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

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Bhushan, B. “Boundary Lubrication Studies Using Atomic Force/Friction ...”

Handbook of Micro/Nanotribology.

Ed. Bharat Bhushan

Boca Raton: CRC Press LLC, 1999

Boundary Lubrication

Studies Using Atomic

Force/Friction

Force Microscopy

Bharat Bhushan

8.1 Introduction

8.2 Nanodeformation, Adhesive Forces, and Molecular

Conformation

8.3 Boundary Lubrication Studies

Liquid Lubricants • LB and Self-Assembled Monolayers

8.4 Closure

References

8.1 Introduction

Boundary ﬁlms are formed by physical adsorption, chemical adsorption, and chemical reaction. The

physisorbed ﬁlm can be either monomolecular or polymolecular thick. The chemisorbed ﬁlms are

monomolecular, but stoichiometric ﬁlms formed by chemical reaction can have a large ﬁlm thickness.

In general, the stability and durability of surface ﬁlms decrease in the following order: chemical reaction

ﬁlms, chemisorbed ﬁlms, and physisorbed ﬁlms. A good boundary lubricant should have a high degree

of interaction between its molecules and the sliding surface. As a general rule, liquids are good lubricants

when they are polar and thus able to grip solid surfaces (or be adsorbed). Polar lubricants contain reactive

functional groups with low ionization potential or groups having high polarizability (Bhushan, 1993).

Boundary lubrication properties of lubricants are also dependent upon the molecular conformation and

lubricant spreading (Novotny et al., 1989; Novotny, 1990; Mate and Novotny, 1991; Mate, 1992a).

This chapter presents an overview of lubrication studies of polar and nonpolar lubricants and Lang-

muir–Blodgett and chemically grafted ﬁlms, using atomic force/friction force microscopy.

8.2 Nanodeformation, Adhesive Forces,

and Molecular Conformation

Nanodeformation behavior of the bonded lubricant was studied using atomic force microscopy (AFM)

by Blackman et al. (1990a). They used Si(100) substrate with about 1.5 nm of native oxide. Just prior to

the application of lubricant, the surface was cleaned with methylene chloride, spun dried, and followed

by exposure to ultraviolet-created ozone for several minutes to remove the remaining adsorbates. Liquid

ﬁlms of the perﬂuoropolyether Z-Dol of about 4-nm thickness were deposited by a dip-coating method.

The lubricant molecules were bonded to the substrate via the reactive end groups by heating at 150°C

for 1 h, followed by rinsing with a freon solvent to remove any unbonded molecules, leaving behind

about 2-nm-thick ﬁlm. Before bringing a tungsten tip into contact with a molecular overlayer, it was

ﬁrst brought into contact with a bare clean-silicon surface, Figure 8.1. As the sample approaches the tip,

the force initially is zero, but at point A the force suddenly becomes attractive (top curve) which increases

until at point B where the sample and tip come into intimate contact and the force becomes repulsive.

As the sample is retracted, a pull-off force of 5

–8

N (point D) is required to overcome adhesion

between the tungsten tip and the silicon surface. The deformation is reversible (elastic) since the retracting

(outgoing) portion of the curve (C to D) follows the extending (ingoing) portion. When an AFM tip is

brought into contact with a molecularly thin ﬁlm of a nonreactive lubricant, a sudden jump into adhesive

contact is observed. The adhesion is initiated by the formation of a lubricant meniscus surrounding the

tip pulling the surfaces together by Laplace pressure. However, when the tip was brought into contact

with a lubricant ﬁlm which was ﬁrmly bonded to the surface, the liquidlike behavior disappears. The

initial attractive force (point A) is no longer sudden as with the liquid ﬁlm, but, rather, gradually increases

as the tip penetrates the ﬁlm. Meniscus formation is suppressed because the polymer molecules are no

longer free to move about on the surface as at least one end is attached.

According to Blackman et al. (1990a), if the substrate and tip were inﬁnitely hard with no compliance

in the tip and sample supports, the line for B to C would be vertical with an inﬁnite slope. The tangent

to the force–distance curve at a given point is referred to as the stiffness at that point and was determined

by ﬁtting a least-squares line through the nearby data points. For bonded lubricant ﬁlm, at the point

where slope of the force changes gradually from attractive to repulsive, the stiffness changes gradually,

indicating compression of the molecular ﬁlm. As the load is increased, the slope of the repulsive force

eventually approaches that of the bare surface. The bonded ﬁlm was found to respond elastically up to

the highest loads of 5 µN which could be applied. Thus, bonded lubricant behaves as a soft polymer solid.

FIGURE 8.1

Wire deﬂection (normal load) as a function of tip–sample separation distance curves comparing the

behavior of clean Si(100) surface to a surface lubricated with free and unbonded PFPE lubricant, and a surface where

the PFPE lubricant ﬁlm was thermally bonded to the surface. (From Blackman, G. S. et al. (1990),

Phys. Rev. Lett.

65, 3189–3198. With permission.)

The attractive adhesive forces at different parts of the surface can be estimated by bringing the sample

into contact with the tip and then measuring the maximum force needed to pull the tip and sample

apart, Mate (1993) and Bhushan and Ruan (1994). Figure 8.2 shows typical normal load as a function

of separation distance (

) curves for unlubricated and lubricated (with about 2-nm thickness of a

perﬂuoropolyether lubricant with an alcohol group, Z-Dol) magnetic disk samples. In these experiments,

the disks are ﬁrst brought into contact and then withdrawn from the tip. The presence of the water vapor

for the unlubricated disk and along with the lubricant ﬁlm for the lubricated disk causes a sudden

attractive force to occur at point A due to a meniscus of liquid forming around the top of the liquid ﬁlm

and a long break-free distance out to point B where the liquid meniscus breaks as the sample is withdrawn.

The major difference between the two curves is that the pull-off force is lower for the unlubricated surface

compared with lubricated surface. Pull-off force is determined by multiplying the cantilever spring

constant (0.4 N/m) by the horizontal distance between points C and D, which corresponds to the

maximum cantilever deﬂection toward the disks before the tip is disengaged. The horizontal distance/pull-

off force is 105 nm/42 nN for the unlubricated disk and 160 nm/64 nN for the lubricated disk. The higher

value of the pull-off force in the case of lubricated disk arises from the larger meniscus contribution from

the liquid ﬁlms (Bhushan and Ruan, 1994).

Figure 8.3 illustrates two extremes for the conformation on a surface of a linear liquid polymer without

any reactive end groups and at submonolayer coverages (Novotny et al., 1989; Mate and Novotny, 1991).

At one extreme, the molecules lie ﬂat on the surface, reaching no more than their chain diameter

above

the surface. This would be the case if a strong attractive interaction exists between the molecules and the

FIGURE 8.2

Tip deﬂection (normal load) as a function of Z (separation distance) curve for (a) unlubricated and

(b) lubricated thin-ﬁlm magnetic rigid disks. The pull-off force is 42 nN for the unlubricated disk and 64 nN for

the lubricated disk calculated from the horizontal distance between points C and D and the cantilever spring constant

of 0.4 N/m. (From Bhushan, B. and Ruan, J. (1994),

ASME J. Tribol.

116, 389–396. With permission.)

solid. On the other extreme, when a weak attraction exists between polymer segments and the solid, the

molecules adopt conformation close to that of the molecules in the bulk, with the microscopic thickness

equal to about the radius of gyration

. Mate and Novotny (1991) used AFM to study conformation of

0.5- to 13-nm-thick perﬂuoropolyether molecules on clean Si(100) surfaces. They found that the thickness

measured by AFM is thicker than that measured by ellipsometry with the offset ranging from 3 to 5 nm.

They found that the offset was the same for very thin submonolayer coverages. If the coverage is

submonolayer and inadequate to make a liquid ﬁlm, the relevant thickness is then the height (

)

molecules extended above the solid surface. The offset should then equal 2

, assuming that the molecules

extend the same height above both the tip and silicon surfaces. They thus concluded that the molecules

do not extend more than 1.5 to 2.5 nm above a solid or liquid surface, much smaller than the radius of

gyration of the lubricants ranging between 3.2 and 7.3 nm, and to the approximate cross-sectional

diameter of 0.6 to 0.7 nm for the linear polymer chain. Consequently, the height that the molecules

extend above the surface is considerably less than the diameter of gyration of the molecules and only a

few molecular diameters in height, implying that the physisorbed molecules on a solid surface have an

extended, ﬂat conformation. They also determined the disjoining pressure of these liquid ﬁlms from

AFM measurements of the distance needed to break the liquid meniscus that forms between the solid

surface and the AFM tip. (Also see Mate, 1992a). For a monolayer thickness of about 0.7 nm, the disjoining

pressure is about 5 MPa, indicating strong attractive interaction between the liquid molecules and the

solid surface. The disjoining pressure decreases with increasing ﬁlm thickness in a manner consistent

with a strong attractive van der Waals interaction between the liquid molecules and the solid surface.

FIGURE 8.3

Schematic representation of two extreme liquid conformations at the surface of the solid for low and

high molecular weights at low surface coverage.

is the cross-sectional diameter of the liquid chain and

is the

radius of gyration of the molecules in the bulk. (From Mate, C. M. and Novotny, V. J. (1991),

J. Chem. Phys.

92,

3189–3196. With permission.)

Attempts to measure mechanical properties of self-assembled monolayer ﬁlms on Au(111) ﬁlms have

been made by Salmeron et al. (1993). They have used AFM in the tapping mode. This technique has the

potential of measuring local viscoelastic properties of lubricant ﬁlms.

8.3 Boundary Lubrication Studies

8.3.1 Liquid Lubricants

Mate (1992b), O’Shea et al. (1992), Bhushan et al. (1995a–c), and Koinkar and Bhushan (1996a,b) used

AFM to provide insight into how lubricants function at the molecular level. Mate (1992b) conducted

friction experiments on Si(100) surface lubricated with a lubricant with alcohol end group (Z-Dol). In

these experiments, the sample was moved rapidly back and forth in the

-direction at a velocity of 1 µm/s,

while the normal load on the tip was slowly increased to some maximum value, then decreased back to

zero by moving the sample in the

-direction. Figure 8.4 shows an example of the friction force on the

tip during one complete X oscillation of the sample. Initially, the tip moves with the sample, until, at

point A, the cantilever wire exerts enough force to overcome the static frictional force and the tip starts

to slide across the surface. When the X sample direction is reversed at point B, the tip again moves with

the sample until it starts to slide at point C. The upward shift in the normal load over the cycle comes

with the slight increase in load as the sample is slowly pushed up against the tip. The slight variation in

load during the cycle correspond to a surface roughness of about 0.1 nm.

Figure 8.5 shows the average normal load and friction forces during sliding as a function of the

Z-sample position for the 3-nm-thick ﬁlms of different types of lubricants. Each data point in the ﬁgure

represents the average over the sliding portion of a cycle in the X-direction like the one shown in

Figure 8.4. For the liquid ﬁlm in Figure 8.5a and c, Mate (1992b) noted that, just before the hard wall

contact is made, the normal load during sliding becomes more attractive for nonalcohol end group (Z03)

than that for alcohol end group (Z-Dol) in the same manner as when no sliding occurs. When the sample

is withdrawn, the friction force returns to zero when the hard wall contact is broken. This regime is

called full-ﬁlm lubrication, where shearing of a liquid ﬁlm takes place resulting in a negligible friction

FIGURE 8.4

(a) Friction force and (b) normal load over an oscillation of the X-sample position during sliding of

the tungsten tip on an Si(100) surface coated with perﬂuoropolyether lubricant with alcohol end group. (From Mate,

C. M. (1992),

Phys. Rev. Lett.

68, 3323–3326. With permission.)

(Bowdon and Tabor, 1950). After hard wall contact, one is in boundary lubrication regime, where

solid–solid shearing takes place. The transition between the two regimes is very sharp, requiring a change

in separation distance less than a chain diameter. In the boundary lubrication regime, the friction force

for the liquid ﬁlms initially rises quickly, but soon increases linearly with load (coefﬁcient of friction in

the range of 0.5 to 0.8). Similar values of the coefﬁcient of friction are observed for unlubricated silicon

surfaces, indicating that most of the unbonded liquid lubricant is squeezed out from the rubbing surfaces.

Some liquid molecules may still be trapped among the microasperities of the tip and contribute to the

solid-solid shearing.

Figure 8.5e and f show the results from sliding on the bonded lubricant ﬁlm prepared by conveniently

bonding of the alcohol-ended lubricant (by heating Z-Dol ﬁlm at 150°C for 1 h in order to react the end

groups with hydroxyl groups on the silicon surface with native oxide and washing off nonreactive portion

with freon). As was the case for the unbonded lubricant, no signiﬁcant friction is observed until the hard

wall contact was made. So, even though the ends of the polymer are rigidly attached to the substrate,

the backbone of the polymer apparently has enough ﬂexibility to offer little resistance to the sliding tip

except when rigidly compressed between the two surfaces. For the bonded lubricant, the initial coefﬁcient

of friction is about 0.3, which is about half that for the unbonded liquid ﬁlms. The lower coefﬁcient of

friction indicates that signiﬁcantly more molecules are trapped between the rubbing surfaces than for

the unbonded lubricant. With repeated traversals of the sliding tip, these attached molecules eventually

wear away and the coefﬁcient of friction increases with increasing load.

Mate (1992b) concluded that the liquid ﬁlms have negligible shear stress to applied shear strains until

the molecules are completely compressed or squeezed out from between the sliding surfaces, showing

that hydrodynamic lubrication can occur for surfaces separated by only a few cross-sectional diameters

of the polymer backbone. (Also see O’Shea et al., 1992, 1994). The addition of alcohol end groups greatly

improves the resistance of the molecules to being squeezed out from between the sliding surfaces.

Koinkar and Bhushan (1996a,b) studied the friction and wear performance of Si(100) sample lubri-

cated with about 2-nm-thick Z-15 and Z-Dol perﬂuoropolyether (PFPE) lubricants. Z-Dol ﬁlm was

thermally bonded at 150°C for 30 min and washed off with a solvent to provide a chemically bonded

layer of the lubricant ﬁlm. Data showing the effect of environment on the unlubricated and lubricated

samples are summarized in Figure 8.6. Note that lubricated silicon samples show a lower value of

FIGURE 8.5

The friction force and normal load as a function of the Z sample position for 3-nm-thick ﬁlms on

Si(100) of (a), (b) unbonded lubricant with unreactive end groups, (c), (d) unbonded lubricant with alcohol end

groups and (e), (f) bonded lubricant. The open circles in (f) show the friction force when the experiment is repeated

in the same spot on the bonded lubricant. (From Mate, C. M. (1992),

Phys. Rev. Lett.

68, 3323–3326. With permission.)

coefﬁcient of friction than that of unlubricated sample. Furthermore, sample lubricated with Z-Dol

exhibits a lower value of the coefﬁcient of friction than that of the Z-15 lubricated sample. For the

unlubricated and lubricated samples, the coefﬁcient of friction in a dry environment is lower than at

ambient of about 50% relative himidity. At high humidity, the condensed water ﬁlm from the environ-

ment results in dewetting of the lubricant ﬁlm (or water ﬁlm for the unlubricated sample) resulting in

poorer lubrication performance. Figure 8.7 shows the effect of scanning velocity on the coefﬁcient of

FIGURE 8.6

Coefﬁcient of friction for unlubricated and lubricated samples in ambient (~50% RH), dry nitrogen

(~5% RH), and dry air (~5% RH). (From Koinkar, V. N. and Bhushan, B. (1996),

J. Vac. Sci. Technol. A

14, 2378–2391.

With permission.)

FIGURE 8.7

Friction force as a function of scanning velocity for unlubricated and lubricated samples in ambient

and dry nitrogen environments. Normal loads used are given in the ﬁgure. (From Koinkar, V. N. and Bhushan, B.

(1996),

J. Vac. Sci. Technol. A

14, 2378–2391. With permission.)

fricition in ambient and dry nitrogen environments. The coefﬁcient of friction of unlubricated silicon

sample and lubricated with Z-15 decreases with an increase of the scanning velocity in the ambient

environment, whereas the sample lubricated with Z-15 in the dry nitrogen and the sample lubricated

with Z-Dol in both ambient and dry nitrogen environments do not show any velocity dependence. We

believe that alignment of free liquid molecules at higher scanning velocities results in lower values of the

coefﬁcient of friction.

To study lubricant depletion during microscale measurements, Koinkar and Bhushan (1996a,b) con-

ducted nanowear studies using Si

tips. They measured friction as a function of the number of cycles

for virgin silicon and silicon surfaces lubricated with Z-15 and Z-Dol lubricants, Figure 8.8. An area of

1 µm was scanned at a normal force of 300 nN. Note that the friction force in a virgin silicon surface

decreases in a few cycles after the natural oxide ﬁlm present on silicon surface gets removed. In the case

of Z-15-coated silicon sample, the friction force starts out to be low and then approaches that of an

unlubricated silicon surface after a few cycles. The increase in friction of the lubricated sample suggests

that the lubricant ﬁlm gets worn and the silicon underneath is exposed. In the case of the Z-Dol-coated

silicon sample, the friction force starts out to be low and remains low during the 100-cycles test. It suggests

that Z-Dol does not get displaced/depleted as readily as Z-15. (Also see Bhushan et al., 1995a.) Microwear

studies were also conducted using the diamond tip at various loads. Figure 8.9 shows the plots of wear

depth as a function of normal force and Figure 8.10 shows the wear proﬁles of worn samples at 40 µN

normal force. The Z-Dol-lubricated sample exhibits better wear resistance than the unlubricated and

Z-15-lubricated silicon samples. Wear resistance of the Z-15-lubricated sample is little better than that

of the unlubricated sample. The Z-15-lubricated sample shows debris inside the wear track. Since Z-15

is a liquid lubricant, debris generated is held by the lubricant and it becomes sticky, which moves inside

the wear track and does damage, Figure 8.10.

8.3.2 LB and Self-Assembled Monolayers

Organized and dense molecular-scale layers of, preferably, long-chain molecules have shown to be

superior lubricants on both macro- and microscales as compared with freely supported multimolecular

layers (Bhushan et al., 1995b,c). Common methods to produce monolayers and thin ﬁlms (Ulman, 1991)

are the Langmuir–Blodgett (LB) deposition (Roberts, 1990) and self-assembled ﬁlms by chemical grafting

of molecules (Jaffrezic-Renault and Martelet, 1992). The LB ﬁlms are bonded to the substrate by weak

FIGURE 8.8

Friction force as a function of number of cycles using an Si

tip at a normal load of 300 nN for

unlubricated and lubricated samples in ambient environment. Arrows in the ﬁgure indicate signiﬁcant changes in

the friction force because of removal of surface or lubricant ﬁlm. (From Koinkar, V. N. and Bhushan, B. (1996),

J. Vac. Sci. Technol. A

14, 2378–2391. With permission.)