Tribological transfer of polytetrafluoroethylene onto silicon oxide and polymer surfaces

Abstract

The main objective of this study is to understand the nature of tribological transfer of polytetrafluoroethylene (PTFE) onto counter thermally-oxidized silicon, polystyrene (PS), polyvinylchloride (PVC) and poly(methyl methacrylate) (PMMA) surfaces, as well as investigating the possible formation of chemical bonds arising at polymer-semiconductor and polymer-polymer interfaces, while or after tribological material transfer. Tuning the wettability characteristics of PTFE transferred surfaces is also aimed. Within these purposes, first part of the research focused on the preparation of silicon oxide and polymer substrates and the utilization of tribological transfer method in order to form desired PTFE patterns on these surfaces. The realization of this transfer was provided by the design of a simple rig to bring about a friction between the surfaces via sliding a piece of PTFE on silicon oxide and polymer specimens. In order to monitor the tribological interaction in a gradual manner as a function of increasing contact force, a very mild inclination (∼0.5◦) along the sliding motion was also employed in some specimens mounted on the inclined aluminum supports. In addition, procedures used to explore the stability of specimens against time and washing / cleaning practices using various organic solvents and boiling water are given within this part. In the second part of the work, characterization of PTFE-contacted silicon oxide and polymer surfaces was carried out using X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy and Atomic Force Microscopy (AFM). XPS results were obtained, which revealed that PTFE was faithfully transferred onto the silicon oxide and polymer surfaces upon even at the slightest contact; SEM and AFM images reinforced that stable morphological changes could be imparted onto the counter silicon oxide surfaces. In experiments where the inclined aluminum supports were used to create gradual tribological transfer of PTFE onto counter silicon oxide surfaces, use of relation between the increase in contact force with respect to transferred amount of PTFE helped us to estimate the minimum apparent contact pressure needed to realize the PTFE transfer, which was found to be about 5 kPa. Stability of the patterns imparted towards time and various chemical washing processes lead us to postulate that the interaction was most likely occurred with formation of chemical bonds. Contact angle measurements, which were carried out to monitor the wettability of the silicon oxide surface, showed that upon PTFE transfer the hydrophobicity of the SiO2 surface could be significantly enhanced, depending on the pattern sketched onto the surface. All of these findings show that tribological transfer of PTFE onto various counter surfaces is possible by a simple procedure, which has both academical and commercial importance.