How can you see
hardness?
Hardness is not only a material property that
can be tested for, we now have the technology to SEE the hardness
of different materials. Using a scanning probe microscope,
it is now possible to see the different hardnesses of a composite
material and visually distinguish between the hard and soft
regions in that material. A new breed of scanning force
microscopes enables one to obtain images, in which resolution
is based solely on the differences between the hard and soft
regions of the material.
This image was created by imaging the differences
of the hardness of the carbon fibers embedded in softer epoxy.
The circular features are the carbon fibers, the area around
the fibers is the epoxy matrix. The carbon fibers are
much harder than the surrounding epoxy, which is sensed by
the scanning probe of the microscope. This difference
in hardness can be exploited to obtain an image.
SPM
Nanoindenter and other Hardness
Tests
A nanoindenter works with the same principle
as the macroscale hardness tests. However the nanoindenter
uses an extremely small probe that indents only slightly into
the material's surface. The nanoindenter measures hardness
at the nanoscale (10-9). The nanoindenter
is used to measure surface or near surface hardness of extremely
thin or very soft samples. The utility of nanohardness
tests is that the hardness at a precise location can be measured.
Other tests used to measure hardness include
a scratch test with a file to measure the resistance of the
material to scratching, and a scleroscope test that measures
the rebound of a weight that is bounced off the surface of
a material.
The hardness tests described here are used on
materials that are too soft or too thin to be measured using
traditional hardness measurements.
Elasticity
Force modulation imaging is an offspring of
the atomic force microscope (AFM), one of the scanning probe
microscopes. Force modulation uses differences in surface
hardness to produce an image. Force modulation can be
used in applications such as imaging composition changes in
a composite material, analyzing polymer homogeneity, and detecting
contaminants in manufacturing processes. At the heart
of imaging hardness is a specially designed tip holder, as
seen here.
In the standard contact mode SPM, the probe
attached to a cantilever, is scanned over the sample in a
X-Y raster pattern. A feedback loop is used to maintain
a constant force between the sample and tip. With force
modulation imaging, the probe is not only scanned in an X-Y
pattern as in the standard SPM mode, but the probe is also
scanned with a small vertical oscillation, in the Z-direction.
As the probe scans across the sample surface, hard areas on
the sample cause greater cantilever deflections as compared
to a softer area. The deflections of the cantilever
(i.e. topographical information) are processed by the computer,
and an image is subsequently generated. Therefore harder
regions appear dark, while soft regions on the sample surface
appear light.
Elasticity
Force modulation microscopy (FMM) is an extension
of AFM imaging that includes characterization of a sample's
mechanical properties. Like LFM and MFM, FMM allows simultaneous
acquisition of both topographic and material-properties data.
In FMM mode, the AFM tip is scanned in contact
with the sample, and the z feedback loop maintains a constant
cantilever deflection (as for constant-force mode AFM). In
addition, a periodic signal is applied to either the tip or
the sample. The amplitude of cantilever modulation that results
from this applied signal varies according to the elastic properties
of the sample, as shown in Figure 6.
The amplitude of cantilever
oscillation varies according to the mechanical properties
of the sample surface. (bottom).
The system generates a force modulation image,
which is a map of the sample's elastic properties, from the
changes in the amplitude of cantilever modulation. The frequency
of the applied signal is on the order of hundreds of kilohertz,
which is faster than the z feedback loop is set up to track.
Thus, topographic information can be separated from local
variations in the sample's elastic properties, and the two
types of images can be collected simultaneously. Figure 7
shows a topographic contact-AFM image (left) and an FMM image
(right) of a carbon fiber/polymer composite.
Contact-AFM (top) and FMM (bottom) images
of a carbon fiber/polymer composite collected simultaneously
Field of view 5µm
Friction
Lateral force microscopy (LFM) measures lateral
deflections (twisting) of the cantilever that arise from forces
on the cantilever parallel to the plane of the sample surface.
LFM studies are useful for imaging variations in surface friction
that can arise from inhomogeneity in surface material, and
also for obtaining edge-enhanced images of any surface.
As depicted in Figure 4, lateral deflections
of the cantilever usually arise from two sources: changes
in surface friction and changes in slope. In the first case,
the tip may experience greater friction as it traverses some
areas, causing the cantilever to twist more strongly. In the
second case, the cantilever may twist when it encounters a
steep slope. To separate one effect from the other, LFM and
AFM images should be collected simultaneously.
Figure 4.Lateral deflection of the cantilever
from changes in surface friction (top) and from changes in
slope (bottom). LFM uses a position-sensitive photodetector
to detect the deflection of the cantilever, just as for AFM.
The difference is that for LFM, the PSPD also senses the cantilever's
twist, or lateral deflection. Figure5 illustrates the difference
between an AFM measurement of the vertical deflection of the
cantilever, and an LFM measurement of lateral deflection.
AFM uses a "bi-cell" PSPD, divided into two halves, A and
B. LFM requires a "quad-cell" PSPD, divided into four quadrants,
A through D. By adding the signals from the A and C quadrants,
and comparing the result to the sum from the B and D quadrants,
the quad-cell can also sense the lateral component of the
cantilever's deflection. A properly engineered system can
generate both AFM and LFM data simultaneously.
The PSPD for AFM (top) and
LFM (bottom).
Magnetism
Magnetic force microscopy (MFM) images the spatial
variation of magnetic forces on a sample surface. For MFM,
the tip is coated with a ferromagnetic thin film. The system
operates in non-contact mode, detecting changes in the resonant
frequency of the cantilever induced by the magnetic field's
dependence on tip-to-sample separation (See Figure8). MFM
can be used to image naturally occurring and deliberately
written domain structures in magnetic materials.
MFM maps the magnetic domains
of the sample surface.
An image taken with a magnetic tip contains
information about both the topography and the magnetic properties
of a surface. Which effect dominates depends upon the distance
of the tip from the surface, because the interatomic magnetic
force persists for greater tip-to-sample separations than
the van der Waals force. If the tip is close to the surface,
in the region where standard non-contact AFM is operated,
the image will be predominantly topographic. As you increase
the separation between the tip and the sample, magnetic effects
become apparent. Collecting a series of images at different
tip heights is one way to separate magnetic from topographic
effects.
