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TECHNIQUES- Microscopy
Sample Selection | Sectioning | Epoxy Mounting | Grinding & Polishing | Etching | Microscopy
Optical
microscopy of metals requires the use of
a reflected-light microscope. I use a
Reichert PolyVar SC microscope, which
offers a wide field of view and superb
optics. This microscope comes in
many configurations, and my model is equipped with
a large stage and offers two
illumination modes: 1) incident light
for reflected-light metallography, and 2) transmitted
light for thin section analysis.
In each mode of illumination, the
operator can select either
plane-polarized light or cross-polarized
light. The microscope is also
equipped with differential interference
contrast (DIC, also known as Nomarski
microscopy). This mode of
illumination is effective in amplifying
minor topographic variations of the
specimen's surface.
The
following three images show a troilite
inclusion in the iron meteorite NWA6259, viewed using the
three modes of reflected-light
illumination (FOV = 688 microns).
The photo on the left was taken in plane-polarized
light and shows troilite nodule
surrounded by cloudy zone metal. The thin
white mineral at the top of the troilite is
probably schreibersite (isotropic). The
center photo, taken in
cross-polarized light, reveals
the presence of troilite twinning.
The third photo was taken using
differential interference contrast. The troilite substructure is visible due to minor differential
polishing.
For reflected-light work,
it is important that the
specimen's surface is precisely
perpendicular to the microscope
objective. This can be
accomplished using a metallographic
specimen press (shown next to the
microscope in the first photo).
The specimen is placed on a glass slide, with PlasticineÒ
sandwiched between, and then pressed
to achieve parallelism of the
glass slide and specimen surface.
For photography, I use Nikon Coolpix series cameras
(especially the Coolpix 990 and 4500
models). These older cameras are very well-suited for microscopy due to the
small lens size (28 mm) and internal
zoom mechanism. The camera is
attached directly to one of the
microscope ocular ports using a 10x
digital camera adapter lens. The
camera is controlled remotely using a
Nikon MC-EU1 cable. The camera is
output to a LCD monitor, and this
monitor image is used for focusing. |
