The effect of density and surface topography on the coefficient of friction of polytetrafluoroethylene films
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.330
The effect of density and surface topography on the coefficient of friction of polytetrafluoroethylene films Mathew Brownell1 and Arun K. Nair1,2,* 1
Multiscale Materials Modeling Lab, Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR, USA.
2
Institute for Nanoscience and Engineering, 731 W. Dickson Street, University of Arkansas, Fayetteville AR-72701.
* Corresponding author, electronic address: [email protected]; Phone: +479-575-2573, Fax: +479-575-6982
ABSTRACT
Polytetrafluoroethylene (PTFE) film is observed to increase surface roughness during annealing. Longer annealing times leads to greater surface roughness. The coefficient of friction of PTFE film is affected by the shape of microscale sized particles on the film surface. In this study, we investigate the coefficient of friction of PTFE films using a coarse-grained molecular dynamics model based on experimental observations. We observe how the variation in PTFE chain length and film density affect the topography of PTFE films. We also investigate how these properties of PTFE, and the indenter radius affect the coefficient of friction observed during surface scratch. We find that short PTFE chain lengths create a dense film with greater particle spacing, but longer chains form a mesh structure which reduces the density and creates overlapping portions of particles in the film. We develop a convolutional neural network to classify PTFE film surface and predict the coefficient of friction of a modeled film based solely on the equilibrated film topography. The accuracy of the network was seen to increase when the density and images of internal fiber orientation were added as input features. These results indicate that the coefficient of friction of PTFE films in part is governed by the internal structure of the film. 1
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INTRODUCTION Polytetrafluoroethylene (PTFE) has been widely investigated for its low coefficient of friction [1-4]. Experiments have improved wear of PTFE by incorporating filler particles, intermediate surface coatings, and forming other PTFE composites, making PTFE a more viable solution for some mechanical applications [4-7]. More recent research on PTFE films showed that during annealing, PTFE film surface topography changes and begins to from spindle shaped particles, and the wear of the films also changes [8]. Molecular dynamics and density functional theory have been previously used to investigate nanoscale properties of PTFE films [9-11]. Using a recently developed coarse-grained (CG) model of PTFE, experimental length scales can be modeled and mechanical properties can be predicted based on PTFE particle shapes [12]. Using coarsegrained models, PTFE film properties can be studied from nanoscale to m
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