Introduction
The term tribology is derived from the Greek word "tribo" meaning rubbing
and "logy" meaning knowledge. The original applications by the Greeks of
tribology were in trying to understand the motion of large stones across
the earth's surface. Today tribology has grown to include the methodical
study of friction, lubrication, and wear.
Tribology plays a critical role in diverse technological areas. In the
advanced technological industries of semiconductor and data storage,
tribological studies help optimize polishing processes and lubrication
of data storage substrates. In traditional industries such as automotive
and aerospace, tribological studies help increase the lifespan of
mechanical components.
Many industrial processes require a detailed understanding of tribology
at the nanometer scale. The development of lubricants in the automobile
industry depends on the adhesion of nanometer layers (mono layers) to a
material surface. Assembly of components can depend critically on the
adhesion of materials at the nanometer length scale.
There are a number of traditional tools for characterizing friction,
lubrication and wear. The most common characterization tool is the tribometer
having several configurations such as pin-on-disk, ball on flat, and flat on
flat, etc. Generating motions at the nanometer scale is extremely challenging.
New characterization techniques are required to understand tribology at the
nanometer scale.
The atomic force microscope is now being routinely applied for studying
nanoscale tribology. The natural extension of the AFM for tribology
applications is derived from the motion of a nanometer-sized stylus in
the AFM over a surface. Although traditional tribology testing is not
done with an AFM, many new types of applications are possible.
Examples of the application of AFM to tribology include:
- Direct three-dimensional visualization of wear tracks, or scars on a surface.
- Measurement of the thickness of solid and liquid lubricants having nanometer or even monolayer thickness.
- Measurement of frictional forces at the nanometer scale.
- Surface characterization of morphology, texture, and roughness.
- Evaluation of mechanical properties such as hardness and elasticity, and plastic deformation at the nanometer scale.
A major advantage of the AFM for tribological studies is that the AFM can be
routinely used on all types of materials. Materials commonly studied include:
ceramics, metals, polymers, semiconductors, magnetic, optical, and biomaterials.
AFM investigations are usually made in ambient air environment. It is possible
to make AFM studies in a vacuum or liquid environment.
Nanoscale Wear Analysis
The effects of wear at the nanometer scale become critical to the optimization
and stability of machines as the tolerances in precision machines become
smaller and smaller. Traditional microscopes such as the optical and scanning
electron microscopes facilitate visualization of wear in 2-dimensions. For
example, with the SEM it is possible to get a magnified view of wear tracks
in the x-y axis but cross sectioning is required for measuring the depth of
wear tracks.
The AFM allows direct 3-dimensional visualization of wear tracks and scars.
The images may be displayed in a 2-D projection and a 3-D projection. Direct
measure of wear track depth can be easily measured with a line profile derived
from the AFM image.
Figure 1: Three-dimensional atomic force microscope (AFM) image of a
polish mark on a piece of steel. The scan range in X and Y is 3
micrometers and the entire Z range is 40 nanometers.
Figure 2: Metrological study of a scratch mark in the surface of
a polished material (stainless steel). Because the AFM directly
measures three-dimensional data, the depth of the scratch mark
is easily quantified. In this case the mark is 8.6 nm deep. Z= 8.6 nm
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.
Frictional Forces
Friction between two surfaces depends on the chemical and mechanical
interaction between the surfaces. Changes in chemical composition giving
rise to friction are measurable with the AFM. The technique for measuring
these forces is called lateral force, or frictional force microscopy.
As the probe moves over a surface in the AFM, changes in the chemical
composition of the surface can give rise to torsions of the cantilever
on which the probe is mounted. The torsion of the cantilever is then
proportional to the friction between the probe and the surface.
Figure 4: Illustration of the principle that allows lateral force or
frictional fore microscope images to be measured. The cantilever will
twist as the probe interacts with the surface while scanning.
In an AFM it is possible to simultaneously measure topography and frictional
force images. The topography image is derived from monitoring the vertical
forces on the cantilever and the friction image is acquired simultaneously
by monitoring the lateral motions of the cantilever. Below is a FFM image of
a sample illustrating changes in the friction.
Figure 5: (left) Two dimensional view of a composite material measured
with the AFM. Barely visible in this image are striation marks derived
from a change in chemical composition at the surface.
(right) The lateral force or frictional force image of the composite
material clearly shows changes in chemical composition at the surface
of the composite material. Both the topography and frictional force
image are measured simultaneously with the AFM.
Surface Texture / Morphology / Roughness
The AFM gives extremely high contrast on surfaces that are flat at the
nanometer scale. Optical and electron microscopes are not able to resolve
surface texture that is easily measured with the AFM. Applications include
the visualization of surface topography in both 2-d and 3-d perspectives,
line roughness measurements, and area roughness measurements. All of the
traditional area and surface roughness parameters can be calculated after
the AFM image is acquired.
Figure 6: (A) AFM image of a metal bonding pad on a semiconductor
device (B) Use of the AFM to measure line roughness of a polymer
sample (C) AFM image of polished silicon and the area roughness
of the silicon.
Nanoscale Mechanical Properties
Mechanical properties such as hardness, elastic modulus, stiffness and
compressibility as well as material behavior such as plastic deformation,
and fracture can be studied with the AFM.
It is possible to study nano-hardness by directly pressing an AFM probe
into a sample's surface; however, it is advantageous to use an instrument
that is optimized for nano-indentation. The primary advantage of the
nano-indenter over an AFM for nano-hardness measurements is that it is
easier to get calibrated measurements with the nano-indenter. It is useful
to use the AFM to measure the three-dimensional topography of indentations
made with a nano-indenter. AFM images allow direct visualization of material
deformation or fracture behavior.
Figure 7: Three-dimensional view of nano-indents in a material surface.
From the AFM image it is possible to see the depth of the indentation
as will as visualizing the material deformation.
Using techniques such as pulsed force mode, the stiffness of a sample at a
matrix of locations is measurable. From this data it is possible to create
a stiffness mapping of a surface. Stiffness maps can only be made on samples
where the stiffness of the surface is lower than the stiffness of the
cantilever. Such stiffness images are routinely measured on polymer samples.
Adding a fixture to the stage of the AFM makes the study of material
behavior such as plastic deformation and fracture possible. The fixture
permits creating forces on a sample while AFM images are being taken.
A variety of materials may be studied with such a technique.
Acknowledgment
This article was written with the assistance of Dr. Hong Liang. Dr Liang
is an Assistant Professor in the Mechanical Engineering Department at the
University of Alaska Fairbanks. Her specialty is the application of
tribological studies to several areas of science and technology including
surface properties of advanced materials and nanomanufacturing processes.
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