Using self-assembly techniques for green lubrication solutions in tribological contacts

Abstract

This thesis presents a novel lubrication solution for silicon nitride hybrid bearings developed through the use of polymer brush technology, specifically brushes created using surface initiated atom transfer radical polymerisation. This work also details a novel testing regime utilising custom colloidal probes to replicate, for the first time, this hybrid bearing under atomic force microscopy in both dry and lubricated conditions. Due to their promising tribological properties polymer brushes have the potential to be a lubrication solution for the hybrid bearing system where current lubrication solutions are not tailored to the surfaces and contain harmful components such as sulphur and phosphorus. Polymer brush systems have generated considerable interest in the academic community as a possible new greener lubrication solution. To further understand the mechanism by which an effective polymer brush can be employed in a tribological contact this study was initiated. As the first known study to investigate the effect of the polymer brushes on the silicon nitride-steel contact, previous literature findings have been reapplied to a novel material for a novel application. Grafting from the silicon nitride surface ensures that less additive competition will occur. Poly(methyl methacrylate) (PMMA) brushes were chosen for the reduction of steric hindrance within the polymer chain therefore allowing a higher density brush and better load carrying capacity in a tribological sense. These brushes act synergistically with a poly-alpha-olefin, a high quality base oil lubricant present in the type of engine where these hybrid bearing operate. The synergy here refers to the swelling effect in which the anchored macromolecule and base oil work as one to repel the asperity contacts, reducing friction whilst the brush system protects itself. The formation of polymer brushes on a silicon nitride surface utilises atom transfer radical polymerisation (ATRP) and activators regenerated by electron transfer (ARGET) coupled with a surface initiation step. Initiating from the surface allows a strong covalent bond to the contact surface ensuring stability when the final brush is subjected to physical interactions, the main advantage being that by adding monomer molecules individually in situ the steric interaction of the chain-chain iterations in the growing brush is reduced so denser films can be formed, especially with small molecules such as MMA. By applying recent developments such as ARGET synthesis of the polymer brush is made much easier, as this technique allows reactions to occur with limited amounts of oxygen present as well as reducing the quantity of the copper complex needed for the reaction. By investigating the chemical and mechanical properties of the polymer brush with techniques such as ellipsometry, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS), it is possible to suggest explanations for the tribological properties of the polymer brush system. Detailed XPS analysis shows that bromine is still present at the surface, a key indicator that the functional sites of the polymer process are still available to be bonded for increasing the chain length while also indicating that fewer termination reactions had occurred. With a lack of silicon visible in the XPS sample spectra it is clear that the polymer has achieved good surface coverage and should therefore exhibit better tribological characteristics. By using novel, custom made stainless steel colloidal probes, it has been possible for the first time to replicate the hybrid contact on the nanoscale, which allows high quality testing by accurately replicating the materials in contact and thus an effective evaluation of the lubrication solution. The importance of the polymer thickness, measured by ellipsometry, and the liquid in which they are solvated, is clearly elucidated by testing in multiple fluids, when highly synergetic fluids like the poly-alpha-olefin result in a significant reduction in friction whereas poor solvents like water can even be detrimental when compared to the bare surfaces in contact. In the worst case scenario under the highest load using the novel probes the lubricated polymer brush reduced the friction force successfully from 3.3 nN to 1.3 nN when compared to the bare nitride surface. Preliminary work has been completed in respect to the transition from the nanoscale to the macroscale, and the polymerisation reaction has been scaled from 1 cm2 silicon nitride wafers up to a 10 cm diameter silicon nitride discs. One of the reasons why the polymerisation can be scaled in such a way is due to the ARGET technique which allows polymerisations to occur in the presence of limited amounts of air. A tribological study of these PMMA modified disc surfaces using a pin-on-disc setup shows favourable results and on average a reduction of friction of 15% when comparing PMMA modified surfaces with unmodified ones in an oil lubricated environment.

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ORCID for Ling Wang: orcid.org/0000-0002-2894-6784

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