Friction Induced Plastic Deformation of High Polymer Surfaces
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University of Wollongong Research Online University of Wollongong Thesis Collection 1954-2016 University of Wollongong Thesis Collections 2004 Friction induced plastic deformation of high polymer surfaces Philip Gregory Whitten University of Wollongong Follow this and additional works at: https://ro.uow.edu.au/theses University of Wollongong Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of this work may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of the author. Copyright owners are entitled to take legal action against persons who infringe their copyright. 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For further information contact the UOW Library: [email protected] Friction Induced Plastic Deformation of High Polymer Surfaces A thesis submitted in fulfilment of the requirements for the award of the degree Doctor of Philosophy From University of Wollongong By Philip Gregory Whitten, B. Eng Department of Materials Engineering 2004 Certification I, Philip G. Whitten, declare that this thesis, submitted in fulfilment of the requirements for the award of Doctor of Philosophy, in the Department of Materials Engineering, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution. Philip G. Whitten 2004 Abstract Four standard glassy polymers are subjected to a classical single pass friction test by a spherical glass indenter. The plastic deformation of the polymers surfaces resulting from a single sliding pass was examined by scanning probe microscopy using intermittent contact mode. Debris were present on the surface of polymers with a low entanglement density following a single sliding pass. No debris were present on the surface of polymers with a high entanglement density following a single sliding pass. A relationship between entanglement density and debris production was confirmed by studying different crosslink densities of polystyrene. Localised plastic deformation occurring at the exposed surface region was proposed to explain the formation of debris. The friction of different crosslink densities of polystyrene was shown to be indifferent when measured by a glass indenter and by a silicon nitride scanning probe microscope tip. Micro-Raman spectroscopy was employed to detect molecular orientation in thin polymer films resulting from friction processes. i ii Acknowledgements The following people were instrumental to the formation of this thesis. Professor Hugh Brown for teaching me almost all I know about polymers and their physical properties. Hugh also provided a critical sounding board for all my poorly thought out explanations of what processes were occurring at the sliding interface. Dr. Robert Oslanec and Ms. Michaela Tymichova for teaching the preparation techniques of smooth polymer films. Robert also designed and built the friction apparatus that was subsequently modified for the research contributing to this thesis. Dr. Christian Fretigny for help with SPM imaging, and friction measurements. Christian motivated me when I was feeling lost in the PhD puzzle, and gave me the belief that I could finish the thesis. Mr. Bob Dejong for training soldering techniques and always helping when an electrical type problem occurred. Bob is a great person to have around the lab. Mr. Nick Mackie for helping me to develop a PS crosslinking procedure for the SEM. Mr. Daniel McCubbery for performing and interpreting XPS analysis. Ms. Lorelle Pollard for her wisdom. Dr. Greg Tilman for help with sample preparation for optical microscopy. Engineering workshop for milling the aluminium cantilever used within the friction apparatus. Finally, my permanent girlfriend Belinda Bright for supporting me, and having so much faith that I would complete this thesis. Belinda would not let me loose focus of why I had wanted to obtain a PhD, and provided the rationale thoughts when I was struggling. iii iv Contents 1 Introduction .............................................................................. 1 2 Polymer friction – a literature review ..................................... 3 2.1 Historical perspective ...............................................................................4 2.2 Contact mechanics of friction ...................................................................7 2.3 Pressure dependence of friction.............................................................11 2.4 Plastic deformation during friction processes .........................................14 2.5 How do sliding surfaces move relative to each other? ...........................28 3 Experimental........................................................................... 35 3.1 Objectives ..............................................................................................35 3.2 Polymers studied....................................................................................35 3.3 Preparation of polymer samples for friction experiments........................37 3.4 Bulk polymer sample preparation...........................................................39 3.5 SEM - Crosslinking technique ................................................................39 3.6 Friction apparatus ..................................................................................41 3.7 Scanning probe microscopy (SPM)........................................................44 3.8 X-ray photoelectron spectroscopy (XPS) ...............................................46 3.9 Bulk polymer testing (tensile testing, DSC and DMA) ............................46 3.10 Micro-Raman spectroscopy....................................................................47 4 Results .................................................................................... 49 4.1 Macroscopic friction measurements.......................................................49 4.2 Plastic deformation of polymer surfaces.................................................60 4.3 PMMA film thickness and friction .........................................................101 4.4 Friction force dependence on surface deformation ..............................103 5 Discussion ............................................................................ 109 5.1 Analysis of the macroscopic friction data .............................................109 5.2 Analysis of the plastic deformation of the rubbed surfaces ..................117 5.3 Polymer film thickness and friction .......................................................153 5.4 Critical shear stress required to plastically deform a polymer surface..154 5.5 Contact between a smooth glass indenter and a polymer....................156 5.6 Strain localisation and the sliding interface ..........................................163 6 Conclusion............................................................................ 165 7 Recommendations ............................................................... 167 v Appendix A. Dimensions of the double cantilever used in the friction experiments....................................................................169 Appendix B. Physical properties of the indenters and the substrates used in the calculation of the Hertzian Contact areas. 170 Appendix C. Parameters used for fitting XPS analysis.........171 vi List of Tables TableUT 2-1. Comparison of thin film (~200 nm) and bulk friction coefficients.[16]T .........6 TableT 2-2. Values of T τ andT T α T taken from the literature.T ...............................................13 TableT 2-3. Critical conditions for surface cracking of glassy polymers exposed to a 6.3 T mm T radius steel indenter sliding at a speed of 1.5 T mm min .[36]T T .................20 TableT 3-1: Polymers studied within this thesis.T .............................................................36 TableT 4-1. Quantitative elemental analysis (XPS) of different indenters as used in the friction experiments.T .........................................................................................57 TableT 4-2. Quantitative elemental analysis (XPS) of the indenters presented in Table 4-1 following argon ion etching.T ...................................................................58 TableT 4-3. The surface roughness of the same T M w T 200,000 T g mol T PS sample following annealing