The relationships between interfacial friction and the conformational order of organic thin films

This report describes studies of the relationships between the structures of organic monolayers and their molecular-scale frictional properties. Several distinct self-assembled monolayers (SAMs) were formed by the adsorption of a series of spiroalkanedithiols and a single structurally related normal alkanethiol. Measurements of hexadecane wettability, infrared vibrational spectroscopy, and X-ray photoelectron spectroscopy revealed that the films possessed a wide range of interfacial structures and conformational orders. Atomic force microscopy was used to measure the frictional properties of the well-characterized SAMs on the molecular scale. Comparison of the frictional data with structural information derived from complementary analytical techniques revealed a high correlation between the conformational order of the films and the observed frictional response.     

Effect of Water Swelling on the Tribological Properties of PMMA Spin-Cast Film and Brush in Aqueous Environment

Due to their light weight, low corrosion and good tribological properties, polymer films have been widely studied in dry condition as well as recently in aqueous environment. Though the presence of water can further reduce the friction, it promotes the wear rate of the polymer films. As a remedy to decrease the wear rate of polymer films under aqueous condition, in this study, we used PMMA brush which is chemically anchored to a substrate and compared its friction and wear properties with those of conventional PMMA spin-cast film. Ellipsometry, contact angle measurements and atomic force microscopy are used to study the surface properties, e.g., wear mechanisms and wear depths of PMMA films. Under different sliding speeds and applied loads, PMMA brush showed lower friction than PMMA spin-cast film in aqueous. Moreover, it was shown that the swelling of water molecules is a dominant factor in determining the wear durability of PMMA films in which PMMA brush showed better wear performance than PMMA spin-cast film.     

Investigation of the Tribological Behaviors of Polymer Molecular Deposition Films by Tribometer Based on Interferometer

The polymer molecular deposition films including polyelectrolyte molecular deposition (PEMD) film and nanoparticles composite molecular deposition (NPs/MD) film have been prepared using the molecular deposition method and the in situ synthesize method. The polymer molecular deposition films were characterized by atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The tribological behaviors of the substrate and polymer molecular deposition films were investigated by a tribometer based on interferometer. It is found that the NPs/MD film has a lower friction force and a better anti-wear property than the PEMD film under the dry friction. The poly alpha olefin (PAO2) and water films confined between samples and steel ball surfaces have been investigated using thin film interferometry. The friction force of substrate was lower than the polymer molecular deposition films under PAO2 lubrication. The friction forces alteration of PEMD film and NPs/MD film were similar and consistent, and lower than that for substrate under water lubrication.     

A biomimetic approach for effective reduction in micro-scale friction by direct replication of topography of natural water-repellent surfaces

In this paper, we report on the replication of surface topographies of natural leaves of water-repellent plants of Lotus and Colocasia onto thin polymeric films using a capillarity-directed soft lithographic technique. The replication was carried out on poly(methyl methacrylate) (PMMA) film spin coated on silicon wafer using poly(dimethyl siloxane) (PDMS) molds. The friction properties of the replicated surfaces were investigated at micro-scale in comparison with those of PMMA thin film and silicon wafer. The replicated surfaces exhibited superior friction property when compared to those of PMMA thin film and silicon wafer. The superior friction behaviour of the replicated surfaces was attributed to the reduced real area of contact projected by them.     

Phase separation in polyfluorene – polymethylmethacrylate blends studied using UV near-field microscopy

In this paper we present a near‐field microscopy study of thin films of a phase‐separated blend of the fluorescent conjugated‐polymer poly(9,9‐dioctylfluorene) [PFO] with the non‐fluorescent polymer polymethylmethacrylate [PMMA]. A scanning near‐field optical microscope (NSOM) was used to generate (blue) fluorescence from the PFO following UV excitation at 362 nm. A range of different concentrations of PFO in PMMA were studied ranging from 1 to 50% PFO in PMMA by mass. By studying both the shear force and fluorescence images we were able accurately to determine the distribution of PFO in the PMMA. We found that phase separation occurs over a number of different length‐scales between 5 µm and 250 nm. We show that at PFO concentrations of 1%, the PFO lies on top of the PMMA. At a PFO relative concentration of 50%, the PMMA phase extends through the whole thickness of the film to the underlying substrate. We use such samples to discuss the resolution of NSOM when imaging thick organic films. Furthermore, we confirm that the length‐scales of phase separation can be modified via control over spin‐casting protocols.     

Friction of Poly(n-Alkyl Methacrylates)

The friction coefficient of solid poly(n-alkyl methacrylates) decreases progressively with increasing length of the alkyl group. This behavior parallels the behavior of adsorbed films of these polymers from solution on solid surfaces. Durability of thin films of these polymers (approximately 3,500 Å thick) increases with increasing length of alkyl group. For example, when the alkyl is C₂₂ the durability is four times greater than when the alkyl is C₁₂ (polymer film on a 1020 steel substrate). The film durability is strongly substrate dependent, e.g., durability of poly(n-dodecyl methacrylale) is three times greater on glass than on 1020 steel when rubbed with a 440 C rider. The friction and durability are attributed to side chain crystallinity of the polymer. Side chain crystallinity has been observed by others using x-ray and other techniques when the alkyl group is at least 10 or 12 carbons long. Friction measurements may be a more sensitive way of detecting side chain crystallinity. The friction behavior of adsorbed films from solution of the poly(n-alkyl methacrylates) suggests that side chain crystallinity also pertains to suck adsorbed films.     

Small amplitude reciprocating wear performance of diamond-like carbon films: dependence of film composition and counterface material

Small amplitude (50 μm) reciprocating wear of hydrogen-containing diamond-like carbon (DLC) films of different compositions has been examined against silicon nitride and polymethyl-methacrylate (PMMA) counter-surfaces, and compared with the performance of an uncoated steel substrate. Three films were studied: a DLC film of conventional composition, a fluorine-containing DLC film (F-DLC), and silicon-containing DLC film. The films were deposited on steel substrates from plasmas of organic precursor gases using the Plasma Immersion Ion Implantation and Deposition (PIIID) process, which allows for the non-line-of-sight deposition of films with tailored compositions. The amplitude of the resistive frictional force during the reciprocating wear experiments was monitored in situ, and the magnitude of film damage due to wear was evaluated using optical microscopy, optical profilometry, and atomic force microscopy. Wear debris was analyzed using scanning electron microscopy and energy dispersive spectroscopy. In terms of friction, the DLC and silicon-containing DLC films performed exceptionally well, showing friction coefficients less than 0.1 for both PMMA and silicon nitride counter-surfaces. DLC and silicon-containing DLC films also showed significant reductions in transfer of PMMA compared with the uncoated steel. The softer F-DLC film performed similarly well against PMMA, but against silicon nitride, friction displayed nearly periodic variations indicative of cyclic adhesion and release of worn film material during the wear process. The results demonstrate that the PIIID films achieve the well-known advantageous performance of other DLC films, and furthermore that the film performance can be significantly affected by the addition of dopants. In addition to the well-established reduction of friction and wear that DLC films generally provide, we show here that another property, low adhesiveness with PMMA, is another significant benefit in the use of DLC films.     

Effects of Temperature on the Microscale Adhesion Behavior of Thermoplastic Polymer Film

Adhesion tests were carried out under various temperature conditions to investigate the effects of temperature on adhesion behavior between a fused silica lens and a poly(methylmethacrylate) (PMMA) film, materials used for molds and as thermoplastic polymer films in nanoimprint lithography (NIL). The pull-off force and the contact area were measured as the temperature of the PMMA film was increased from 300 to 443 K, and as it was decreased back to 300 K. As the temperature increased, the PMMA film changed from a glassy state to a viscous flow state, and the adhesion behavior showed significant variation corresponding to this change in state. In the glassy state (below 363 K), the pull-off force did not change noticeably as the temperature increased. In the rubbery state (383–413 K), the pull-off force increased significantly as the temperature increased. In addition, the contact area was enlarged. In the viscous state (above 423 K), fingering instability was observed in the contact area and the pull-off force decreased, while the contact area increased, as the temperature increased. The adhesion behavior was also found to vary markedly with thermal history of the PMMA film. The causes of these changes are discussed.     

Effect of Molecular Structure on Friction and Wear of Polymer Thin Films Deposited on Si Surface

In this study, the influence of the molecular structure (linear or with bulky side groups) of polymer films covalently attached to Si surface on tribological properties is investigated. Two polymers, PE (polyethylene) and PS (polystyrene), are selected where PE has simple linear molecular structure whereas PS has linear molecular structure but contains bulky benzene groups located at the sides of the linear chain. PE and PS molecules, both with reactive maleic anhydride groups, are chemisorbed onto Si via an intermediate APTMS SAM (3-aminopropyltrimethoxysilane self-assembled monolayer). Water contact angle measurements, AFM (atomic force microscopy), ellipsometry, and XPS (X-ray photoelectron spectroscopy) are used to identify and characterize the polymer films. Tribological properties are studied using a microtribometer where a 4 mm diameter Si₃N₄ ball is used as the counterface. Among the two polymer films investigated, Si/APTMS/PE has shown very low coefficient of friction (0.08) and high wear life (∼4,400 cycles) than those of Si/APTMS/PS. Surprisingly, Si/APTMS/PS did not show any improvement in tribological properties when compared to that of bare Si. The present study proves that the polymer with linear molecular structure without the bulky side groups show good tribological properties even when it is coated as a thin film and hence such polymers can be used as thin-films for reducing friction and wear of substrates such as Si or other materials.     

Effects of film characteristics on frictional properties of carboxylic acid monolayers

Monolayers of long-chain carboxylic acids, with various fluorination ratios, were deposited on solid substrates to investigate the effects of surface properties on frictional behavior. It is found that a lower surface free energy correlates to a lower friction coefficient for the monolayers of partially fluorinated carboxylic acids. However, a stearic acid (C₁₇H₃₅COOH) monolayer shows the lowest friction coefficient, although its surface free energy is relative high. The two-dimensional elastic modulus, which might be used as a parameter to quantitatively characterize the film strength, was developed. Friction coefficients of the monolayers show a strong correlation with their elastic modulus, that is, the higher the elastic modulus, the lower the friction coefficient. The research results indicate that frictional properties of ultrathin films are mainly dependent on film strength. Enhancement of intermolecular attractive force might be a more effective way to improve the lubricating properties of ultrathin films.     

A study of the durability of boundary films

The frictional properties of solid surfaces are modified by the adsorption of organic molecular films. The lifetime of a monomolecular film of a fatty acid which was deposited on a surface to form a solid/monolayer/air interface has been investigated.The continual presence of an agglomerate has been observed during frictional contact under boundary lubrication conditions. This agglomerate is composed of several substances resulting from the deterioration and recombination of fatty acid molecules with each other and/or the surrounding media.     

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.     

Anisotropic stick-slip friction of highly oriented thin films of poly(tetrafluoroethylene) at the molecular level

Lateral force microscopy (LFM) studies of poly(tetrafluoroethylene) (PTFE) films with molecular resolution are reported. Thin PTFE layers with a high degree of orientation were obtained by pressing and sliding a block of polymer on a clean, heated muscovite mica substrate. LFM nanographs obtained on these films by scanning at directions between ca. 40 and 90° with respect to the film orientation direction, i.e. with respect to the direction of the polymer chains, showed a stick-slip type frictional motion of the LFM probe tip at the molecular level. The friction force observed at constant load decreased with decreasing scan angles. Chain-chain packing distances obtained by LFM and contact-mode atomic force microscopy were the same to within the experimental error and had a value of 5.8 Å. Dual-mode contact AFM/LFM imaging was also performed by scanning in the chain direction. Here LFM nanographs showed no distinct stick-slip phenomenon. The contact mode AFM images, however, exhibited clear molecular resolution with the expected chain-chain periodicity. The disappearance of the stick component in LFM scans performed in the chain direction was attributed to the smooth surface of PTFE on the molecular scale.     

载荷、相对湿度和气氛对硅表面及金表面纳米摩擦特性的影响

利用分子沉积技术 ,在不同修饰的硅表面上考察了载荷对纳米摩擦学性能的影响 ,同时在阴、阳离子化金表面上考察了湿度、气氛对纳米摩擦学性能的影响。结果表明 ,不同修饰的硅表面在较低的载荷下 ,摩擦力均随着载荷成线性增加 ,同时发现单层磺化酞菁铜MD膜修饰的硅表面没有改变硅的摩擦特性 ;试验表明 ,在阳离子化金表面上 ,相对湿度越大 ,摩擦力和摩擦系数越低 ;阴离子化金表面在氮气保护下 ,体系的摩擦学特性不同于大气环境。     

Investigation of transfer phenomenon by X-ray photoelectron spectroscopy and tribological properties of polymers sliding against polymers

Tribological properties of polymeric materials were investigated with various polymer-polymer combinations by means of a pin-on-disk wear testing apparatus. The specific wear rate was related to the cohesive energy density of the polymeric materials, but hat of a given polymer slider was dependent on the mated polymeric materials, and a high wear rate was observed in the sliding against a polymer counterface with higher cohesive energy than that of the slider. By means of X-ray photoelectron spectroscopy it was confirmed that the polymer transfer did occur on the mated polymer, and the degree of covering of the track with transfer films could be estimated. The covering ratio with transfer films was dependent on the polymer-polymer combinations. Poly(tetrafluoroethylene) (PTFE) transfer film on various polymers was very effective in reducing friction irrespective of the covering with the transfer films. With the other polymers, high density poly(ethylene) (HDPE), poly(propylene) (PP), and polyacetal (PAc), the transfer film was less effective in reducing friction than PTFE transfers and friction in the steady state dependent on the polymer-polymer combinations.     

复合钒钼酸凝胶薄膜的湿敏特性研究

本文研究了复合钒钼酸凝胶薄膜的湿敏特性。复合钒钼酸凝胶薄膜的湿敏性能较复合钒酸好,而且基电阻随相对湿度的增加而减小,添加正硅酸乙酯（TEOS）对其湿滞有明显的改进作用,含5wt%TEOW的薄膜的湿滞加差最小。同时器件的响应恢复较快。XRD分析表明复合钒钼酸干凝胶薄膜具有层状结构。     

Friction of copolymers of methyl α-n-alkylacrylates (alkyl ⩾ C16H33) and n-octadecyl methacrylate with methyl methacrylate

A comparative study was made of the friction and wear properties of homopolymers and copolymers (with methyl methacrylate (MMA)) of methyl α-n-hexadecylacrylate (MHDA), methyl α-n-octadecylacrylate (MODA) and n-octadecyl methacrylate (ODMA). In the first two compounds the long alkyl side chain is attached to the polymer backbone, whereas in the last it is adjacent to the carboxyl group that is attached to the polymer backbone. The friction data obtained for homopolymers and copolymers (thin films on glass or steel) of MHDA, MODA and ODMA were similar. The coefficient of friction μ decreased rapidly at a low concentration of MHDA, MODA or ODMA in the copolymers, then decreased more slowly. At about 50 mol.% μ (copolymer) ≈ μ(homopolymer) and further increases in the concentrations of these monomers in the copolymer led to a slight decrease in μ to the limit of the μ value for the homopolymer (μ ≈ 0.10). The durability of thin films (about 3700–4100 Å thick) of the copolymers increased with the concentration of MHDA, MODA or ODMA up to 21–25 mol.%. However, 100% MHDA and MODA have lower durabilities than some of the copolymers. All homopolymers and copolymers show higher durabilities than poly-MMA and the durabilities of some of the copolymers approach that of poly-ODMA, which has the highest durability in the series of polymers studied. The lower durabilities of poly-MHDA and poly-MODA ($\text{M}\text{?}{}_{\mathrm{n}}$ values 7083 and 1320) compared with that of poly-ODMA ($\text{M}\text{?}{}_{\mathrm{n}}\text{= 1.9 × 10}{}^5$) may be related to their low molecular weights. In all cases the durability of the homopolymers and copolymers was greater on glass than on steel. Available evidence indicates that the decrease in μ and the increase in durability of the copolymers studied is attributable to the long alkyl side chain and not to the polymer backbone. Further, the effect of the long alkyl side chain on μ and on durability is related to its flexibility and capability of orientation during the rubbing process.     

Studies on the friction and transfer of semicrystalline polymers

The friction and transfer of various semi-crystalline polymers were studied in several experiments. The cylindrical surfaces of polymers were slid over glass plates at low speed and under constant load. The kinetic friction of PTFE in repeated traverses did not vary with the number of traverses and the transfer of PTFE occurred successively on previously transferred PTFE films. The film transferred at each traverse was extremely thin (< 50 Å). The friction of PTFE decreased with increased humidity in the environment and appeared to be independent of crystalline transitions. Other polymers exhibited higher friction than PTFE and their transfer was generally as small lumps or short streaks. HDPE displayed a very low friction, although the friction of ultrahigh molecular weight polyethylene (UHDPE) was somewhat higher than that of HDPE. With HDPE and UHDPE, as well as with PTFE, long films stretched from one side of the abrasion grooves produced on the polymer frictional surfaces to the other side, like a bridge. The static friction of the three different polymers was very sensitive to the direction of prerubbing on the frictional surfaces and the static friction in sliding parallel to the pre-rubbing direction was much smaller than that perpendicular to it. The roles of the molecular profile and of the banded or spherulitic structure of the polymers in the polymer transfer mechanism are discussed on the basis of the experimental results obtained.     

硅基微机械表面粘附及摩擦性能的AFM试验研究

在Si（100）基片上制备了十八烷基三氯硅烷（OTS）分子润滑膜，并用原子力显微镜（AFM）对比研究了施加OTS膜前后的硅表面的粘附、摩擦磨损性能。试验考虑了相对湿度和扫描速度对粘附、摩擦性能的影响。结果表明，相对于硅构件来讲，OTS膜表面粘附力较小，具有较小的摩擦因数，呈现较好的润滑性能；硅构件受湿度变化的影响比OTS膜明显。微构件的摩擦性能由于水合化学作用生成Si（OH）。润滑膜，使得其受相互间运动速度影响很大。OTS膜不仅是一种耐磨性较好的润滑膜，而且有良好的稳定性。     

Microtribology,squeeze, and friction of the colloidal layer

Lubricant films containing colloidal overbased calcium salts have particular effects on detergency and on the wear process. The mechanical properties of such boundary films are very dependent on the process conditions, and are essentially a result of physico-chemical transformations of the colloid. Three types of films were compared: a film obtained by the drying of the base solvent, a film due to the compaction of the colloid between two steel surfaces, and a friction film obtained with this type of lubricant. Adsorption, compaction, and shear transformations induce a solid film. The pressure effect on the frictional behaviour of these colloidal films has been investigated using a surface force apparatus for low contact pressure (10⁴ to 10⁶ Pa), and using tribometers supporting heavy loads for the analysis of the high pressure domain. Above a critical pressure, evaluated at 10⁶ Pa, the colloidal film does not flow, but forms a compacted mattress sliding on the surface plane and squeezing a molecular layer of lubricant.