Doctor of Philosophy
School of Mechanical, Materials and Mechatronics Engineering
Lin, Bingjing, Adsorption structure and tribological performance of aqueous copolymer lubricants on Si and Ti surfaces, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4266
Poly (propylene oxide)-poly (ethylene oxide)-poly (propylene oxide), PPO-PEO-PPO copolymer-based lubricant with phosphate ester (an extreme pressure additive) was proposed as a potential lubricant in metal forming. Before it is accepted and applied in industries, it is necessary to understand its lubrication behaviour. In this thesis, the adsorption structure and the tribological performance of this lubricant on Si and Ti surfaces was experimentally investigated.
To perform its lubrication functions, the copolymer and the additive must be adsorbed onto solid surfaces. Many experimental facilities have been employed to investigate the adsorption behaviour, such as Ellipsometry, atomic force microscopy (AFM) and neutron reflectometry (NR). The roles of PEO blocks and PPO blocks during the adsorption were discerned, and the influences of molecular structure, concentration and EP additive were measured. Results showed that hydrophobic PPO blocks anchored on the surfaces with PEO chains extending into the bulk liquid, and a stable copolymer film formed after adsorption. Additionally, the copolymer with higher hydrophobic contents formed thicker films on surfaces. When phosphate ester EP additive was added to the lubricant, phosphate head was adsorbed on surfaces by the electrostatic interaction, and PPO blocks of copolymer mixed with the phosphate ester to form an inner layer due to the hydrophobic interaction, while the PEO blocks of copolymer were driven into solution to form an outer layer.
The adhesion strength of lubricants to surfaces was detected by micro-scratch tests. Higher critical load, an indicator of better scratch resistance of copolymer film, was observed on surface covered by copolymer with a longer chain and higher weight percentage of PPO. The scratch tests results also showed that the EP additive significantly enhanced the adsorption strength of the lubricant film.
Finally, the tribological performance of lubricants was investigated by the pin-on-disc tribometer, where it was found that the hydrophobic PPO block played an important role in the lubricated contacts. Copolymer with a longer length of PPO chain was able to exhibit a lower friction. The addition of phosphate ester had a significantly effect on the lubrication, since it not only could reduce the friction, but also could provide more effective anti-wear behaviours.
The work in this thesis covered the main aspects of the adsorption and the lubrication of PPO-PEO-PPO copolymer-based lubricant with EP additive. The results offered an enlightened view on the mechanism of the adsorption behaviour and the tribological performance of the tri-block copolymer, and they can help to promote effective applications of this new lubricant in metal forming.