Atomic scale investigation of interfacial friction: The role of chemical composition and structure.

摘要

Atomic force microscopy (AFM) has been used to study the interfacial frictional properties of well-characterized surfaces in order to establish the molecular scale origins of their frictional properties.; The atomic-scale studies involving SAMs have addressed a number of different issues related to the fundamental origins of friction. The influence of packing density/surface order of SAMs on frictional properties has been investigated by employing a unique series of spiroalkanedithiol molecules. The studies indicate that the more densely packed and highly crystalline films possess lower frictional properties. The SAMs terminated with aromatic C60 and phenyl terminal groups have been investigated and exhibit significantly higher frictional properties than methyl-terminated SAMs or graphite. In a separate study, methyl-, trifluorimethyl-, phenyl-, and isopropyl-terminated SAMs have been investigated as a function of chain length and have illustrated the contributions of terminal group orientation and interfacial molecular structure to wettability and friction properties.; The frictional properties of TiC(100), TiN(100) and VC('100) surface have been measured under ambient conditions. The TiC(100) surface exhibited inherently lower friction than the TiN(100) and the VC(100), regardless of the contacting material. A variation in the counterface composition (tip coating) revealed a clear dependence of frictional properties on interfacial composition, especially for TiN and VC where higher friction for interfaces composed of chemically similar materials was observed. In a separate study, ethanol adsorption on VC(100) surface under ultrahigh vacuum (UHV) conditions resulted in the reduction of frictional and adhesive properties due to chemical modification of the surface through the decomposition of ethanol.; AFM friction studies also demonstrated lower frictional and adhesive forces on hydrogen-terminated silicon surfaces as compared with oxide-covered surfaces, regardless of crystallographic orientation, dopant type, or level.; Finally, the influence of humidity and on the frictional properties of mica, silicon oxide, hydrogen-terminated silicon, and amorphous carbon was found to be systematically correlated with the hydrophilic character of the sample surfaces. The results indicate that multiple interfacial forces affect the contact asperities in the presence of condensed liquids and collectively determine the frictional properties of the interface under humid conditions.