Bhushan, B. “Boundary Lubrication Studies Using Atomic Force/Friction ...”
Handbook of Micro/Nanotribology.
Ed. Bharat Bhushan
Boca Raton: CRC Press LLC, 1999© 1999 by CRC Press LLC
© 1999 by CRC Press LLC

8

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

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

×

10

–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.

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

δ

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.)

© 1999 by CRC Press LLC

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

R

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.

δ

is the cross-sectional diameter of the liquid chain and

R

g

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.)

© 1999 by CRC Press LLC

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.

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.)