Nano-tribological characteristics of lanthanum-based thin films on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

Silane coupling reagent （3-mercaptopropyl trimethoxysilane （MPTS）） was prepared on silicon substrate to form two-dimensional Self-Assembled Monolayer （SAM） and the terminal -SH group in the film was in situ oxidized to -SO3H group to endow the film with good chemisorption ability. Thus, lanthanum-based thin films were deposited on oxidized MPTS-SAM to form rare earth composite thin films （RE thin films）, making use of the chemisorption ability of the -SO3H group. Atomic Force Microscope （AFM）, X-ray Photoelectron Spectrometry （XPS）, and contact angle measurements were used to characterize the RE thin films. Adhesive force and friction force of the RE thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. The results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. To study the effect of capillary force, tests were performed in various relative humidities. The results showed that the adhesive force of silicon substrate increased with relative humidity and the adhesive force of RE thin films only increased slightly with relative humidity. Research showed that surfaces with higher hydrophobic property reveal lowered adhesive and friction forces.

NANO-TRIBOLOGICAL STUDY ON A SUPER-HYDROPHOBIC FILM FORMED ON ROUGH ALUMINUM SUBSTRATES

A novel super-hydrophobic stearic acid(STA)film with a water contact angle of 166° was prepared by chemical adsorption on aluminum wafer coated with polyethyleneimine(PEI)film. The micro-tribological behavior of the super-hydrophobic STA monolayer was compared with that of the polished and PEI-coated Al surfaces.The effect of relative humidity on the adhesion and friction was investigated as well.It was found that the STA monolayer showed decreased friction,while the adhesive force was greatly decreased by increasing the surface roughness of the Al wafer to reduce the contact area between the atomic force microscope(AFM)tip and the sample surface to be tested.Thus the friction and adhesion of the Al wafer was effectively decreased by generating the STA monolayer, which indicated that it could be feasible and rational to prepare a surface with good adhesion resistance and lubricity by properly controlling the surface morphology and the chemical composition.Both the adhesion and friction decreased as the relative humidity was lowered from 65% to 10%,though the decrease extent became insignificant for the STA monolayer.

Nano-tribology through molecular dynamics simulations

The solidification and interfacial slip in nanometer-scale lubricating films as well as the contact and adhesion of metal crystals have been studied via molecular dynamics simulations. Results show that the critical pressure for the solid-liquid transition declines as the film thickness decreases, in-dicating that the lubricant in the thin films may exist in a solid-like state. It is also found that the interfa-cial slip may occur in thin films at relatively low shear rate, and there is a good correlation between the slip phenomenon and the lubricant solidification. The simulations reveal that a micro-scale adhesion may take place due to the atomic jump during the process of approaching or separating of two smooth crystal surfaces, which provides important information for understanding the origin of interfacial friction.

Progresses and problems in nano-tribology

The recent development in the four areas of nano-tribology is analysed. They are micro- and nano-friction and wear, micro- and nano-tribological engineering. The research methods in nano-tribology and thin film lubrication in which five branches are included: equivalent viscusity and flowing characteristics of micro fluid, the transition between ordered molecular film and disordered one, the essence of matrix influence on thin film lubrication, the lubrication mechanism of high polymer lubricants and the problem of superlubricity. Meanwhile, more than 35 problems are proposed which should be solved in the future.

Contact ratio and deformation of asperity in nano-partia lubrication

Partial lubrication or mixed lubrication in the nano-scale is discussed, which is consti-tuted from dry contact, boundary lubrication, thin film lubrication. A dynamic contact ratio has beenused to describe such lubrication, and the relationship between the contact ratio and its influencefactors was investigated. Experimental results indicate that the dynamic contact ratio increaseswith the decrease of film thickness by the exponential function. The decrease of speed and lubri-cant viscosity, and the increase of loads will enlarge the value of the contact ratio. When the polaradditives are added into the basic oil, the contact ratio decreases. In addition, the contact ratio ofthe surfaces with small roughness is larger than that of the surfaces with large roughness at verylow speed. However, the contact ratio of smooth surfaces decreases more quickly with speed thanthat of rough surfaces, and therefore, it will become smaller than that of rough surfaces after speedincreases over a certain degree.