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Research Collection Research Collection Doctoral Thesis Surface modifications for improved aqueous lubrication under low-contact-pressure conditions Author(s): Heeb, Raphael Emanuel Publication Date: 2009 Permanent Link: https://doi.org/10.3929/ethz-a-006001889 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library DISS. ETH No. 18431 Surface Modifications for Improved Aqueous Lubrication under Low-Contact-Pressure Conditions A dissertation submitted to ETH ZURICH for the degree of Doctor of Sciences (Dr. sc. ETH Zurich) presented by RAPHAEL EMANUEL HEEB Dipl. Werkstoff-Ing. ETH born on January 16, 1980 citizen of Altst¨atten(SG) accepted on the recommendation of Prof. Dr. Nicholas D. Spencer, examiner Prof. Dr. Seunghwan Lee, co-examiner Prof. Dr. Hugh Spikes, co-examiner 2009 FOR KATHRIN AND MY FAMILY The scientist is not a person who gives the right answers, he's one who asks the right questions. CLAUDE LEVI-STRAUSS´ Abstract The reduction of the interfacial friction between two surfaces in relative motion is a prerequisite for the proper functioning of many systems, ranging from machine parts to human joints. While the lubrication of two contacting surfaces primarily aims at the reduction of friction and wear, either by a separation of the surfaces by means of a fluid film or by introducing a layer of low shear strength between them, additional requirements such as the environmental compatibility of a lubricant or the energy efficiency of tribological systems have become important during recent years. From this perspective, the substitution of traditional oil-based lubricants with aqueous systems would be highly beneficial for specific tribological systems. Although wa- ter cannot significantly increase its inherently low viscosity under pressure, Nature demonstrates that aqueous lubrication is feasible and highly effective under certain circumstances. The objective of this thesis is to investigate surface modifications that promote the reduction of the macroscopic interfacial friction in aqueous environments. With the focus on mild contact-pressure conditions, it was feasible to investigate strongly attached self-assembled monolayers (SAMs) as well as polymer brushes with respect to their aqueous lubricating properties on a macroscopic scale. In the first part of the thesis, the basic surface-chemical as well as structural pa- rameters that were considered important in aqueous lubrication, were investigated by means of different thiol SAMs on gold surfaces. The setup developed for tribo- logical experiments consisted of a conventional pin-on-disk tribometer with a soft elastomeric rather than a rigid slider, which additionally allowed for the ex situ inves- tigation of the tribologically stressed area by means of spectroscopic techniques. The macroscopic sliding friction as experienced from alkanethiol SAMs against an oxi- vii viii dized and therefore hydrophilic poly(dimethyl siloxane) (ox-PDMS) slider revealed that surface-chemical as well as structural properties of the SAMs are parameters that largely influence the aqueous lubrication performance. The spectroscopic anal- ysis of the sample areas that were exposed to tribological stress further revealed that the SAMs remain intact after pin-on-disk experiments. Based on these preliminary findings, the experimental work was extended in two different directions. Firstly, the aqueous lubrication performance of monolay- ers formed from hydrophilic PEG thiols with two different molecular weights was compared to that of previously employed alkanethiol SAMs. Subsequently, the so- lution parameters of the aqueous lubricants, such as pH and ionic strength, were examined with regard to their influence on the lubrication performance of the mono- layers. Besides the tribological characterization, changes in conformation as well as in the hydration properties of the monolayers were investigated. These results re- vealed that hydrophilic, PEG-based monolayers with a high molecular weight serve as promising lubricant additives in various aqueous environments. The acquired knowledge about the parameters influencing the aqueous lubri- cating properties was also transferred to other technologically important systems. In an attempt to further enhance the lubricity of silicon oxide surfaces in aqueous media, high-surface-density poly(methacrylic acid) (PMAA) brushes were prepared by means of a novel \grafting from" approach. The utilization of an ultraviolet light emitting diode (UV-LED) for the surface-initiated photopolymerization led to high-molecular-weight polyelectrolyte brushes within relatively short irradiation times and rendered tedious cleaning steps unnecessary. Macroscopic pin-on-disk experiments involving PMAA brushes under mild contact-pressure conditions re- vealed undetectably low friction coefficients and very good long-term stabilities of the longer brushes. Together with the detailed analysis of bulk and surface properties of the em- ployed PDMS tribopairs, this study has systematically investigated the aqueous lubrication performance of different surfaces under low contact-pressure conditions. It was demonstrated that the interfacial friction in an aqueous environment is de- pendent on surface-chemical and structural properties of both surfaces as well as on their interaction with the lubricant. Zusammenfassung Die Minimierung der Grenzfl¨achenreibung zwischen zwei sich kontaktierenden Ober- fl¨achen in Relativbewegung ist eine Voraussetzung f¨urdas einwandfreie Funktion- ieren vieler Systeme, von Maschienenbauteilen bis hin zu menschlichen Gelenken. W¨ahrenddie Schmierung zum Ziel hat, die kontaktierenden Oberfl¨achen mittels Schmierfilm oder einer geeigneten Schicht mit niedriger Scherfestigkeit zu separi- eren, sind in den letzten Jahren zus¨atzliche Anforderungen an Schmiermittel wichtig geworden, zum Beispiel deren Umweltvertr¨aglichkeit oder die Energieeffizienz des ganzen tribologischen Systems. Unter diesem Aspekt w¨aredie Substitution von tra- ditionellen, ¨olbasiertenSchmiermitteln mit w¨assrigenSystemen f¨urspezifische tri- bologische Systeme vorteilhaft. Obwohl Wasser seine geringe Viskosit¨atunter Druck nicht massgeblich erh¨ohenkann, demonstriert die Natur, dass w¨assrigeSchmierung unter gewissen Umst¨andensehr effizient ist. Das Ziel dieser Dissertation war die Untersuchung von Oberfl¨achenmodifikationen zur Reduktion der makroskopischen Grenzfl¨achenreibung in w¨assrigerUmgebung. Da der Fokus auf milde Kontaktdruckbedingungen gerichtet war, wurde die Unter- suchung der w¨assrigenSchmiereigenschaften von fest adsorbierten, sich selbst or- ganisierenden Monoschichten und b¨ursten¨ahnlichen Polymeren auf makroskopischer Ebene m¨oglich. Im ersten Teil dieser Dissertation wurden anhand monomolekularer Schichten von Thiolen auf Goldoberfl¨achen diejenigen Parameter untersucht, welche f¨urdie w¨assrigeSchmierung als wichtig erachtet wurden. Die daf¨urentwickelte experi- mentelle Apparatur bestand aus einem herk¨ommlichen Stift-Scheibe-Tribometer mit einem elastischen anstatt eines harten Stifts, womit im Anschluss an die Reibungs- experimente die spektroskopische Charakterisierung der triologisch beanspruchten ix x Fl¨ache m¨oglich wurde. Aus der makroskopischen Gleitreibung zwischen Alkanthiol Monoschichten und oxidierten und deshalb hydrophilen Poly(dimethyl siloxan) (ox- PDMS) Oberfl¨achen wurde deutlich, dass oberfl¨achenchemische sowie strukturelle Eigenschaften der Monoschichten deren w¨assrigeSchmiereigenschaften massgeblich beeinflussen. Die spektroskopische Analyse der tribologisch beanspruchten Proben- fl¨achen ergab, dass die Monoschichten nach dem Reibungsexperiment intakt blieben. Basierend auf diesen Resultaten wurden die Experimente in zwei unterschiedliche Richtungen ausgedehnt. Zuerst wurden die Schmiereigenschaften von zwei hy- drophilen Poly(ethylenglykol) (PEG) Monoschichten mit unterschiedlichem Moleku- largewicht mit denjenigen der Alkanthiolschichten verglichen und danach wurde untersucht, welchen Einfluss verschiedene w¨assrigeSchmiermittel auf die Schmier- leistung der verschiedenen Monoschichten haben. Neben der Charakterisierung der tribologischen Eigenschaften wurden die Monoschichten auch auf Unterschiede in deren Konformation sowie in deren Hydrierung untersucht. Diese Resultate haben aufgezeigt, dass hydrophile PEG-basierte Monoschichten mit einem hohen Moleku- largewicht vielversprechende Schmieradditive f¨urzahlreiche w¨assrigeUmgebungen darstellen. Im weiteren Verlauf dieser Dissertation wurden die gewonnenen Kenntnisse ¨uber die Parameter, welche die w¨assrigen Schmiereigenschaften beeinflussen, auf weitere technologisch wichtige Materialien ¨ubertragen. In einem Versuch, die Schmiereigen- schaften von Silikonoxidoberfl¨achen in einer w¨assrigenUmgebung zu verbessern, wurden diese mittels Poly(methacryls¨aure)B¨urstenmit einer hohen Oberfl¨achendich- te modifiziert, wobei eine neue Pfropfmethode angewendet wurde. Durch die Ver- wendung einer Ultraviolet Leuchtdiode (UV-LED) zur oberfl¨acheninitiierten Pho- topolymerisation von Polyelektrolytb¨urstenwurden hohe Molekulargewichte mit relativ kurzen Belichtungszeiten erreicht. Zudem wurden langwierige Reinigungss- chritte nach der Polymerisation ¨uberfl¨ussig. Makroskopische Reibungsexperimente mittels Stift-Scheibe-Tribometer mit Poly(methacryls¨aure)B¨urstenunter milden Kontaktdruckbedingungen haben zu unmessbar niedrigen Reibungskoeffizienten und zu sehr guter Langzeitstabilit¨atder
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