L05D: Observation of defects
• Many methods of observing defects in and on solids, with improvements and
new techniques being developed all the time.
• What technique is best depends on the size of the feature you want to see
and the type of material.
• Surface features seen with the naked eye can reveal a lot, e.g.:
− The symmetry of the crystal lattice may be manifested in the crystal shape.
− The shape may reveal the presence of grain boundaries.
− Very small features can be seen by looking at light reflected off the surface,
such as grains down to ~ 1 mm and the emergence of screw dislocations.
• Roughening the surface mechanically or by etching can accentuate
orientation differences in reflected
light, as well as preferential etching
of grain boundaries to outline them.
For small grains might need
an optical microscope with
reflected light at different angles.
Also known as incident light or
metallurgical microscopy.
Last modified by W.R. Wilcox, Clarkson University, on October 20, 2013
No color.
Optical microscopy by reflected light
(e.g. a metallurgical microscope with light from below)
• Useful up to 1000X magnification. Depth of field decreases as mag increases
• Polishing removes surface features (e.g., scratches)
• Etching or sand blasting changes reflectance depending on grain orientation.
crystallographic planes
Micrograph of
brass (a Cu-Zn alloy)
0.75mm
2
Incident light microscopy
http://en.wikipedia.org/wiki/Optical_microscope
• Basic limitation is the wavelength of light, ~0.2 m = 200 nm
• But interference effects can be used to see much smaller step heights in
incident light.
• For opaque engineering materials, the solid is first cut, polished
mechanically, possibly etched, and then can be viewed by various methods:
e.g., stainless steel http://www.cartech.com/techarticles.aspx?id=1450 :
Brass :
http://www.pearson-studium.de/books/3827370597/cd01/Gallery/Images/Copper/VanderVoort-Readme.pdf
For some materials, the polarization of reflected light depends on orientation,
so a polarizing microscope can be used to reveal grains and twins in color
without a colorizing stain.
Transmitted light microscopy
• For transparent materials, shine the light up through the sample.
• Many variations in the optics are possible.
• The colors obtained in a polarizing microscope reflect both the grain
orientation and thickness, e.g.:

http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artdec11/db-video1.html
• http://www.olympusmicro.com/galleries/polarizedlight/index.html
Alkalic Syenite
Other methods of optical microscopy
• Polarized light
– metallographic microscopes often use polarized light to increase contrast
using reflected (incident) lighting.
– Also used for transparent samples such as some ceramics and polymers
using transmitted light. Can see strain.
• Dark-field
– Light comes in only from an angle. Only the light reflected in the
direction of the objective lens is seen. All else is dark.
• Ways to beat the resolution and depth of field limits imposed by the
wavelength of light and/or the need to focus:
– Interference microscopy (several types)
– Scanning optical microscopy:
http://en.wikipedia.org/wiki/Near-field_scanning_optical_microscope
– Confocal microscopy: http://en.wikipedia.org/wiki/Confocal_microscopy
5
Other types of microscopy
• For higher resolution need shorter wavelength than light:
− X-Rays? Difficult to focus, but x-ray topography useful for dislocations
− Transmitted electrons
– wavelengths about 3 pm (0.003 nm) depending on energy
• (Magnification up to ~1,000,000X)
– Electron beam focused by magnetic lenses
– Requires very thin flat samples for electrons to go through & focus
– Can make carbon replicas of the surface of thick materials or particles
6
Transmission electron microscopy
http://en.wikipedia.org/wiki/Transmission_electron_microscopy
• Very thin samples so that electrons can pass through them.
• High resolution TEM allows resolution of atoms.
• Can see individual dislocation lines,
e.g. in Ti:
Si EBSP
(electron backscatter diffraction)
http://www.sciencedirect.com/science/article/pii/S1359645404004410
Scanning electron microscopy (SEM)
http://en.wikipedia.org/wiki/Scanning_electron_microscope
• A serious limitation of optical and electron microscopy is that the depth of
field is on the order of the resolution. As you go up in magnification the depth
that is in focus becomes smaller and smaller so only view planar surfaces.
• Modern electronics enables imaging of non-planar surfaces.
• In SEM a finely focused electron beam is raster scanned across the surface
in synchronization with a display device such as a monitor.
• A detector is used to detect scattered electrons reflected by the sample, or
secondary electrons or x-rays emitted by the sample. The detector signal is
fed to the display device, thereby yielding
an image of the surface. For example:
http://www.rcsms.auckland.ac.nz/uoa/home/rcsms/rcs
ms-facilities/rcsms-sem
fracture surfaces: http://www.met-tech.com/SEM1.html
Scanning electron microscopy of surfaces
− 1 nm to 20 nm resolution.
•
•
•
•
•
Several detection methods:
Secondary electrons
Back-scattered electrons
Because focusing not required, sample need not be planar.
To avoid charging of the sample, it must either be an electrical
conductor or be coated with a thin conducting film, such as
gold.
Energy-dispersive x-ray spectroscopy
http://en.wikipedia.org/wiki/Energy-dispersive_X-ray_spectroscopy
• Detection of x-rays emitted in an SEM enables analysis of composition
(acronyms EDS, EDX, XEDS).
• For example, Portland cement: Image is 256 µm x 200µm.
C 3S
C 2S
C 3A
C4AF
gypsum
K2SO4
lime and/or calcite
silica
periclase (with Mg)
C=CaO, S=SiO2, A=Al2O3, F=Fe2O3 all hydrated
Scanning probe microscopy
https://en.wikipedia.org/wiki/Scanning_probe_microscopy
• Atomically sharp probe that is raster scanned along a surface in
synchronization with a display.
• Many types of tips, which may tap the surface or be slightly above it.
• Can sense many types of atomic fields – electronic, magnetic, etc.
• Can see individual atoms and even move them around.
• Tend to be slow, cover a small area, and require a very planar surface.
Tilt grain boundary in SrTiO3 [001](210) by STM
http://www.sciencedirect.com/science/article/pii/S0304399111002075
Vacancy on Ge (111) by STM
http://www.sciencedirect.com/science/article/pii/S0039602801010573
Scanning Tunneling Microscopy
(STM or STEM)
• Atoms and molecules can be imaged and also moved on a surface!
CO on platinum (111)
Fe on copper (111).
These Kanji characters
represent the word “atom.”
CO on copper
 http://www.youtube.com/watch?feature=player_embedded&v=oSCX78-8-q0
Many other sensing methods have been developed for surface scanning.
Dislocation etch pits
• With some etchants, pits are formed where dislocations emerge at a surface.
• For example, InP. Line is 0.1 mm.
http://www.sciencedirect.com/science/article/pii/S135964549900292X
• If the numbers are not large and not aligned in low-angle grain boundaries,
the number of pits per unit area can be counted to give an etch pit density,
which is often referred to as the dislocation density.
• It’s equivalent to the total lengths of all dislocations per unit volume.
• May be as high as 1012/cm2
Characterization of grain size
• ASTM International standard methods:
http://materials.mcmaster.ca/faculty/malakhov/4L04/ReferenceMaterials/AST
M%20standards/ASTM%20E112-10%20Average%20Grain%20Size.pdf
• ASTM grain size number, n = (log N/log 2) + 1
where N is the number of grains per square inch
at 100X.
• Methods to measure N:
– Planimetric: Count grains in given area
– Intercept: Count the number of grain boundaries
intersecting a line.
– Above by hand or with image analysis software.
– For equiaxed grains compare image at 100X with
n
N