an expanded PTFE film with epoxy, which provides three unique functions: (1) the epoxy compartmentalizes the PTFE nodes, which is believed to reduce the wear of the PTFE, (2) the epoxy increases the mechanical properties such as elastic modulus and hardness, and (3) the epoxy provides a ready interface to bond the films onto a wide variety of substrates easily and securely. The experimental matrix had normal loads of 1–3 N, sliding speeds from 0.25 to 2.5 m/s, and used a 2.4 mm radius low carbon steel pin in a rotating pin-on-disk tribometer. The skived PTFE films had wear rates on the order of K=10−3 mm3/Nm and friction coefficients around μ=0.2. Both the high density films (70 wt%PTFE) and low density films (50 wt% PTFE) had wear rates on the order of K=10−6 mm3/Nm and friction coefficients around μ=0.15. The neat epoxy films showed significant scatter in the tribological measurements with wear-rates on the order of K=10−4 mm3/Nm and friction coefficients around μ=0.40. The enhanced tribological behavior of these composites is believed to stem from the coatings ability to draw thin PTFE transfer films into the contact from the nodes of PTFE, which act like reservoirs. Nanoindentation mapping of the coatings and the transfer films supports this hypothesis, and accompanies scanning electron microscopy observations of the worn and unworn coatings.

Cryogenic Friction Behavior of PTFE based Solid Lubricant Composites

Abstract: Solid lubricants used in aerospace applications must provide low friction and a predictable operation life over an extreme range of temperatures, environments and contact conditions. PTFE and PTFE composites have shown favorable tribological performance as solid lubricants. This study evaluates the effect of temperature on the friction coefficient of neat PTFE, a PTFE/PEEK composite and an expanded PTFE (ePTFE)/epoxy coating. These experiments evaluate friction coefficient over a temperature span which, to the investigators’ knowledge, has not been previously examined. Results show a monotonic increase in friction coefficient as sample surface temperature was decreased from 317 to 173 K for all three samples. The frictional performance of these and other published solid lubricant polymers was modeled using an adjusted Arrhenius equation, which correlates the coefficient of friction of the polymer materials to their viscoelastic behavior. A model fit of all the polymer data from 173 to 450 K gives an activation energy of 3.7 kJ/mol. This value suggests that breaking of van der Waals bonds is the likely mechanism responsible for the frictional behavior over this temperature range.

A unified shear-thinning treatment of both film thickness and traction in EHD

Abstract: A conclusive demonstration has been provided that the nature of the shear-thinning, that affects both film thickness and traction in EHL contacts, follows the ordinary power-law rule that has been described by many empirical models of which Carreau is but one example. This was accomplished by accurate measurements in viscometers of the shear response of a PAO that possesses a very low critical stress for shear-thinning and accurate measurements in-contact of film thickness and traction under conditions which accentuate the shear-thinning effect. The in-contact central film thickness and traction were entirely predictable from the rheological properties obtained from viscometers using simple calculations. These data should be invaluable to researchers endeavoring to accurately simulate Hertz zone behavior since the shear-thinning rheology is extensively characterized and accurate in-contact data are available to test. In addition, a new model has been introduced that may be useful for the rheological characterization of mixtures.

Reduction of adhesion and friction of silicon oxide surface in the presence of n -propanol vapor in the gas phase

Abstract: The equilibrium adsorption of gas phase alcohol molecules has been proposed as a new means of in-use anti-stiction and lubrication for MEMS devices. Adhesion and friction of silicon oxide surfaces as a function of n-propanol vapor pressure in the ambient gas were invesitigated using atomic force microscopy. As the vapor pressure increases, the adsorbed n-propanol layer thickness increases. The adhesion and friction significantly decrease with very little addition of n-propanol vapor.

In-Contact IR Spectroscopy of Hydrocarbon Lubricants

Abstract: This paper reviews the application of infrared (IR) spectroscopy to in-contact lubrication studies, details improvements to the method and reports initial results from the study of a simple hydrocarbon fluid. IR spectra have been taken from the Hertzian region of a sliding contact lubricated by poly-α-olefin fluids. Single reflection sampling was used with a spot size of 100 μm square. The spectral range was limited to the CH stretch region of 3100--2700 cm−1. The results show that it is possible to obtain good quality IR spectra from hydrocarbon films of less than 50 nm. Spectra sampled from the contact zone show peak intensity changes, frequency shifts and shape changes. Analysis of these results indicate that in the high pressure region there is an increase in gauche defects in the alkyl chain implying a more globular molecule with a lower volume but higher energy. In thin lubricant films (<100 nm) there is evidence of fluid structuring close to the metal surface.

Imaging of the Surface of Human and Bovine Articular Cartilage with ESEM and AFM

Abstract: The surface of human and bovine articular cartilage was imaged with environmental SEM and AFM. The effective modulus of the surface, from force--distance curves obtained with AFM, remained constant at 9±2 kPa in the presence of synovial fluid. Extensive washing of the cartilage surface with phosphate buffered saline (PBS) removed a superficial gel-like layer, leaving a granular layer intact. Force--distance curves showed that the chemical and mechanical properties of the gel exposed to PBS changed over time. The effective modulus at the surface dropped from 481 to 4 kPa over an hour. The results suggest that the gel-like layer, having partly lost water through evaporation on removal from the joint, absorbs water from PBS. It becomes softer and eventually begins to dissolve. The low effective modulus of the gel-like layer in synovial fluid indicates that it is too soft to influence the surface roughness. Imprints of the surface under pressure were taken using a low viscosity dental kit. Imaging of the imprint surface indicated that the topography of the cartilage under pressure was similar to that of the surface after removal of the gel-like layer. In conclusion, imaging of articular cartilage with ESEM and AFM revealed two distinct non-fibrous layers, which are granular and gel-like, and cover the fibrous collagen matrix.

Chemical characterization and nanomechanical properties of antiwear films fabricated from ZDDP on a near hypereutectic Al–Si alloy

Abstract: X-ray absorption near edge structure (XANES) spectroscopy was used to monitor the products formed during the breakdown of the engine additive ZDDP during its action as a protective tribochemical agent. This investigation examines the film formed under various physical conditions on a near hypereutectic Al–Si alloy. For the first time, tribochemical films (tribofilms) formed on a high silicon weight content alloy, with virtually no ferrous component have been studied. Phosphorus K- and L-edge spectroscopies show that under typical engine operating conditions, the tribofilms have similar chemical composition over a range of different test conditions. X-ray photoelectron emission microscopy (X-PEEM) reveals that the polyphosphate glasses formed vary in chain length within localized regions. The mechanical properties of the substrate and the tribofilms were acquired using a Triboscope® from Hysitron Inc. The elastic moduli can be extracted from the indentation curves and show that the tribofilms’ mechanical properties are similar to those of the tribofilms which form on steel under similar conditions.

Tribofilms generated from ZDDP and DDP on steel surfaces:Part 2, chemistry

Abstract: The chemical constitution of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and ashless dialkyldithiophosphate (DDP), has been examined by X-ray Absorption Near Edge Structure (XANES) spectroscopy. The identification of spectral features and interpretation of the results for P, O, Fe, and S species are given, allowing an overall mechanism to be deduced. The role of Fe in these films was investigated in some detail using P L-edge, O K-edge and Fe L-edge XANES spectra. From the P L-edge XANES spectra, the DDP films are uniformly very short chain iron polyphosphates. In contrast, the ZDDP films are formed initially as short chain polyphosphates; but after more rubbing, a bilayer phosphate film is formed with long chain Zn polyphosphates on the surface and shorter chain in the bulk of the film. The O K-edge XANES spectra show that there is, as expected, more Fe in the DDP phosphate films than in the ZDDP phosphate films. The S K-edge spectra of ZDDP films show the presence not only of ZnS as previously observed, but also the presence of FeS for the first time in the early stages of film formation. The predominant S species in the DDP films is FeS.

Friction and Wear Properties of IF–MoS2 as Additive in Paraffin Oil

Abstract: Inorganic fullerene-like (IF) MoS2 nanoparticles with diameters ranging from 70 to 120 nm were synthesized by desulphurizing the MoS3 precursor and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Tribological properties of the IF–MoS2, as lubricating oil additive, were evaluated using a MMW-1 four-ball tribotester. The wear scar was examined with an optical microscope and scanning electron microscopy (SEM). The wear resistance of the paraffin oil was improved and the friction coefficient of the oil was decreased by addition of the IF–MoS2 nanoparticles. The mechanism of friction and wear of the IF–MoS2 nanoparticles was discussed.

Friction and wear of carbon nanohorn-containing polyimide composites

Abstract: Polyimide (PI)-based composites containing single-wall carbon nanohorn aggregate (NH) were fabricated using the spark plasma sintering (SPS) process. For comparison, composites with carbon nanotube (NT) and traditional graphite (Gr) were also fabricated. The NH was produced using CO2 laser vaporization and a graphite target and the NT was produced by a chemical synthesis method. We evaluated the friction and wear properties of the PI-based composites with a reciprocating friction tester in air using an AISI 304 mating ball. NH drastically decreased the wear of PI-based composites; the specific wear rate of composite with NH of only 5 wt% was of the order of 10−8 mm3/Nm, which was two orders of magnitude less than that of PI alone. The wear reduction ability of NT seemed to be slightly inferior to that of NH, although it was considerably better than that of Gr. NH and NT lowered the friction of composites. The friction coefficient of composite with 10 wt% NH was less than 0.25, although it was slightly higher than that of composite with 10 wt% Gr. There was no clear difference in the friction reduction effect of NH and NT. The further addition of Gr to composites with NH or NT rather deteriorated the antiwear property of composites, although the friction coefficient was slightly reduced. The transferred materials existed on the friction surface of the mating ball, sliding against composites with three types of carbon filler. These transferred materials seemed to correlate with the low friction and wear properties of composites.

On the effect of counterface material and aqueous environment on the sliding wear of various PEEK compounds

Abstract: Polyetheretherketone (PEEK) compounds containing carbon fibres (CF), glass fibres (GF), PTFE, and graphite, respectively, were exposed to unidirectional sliding against various counterparts (100Cr6, X5CrNi18-10, alumina, and bronze). Some of these tests were repeated in water. The stainless steel revealed the best results under dry conditions, whereas alumina was the best counterpart in water. The compound containing GF plus PTFE performed best under dry conditions. Under wet conditions, CF were superior to GF, which react very susceptibly to water. The aqueous environment usually accelerated the compound wear. Only in case of CF containing compounds sliding against alumina, the water lubrication reduced the wear rate.

An extension of the Hertz theory for three-dimensional coated bodies

Abstract: This paper presents a work on extending the Hertz theory for circular and elliptical point contact problems involving coated bodies. The extended form of the Hertzian formulae are adopted to express maximum contact pressure, contact radius, and contact approach in terms of applied load, equivalent radius, and an extended equivalent modulus that properly considers the presence of a coating. The extended equivalent modulus is a function of Young’s moduli and Poisson’s ratios of the coating and the substrate, coating thickness, and a parameter, which is obtained through substantial numerical simulation. The extended Hertzian formulae are easy to use and give accurate predictions of contact characteristics.

Tribological behavior and graphitization of carbon nanotubes grown on 440C stainless steel

Abstract: Vertically aligned carbon nanotube (CNT) arrays were directly grown onto 440C stainless steel substrates by plasma-enhanced chemical vapor deposition. Tribological properties of both short and long CNTs samples were studied under normal loads of 10 g, 25 g and 100 g. The CNTs had a steady-state friction coefficient of about 0.2 in humid air. In dry nitrogen, a friction of 0.2 was measured under a load of 10 g while high friction was measured at 25 g and 100 g loads. No significant variation of tribological behavior was measured between the short and long CNTs samples. SEM observations showed that rubbing caused the CNTs to align or lay down along the wear scar. They formed aggregates and were compressed by rubbing, which resulted in layer-structured graphite formations. SEM observation of the wear scars revealed loss of CNT structures accompanied by the appearance of dark areas. Micro Raman spectroscopic studies demonstrated that the dark areas were graphitized CNTs. Shear stress aligned the basal planes of the small graphene sheets in the CNT layers to the low friction orientation and eventually caused formation of more ordered graphite. The tribological formation of interfacial carbon layers increased with increasing stress from higher loads.

Hyperthermal atomic oxygen interaction with MoS2 lubricants and relevance to space environmental effects in low earth orbit – effects on friction coefficient and wear-life

Abstract: Wear-life of the MoS2 film was evaluated by in situ tribological testing under 5 eV atomic oxygen exposures which simulated atomic oxygen environment in low Earth orbit. A combination of a laser-detonation atomic oxygen source and a conventional pin-on-disk friction tester was used to perform tribological tests. It was confirmed that the friction coefficient was not affected by atomic oxygen exposure when atomic oxygen fluence was low; however, the friction coefficient increased with increasing atomic oxygen fluence and it reached as high as 0.05 at the atomic oxygen fluence of 3.4 × 1016 atoms/cm2/cycle (nine times larger than the normal value). Effect of atomic oxygen on the wear-life of the film has much more drastic. With atomic oxygen fluence of 1.7 × 1016 atoms/cm2/cycle, wear-life of the film was reduced less than one-tenth of that during ex situ testing result of the same film. It was also observed that the wear-life of the film was inversely proportional to the atomic oxygen fluence between sliding passes.

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Abstract: Friction and wear behaviors of diamond-like carbon (DLC) film in humid N2 (RH-100%) sliding against different counterpart ball (Si3N4 ball, Al2O3 ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si3N4 ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si3N4 ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al2O3 ball caused the highest wear rate of DLC film when sliding against Al2O3 ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.