Tribology Of Self-assembled Monolayers

List of FiguresList of FiguresPageNumberFigure 1.1Research methodology followed in the research studies.10Figure 2.1Schematic of typical SAM molecule structure andattachment with substrate.25Figure 2.2General classification of the wear of polymers [Briscoeand Sinha 2002].30Figure 2.3Schematic representations of wear mechanisms (N:normal load; V: sliding velocity). (a) Adhesive wear. (b)Abrasive wear.31Figure 2.4A schematic diagram of contact angle measurement.36Figure 3.1Experimental apparatus. (a) Dip-coating machine. (b)Clean air furnace.44Figure 3.2Optima contact angle measurement set-up.46Figure 3.3Experimental instruments. (a) Optical microscope set-up.(b) Ball-on-disk tribometer. (c) Ball-on-disk tribometerstage.47Figure 3.4Ball-on-disk tribometer schematic (R: Track radius; r:Ball radius; F: Normal load; ω: Rotational speed of thedisk).53Figure 4.1Step-height measurement method.55Figure 4.2Measured water contact angle values for differentspecimens. (a) Ti6Al4V. (b) Ti6Al4V/O 2 plasma treated.(c) Ti6Al4V/O 2 plasma treated/UHMWPE. (d)Ti6Al4V/O 2 plasma treated/UHMWPE/PFPE.57Figure 4.3Surface morphology of Ti6Al4V/UHMWPE surfaceusing FESEM. (a) At lower magnification, 100x. (b) Athigher magnification, 500x.57xiii

List of FiguresPageNumberFigure 4.4AFM morphology of surfaces. (a) Polished Ti6Al4Valloy surface (scan area: 40µm 40µm, vertical scale:1µm). (b) Ti6Al4V/UHMWPE surface (scan area:40µm 40µm, vertical scale: 5µm).58Figure 4.5FTIR-ATR spectrum of the UHMWPE coating onTi6Al4V substrate.59Figure 4.6XPS spectra of specimens. (a) UHMWPE coating onTi6Al4V. (b) UHMWPE/PFPE coating on Ti6Al4V.60Figure 4.7Variation of friction coefficient as a function of theslidingcycles(forbareTi6Al4VandTi6Al4V/UHMWPE) using Si 3 N 4 ball as the counterface(track radius: 3 mm, normal load: 0.5 N, spindle speed:200 rpm).62Figure 4.8Variation of friction coefficient vs. sliding cycles (forbare Ti6Al4V and Ti6Al4V/UHMWPE) using Si 3 N 4 ballas the counterface (track radius: 2 mm, normal load: 4 N,spindle speed: 400 rpm).62Figure 4.9Wear track morphology. (a) FESEM morphology of weartrack for bare Ti6Al4V alloy for high normal loadtribology test (track radius 2 mm, normal load 4 N,spindle speed 400 rpm) after 1,000 cycles,magnification 60X. (b) FESEM morphology of weartrack for Ti6Al4V/UHMWPE specimen for high loadtribology test after the completion of 175,000 slidingcycles, magnification 80X. (c) AFM surface morphologyinside the wear track for Ti6Al4V/UHMWPE specimenfor high load tribology test after the completion of175,000 cycles (scan area: 40µm 40µm, vertical scale:500 nm).64xiv

List of FiguresPageNumberFigure 4.10Wear track and counterface analysis. (a) EDX analysis ofwear track for high normal load tribology test (trackradius 2 mm, normal load 4 N, spindle speed 400rpm) after completion of 175,000 cycles. (b) EDXanalysis of Ti6Al4V surface without polymer coating. (c)Optical image of Si 3 N 4 ball for high normal loadtribology test after completion of 175,000 cycles, 100X.(d) Optical image of Si 3 N 4 ball after cleaning withacetone for high normal load sliding tribology test aftercompletion of 175,000 sliding cycles, 100X.65Figure 4.11Effect of PFPE overcoat on wear life (track radius 2mm, normal load 4 N, spindle speed 1000 rpm).66Figure 5.1Water contact angle measurement from representativesamples. (a) Ti6Al4V. (b) Ti6Al4V (after O 2 plasmatreatment). (c) Ti6Al4V/PFPE. (d) Ti6Al4V/PFPE (heattreated).72Figure 5.2Water contact angle measurement from GPTMS/PFPE. (c) Ti6Al4V/GPTMS/PFPE(heat treated).72Figure 5.3AFM imaging (scan area: 5µm 5µm, vertical scale: 100nm). (a) Ti6Al4V. (b) Ti6Al4V/PFPE. (c) l4V/GPTMS/PFPE. (f) Ti6Al4V/GPTMS/PFPE(heat treated).74Figure 5.4Wide scan XPS spectra of Ti6Al4V/GPTMS specimens.76Figure 5.5Comparison of C1s peaks for bare Ti6Al4V/GPTMS andTi6Al4V in C1s scan.76Figure 5.6Wear durability (number of sliding cycles before failure)of tested specimens in the study.77Figure 5.7Variation of friction coefficient as a function of thesliding cycles ((a) Ti6AL4V/PFPE, (b) Ti6AL4V/PFPE(heat treated) and (c) bare Ti6Al4V) using Si 3 N 4 ball asthe counterface (track radius: 2 mm, normal load: 0.2 N,spindle speed: 200 rpm).78xv

List of FiguresPageNumberFigure 5.8Variation of friction coefficient as a function of thesliding cycles ((a) Ti6AL4V/GPTMS/PFPE, (b)Ti6AL4V/GPTMS/PFPE (heat treated) and (c)Ti6Al4V/GPTMS) using Si 3 N 4 ball as the counterface(track radius: 2 mm, normal load: 0.2 N, spindle speed:200 rpm).79Figure 5.9Optical micrographs of Ti6Al4V specimen’s wear trackand counterface surface after completion of 5,000 slidingcycles. (a) Wear track, magnification 50x. (b)Counterface, magnification 100x.80Figure 5.10Optical micrographs of Ti6Al4V/GPTMS specimen’swear track and counterface surface after completion of5,000 sliding cycles. (a) Wear track, magnification 50x.(b) Counterface, magnification 100x.81Figure 5.11Optical micrographs of Ti6Al4V/PFPE specimen’s weartrack and counterface surface after completion of 10,000sliding cycles. (a) Wear track, magnification 50x. (b)Counterface, magnification 100x.81Figure 5.12Optical micrographs of Ti6Al4V/PFPE (heat treated)specimen’s wear track and counterface surface aftercompletion of 10,000 sliding cycles. (a) Wear track,magnification 50x. (b) Counterface, magnification 100x.Figure ’s wear track and counterface surface after100,000 sliding cycles. (a) Wear track, magnification 50x.(b) Counterface, magnification 100x.8282xvi

List of NotationsList of NotationsADLC: Amorphous diamond-like carbonAFM: Atomic force microscopyASTM: American society for testing and materialsCVD: Chemical vapor depositionDLC: Diamond-like carbonEDX: Energy-dispersive x-ray spectroscopyEpoxy SAM: 3-GlycidoxypropyltrimethoxysilaneFE-SEM: Field emission- scanning electron spectroscopyFTIR-ATR: Fourier transform infrared spectroscopy-attenuated total reflectanceGPTMS: GlycidoxypropyltrimethoxysilaneHSS: High speed steelISO: International standards organizationL-B: Langmuir-Blodgett methodMEM: Minimum essential mediumMEMS: Micro-electro-mechanical systemsMPa: Mega PascalNEMS: Nano-electro-mechanical systemsPDMS: PolydimethylsiloxanePE: PolyethylenePEEK: Poly ether ether ketonePFPE: PerfluoropolyetherPI: Polyimidexvii

List of NotationsPMMA: PolymethylmethacrylatePS: PolystyrenePTFE: PolytetrafluoroethylenePVD: Physical vapor depositionRMS: Root mean square roughnessSAMs: Self-assembled monolayersSEM/EDS: Scanning electron microscope equipped with x-ray energy dispersionspectroscopySi 3 N 4 : Silicon nitrideTO: Thermal oxidationUHMWPE: Ultra-high-molecular-weight polyethyleneXPS: X-ray photoelectron spectroscopyxviii

Chapter 1: IntroductionChapter 1Introduction1.1 Importance of tribologyTribology is defined as the discipline to study the science and technologyof interacting surfaces in relative motion and of associated subjects and practices[Jost 1966]. The word “Tribology” was originated from the Greek word “Tribos”which means rubbing [Dowson 1979]. Tribology investigates the principles andrelated practices of friction, lubrication and wear phenomena to understand theinteraction of contact surfaces in a given environment.Friction is defined as the resistance between interacting surfaces underrelative motion. Wear can be described as the material removal phenomenon dueto interaction of surfaces in relative motion. Friction and wear are oftenunavoidable phenomena in sliding and rolling surface contacts. Lubrication is themethod employed to reduce friction and wear of contact surfaces in relativemotion by interposing a material called lubricant. Lubricant can be of anymaterial state such as solid, liquid and gas or a combination of them.Friction and wear play important roles in many places in naturalphenomena as well as man-made devices such as automobile, manufacturing etc.Friction and wear are often undesirable factors in many applications andadversely affect the performance and efficiency of systems thus scientists and1

Chapter 1: Introductionengineers strive to come up with means to minimize friction and wear to increaselife and durability of such systems.Tribology has grown into an important discipline for studying friction,lubrication and wear principles in order to improve the efficiency of mechanicalsystems.1.2 Brief history of tribologyAlthough full appreciation of significance of tribology as an independentdiscipline has been recognized only recently, human civilization had realized theimportance of friction and wear phenomena since ages. Wheel is the mostimportant mechanical invention of human civilizations and has been importantmilestone in the journey to come up with solutions to address friction and wearphenomenon in transportation. Known oldest wheel, discovered in Mesopotamia,dates back to 3500 BC although archaeologists believe that it was invented around8,000 BC. In 1880 BC, the Egyptians used sledges to transport large statues andmade use of water to lubricate sledges. Leonardo Da Vinci (1452-1519) is knownas the first person to study friction systematically as indicated by the sketchesdiscovered several hundred years later. Amonton (1699) stated throughexperiments that friction force is directly proportional to the applied normal loadand is independent of the apparent area of contact. These two laws are known asAmonton’s laws of friction. Charles Augustine Coulomb (1785) discovered thethird law that kinetic friction force is independent of sliding velocity. These three2

Chapter 1: Introductionlaws of friction were discovered on the basis of experimental observations andwere related to dry friction.1.3 Tribological applicationsTribology as a discipline has grown tremendously in sync with thescientific and technical developments in the world. Being a multidisciplinarydiscipline, tribology keeps reinventing itself with developments in science andtechnology. Today, tribology has found place in every aspects of everyday life.Due to the growth of knowledge and interest in tribology for differentapplications, this discipline has been further divided into different areas.1.3.1 Industrial tribologyOne of the important factors affecting the performance of the machines isthe nature of interacting surfaces. Thus friction and wear become importantconsiderations in the functioning of machines. Industrial tribology has foundimportant place in production, manufacturing, fabrication, aviation, aerospace andmarine sectors.1.3.2 MEMS/NEMS tribologyWith the development of MEMS/NEMS applications, it has been observedthat friction and wear at small length scales become limitations for efficiency anddurability of devices. At small length scales, surface forces become predominantcompared to inertial forces. Thus MEMS/NEMS devices require specialized3

Chapter 1: Introductionsolutions to address tribological limitations to reduce friction and increase weardurability.1.3.3 Biomedical tribologyWith the development of biomedical engineering, researchers areinvestigating the application of tribological principles for the improvement offunctioning of medical implants and patients’ comfort.With continued research by tribologists, this area has grown tremendouslyand has been able to make useful contributions to biomedical engineering.Tribology has found importance in improving implants life and in reducingpatient trauma in biomedical applications.1.4 Importance of titanium and titanium alloysBritish mineralogist and chemist, William Gregor, discovered titaniummetal in 1791. A Berlin chemist, Martin Klaporth, independently isolated titaniumoxide in 1795. He named it titanium after Greek mythological name “Titans”. Themost popular titanium alloy Ti6Al4V was developed in the late 1940s in theUnited States [Leyens and Peters 2003].Titanium and its alloys are widely used in biomedical, aerospace, aviation,marine, chemical industry, sports and leisure applications due to its high specificstrength and excellent corrosion resistance.The ASTM defines a number of alloy standards from Grade 1 to 38(ASTM B861 - 10 Standard Specifications for Titanium and Titanium Alloy4

Chapter 1: IntroductionSeamless Pipe). Ti6Al4V is the most commonly used titanium alloy forbiomedical and industrial applications. Its chemical composition consists of 6%aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen andremaining of titanium. It is significantly stronger than pure titanium whilestiffness and thermal properties are same as that of pure titanium (althoughthermal conductivity is about 60% lower in Grade 5 Ti compared to that of pureTi). Grade 5 is heat treatable and is an excellent combination of strength,corrosion resistance, weldability and fabricability. Grade 5 can be used upto 400degrees Celsius temperature [Leyens and Peters 2003]. Ti6Al4V is most widelyused among titanium alloys [Donachie 2000] and is considered as the"workhorse" of the titanium industry.1.4.1 Industrial applicationsTitanium and its alloys are used in industrial applications due to its highspecific strength, fatigue strength and creep resistance at high temperature[Leyens and Peters 2003].The much higher payoff for weight reduction in aircraft and spacecraft isthe driving factor for the usage of titanium and titanium alloys. In jet engine,titanium is the second most common material after Ni-based super-alloys. It iswidely used in airframe and gas turbine engine due to the weight savingconsiderations.5

Chapter 1: IntroductionHighly stressed components of helicopters such as rotor mast and head aremade from titanium alloys. In space applications, titanium alloys are usedextensively due to small payload requirement of space vehicles.Titanium is reactive metal but is extremely corrosion resistant due to itsstable oxide layer at surfaces. Due to its corrosion resistant behavior, titaniumalloys are popular in chemical, process and power generation industries. Heatexchangers, condensers, containers, apparatus and steam turbine blades are madefrom titanium alloys. It is also used in photochemical refineries and flue gasdesulphurization plants.Titanium alloys show excellent corrosion resistance in seawater and sourhydrocarbons, thus they are widely used in marine and offshore applications.Titanium alloys are used in automobile industry to improve performance atreduced weight although their use has been limited to racing and highperformance sports car due to higher cost.1.4.2 Consumer durablesIn sports and leisure, titanium alloys are used in making golf clubs, tennisracquets, baseball bats, pool cues, high speed cycling, scuba diving equipment,expedition and trekking equipment [Leyens and Peters 2003]. Titanium alloyshave also found usage in architecture due to its excellent immunity toenvironmental corrosion and a low coefficient of thermal expansion.In jewellery and fashion industry, titanium alloys are gaining popularitydue to its lightweight, corrosion resistant, hypo-allergic nature and possibility of6

Chapter 1: Introductioncreating a large range of surface finishes by utilizing anodizing and heattreatment. Besides titanium alloys are finding place in musical instruments,optical instruments, information technology and security applications due to itsversatile properties.1.4.3 Medical applicationsExcellent compatibility with the human body makes titanium a keymaterial for biomedical implant materials. It is resistant to corrosion from bodyfluids. Their excellent fatigue property, high specific strength and low modulus ofelasticity make it a preferred material for orthopedic devices. Bone fracture plates,screws, nails and plates for cranial surgery are made from titanium alloys [Leyensand Peters 2003].Shape memory property of Nitinol (a titanium alloy) makes it suitable forsome specialized applications such as stent. Titanium has widespread usage indental implants due to its biocompatibility and low thermal conductivity.1.4.4 MEMS applicationsTitanium is also been proposed as a potential MEMS (Micro-electromechanical systems) material for its physical and mechanical properties. Titaniumand titanium alloy MEMS can be preferably used in biomedical applications dueto its excellent biocompatibility.As a potential MEMS material for its physical and mechanical propertiesas well as biocompatibility, titanium alloy can be used in many MEMS7

Chapter 1: Introductionapplications [Aimi et al. 2004]. In MEMS applications, lubrication is required toreduce adhesion, friction and wear to ensure the reliability and durability ofdevices.The durability and reliability of MEMS/NEMS devices are affected bysurface properties such as adhesion, friction, and wear [Bhushan 2003, 2004,2005]. This requires the application of ultra-thin lubricant films having lowfriction and low adhesion as well as high wear durability to protect the contactsurfaces in MEMS/NEMS devices.1.5 Titanium and titanium alloys tribologyTitanium and titanium alloys have found many applications due to its highstrength-to-weight ratio, excellent corrosion resistance and biocompatibility.Unalloyed titanium is as strong as steel but has 45% less weight. Titanium can bealloyed with aluminium, vanadium, molybdenum and iron to produce lightweightstrong alloys to produce alloys of importance in biomedical, industrial, marine,automotive and aerospace applications.Application of titanium alloys in many areas is limited by its tribologicalproperties such as high friction coefficient, poor wear durability and low surfacehardness. Its poor tribological properties are caused by severe adhesive wear witha strong tendency to seizure, low abrasion resistance and the lack of mechanicalstability of the oxide layer [Budinski 1991; Yildiz et al. 2009]. Titanium tribologyhas found great interest among researchers due to possible application of titaniumalloys with improved tribological properties in many potential areas.8

Chapter 1: IntroductionIn industrial applications, various surface treatments such as thermochemical processes, energy beam surface alloying and duplex treatments havebeen proposed by researchers to address the tribological limitations of titaniumalloys [Bloyce 1998].For biomedical applications, plasma nitriding and bio-ceramic coatingsare widely investigated solutions to improve the tribological properties of alloysin orthopedic implants [Molinari et al. 1997; Yildiz et al. 2008; Fei et al. 2009].1.6 Objectives of the thesisThe objective of this thesis is to evaluate some of the potential solutionsfor surface modifications of titanium and titanium alloys to improve itstribological properties.In the first study, UHMWPE and UHMWPE/PFPE thin film coatings wereevaluated to address Ti6Al4V alloy tribological limitations. Experimentalcharacterizations of the physical, c

This thesis is submitted for the degree of Master of Engineering in the Department of Mechanical Engineering, National University of Singapore under . 2.1 Surface engineering and tribology ; 2.2 Existing tribology solutions