ORGANIC MOLECULAR FILMS UNDER SHEAR FORCES - FLUID AND SOLID LANGMUIRMONOLAYERS

Scanning probe microscope observations of monolayers of a classic boun dary lubricant, stearic acid (STA), reveal long-range dynamics of wear and reconstruction of monomolecular films under the shear forces caus ed by the sliding tip. The STA monolayer in a fluid state displays a f low of material from the worn area and its redistribution resulting in multilayer formation within the range of 80 mu m. Surface diffusion o f mobile organic material is responsible for the observed long-range e ffects of the local shear stresses produced within the contact area. S olid and fluid monolayers have very different velocity dependencies of the friction forces. For solid monolayers, we observe a monotonic inc rease of the friction forces with velocity rising from 0.02 to 1000 mu m/s. In contrast, for the fluid STA monolayers the friction forces be have nonmonotonically with a maximum value around 0.2 mu m/s. We obser ve significant compression of the STA monolayers under the tip reachin g 35% of initial thickness before the fatal damaging. The observed com pression can be related to the collective tilting of the molecules und er normal loads due to a formation of gauche conformers in alkyl chain s. Estimated Young's modulus is in the range of 0.2-0.7 GPa for very s mall deformations (<3% of thickness compression). However, the elastic modulus decreases sharply to tens of MPa at higher deformations.