Date of Graduation

7-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Min Zou

Committee Member

Paul Millett

Second Committee Member

David Huitink

Third Committee Member

Xiangbo Meng

Fourth Committee Member

Jingyi Chen

Keywords

Lubrication, Nanocomposite, PFQNM, Polydopamine, PTFE, Tribology

Abstract

Polytetrafluoroethylene (PTFE) is a popular low friction solid lubricant with high chemical and thermal stability. Thick PTFE coatings have the potential for many tribological applications, such as replacing Tin-based Babbitt materials in journal bearings. However, the weak bonding strength to the substrate and the high wear rate of PTFE coatings are current limiting factors. The lack of understanding of their tribological properties and wear mechanisms in oil-lubricated conditions and how coating thickness affects the tribological performance further hindered the use of PTFE coatings. In this dissertation, polydopamine (PDA), a bio-inspired adhesive, is used as an underlayer for or as a constituent of PTFE coatings to improve the tribological properties of the resulting PDA/PTFE and PDA + PTFE composite coatings. The tribological properties and wear mechanisms of the coatings were studied in dry and oil-lubricated conditions. The wear mechanisms were investigated and correlated with their nanomechanical properties.

The durability of the PDA/PTFE coatings increased drastically when the thickness is over 34 µm due to the reduction in contact pressure. The 42 µm-thick PDA/PTFE coating was four times more durable in dry condition than the PTFE coating of similar thickness due to the better adherence to the substrate and higher load-carrying capacity. PDA mixed with PTFE helped prevent the detachment of PTFE coating from the substrate and improved the durability by 11 times. Hot compaction further increased the durability of the PDA+PTFE coating by 2.6 times by reducing the porosity and preventing delamination. In addition to better adhesion to the SS substrate and better load-carrying capacity, reduced porosity and roughness due to the hot-compaction prevented the local and global deformation of the compacted PDA+PTFE coating during wear tests, which in turn improved the durability of the coating.

Mixing PDA with PTFE increased Young's modulus of the PTFE coating from 0.61 to 0.97 GPa. Adding Cu nanoparticles (NPs) further increased Young's modulus of the PDA+PTFE coating and decreased the adhesion force between the AFM probe and coating surface. The PDA and PTFE particles cross-link themselves during the annealing process, and the presence of Cu NPs helps further cross-linking between the PDA and PTFE particles. The PDA+PTFE coating showed a 62.5% less COF in oil-lubricated conditions than in dry conditions. Moreover, the PDA+PTFE coating is five times more durable than PTFE coating in boundary oil-lubricated conditions. The delamination dominated the wear mechanism of the PDA+PTFE coating in the coating due to the presence of porous and non-annealed PDA+PTFE coating underneath the compacted top surface. The addition of 0.25 wt% of Cu NPs further improved the durability of PDA+PTFE coatings by 60% in boundary oil-lubricated conditions. The improved durability of the PDA+PTFE+0.25 wt% Cu NPs coating can be attributed to the lower adhesion force to the counterface, enhanced cross-linking between the PTFE and PDA in the presence of Cu NPs, better adherence to the substrate, and the better load-carrying capacity of these coatings. The addition of Cu NPs also improved the thermal conductivity of the PDA+PTFE coating by 12%. This research showed that the PDA can enhance the durability and adherence of the PTFE coatings to substrate, which open the avenue to investigate further the potential of the PDA+PTFE coatings in journal bearing applications.

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