Year

2014

Degree Name

Doctor of Philosophy

Abstract

Boundary lubrication involves asperity contact and confined lubricant support. To understand the molecular origin of the friction and lubrication in boundary lubrication, it is necessary to investigate the related problems from the atomic perspective. In this thesis, molecular dynamics (MD) simulations were carried out to investigate boundary lubrication at nano-scale.

MD simulations of thin film lubrication at nano-scale aimed at the status of confined lubricant with a thickness of several angstroms. Lubricant of n-alkanes with different chain length and branch was introduced into the atomic flat surfaces with various liquid-solid interactions. Results showed that the friction force was a combined result of the interfacial slip, the properties of lubricant, and the confinement. With the decrease of film thickness, explicit layering structure occurred, such as the bi-layer structure without bridging molecules.

Lubricated asperity contact was crucial in boundary lubrication. The lubricated single asperity contact showed the dynamic process of lubricant being squeezed out at contacting interface. It demonstrated that long chain molecules can remain at the interface and minimize direct asperity contact. More importantly, it is the first time that a model of the lubricated 3D multi-asperity contact through MD simulation was carried out. The results showed the friction force depended on the surface roughness, the amount of lubricant, and adhesion force.

Finally, the flat surfaces with charge were used to investigate the effect of polar molecules (short chain PEO) on thin film lubrication, which observed the increase in friction due to the extra roughness from adsorbed molecules. For the charged rough surfaces, the firmly adsorbed PEO layers separated the asperities; hence the rough surfaces actually slid through two molecular layers, which resulted in a small friction force.

The work in this thesis covered the main aspects of boundary lubrication at nano-scale. The results offered an enlightened view on the mechanism of boundary lubrication, some of which can help to understand the experimental observation.

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