NANOTRIBOLOGICAL PROPERTIES OF ORGANIC BOUNDARY LUBRICANTS - LANGMUIRFILMS VERSUS SELF-ASSEMBLED MONOLAYERS

Frictional characteristics of several types of boundary lubricants wer e tested using scanning probe microscopy;(SPM). These include Langmuir monolayers of stearic acids (STA), their cadmium salts (STCd), self-a ssembling monolayers (SAMs) of alkylchlorsilanes, and complexes of STA with rigid naphthoylene benzimidazole (x-NBI) fragments. We observed that a Langmuir monolayer deposited on a silicon surface had a very lo w friction coefficient against a silicon nitride tip (about 0.01-0.05) but also low mechanical stability. SAMs were found to be much more st able but had the drawback of growth in the friction coefficient at hig h sliding velocities. Composite NBI/STA monolayers were much more stab le and were not damaged by the highest normal load applied. The fricti onal behavior of different monolayers was analyzed in relation to thei r structural organization (the type of tethering to the surface and pa cking density). We introduced a figure of merit (FOM) parameter which allowed comparison of frictional properties of very different lubrican t materials to those of the supporting substrate. For Langmuir monolay ers the FOM increased strongly with surface packing density whereas fo r SAMs and x-NBr/STA complexes it possessed a maximum at surface densi ties in the range 3.5-4.5 molecules per nm(2). Because of the possibil ity of tailoring the surface packing density of aliphatic tails in the complexes, they are a promising alternative to both LB films and SAMs . For such composite monolayers, the surface packing density can be op timized to give a desired frictional behavior.