Bhushan B. Handbook of Micro/Nano Tribology, Second Edition

Подождите немного. Документ загружается.

van der Waals attraction; whereas the grafting process forms directly covalently bonded dense ﬁlms of

long-chain molecules.

Bhushan et al. (1995b,c) conducted micro- and macroscale friction and wear studies on single-grafted

and double-grafted octodecyl (C

) and LB ﬁlms and their subsurfaces. C

ﬁlms were produced by

grafting of long-chain organic molecules onto an Si(100) wafer covered with a thermally grown SiO

layer, Figure 8.11. The structure of the LB ﬁlm consisted of an octadecylthiol (ODT) coated gold sample

(gold ﬁlms thermally evaporated onto single-crystal silicon) on top of which a single, upper inverted

bilayer of zinc arachidate (C

, ZnA) was deposited by the LB technique, Figure 8.12. Macro- and

microscale friction and wear data are summerized in Table 8.1 and Figure 8.13. We note that C

double-

grafted ﬁlm exhibits the lowest coefﬁcient of friction of 0.018 as compared with other samples measured

in this study (the coefﬁcient of friction of ZnA ﬁlms is ~0.03). The coefﬁcient of friction for LB ﬁlm is

comparable to that of the ODT layer and lower than the Au ﬁlm. The wear resistance of C

double-

grafted ﬁlm is much better than C

single-grafted, LB, and Au ﬁlms and is comparable with that of SiO

ﬁlm. The C

double-grafted ﬁlms can withstand much higher normal force of 40 µN as compared with

200 nN for the case of LB ﬁlms. Surface proﬁles showing the worn regions after one scan cycle for C

double-grafted and ZnA ﬁlms are shown in Figure 8.14. Nanoindentation studies conducted by Bhushan

et al. (1995c) showed that the C

double-grafted ﬁlms are more rigid than the LB ﬁlms which may be

responsible for the high wear resistance of C

ﬁlms. Flexibility of choosing the alkyl chain length,

functional terminal group, and cross-linking enables the adaptability of the grafting process for lubrica-

tion of microcomponents.

8.4 Closure

In nanodeformation experiments, bonded lubricants behave as soft polymer solids. AFM/friction force

microscopy friction experiments show that lubricants with polar (reactive) end groups dramatically

increase the load or contact pressure that a liquid ﬁlm can support before solid–solid contact and these

exhibit long durability. Chemically grafted ﬁlms may be suitable for lubrication of microcomponents.

FIGURE 8.9

Wear depth as a function of normal force using a diamond tip for unlubricated and lubricated samples

after one cycle. (From Koinkar, V. N. and Bhushan, B. (1996),

J. Vac. Sci. Technol. A

14, 2378–2391. With permission.)

FIGURE 8.10

Wear proﬁles for unlubricated and lubricated samples after microwear using diamond tip. Normal

force used and wear depths are listed in the ﬁgure. (From Koinkar, V. N. and Bhushan, B. (1996),

J. Vac. Sci. Technol. A

14, 2378–2391. With permission.)

FIGURE 8.11

Schematic of the ODT (C

) and docosyl (C

) grafted ﬁlm on silica.

FIGURE 8.12

Schematic of the zinc arachidate LB ﬁlm on ODT with a gold underlayer.

TABLE 8.1

Typical rms Roughness and Coefﬁcients of Microscale and Macroscale

Friction Values of Various Samples

rms Roughness Coefﬁcient of Coefﬁcient of

Sample (nm)

microscale friction

macroscale friction

Si(100) 0.12 0.03 0.18

SiO

/Si 0.21 0.03 0.19

single-grafted/SiO

/Si 0.16 0.04 0.23

double-grafted/SiO

/Si 0.16 0.018 0.07

Au/Si 1.16 0.04 0.13

ODT/Au/Si 0.92 0.03 0.14

ZnA/ODT/Au/Si 0.55 0.03 0.16

Measured on 1

1 µm scan area using AFM.

tip with a radius of about 30–50 nm at normal load in the range of 10–200 nN and

scanning speed of 4 µm/s on a 1

1 µm scan area.

Alumina ball with 3-mm radius at normal loads of 0.1 N and average sliding speed of

0.8 mm/s.

FIGURE 8.13

(a) Wear depth as a function of normal force after one scan cycle (b) wear depth as a function of

number of scan cycles at the normal force indicated for SiO

/Si, C

single-grafted, C

double-grafted, Au/Si,

ODT/Au/Si, and ZnA/ODT/Au/Si. (From Bhushan, B. et al. (1995),

Langmuir

11, 3189–3198. With permission.)

References

Bhushan, B. (1993), “Magnetic Recording Surfaces,” in

Characterization of Tribological Materials

(W.A.

Glaeser, ed.), pp. 116–133, Butterworth-Heinemann, Boston.

Bhushan, B. and Ruan, J. (1994), “Atomic-Scale Friction Measurements Using Friction Force Microscopy:

Part II — Application to Magnetic Media,”

ASME J. Tribol.

116, 389–396.

Bhushan, B., Miyamoto, T., and Koinkar, V. N. (1995a), “Microscopic Friction between a Sharp Diamond

Tip and Thin-Film Magnetic Rigid Disks by Friction Force Microscopy,”

Adv. Info. Storage Syst

. 6,

151–161.

Bhushan, B. Israelachvili, J. N., and Landman, U. (1995b), “Nanotribology: Friction, Wear and Lubrica-

tion at the Atomic Scale,”

Nature

374, 607–616.

Bhushan, B., Kulkarni, A. V., Koinkar, V. N., Boehm, M., Odoni, L., Martelet, C., and Belin, M. (1995c),

“Microtribological Characterization of Self-Assembled and Langmuir-Blodgett Monolayers by

Atomic and Friction Force Microscopy,”

Langmuir

11, 3189–3198.

Blackman, G. S., Mate, C. M., and Philpott, M. R. (1990a), “Interaction Forces of a Sharp Tungsten Tip

with Molecular Films on Silicon Surface,”

Phys. Rev. Lett.

65, 2270–2273.

Blackman, G. S., Mate, C. M., and Philpott, M. R. (1990b), “Atomic Force Microscope Studies of Lubricant

Films on Solid Surfaces,”

Vacuum

41, 1283–1286.

Bowden, F. P. and Tabor, D. (1950),

The Friction and Lubrication of Solids

, Part 1, Clarendon Press, Oxford.

Jaffrezic-Renault, N. and Martelet, C. (1992), “Preparation of Well-Engineered Thin Molecular Layer on

Semiconductor–Based Transducers,”

Sensors Actuators

A 32, 307–312.

Koinkar, V. N. and Bhushan, B. (1996a), “Microtribological Studies of Unlubricated and Lubricated

Surfaces Using Atomic Force/Friction Force Microscopy,”

J. Vac. Sci. Technol. A

14, 2378–2391.

Koinkar, V. N. and Bhushan, B. (1996b), “Micro/Nanoscale Studies of Boundary Layers of Liquid Lubri-

cants for Magnetic Disks,”

J. Appl. Phys.

79, 8071–8075.

Mate, C. M. (1992a), “Application of Disjoining and Capillary Pressure to Liquid Lubricant Films in

Magnetic Recording,”

J. Appl. Phys.

72, 3084–3090.

FIGURE 8.14

Surface proﬁle showing the worn region (center 1

1 µm) after one scan cycle for C

double-grafted

and ZnA ﬁlms. Normal force and the wear depths are indicated in the ﬁgure.

Mate, C. M. (1992b), “Atomic-Force-Microscope Study of Polymer Lubricants on Silicon Surface,”

Phys.

Rev. Lett.

68, 3323–3326.

Mate, C. M. (1993), “Nanotribology of Lubricated and Unlubricated Carbon Overcoats on Magnetic

Disks Studied by Friction Force Microscopy,”

Surf. Coat. Technol.

62, 373–379.

Mate, C. M. and Novotny, V. J. (1991), “Molecular Conformation and Disjoining Pressures of Polymeric

Liquid Films,”

J. Chem. Phys.

94, 8420–8427.

Novotny, V. J. (1990), “Migration of Liquid Polymers on Solid Surfaces,”

J. Chem. Phys.

92, 3189–3196.

Novotny, V. J., Hussla, I., Turlet, J. M., and Philpott, M. R. (1989), “Liquid Polymer Conformation on

Solid Surfaces,”

J. Chem. Phys.

90, 5861–5868.

O’Shea, S. J., Welland, M. E., and Rayment, T. (1992), “Atomic Force Microscope Study of Boundary

Layer Lubrication,”

Appl. Phys. Lett.

61, 2240–2242.

O’Shea, S. J., Welland, M. E., and Pethica, J. B. (1994), “Atomic Force Microscopy of Local Compliance

at Solid-Liquid Interface,”

Chem. Phys. Lett.

223, 336–340.

Roberts, G. G. (1990),

Langmuir-Blodgett Films,

Plenum, New York.

Salmeron, M., Neubauer, G., Folch, A. Tomitori, M., Ogletree, D. F., and Sautet, P. (1993), “Viscoelastic

and Electrical Properties of Self-Assembled Monolayers on Au(111) Films,”

Langmuir

9, 3600–3611.

Ulman, A. (1991),

An Introduction to Ultrathin Organic Films

, Academic Press, Boston.

Israelachvil, J. N. et al.“Surface Forces and Microrheology of ...”

Handbook of Micro/Nanotribology.

Ed. Bharat Bhushan

Boca Raton: CRC Press LLC, 1999

Surface Forces and

Microrheology of

Molecularly Thin

Liquid Films

Jacob N. Israelachvili and

Alan D. Berman

9.1 Introduction

9.2 Methods for Measuring Static and Dynamic Surface

Forces

Adhesion Forces • Force Laws • The Surface Force Apparatus

and the Atomic Force Microscope

9.3 van der Waals and Electrostatic Forces between

Surfaces in Liquids

van der Waals Forces • Electrostatic Forces

9.4 Solvation and Structural Forces: Forces Due to

Liquid and Surface Structure

Effects of Surface Structure • Effect of Surface Curvature and

Geometry

9.5 Thermal Fluctuation Forces: Forces between Soft,

Fluidlike Surfaces

9.6 Hydration Forces: Special Forces in Water and

Aqueous Solutions

Repulsive Hydration Forces • Attractive Hydrophobic

Forces • Origin of Hydration Forces

9.7 Adhesion and Capillary Forces

Adhesion Mechanics

9.8 Nonequilibrium Interactions: Adhesion Hysteresis

9.9 Rheology of Molecularly Thin Films:

Nanorheology

Different Modes of Friction: Limits of Continuum Models •

Viscous Forces and Friction of Thick Films: Continuum

Regime • Friction of Intermediate Thickness Films

9.10 Interfacial and Boundary Friction: Molecular

Tribology

General Interfacial Friction • Boundary Friction of Surfactant

Monolayer-Coated Surfaces • Boundary Lubrication of

Molecularly Thin Liquid Films • Transition from Interfacial

to Normal Friction (with Wear)

9.11 Theories of Interfacial Friction

Theoretical Modeling of Interfacial Friction: Molecular

Tribology • Adhesion Force Contribution to Interfacial

Friction • Relation between Boundary Friction and Adhesion

Energy Hysteresis • External Load Contribution to Interfacial

Friction • Simple Molecular Model of Energy Dissipation

9.12 Friction and Lubrication of Thin Liquid Films

Smooth and Stick-Slip Sliding • Role of Molecular Shape and

Liquid Structure

9.13 Stick-Slip Friction

Rough Surfaces Model • Distance-Dependent Model •

Velocity-Dependent Friction Model • Phase-Transition

Model • Critical Velocity for Stick-Slip • Dynamic Phase

Diagram Representation of Tribological Parameters

Acknowledgment

References

9.1 Introduction

In this chapter the most important types of surface forces are described and the relevant equations for

the force laws given. A number of attractive and repulsive forces operate between surfaces and particles.

Some of these occur only in vacuum, for example, attractive van der Waals and repulsive hard-core

interactions. Others can arise only when the interacting surfaces are separated by another condensed

phase, which is usually a liquid medium. The most common types of surface forces and their main

characteristics are list in Table 9.1.

vacuum

, the two main long-ranged interactions are the attractive van der Waals and electrostatic

(coulombic) forces, while at smaller surface separations — corresponding to molecular contacts at surface

separations of

≈

0.2 nm — additional attractive forces can come into play, such as covalent or metallic

bonding forces. These attractive forces are stabilized by the hard-core repulsion, and together they

determine the surface and interfacial energies of planar surfaces as well as the strengths of materials and

adhesive junctions. Adhesion forces are often strong enough to deform the shapes of two bodies or

particles elastically or plastically when they come into contact for the ﬁrst time.

When exposed to

vapors

(e.g., atmospheric air containing water and organic molecules), two solid

surfaces in or close to contact will generally have a surface layer of chemisorbed or physisorbed molecules,

or a capillary condensed liquid bridge between them. These effects can drastically modify their adhesion.

The adhesion usually falls, but in the case of capillary condensation the additional Laplace pressure or

attractive “capillary” force between the surfaces may make the adhesion stronger than in inert gas or

vacuum.

When totally immersed in a

liquid

, the force between two surfaces is once again completely modiﬁed

from that in vacuum or air (vapor). The van der Waals attraction is generally reduced, but other forces

can now arise which can qualitatively change both the range and even the sign of the interaction. The

overall attraction can be either stronger or weaker than in the absence of the intervening liquid medium,

for example, stronger in the case of two hydrophobic surfaces in water, but weaker for two hydrophilic

surfaces. Since a number of different forces may be operating simultaneously in solution, the overall

force law is not generally monotonically attractive, even at long range: it can be repulsive, oscillatory, or

the force can change sign at some ﬁnite surface separation. In such cases, the potential energy minimum,

which determines the adhesion force or energy, occurs not at true molecular contact but at some small

distance farther out.

The forces between two surfaces in a liquid medium can be particularly complex at

short range

, i.e.,

at surface separations below a few nanometers or 5 to 10 molecular diameters. This is partly because,

with increasing conﬁnement, a liquid ceases to behave as a structureless continuum with properties

TABLE 9.1

Types of Surface Forces

Type of Force

Subclasses and

Alternative Names Main Features

Attractive

van der Waals Dispersion force (v & s)

Induced dipole force (v & s)

Casimir force (v & s)

Ubiquitous force, occurs both in vacuum and in liquids

Electrostatic Coulombic force (v & s)

Ionic bond (v)

Hydrogen bond (v)

Charge–transfer interaction (v & s)

“Harpooning” interaction (v)

Strong, long-ranged force arising in polar solvents; requires

surface charging or charge-separation mechanism

Quantum

mechanical

Covalent bond (v)

Metallic bond (v)

Exchange interaction (v)

Strong short-ranged forces responsible for contact binding

of crystalline surfaces

Hydrophobic Attractive hydration force (s) Strong, apparently long-ranged force; origin not yet

understood

Ion–correlation van der Waals force of

polarizable ions (s)

Requires mobile charges on surfaces in a polar solvent

Solvation Oscillatory force (s)

Depletion force (s)

The oscillatory force generally alternates between attraction

and repulsion; mainly entropic in origin

Speciﬁc binding “Lock and key” binding (v & s)

Receptor–ligand interaction (s)

Antibody–antigen interaction (s)

Subtle combination of different noncovalent forces giving

rise to highly speciﬁc binding; main “recognition”

mechanism of biological systems.

Repulsive

Quantum

mechanical

Hard-core (v)

Steric repulsion (v)

Born repulsion (v)

Forces stabilizing attractive covalent and ionic binding

forces, effectively determine molecular size and shape

van der Waals van der Waals disjoining

pressure (s)

Arises only between dissimilar bodies interacting in a

medium

Electrostatic Arises only for certain constrained surface charge

distributions.

Solvation Oscillatory solvation force (s)

Structural force (s)

Hydration force (s)

Monotonically repulsive forces, believed to arise when

solvent molecules bind strongly to surfaces.

Entropic Osmotic repulsion (s)

Double-layer force (s)

Thermal ﬂuctuation force (s)

Steric polymer repulsion (s)

Undulation force (s)

Protrusion force (s)

Forces due to conﬁnement of molecular or ionic species

between two approaching surfaces. Requires a mechanism

which keeps trapped species between the surfaces.

Dynamic Interactions

Nonequilibrium Hydrodynamic forces (s)

Viscous forces (s)

Friction forces (v & s)

Lubrication forces (s)

Energy-dissipating forces occurring during relative motion

of surfaces or bodies.

Note:

v, Applies only to interactions in

vacuum

; s, applies only to interactions in

solution

, or to surfaces separated

by a liquid; v & s, applies to interactions occurring both in vacuum and in solution.