Molecular Tribology

Abstract

Surface-active agents play an important role in lubrication technology and are often used as additives in liquid lubricants films to reduce the friction and wear. Under high loads, in asperity contacts or when there is no relative motion between two surfaces, fluid lubrication breaks down and boundary lubricants are essential to prevent wear and seizure. In other applications where the use of ‘thick’ films of liquid lubricants is not possible or desirable (i.e. micromotors, hard disk drives), lubrication between contacting surfaces is exclusively in the boundary regime. Despite the use of boundary lubricants in engineering applications for centuries, our understanding of how boundary lubricants work at the molecular level remains unclear. My thesis describes the use of total internal reflection (TIR) Raman scattering to characterise model boundary lubricants both ex situ
and in situ, under realistic conditions of pressure and shear.

The model systems comprise either Langmuir-Blodgett (LB) monolayers of long chain fatty acids (e.g. Zn arachidate) and phospholipids (e.g. DPPC) deposited on silica and SF10 glass, or phospholipid bilayers (e.g. DMPC) fused to silica and SF10 glass surfaces in water. TIR Raman scattering is a form of vibrational spectroscopy with sub-nanometer sensitivity and spatial resolution of a few microns. Control of the polarisation of the incoming and scattered light allows us to probe the orientation of adsorbed molecules and how that orientation changes under pressure and shear. The resonant frequency and intensity of different molecular vibrations is also sensitive to the packing and conformational order in the lubricant film. LB monolayers of Zn arachidate and DPPC are first characterised ex situ and then subjected to increasing load (upto ~750 MPa) in a contact between a fused silica ball and the flat surface of an SF10 hemisphere. A better
packing or a higher orderliness of the molecules are observed at higher pressure without the monolayers being squeezed out. In contrast, application of load to the DMPC bilayer appears to squeeze some of the lipid materials out of contact. The designs of two Raman tribometers are described that allow Raman measurements in a sheared contact with simultaneous measurements of friction and load. Elasto-hydrodynamic and boundary lubrication regimes are studied with the tribometers overcoming the engineering
difficulties up to a significant extent.