Date of Graduation

12-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Nair, Arun K.

Committee Member

Zou, Min

Second Committee Member

Chen, Jingyi

Third Committee Member

Millett, Paul C.

Fourth Committee Member

Leylek, James H.

Keywords

adhesion; deformation mechanisms; friction; indentation test; scratch test; wear

Abstract

Polydopamine (PDA) has been shown to bond via covalent bonding, van der Waals forces, and hydrogen bonding and is known to adhere strongly to almost any material. The application of PDA between a substrate and a PTFE surface coating has resulted in low friction and a greatly reduced wear rate. Previous research probing the capabilities and limitations of PDA/PTFE films have studied the wear and mechanical properties of the film, but the overall adhesive and deformation mechanisms remain unclear.

In this research, we investigate the tribological properties of PDA and PTFE molecules and composites from the atomic to the microscale using computational modeling. Molecular dynamics is used to investigate the mechanical properties of individual PTFE chains. The elastic moduli of varying lengths of PTFE molecules are tested in vacuum and in water and at varying temperatures, showing how the chain length and the surrounding environment affect the elastic strength of PTFE molecules. The deformation mechanisms of a nanoscale PTFE film are observed during an indentation and scratch test, and various scratch angles are used during the scratch tests to elucidate the deformation mechanisms of individual PTFE chains within a film. Based on molecular dynamics simulations, a coarse-grained model of PTFE is developed which allows modeling of PTFE particles and films up to the microscale. Micrometer sized PTFE particles are then modeled which show that frictional values of PTFE are dependent on the surface topography.

Individual properties of PDA and PTFE molecules are investigated with density functional theory and molecular dynamics simulations. The adhesive properties of each molecule are tested as well as the deformation mechanisms. The primary source of adhesion between the PDA and PTFE molecules was observed to be van der Waals interactions, although, hydrogen bonding was also observed between PDA-PDA interactions. A PDA/PTFE thin film composite is studied, and an indentation and scratch test are performed to uncover deformation mechanisms. During scratch tests of the PDA/PTFE composite, a tenacious layer of PTFE is observed to adhere to the PDA substrate similar to experimental observations of PDA/PTFE composite films. Due to PTFE molecules penetrating the PDA substrate, and the unique deformation mechanisms of PDA oligomers, peeling is highly unlikely at the PDA/PTFE interface which increases wear resistance of the film.

To continue the investigation of PTFE films and particles, the coarse-grained model was used to investigate PTFE films annealed for 0-, 4- and 8-minutes. PTFE films were created using images of experimental PTFE films taken by atomic force microscopy. The properties of PTFE films are investigated to understand how the chain length and film density affect the formation of the film, and the coefficient of friction. A machine learning algorithm is developed and used to evaluate whether the numerous models created can affectively predict the coefficient of friction prior to testing. Friction was seen to be dependent on the internal fiber orientation of the films and not just the surface topography.

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