Lubricant Studies
It is well known that layers of lubricants on surfaces that are less than
100 nm can dramatically affect lubrication behavior. Characterization of
such films is necessary for developing optimized lubricating films. However,
nanometer scale characterization of lubrication films offers a substantial
challenge. Optical techniques such ellipsometers can be used for measuring
lubrication thickness of large sections, (greater than 10 square micrometers),
of a surface. Measurement of the localized (less than 1 micron) film thickness
is not possible with the ellipsometer.
The probe is mounted at the end of a cantilever in an AFM making it possible
to measure interaction forces between the probe and the surface by monitoring
the deflection of the cantilever. A graph, called a force/distance curve,
shows the forces on the probe as the distance between the probe and the
surface are reduced. The nature of the force/distance curve depends on the
force constant of the cantilever, the lubrication density, probe geometry,
and the lubrication thickness.
By measuring the changes in force/distance curves in an AFM it is possible
to directly ascertain the thickness of lubrication films. Below is an example
of a force/distance curve for a surface with no lubrication film compared to
one with a lubrication film. The thickness of the film is established from
the force/distance curve.
Figure 3: Video optical microscope image of a silicon surface coated
with a lubricant. Force/Position curves were measured at locations on
the surface indicated with the letters A, B, and C. The red light in
the video microscope image is from the laser used for the force sensor
in the atomic force microscope.
A: Force/Position curve measured on one of the droplets of
lubricant. The slope represents the force required to move
the AFM probe through the liquid surface.
B: Force/Position curve at a place on the surface where there
is apparently a thin film of lubricant material.
C: Force/Position curve on a section of the silicon surface
that appears to have no lubrication.
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