Tribology on the Small Scale: A Bottom Up Approach to Friction, Lubrication, and Wear

Introduction     1Why is it called tribology?     2Economic and technological importance of tribology     3Some tribology success stories     4Reducing automotive friction     4MEMS and solving adhesion in Digital Micro-mirror Devices     5Slider-disk interfaces in disk drives     7A brief history of modern tribology     10Scientific advances enabling nanoscale tribology     12Breakthrough technologies relying on tribology at the small scale     14Nanoimprinting     16IBM's millipede for high density storage     18Nanotechnology     19References     20Characterizing surface roughness     24Types of surface roughness     24Roughness parameters     26Variations in Z-height     26Asperity summits roughness parameters     28Surface height distributions     29Measuring surface roughness     30Atomic force microscopy (AFM)     30Example: Disk surfaces in disk drives     33References     37Mechanical properties of solids and real area of contact     39Atomic origins ofdeformation     39Elastic deformation     43Basic relations     43Elastic deformation of a single asperity     44Approximating a single asperity contact     44Elastic contact area for a sphere on a flat     45Example: Spherical steel particle sandwiched between two flat surfaces     46Plastic deformation     48Basic relations     48Hardness     49Real area of contact     50Greenwood and Williamson model     51Example: TiN contacts     53Real area of contact using the Greenwood and Williamson model     54Example: Recording head on a laser textured disk surface     55Inelastic impacts     59References     61Friction     63Amontons' and Coulomb's laws of friction     63Adhesion and plowing in friction     66Adhesive friction     66Plowing friction     68Work hardening     70Junction growth     70Static friction     72Stick-slip     74Velocity-controlled stick-slip     75Time-controlled stick-slip      77Displacement-controlled stick-slip     78References     81Surface energy and capillary pressure     82Liquid surface tension     82Capillary pressure     85Capillary pressure in confined places     87The Kelvin equation and capillary condensation     90Example: Capillary condensation of water in a nanosized pore     91Example: Capillary condensation of an organic vapor at a sphere-on-flat geometry     91Interfacial energy and work of adhesion     92Surface Energy of Solids     93Why solids are not like liquids     93Experimental determination of a solid's surface energy     95Contact angles     96Estimating interfacial energies     97Zisman method for estimating surface energy for a solid     98Types of wetting     101Contact angle measurements     101Contact angle hysteresis     103Adhesion hysteresis     104References     110Surface forces derived from surface energies     113The Derjaguin approximation     113Dry environment     114Force between a sphere and a flat      114Example: Adhesion force between two polystyrene spheres     115Example: Adhesion force between a polystyrene sphere and a PTFE Flat     115Example: Adhesion force for an atomically sharp asperity     116Adhesion-induced deformation at a sphere-on-flat contact     117The Johnson-Kendall-Roberts (JKR) theory     117The Derjaguin-Muller-Toporov (DMT) theory     121Adhesion deformation in nanoscale contacts     121Wet environment     122Force for a sphere-on-flat in a wet environment     122Example: Lubricant meniscus force on an AFM tip     123Solid-solid adhesion in the presence of a liquid meniscus     125Water menisci in sand     126Meniscus force for different wetting regimes at contacting interfaces     128Toe dipping regime     128Example: Toe dipping adhesion with exponential distribution of summit heights     129Pillbox and flooded regimes     131Immersed regime     132Example: Liquid adhesion of a microfabricated cantilever beam     133References     135Physical origins of surface forces     137Normal force sign convention     137Repulsive atomic potentials      138Van der Waals forces     139Van der Waals forces between molecules     139Retardation effects for dispersion forces     142Van der Waals forces between macroscopic objects     142Molecule-flat surface interaction     142Flat-Flat interaction     144Sphere-flat interaction     145The Hamaker constant     145Determining Hamaker constants from Lifshitz's theory     146Example: Van der Waals force on a polystyrene sphere above a Teflon flat     151Surface energies arising from van der Waals interactions     152Van der Waals adhesive pressure     153Van der Waals interaction between contacting rough surfaces     154Example: Stuck microcantilevers     156Example: Gecko adhesion     158Van der Waals contribution to the disjoining pressure of a liquid film     160Liquid-mediated forces between solids     162Solvation forces     162Example: Squalane between smooth mica surfaces     164Oscillatory solvation forces at sharp AFM contacts     166Forces in aqueous medium     167Electrostatic double-layer force     167Hydration repulsion and hydrophobic attraction      169Contact electrification     171Mechanisms of contact electrification     172Conductor-conductor contact     172Example: Recording head slider flying over a disk in a disk drive     175Metal-insulator and insulator-insulator Contacts     177AFM studies of contact electrification     179References     181Measuring surface forces     186Surface force apparatus     188Atomic force microscope     192Examples of forces acting on AFM tips     195Van der Waals forces under vacuum conditions     195Capillary condensation of contaminants and water vapor     197Bonded and unbonded perfluoropolyether polymer films     200Electrostatic double-layer force     202References     204Lubrication     207Lubrication regimes     207Viscosity     209Definition and units     209Non-Newtonian behavior and shear degradation     211Temperature dependence     214Fluid film flow in confined geometries     214Slippage at liquid-solid interfaces     216Definition of slip length     217Measuring slip at liquid-solid interfaces     218Pressure drop versus flow rate method     218Drainage versus viscous force     219Mechanisms for slip at liquid-solid interfaces     220Molecular slip     220Molecular slip at low energy surfaces     220Slippage of polymers melts     222Apparent slip     222Example: Shear stress in the presence of slip     225Why does the no-slip boundary condition work so well?     225Fluid film lubrication     226Hydrodynamic lubrication     228Inclined plane bearing     229Rayleigh step bearing     229Journal bearings     230Gas bearings     232Slip flow in gas bearings     234Elastohydrodynamic lubrication     235Pressure dependence of viscosity     235Pressure-induced elastic deformation     236Example: Minimum film thickness between sliding gear teeth     238Experimental measurements of elastohydrodynamic lubrication     239Important physical and chemical properties of lubricants     241Surface tension     241Thermal properties     242References      243Lubrication in tight spots     246Confined liquids     246Boundary lubrication     255Molecular mechanisms of boundary lubrication     256Molecularly thin liquid boundary lubricant layers     260Example of the importance of end-groups in a liquid lubricant film     262Capillary and disjoining pressures     265Disjoining pressure     265Distribution of a liquid film around a pore opening     267Example: Measurement of the disjoining pressure of a perfluoropolyether lubricant     269Lubricant distribution between contacting surfaces     270Meniscus force     272Example: Stiction of a recording head slider     272Calculating meniscus force     273Example: Calculation of stiction force of disk drive sliders in the pillbox regime     275Padded or stiction-free slider     276Liquid menisci at high speeds     278References     279Atomistic origins of friction     284Simple models for adhesive friction     284Atomistic models for static friction     286Frenkel-Kontorova model     287Experimental realizations of ultra-low friction in incommensurate sliding systems     289Tomlinson model     290Example: An AFM tip sliding across an NaCl crystal at ultra-low loads     291Molecular dynamic simulations     295Example: Cold welding     295Why static friction occurs in real-life situations     295Atomic origins of kinetic friction     297Sliding isolated molecules and monolayers across surfaces     297Quartz crystal microbalance     299Example: Xe on Ag(111)     300Movement of a liquid film on a surface with the blow-off technique     301Example: Wind-driven flow of perfluoropolyether lubricants on silicon wafers     302Pinning of an absorbed layer     307References     308Wear     313Simple model for sliding wear     314Major influences on wear rates     317Wear maps     318Mechanisms of wear     319Wear from plastic deformation     319Adhesive wear     320Example: An atomic level simulation of adhesive wear     321Abrasive wear     321Oxidative wear     325Metals     325Carbon overcoats     326Ceramics      326Plasticity at the nanoscale     327References     329Index     331