Date of Graduation

5-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Min Zou

Committee Member

Jingyi Chen

Second Committee Member

Paul Millet

Third Committee Member

Xiangbo Meng

Fourth Committee Member

Han Hu

Keywords

Friction, Laser Surface Texturing, Polytetrafluoroethylene Coatings, Tribology, Wear

Abstract

Polytetrafluoroethylene (PTFE) is a widely used polymer that has unique properties such as chemical and temperature resistance, corrosion resistance, low maintenance cost, and a very low coefficient of friction (COF). These distinctive properties have made PTFE a promising candidate for industrial applications where surfaces are in relative motion. PTFE can be utilized as a thin film solid lubricant to significantly reduce the friction between moving surfaces. However, PTFE coatings have low cohesion and poor adhesion to the substrate resulting in its low wear resistance. This fact limits the application of PTFE coatings in load-bearing situations. To address these limitations, an adhesive polydopamine (PDA) underlayer was used to increase PTFE coating adhesion to stainless steel substrates and the resulting coating was labeled as PDA/PTFE. To prolong the wear life of PDA/PTFE coatings on stainless steel substrates, the coating cohesion and adhesion was improved by incorporating three different low concentration of Cu-SiO2 core-shell nanoparticles (NPs) into the PTFE coatings. Also, the Hilbert curve patterns were laser textured on both the hard stainless steel substrate and the soft PTFE coating to improve the wear life. The mechanical and tribological properties of PDA/PTFE + Cu-SiO2 NP coatings were investigated, and the wear mechanisms of smooth and laser-textured PDA/PTFE coatings on smooth and laser-textured stainless steel substrates were analyzed. The effects of laser power and texture density on the tribological behavior of PDA/PTFE coatings were also studied. Adding low concentrations of Cu-SiO2 core-shell NPs into the PTFE top layer increased the wear life of PDA/PTFE coatings, prevented the global delamination inside the wear track, improved the surface morphology of the coatings, and made the coating more coherent and interconnected and less porous. Also, the adhesion of the transferred film to the counterface ball was improved after adding the NPs. Laser texturing the Hilbert curve pattern on the surface of stainless steel substrates provided reservoirs to store the PTFE lubricant and replenish it inside the wear track. Also, laser texturing the stainless steel substrate prevented the global delamination by providing interlocking for the PTFE coating. Altogether, it significantly increased the wear life of PDA/PTFE coatings. Higher laser powers contributed to deeper and wider texture grooves, providing larger reservoirs for PTFE lubricant. Also, laser texturing the PTFE coating made it softer without changing its chemical properties, and the softer coating experienced better compaction during the tribology tests. Therefore, laser-textured PTFE coating had much longer wear life, did not experience global delamination, and showed better adhesion at higher loads during the nanoscratch tests. Overall, this dissertation provides different methods to improve the wear resistance of PTFE coatings which increases their wear life. These advancements can result in cost savings and improved performance for industries that use PTFE coatings.

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