Investigation On The Tribological Behavior Of The Alkylsilane Self-Assembled Molecular Films

The investigation of self-assembled molecular films is of great importance not only in theoretical aspects, but also for application in micro/nano-electromechanical systems (MEMS/NEMS). In the present work, the focus are fluorinated and non-fluorinated alkylsilane molecular films, which currently serve as lubricants in micro/nano devices. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurement are used to show the morphology, component and wettability of the films. The tribological properties of the perfluo and non-perfluo self-assembled molecular films have been studied and compared detailedly by a universal micro-tribometer (UMT-2). Also, the difference of growth mechanism and friction between single and mixed self-asembled molecular films have been analysed.The roughness of the substrates places a significant dependence on the wettability and frictional characters of the molecular films. Since the alkylsilane self-assembled molecular films are hydrophobic, once the smooth silicon substrates were replaced by porous silicon, the water contact angles of these films could exceed 160o, which meant they were super-hydrophobic. The reason is that the contact area between the water drop and the surface of porous silicon is a composite area, which is consisted of solid-liquid and air-liquid portion. Furthermore, the air-liquid portion dominates over the whole interface, therefore the alkylsilane molecular films modified porous silicon surfaces are super-hydrophobic.The characters of the fluorinated alkylsilane molecular films are different from the non-fluorinated ones. The fluorinated films have lower surface tension than the non-fluorinated ones which leads to the higher water contact angles. It is known that perfluocarbon chains adopt a helical conformation and consequently possess larger cross sections than the simple hydrocarbon chains. And perfluorocarbon chains are more rigid than hydrocarbon chains, resistant to oxidation and corrosion. Hence the fluorinated molecular films have orderly structure and little defects, which make them good candidates to decrease friction. The carbon chain length could influence the frictional properties of the alkylsilane molecular films prominently. Due to the long chain length, the Van der Waals interactions between the molecular films with longer carbon chain length are stronger than the films with shorter ones, which leads to orderly film structure and low friction coefficient.The growth mechanism of the single component fluorinated alkylsilane molecular films and the mixed ones consisted of fluorinated and non-fluorinated alkylsilane monolayers have been investigated in this paper. The experimental results showed that the growth of the single component SAMs on the silicon substrates was faster than the mixed ones. Due to the more ordered and compact film structure, the the roughness and the friction coefficients of the single component alkylsilane molecular films were lower than the ones of the mixed self-assembled monolayers. In addition, the single molecular films were more hydrophobic than the mixed ones.