Effect of solid surface on the formation of thin confined lubricating film of water with micro-content of oil

The formation of confined film between two contacting surfaces is significant for evaluating the lubricating ability of liquid. A micro-content of oil in water was experimentally demonstrated to be significantly effective to the film formation of water, which was much thicker than predicted by elastohydrodynamic lubrication theory. The effect of solid surface characteristics on the liquid film confined in a nanogap has been investigated. The film forming performances of such films were presented. The work of adhesion between two different phases was calculated, and the competitive wetting behaviours of water and oil on different solid surfaces were employed to understand the film formation mechanism.     

Capillary force on colloidal particles in a freely suspended liquid thin film

The dynamic behavior of micron-sized polystyrene latex particles confined in a free-standing liquid film is experimentally studied. When the thickness of the film is less than the particle diameter and varies depending on position, the particles are accelerated toward the thicker region. Using a simple geometrical model and hydrodynamic theory, we calculate the capillary force on the particle. The drag coefficient of the particle is found to depend on the thickness of the film from the value of near zero to the Stokes' drag coefficient.     

Effect of medium on friction and wear properties of compacted graphite cast iron processed by biomimetic coupling laser remelting process

Stimulated by the cuticles of soil animals, an attempt to improve the wear resistance of compact graphite cast iron (CGI) with biomimetic units on the surface was made by using a biomimetic coupled laser remelting process in air and various thicknesses water film, respectively. The microstructures of biomimetic units were examined by scanning electron microscope and X-ray diffraction was used to describe the microstructure and identify the phases in the melted zone. Microhardness was measured and the wear behaviors of biomimetic specimens as functions of different mediums as well as various water film thicknesses were investigated under dry sliding condition, respectively. The results indicated that the microstructure zones in the biomimetic specimens processed with water film are refined compared with that processed in air and had better wear resistance increased by 60%, the microhardness of biomimetic units has been improved significantly. The application of water film provided finer microstructures and much more regular grain shape in biomimetic units, which played a key role in improving the friction properties and wear resistance of CGI.     

Boundary lubrication properties of materials with expansive freezing

We have performed molecular dynamics simulations of solid-solid contacts lubricated by a model fluid displaying many of the properties of water, particularly its expansive freezing. Near the region where expansive freezing occurs, the lubricating film remains fluid, and the friction force decreases linearly as the shear velocity is reduced. No sign of stick-slip motion is observed, even at the lowest velocities. We give a simple interpretation of these results, and suggest that, in general, good boundary lubrication properties will be found in the family of materials with expansive freezing.     

Water monolayer and multilayer adsorption on Ni(1 1 1)

Water adsorbed on Ni(1 1 1) forms an ordered, hydrogen bonded ice structure with a (2√7 × 2√7)R19° unit cell. The 2√7 wetting structure forms as islands and persists up to saturation of the first layer. Adsorption of a fraction of a monolayer more water into a second layer destroys the 2√7 registry and creates a disordered ice film. Gas adsorption measurements indicate that the wetting layer is completely covered by a second layer of water before thicker multilayer ice forms. As the second layer is completed the film orders to form an incommensurate crystalline ice film with a hexagonal LEED pattern, oriented to the Ni close packed rows. This ordered, incommensurate structure persists as the ice multilayer grows thicker.     

Frictional properties of confined polymers

We present molecular dynamics friction calculations for confined hydrocarbon solids with molecular lengths from 20 to 1400 carbon atoms. Two cases are considered: a) polymer sliding against a hard substrate, and b) polymer sliding on polymer. In the first setup the shear stresses are relatively independent of molecular length. For polymer sliding on polymer the friction is significantly larger, and dependent on the molecular chain length. In both cases, the shear stresses are proportional to the squeezing pressure and finite at zero load, indicating an adhesional contribution to the friction force. The friction decreases when the sliding distance is of the order of the molecular length indicating a strong influence of molecular alignment during run-in. The results of our calculations show good correlation with experimental work.     

Electrical properties of water drops inside the dropwise cluster

The Letter shows that inside a dropwise cluster formed over the heated water surface, water drops are electrically charged. The charge of a separate drop reaches 10³ units of an elementary charge. The drops are positioned from each other at the distance of double Debye radius length. It is fixed up that drops levitate over the water surface in consequence of the Stokes force acting from the side of gas-vapor flow rising from water surface. The Stokes force thousand times exceeds the Coulomb drop repulsion force from the water surface.     

Fabrication of durable hydrophobic surfaces through surface texturing

Low surface energy polymer thin-films can be applied to surfaces to increase hydrophobicity and reduce friction for a variety of applications. However, wear of these thin films, resulting from repetitive rubbing against another surface, is of great concern. In this study, we show that highly hydrophobic surfaces with persistent abrasion resistance can be fabricated by depositing fluorinated carbon thin films on sandblasted glass surfaces. In our study, fluorinated carbon thin films were deposited on sandblasted and as-received smooth glass using deep reactive ion etching equipment by only activating the passivation step. The surfaces of the samples were then rubbed with FibrMet abrasive papers in a reciprocating motion using an automatic friction abrasion analyzer. During the rubbing, the static and kinetic friction forces were also measured. The surface wetting properties were then characterized using a video-based contact angle measuring system to determine the changes in water contact angle as a result of rubbing. Assessment of the wear properties of the thin films was based on the changes in the water contact angles of the coated surfaces after repetitive rubbing. It was found that, for sandblasted glass coated with fluorinated carbon film, the water contact angle remained constant throughout the entire rubbing process, contrary to the smooth glass coated with fluorinated carbon film which showed a drastic decrease in water contact angle with the increasing number of rubbing cycles. In addition, the static and kinetic friction coefficients of the sandblasted glass were also much lower than those of the smooth glass.     

Transducer properties of AFM microprobes conformally covered by ALE-deposited SnO2 thin films

2 thin films. The study showed that by applying very thin film (30 nm SnO₂ in this study) the transducer influences on force sensing can be neglected. However, even much thicker (470 nm SnO₂) overlayers can be used if the coating influence on the optical and mechanical properties of the cantilever is sufficiently taken into account. Received: 3 August 1998 / Accepted: 3 November 1998     

Tribological properties of chemically bonded polyimide films on silicon with polyglycidyl methacrylate brush as adhesive layer

Polyimide thin films, which possess good stability and film uniformity, are successfully fabricated on single crystal silicon wafers coated with a thin polymer brush by suface-initiated polymerization (SIP) as an adhesive layer. The growth kinetic of polyglycidyl methacrylate (PGMA) brush was studied by the means of ellipsometry. The nano-scale morphology and chemical composition of PGMA brush and polyimide film were studied with atomic force microscopy (AFM), Fourier transform infrared spectrum (FT-IR), and X-ray photoelectron spectroscopy (XPS). The tribological behaviors of the thin films sliding against AISI-52100 steel ball were examined on a static-dynamic friction precision measurement apparatus and UMT-2MT tribometer. The worn surface of the polyimide thin films was investigated with scanning electron microscopy (SEM). The results indicated that the chemically bonded polyimide films exhibited better friction reduction and antiwear behavior compared to the polymide films on bare silicon surface. At a load of 0.5 N and sliding speed of 20 mm s⁻¹, the durability life of the polyimide thin films is over 25,000 sliding cycles and the friction coefficient is about 0.08.     

Synthesis and tribological behaviors of diamond-like carbon films by electrodeposition from solution of acetonitrile and water

Diamond-like carbon (DLC) films were prepared on silicon substrates by liquid phase electrodeposition from a mixture of acetonitrile and deionized water. The deposition voltage was clearly reduced owing to the presence of deionized water in the electrolyte by changing the basic properties (dielectric constant and dipole moment) of the electrolyte. Raman spectra reveal that the ratio of sp³/sp² in the DLC films is related to the concentration of acetonitrile. The surface roughness and grain morphology determined by atomic force microscopy are also influenced by the concentration of the acetonitrile. The UMT-2 universal micro-tribometer was used to test the friction properties of the DLC films obtained from electrolytes with different concentration. The results convey that the DLC film prepared from the electrolyte containing 10 vol.% acetonitrile has the better surface morphology and friction behavior comparing with the other. In addition the growth mechanism of the film was also discussed.     

碳氢烧蚀层的厚度对聚乙烯醇薄膜阻气性能的影响

 以ICF实验塑料靶丸保气的需求为背景,研究了水蒸气在低压等离子体聚合碳氢（CH）膜、聚乙烯醇（PVA）膜、聚苯乙烯（PS）膜中的扩散渗透行为。根据实验数据,算出在40℃ 90%相对湿度下,水蒸气在三种样品膜中的渗透系数分别为1.906×10^-13,5.950×10^-15,.432×10 ^-14mol·m/(m²·s·Pa),并借助多层复合膜模型的近似算法,算出在40 ℃ 90%相对湿度的外界环境下,类似多层塑料微球结构的三层复合膜中,PVA阻气层所处环境的相对湿度为53.06%,推导出PVA所处环境的相对湿度与外层CH层厚度的关系式。研究表明：CH烧蚀层越厚,PVA所处环境的相对湿度越小,保气性能越好。     

Mechanical properties of hexagonal boron nitride synthesized from film of Cu/BN mixture by surface segregation

Hexagonal boron nitride (h-BN) has a low friction coefficient and weak surface attractive force similar to graphite. Furthermore, while graphite is conductive, BN is a good insulator. These properties make it suitable for application like lubricating coating or as an insulator/buffer layer in electronic devices. The synthesize of h-BN layer by surface segregation phenomena and mechanical properties of the h-BN surface segregated on Cu substrate have been investigated. During in situ annealing, the surface segregation of BN occurred on Cu/BN film deposited by deposition process with a rf magnetron co-sputtering system. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis showed that though the h-BN layer synthesized was not covered whole area of substrate but the h-BN layers partially covered substrate. And the concentration of oxygen on the surface after exposure in air is decreased with increase of BN concentration. The topography of atomic forces microscopy (AFM) showed that h-BN phases surface segregated are discontinuous droplet shape. The force curves of AFM and friction force of lateral force microscopy (LFM) showed that the h-BN droplet surface segregated have very weak attractive force and low friction coefficient equal to h-BN sintered plate.     

生长初期Ta膜的表面动态演化行为

用磁控溅射方法在单晶Si衬底上沉积膜厚为15—250nm的Ta膜.基于原子力显微镜获得的薄膜表面形貌，用动力学标度理论量化表征薄膜表面动态演化行为.结果表明：当膜厚d<50nm时，薄膜生长指数β≈0.17，而d>50nm后β≈0.45；随着d增加，粗糙度指数α由0.24逐渐增加到0.69，且在d>50nm后趋于稳定.Ta膜的表面动态演化行为揭示了其由小岛聚合结构向连续膜演化的生长过程.与自阴影等非局域效应引起的非稳定行为不同的是，当d<50nm时，薄膜表面动态演化的非稳定行为来源于生长初期的小岛聚合，表面小岛沿膜面切向的生长优于沿法向的生长.随着d继续增加，薄膜以连续膜形式生长，表面动态演化趋于稳定.     

多层石墨烯作为液体石蜡添加剂的摩擦学性能

以膨胀石墨为原料,采用不同溶剂,通过液相超声直接剥离法制备多层石墨烯,利用透射电子显微镜、原子力学显微镜对其形貌进行了表征,在多功能往复摩擦磨损试验仪上研究了石墨烯石蜡分散体系的摩擦学性能。通过扫描电镜、能谱仪分析了磨痕形貌及表面元素组成。结果表明:多层石墨烯作为液体石蜡添加剂表现出良好的减摩抗磨性能,主要是因为多层石墨烯在磨损表面形成的物理吸附膜与摩擦化学反应膜的共同作用。     

Comparison of tribological properties of CrN, TiCN and TiAlN coatings sliding against SiC balls in water

CrN, TiCN and TiAlN coatings were deposited on WC cemented carbide disks using enhanced cathodic arc magnetron sputtering and their topographies and structures were observed and analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The friction and wear properties of CrN, TiCN and TiAlN coatings sliding against SiC balls in water were investigated and compared synthetically using ball-on-disk tribometer. The results showed that the CrN/SiC tribopairs showed the lowest friction coefficient of 0.076, while the TiCN/SiC tribopairs displayed the highest friction coefficient of 0.264. For the CrN/SiC tribopairs the specific wear rate of CrN coatings was lowest while that of SiC balls became highest. But for the TiAlN(TiCN)/SiC tribopairs, the specific wear rate of TiAlN coatings was highest while that of SiC balls for the TiCN/SiC tribopairs became lowest. This indicated that the friction and wear behaviors of nitride coatings/SiC balls tribo-systems in water were more strongly influenced by the anti-oxidative ability of tribomaterials in water than by their mechanical properties.     

Nanofluidics: viscous dissipation in layered liquid films

We studied the layer-by-layer collapse of molecularly thin films of a model lubricant confined between two atomically smooth substrates. The dynamics of the consecutive expulsion of four molecular layers were found to slow down with decreasing film thickness but showed no evidence for confinement-induced solidification. Using a hydrodynamic model, we show that the sliding friction of liquid layers on top of the solid substrates is approximately 18 times higher than the mutual friction between adjacent liquid layers. The latter was independent of film thickness and in close agreement with the bulk viscosity.     

掺硅类金刚石薄膜摩擦过程的分子动力学模拟

掺硅类金刚石(Si-DLC) 薄膜表现出优异的摩擦学性能, 在潮湿空气和高温中显示出极低的摩擦系数和很好的耐磨性, 但是许多实验表明Si-DLC膜的摩擦性能受其硅含量的影响很大. 因此, 本文采用分子动力学模拟的方法分别研究干摩擦和油润滑两种情况下不同硅含量的Si-DLC膜的摩擦过程. 滑移结果表明干摩擦时DLC膜和掺硅DLC膜之间生成了一层转移膜, 而油润滑时则为边界膜. 因此干摩擦时的摩擦力明显大于油润滑时的摩擦力. 少量添加硅确实能降低DLC膜的摩擦力, 但是硅含量大于20%后对DLC膜的摩擦行为几乎无影响. 干摩擦时硅含量对转移膜内键的数量影响很大, 转移膜内CC键和CSi键都先增加后减少, 滑移结束时几乎不含CSi键.     

Tribological Properties of MACs-APS Films

Multiply-alkylated cyclopentanes (MACs) with different molecular structure were deposited on single crystal silicon wafers coated with a thin aminopropyltrimethoxylsilane (APS) film as an adhesive layer to form MACs-APS films. The thickness, wetting behavior and nano-scale morphologies of the films were characterized by means of ellipsometry, contact angle measurement, and atomic force microscopy (AFM). The friction and wear behaviors of the thin films sliding against a Si₃N₄ ball were examined on a UMT-2MT tribometer in a ball-on-disk contact mode. The worn surfaces of the MACs-APS films and the counterpart Si₃N₄ balls were investigated with a scanning electron microscope (SEM). It was found that the water contact angles on the MACs-APS film increased with the MACs alkyl chain-length. The MACs-APS film exhibited higher load-carrying capacity and better friction reduction and anti-wear behavior as compared with the APS film. This is suggested to occur because the APS acts as a strongly bonded lubricant phase and MACs as a mobile lubricant phase in the MACs-APS film. The increase of the chain-length of the alkyl substituent in the MACs compounds resulted in improved tribological properties of MACs-APS film. It is suggested that the longer alkyl chains are much more flexible and can dissipate the mechanical energy during the shearing process more easily than the short chain compounds. MACs with the longer chains have stronger chain-chain interactions and the larger MAC molecules have stronger intermolecular interactions, resulting in the good tribological properties of MACs-APS film.     

Effects of confinement on static and dynamical properties of water

Molecular-dynamics results on water confined in a silica pore are reviewed and discussed in connection with experiments performed on water in Vycor and with studies of water in contact with proteins. The properties of confined water are studied as a function of both temperature and hydration level. The interaction of water in the film close to the substrate with the silica atoms induces a strong distortion of the hydrogen bond network. At high hydration levels a double dynamical regime is observed. At low hydration an anomalous diffusion is found upon supercooling with a transition from a Brownian to a non-Brownian regime on approaching the substrate in agreement with results found in studies of water in contact with globular proteins.Received: 1 January 2003, Published online: 14 October 2003PACS: 61.20.-p Structure of liquids - 61.20.Ja Computer simulation of liquid structure