Tribological Properties of Langmuir–Blodgett Films on Silicon Surface in Microscale Sliding Contact

Microtribological properties of Langmuir–Blodgett (LB) films transferred from behenic acid (BehA), 2,4-heneicosanedione (HD) and its copper complex ((HD)₂Cu) onto silicon surface were examined. To better understand the wear resistance performance of these LB monolayers, a comparison was made with a chemically grafted octadecyltrichlorosilane (OTS) monolayer. Auger electron spectroscopy (AES) was used for identification of the chemical composition of the monolayers, worn areas and counterpart surfaces. We observed that the studied LB films in microscale sliding contact exhibited stable friction properties comparable to OTS, and better wear performance than OTS at high contact pressure. The tribological properties of these LB monolayers were explained in terms of molecular packing density and molecular transfer to the counterpart surface. The relationship between the wear resistance of the studied LB films and the degree of molecular packing of the surfactants indicated that the wear properties of the LB films are strongly associated with the degree of molecular packing. We suggest that the steady low friction and high wear resistance of the BehA monolayer may partly be attributed to the transfer of the amphiphilic molecules to the counterpart surface in the contact region.     

Probing nanotribological and electrical properties of organic molecular films with atomic force microscopy

Structural aspects of organic molecular films, such as disordering, packing density, molecular bending or tilts, and phase separation, influence electrical properties as well as friction and adhesion. This indicates a correlation between nanomechanical and charge transport properties of molecular films at the molecular scale. In this review, we highlight the recent studies on correlations between charge transport and nanomechanical properties probed with atomic force microscopy. We discuss the key issues that determine charge transport and nanomechanical properties on several organic molecular films, including self-assembled monolayers formed by saturated hydrocarbon molecules conjugated molecules, and hybrid molecules as well as polymer and polymer blend films. We address the role of molecular deformation and bending in friction and conductance measurements.     

The structural origin of the complex rheology in thin dodecane films: Three routes to low friction

The effective viscosity of confined lubricant films less than 6–7 molecular layers is usually enhanced by many orders of magnitude. For dodecane the high friction film has a strong in-plane order with “mosaic-like” structures that extend across the film and effectively form “crystalline bridges” [Jabbarzadeh A, Harrowell P, Tanner RI. Crystal bridge formation marks the transition to rigidity in a thin lubrication film. Phys Rev Lett 2006; 96: 206102-1/4] resulting in high friction. Using molecular dynamics simulations, we have identified three routes to lower the friction. We show that the structure of confined films and their response to shearing are affected by atomic in-plane order and smoothness of the confining surfaces, the relative orientation of two crystalline surfaces and the direction of shear. We show a small increase in surface roughness in going from crystalline to amorphous surfaces can lead to a much lower friction. We demonstrate that misaligning (twisting) one surface with respect to the other by 45° results in a much lower effective viscosity. Application of shear for extended times induces alignment of lubricant molecules into a nematic-like order with ultra-low effective viscosity. The magnitude of reduction in the friction and the physical process through which it happens varies for each of these three routes. Depending on the method used, destruction of crystalline bridges, multilayer or fault plane slip provides a route for dramatic reduction in friction.     

Developments in tribological research on ultrathin films

A review of studies of the tribology of ultrathin films is presented, which focuses primarily on the tribological properties of the Langmuir–Blodgett (LB) films, self-assembled monolayers, and the molecular deposition films investigated by the authors and their co-workers. The emergence of the atomic force microscope has helped the development of studies of ultrathin films; particularly LB films which have been studied extensively. Firstly, the results of research into the various factors affecting the tribological properties of LB films and progress in the application of molecular dynamics simulations to study the mechanisms of friction and lubrication are introduced. Then a review of the experimental and theoretical research into self-assembled monolayers is given. Finally, recent advances in the investigation of tribological properties of molecular deposition films on different substrates (Au, Si and silica rock surfaces) are presented and the prospects for the tribological applications of such ultrathin films are addressed.     

Towards self-lubricated nanocoatings

The microtribological performance of molecularly thick (<10 nm) thermoplastic elastomeric films grafted to a silicon surface was enhanced by adding a minute amount of paraffinic oil, which was adsorbed from vapor phase and held by the rubber matrix. We studied the kinetics of polymer swelling in oil and the formation of polymer gels. We observed that a vast majority of adsorbed oil evaporated from the ultrathin polymer coating leaving a minute amount of oil trapped within the rubber phase. This resulted in a dramatic enhancement of the microtribological performance of the grafted polymer gel layers. These polymer gel layers exhibited a very steady friction response and a small value of the coefficient of friction as well as greater wear-resistance as compared to the initial polymer coating. The performance of polymer gel coatings was much better than the performance of a classic ‘boundary lubricant’ for silicon surfaces, an alkylsilane self-assembled monolayer. The approach proposed demonstrated a new efficient route towards enhanced tribological performance of ultrathin polymer coatings.     

Generation of defects in model lubricant monolayers and their contribution to energy dissipation in friction

The structural, mechanical (friction) and spectroscopic properties of model lubricant films made of self-assembled and Langmuir–Blodgett monolayers on quartz, mica and gold have been investigated with atomic force microscopy, the surface forces apparatus and sum-frequency generation. In these films, the molecules tend to form densely packed structures, with the alkane chains mostly vertical and parallel to each other. The SFG results suggest that under moderate pressures of a few tens of MPa, the methyl end group of the alkane chains is rotated to accommodate a terminal gauche distortion. The molecule, however, retains its upright close-packed structure with a lattice periodicity when ordered, which can be resolved by AFM. At pressures above 0.1 GPa, changes in the form of collective molecular tilts take place that lower the height of the monolayer. Only certain angles of tilt are allowed that are explained by the interlocking of methylene units in neighboring chains. The discrete angular tilts are accompanied by increases in friction. A model based on the van der Waals attractive energy between chains is used to explain the stability of the films and to estimate the cohesive energy changes during tilt and, from that, the increases in friction force.     

Nanoindentation and nanowear of extremely thin protective layers of C–N and B–C–N

Carbon nitride (C–N) and boron and carbon nitride (B–C–N) films, 1 and 3 nm thick, were deposited on magnetic disks by means of a complex treatment method involving plasma irradiation and CN or BCN reactive sputtering in nitrogen and helium mixed gas using two targets of h-BN and graphite. The properties of these extremely thin coatings were evaluated by indentation using an atomic force microscope and nanowear tests using a lateral modulation friction force microscope. The extremely thin B–C–N coatings show highest indentation hardness and good wear resistance properties. It is proposed that this is due to their graduated composition and interfacial properties.     

Macro- and nanotribological properties of organosilane monolayers prepared by a chemical vapor adsorption method on silicon substrates

Organosilane monolayers are novel ultrathin films used to control the physicochemical properties, such as friction and wear, of solid surfaces. In this study, the authors prepared alkylsilane and fluoroalkylsilane monolayers with a series of chain lengths by a chemical vapor adsorption method. The monolayers tribological properties were investigated by lateral force microscope (LFM) and friction tester. LFM nanoscale measurements of tribological properties showed that alkylsilane monolayer gave lower lateral force than the Si substrate surface. The lateral force decreased as the length of the alkyl chain increased. On the macroscale, friction test revealed that the organosilane monolayers gave lower dynamic friction coefficients than the Si substrate surface in air at room temperature. The longer the alkyl chain, the greater the wear resistance of the organosilane monolayers. Friction experiments using tetradecane as a lubricant showed better tribological properties than were obtained in air. Furthermore, microscopically line-patterned two-component organosilane monolayers were prepared and their macroscopic friction behavior was investigated. Even though the height difference between the two-components was less than 1 nm, friction force anisotropy between the parallel and perpendicular directions against the line pattern was observed.     

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.     

Frictional dynamics of perfluorinated self-assembled monolayers on amorphous SiO<Subscript>2</Subscript>

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.     

Measurements of tribological properties of poly(pyrrole) thin film bearings

We report the results of a recent study on the tribological properties of electropolymerised thin films at light loads and low speeds. Poly(pyrrole) films incorporating different counter-ions have been electrochemically deposited onto gold electrodes on the plano-convex glass substrates and studied extensively. The measuring apparatus has been greatly improved from that reported earlier and now provides simultaneous monitoring of frictional force and wear. High precision capacitive gauging is employed to provide high resolutions of frictional force of better than 100 μN and height variation (wear) of 2 nm. A large number of specimens of poly(pyrrole) grown from five different counter-ions were prepared and their performances evaluated. The film morphology of each type of film was examined by atomic force microscopy (AFM) for control of the variability of film formation. Results are presented for the friction coefficients and wear rates observed for the films typically at a load of 2 N and a sliding speed of 5 mm s⁻¹. The effects of normal loading force and sliding speed on the friction coefficient are also discussed with a load range of 0.2–5 N and a sliding speed up to 30 mm s⁻¹.     

Lubrication properties of non-adsorbing polymer solutions in soft elastohydrodynamic (EHD) contacts

The in-use performance and processing of many consumer products in the food, home and personal care industries are dependent on their tribological properties. A major component of these products is often a high molecular weight polymer, which is typically used to thicken aqueous systems. Polymer solutions tend to be non-Newtonian, and in particular their viscosity varies with shear rate, such that it is difficult to predict their friction or hydrodynamic film-forming behaviour. The present work relates the tribology of aqueous polymer solutions to their rheological properties in thin films in ‘soft’ contacts at high shear rates. The friction properties of three types of polymers in aqueous solution, polyethylene oxide, PEO; xanthan gum, XG; and guar gum, GG, have been studied as a function of polymer concentration over a wide range of entrainment speeds in a point contact formed between silicone rubber and steel. This has enabled the boundary lubrication and isoviscous-elastic lubrication properties of the solutions to be investigated using both hydrophilic and hydrophobic silicone surfaces.It is found that the friction vs. entrainment speed dependence follows the shape of a classical Stribeck curve. In general, a lower friction is observed with increasing polymer concentration in the mixed-regime. Using scaling factors for the entrainment speed, we have shown that this decrease in friction is likely to be due to viscous effects and that the scaling factors represent effective high shear rate viscosities. In the case of PEO and XG, and GG at low concentrations, a good correlation is found between this effective viscosity and the apparent viscosity measured at the highest shear rates attainable with the available rheometer. However, for GG at concentrations above 0.2%, the effective viscosity decreases with increasing polymer content.The three polymers do not significantly reduce friction in the boundary regime and in general give essentially the same response as water when an effective viscosity is taken into account. However, a slight increase in friction in comparison to pure water has been observed for XG and GG on hydrophobic surfaces. It is suspected that this may be due to a blocking of fluid entrainment, or possibly exclusion of polymer from the contact, due to the large hydrodynamic volume and rigid nature of the two biopolymers. Finally, for PEO solutions with full-film elastohydrodynamic conditions were reached, the measured friction coefficient of the film correlated quite well with the value calculated from the effective viscosity.     

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.     

Simulations on atomic-scale friction between self-assembled monolayers: Phononic energy dissipation

Atomic-scale friction between self-assembled monolayers (SAMs) on Au (1 1 1) has been studied through molecular dynamics simulations, with emphasis on the mechanism of energy dissipation. Results show that the shear stress and chain angle on commensurate SAMs exhibit a clean periodic pattern and atomic stick–slip friction, which manifests a gradual storage and sudden release of energy. Using a simple model of two atoms, analysis shows that the atomic stick–slip originates from the dynamic instability of molecule motion. Energy has been built up during the stick, followed by a sudden separation as the equilibrium becomes unstable, and most energy dissipates at the time of slip. Moreover, the simulations reveal that more energy is stored and released in commensurate sliding, resulting in much higher friction than that in incommensurate cases. The contradictory frictional behavior can be traced to the difference in the number and strength of the Van der Waals bonds, formed in the two types of contacts.     

Friction properties of acrylic-carboxylated latex films—1: Effects of acrylic acid concentration and pH

The aim of this work was to investigate how specificities of thin films prepared from aqueous polymer colloids (latexes) influence their friction properties, tested with a sliding stainless steel spherical tip. Two acrylic latexes containing either 1 wt% or 4 wt% of acrylic acid (AA) were used at pH 2 or 10. Bulk mechanical properties were also studied in order to improve the interpretation of friction results. Increasing AA concentration and pH increases the overall film rigidity, pH being more effective than AA concentration. Contact pressures are directly correlated to bulk mechanical properties whereas surface shear stresses are also strongly influenced by molecular interactions with the sliding tip. Friction coefficients are rather high, peaking at 5, because of important viscoelastic dissipation in the films as well as strong polar interactions introduced by AA, especially at high pH. The paper also addresses the question of the relevant characteristic length in this kind of friction study.     

Modification of Boundary Lubrication by Oil-Soluble Friction Modifier Additives

The molecular-level function of model and commercial friction modifier additives in lubricants of the type used at the wet clutch interface in automatic transmissions has been studied using a surface forces apparatus (SFA) modified for oscillatory shear. The nanorheological properties of tetradecane with and without a model friction modifier additive (1-hexadecylamine) were examined in the boundary lubrication regime and compared to a fully-formulated automatic transmission fluid (ATF). 1-Hexadecylamine adsorbed as a single layer on the sliding surfaces, reduced the static frictional force and the limiting shear stress, and eliminated the stick–slip transition that exists in pure tetradecane. The ATF, which contains commercial-grade friction modifiers, showed nanorheological properties similar to those observed for tetradecane containing 0.1–0.2 wt% 1-hexadecylamine.     

On the scratch behaviour of self-lubricating WSe2 films

Transition metal dichalcogenides films are well known for their self-lubricating properties. These films are having lamellar structure – whereby weak “van der Walls” forces act between the layers – commonly believed to be responsible for their excellent self-lubricating properties. Among these films, diselenoids have shown less sensitivity to humidity and they are more oxidation resistant in humid environment than sulphides. In view of the above, a comprehensive work is undertaken to study friction properties in macro- and micro-scale. The present work deals with preliminary study and critical examination of friction due to scratching of WSe₂ film. WSe₂ film is deposited using sputtering technique. The composition of the film is determined by means of energy dispersive spectrometry (EDS) and X-ray photoelectron spectrometry (XPS). The micro-structural features, topography and mechanical properties of the film are evaluated using transmission electron microscopy (TEM), atomic force microscopy (AFM) and nano-indenter. The film is scratched at different constant loads and also with increasing load using a scratch tester with a spherical indenter in macro-loading regime. A 3D confocal microscope is used to study the scratched portion. In micro-loading regime the film is scratched with an angular indenter. The results show that even though self-lubricating effect comes into play in macro-loading regime, this effect cannot be seen in micro-loading regime. Further coefficient of friction in different loading regime is independent of applied load.     

Shear-induced dilation of confined liquid films

We demonstrate that sheared molecularly-thin fluid films dilate at the point of stick-to-slip (which is the transition from static to kinetic friction), indicating that density decreases when sliding occurs. This contrasts with incompressibility characteristic of bulk fluids when they are deformed. The magnitude of dilation was less than the size of the molecule and was larger in a polymer system (large molecules) than for small-molecule fluids. The experiments employed a surface forces apparatus modified to measure, using piezoelectric methods, sub-angstrom variations of film thickness during dynamic shear excitations that were performed at rates too rapid to allow fluid to enter and exit the zone of shear contact during the period of shear excitation. To demonstrate generality of the dilation effect, the specific systems studied included nonpolar fluids whose complexity was varied (a globular-shaped molecule, OMCTS; a branched alkane, squalane; a tethered diblock copolymer, polyvinylpyridine–polybutadiene) and also an aqueous electrolyte, MgCl₂ dissolved in water. Extensive analysis is also presented of the piezoelectric methods that were employed to detect volume changes too small to observe by the methods of multiple beam interferometry that are traditional for thickness measurement in a surface forces apparatus.     

Friction and Wear of Various DLC Films in Water and Air Environments

Three types of diamond-like carbon (DLC) films, pure DLC, F-containing DLC, and a Si-containing DLC film, were deposited on a WC–Co substrate by a plasma-enhanced CVD technique. Friction and wear properties were determined using a ball-on-plate type reciprocating friction tester in water, comparing the water results to those in ambient air. The friction coefficient of DLC and F–DLC films in water was considerably lower than that in air. With Si–DLC, the friction was almost the same level in both water and air, and was less than 0.1. The specific wear rate of films in water was much smaller than that in air and varied around the low level of 10^–8 mm³/Nm in water, The mating ball wear was also less than 10^–8 mm³/Nm. With DLC and F–DLC films, the transferred amount of material on the friction surface of a mating ball was larger in a water environment than that in air. With a Si–DLC film, the difference in the transferred amount when exposed to either the water or air environment was negligible.     

Tribological behaviour of duplex treated Ti–6Al–4V: combining nitrogen PSII with a DLC coating

The chemical structure and tribological behaviour of Ti–6Al–4V plasma source ion implanted with nitrogen then DLC-coated in an acetylene plus hydrogen-glow discharge (bias voltage −10 to −30 kV) were investigated. The as-modified samples have a TiN/H:DLC multilayer architecture (coating resistivity 1.6×10⁹ to 2.4×10¹¹ Ω/cm) and exhibit higher hardness, especially at low loads or plastic penetrations in the order of deposition bias voltage −10, −20 and −30 kV. At a lower contact load (1 N) and higher sliding speed (0.05 m/s), frictional properties in most cases improved, as did wear properties. At a higher contact load (5 N) and lower sliding speed (0.04 m/s), friction showed almost no improvement, and wear properties deteriorated. When the material of the counterbody was then changed from AISI 52100 to Ti–6Al–4V modified as the disc (contact load 5 N unchanged, sliding speed decreased), the friction coefficient decreased (but showed no improvement compared with the unmodified sample), while wear properties deteriorated further, and wear was changed from just the disc to both disc and ball, abrasive and adhesive dominated. Transfer films, mainly made up of wear debris transferred from the disc wear surfaces, were formed on the wear scars of the counterbodies. The deterioration of wear properties of the modified samples at the higher contact load is considered to be caused by the “thin ice” effect.