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Department of Materials Eng. University Of Technology. Structural Characterizations Optical, Laser Scanning, and ELECTRON MICROSCOPE. BY Assistant Professor Dr. Akram R. Jabur. Microstructure Evaluation. Learning objectives Learn sample preparation for microstructure evaluation - PowerPoint PPT Presentation
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BYAssistant Professor Dr. Akram R.
Jabur
Department of Materials Eng. University Of Technology
Microstructure EvaluationLearning objectives
1.Learn sample preparation for microstructure evaluation
2.Learn about the microscopes used to view microstructures
Microstructure evaluation
Steps for sample preparation not complicated, but skill is required
Not just a research tool
•Reveals a great deal of information about a metal and the mechanical and thermal processes to which it has been exposed
Microstructure information useful for
•Supplier evaluation
•Process development
•Competitive analysis
•Root cause analysis
Resin
Mount Sample
Sample cut up if too large to fit inside container
Containers 25 to 50 mm (1 to 2 inches) diameter
25 mm
ContainerS
am
ple
Polishing Wheels
Grind and Polish1. Sample surface ground
2. Sample polished to mirror finish
• Polishing media is paste or slurry
Side view
Top view
Mounted sample
SampleEtchant
Etch to reveal microstructure features
Etchant chemical used depends on the metal
Optical Microscope
Use light (optical) microscopes to observe microstructures
Up to about 2000x magnification
At 1000x 10 m appears as 10 mm
Cold rolled and annealed brass
Micrographs
Cold rolledAnnealed
550 C (1004 F)Annealed
650 C (1202 F)
0.008 in.
TEM constructed in 1931Von Ardenne first STEM in 1938 by rastering the
electron beam in a TEMZworykin et al. 1942, first SEM for bulk samples1965 first commercial SEM by Cambridge Scientific
Instruments
Resolution at that time ~ 50 nm : Today < 1 nm
Morphology only at that time : Today analytical instrument
It is a microscope that produces an image by using an electron beam that scans the surface
of a specimen inside a vacuum chamber.
What can we study in a SEM?Topography and morphologyChemistryCrystallographyOrientation of grainsIn-situ experiments:
Reactions with atmosphereEffects of temperature
“Easy” samplepreparation!!
“Big” samples!
Scanning Electron Microscope (SEM)
Image features less than 1 m (0.00004”)
Uses electrons to form an image
100x to 100,000x magnification Vacuum chamber
AFM Cantilever Tip Ant Head Blood Cells
Diamond Thin Film(Numerous Multifaceted
Micro-crystals)
Microstructure of a plain carbon steel that
contains 0.44 wt% of carbon
Calcium Phosphate Crystal
The instrument in brief
• electron gun (filament)• electromagnetic optics• scan coils• sample stage• detectors• vacuum system• computer hardware and software (not trivial!!)
We want many electrons per time unit per area (high current density) and as small electron spot as possible
Traditional guns: thermionic electron gun (electrons are emitted when a solid is heated)W-wire, LaB6-crystal
Modern: field emission guns (FEG) (cold guns, a strong electric field is used to extract electrons)Single crystal of W, etched to a
thin tip
Secondary electron detector:(Everhart-Thornley)
Backscattered electron detector:(Solid-State Detector)
Our traditional detectors
Secondary electrons: Everhart-Thornley DetectorBackscattered electrons: Solid State DetectorX-rays: Energy dispersive spectrometer (EDS)
The SEM uses electrons instead of light to form an
image.
A beam of electrons is produced at the top of the
microscope by heating of a metallic filament.
The electron beam follows a vertical path through
the column of the microscope. It makes its way through
electromagnetic lenses which focus and direct the beam
down towards the sample.
Once it hits the sample, other electrons
( backscattered or secondary ) are ejected from the
sample. Detectors collect the secondary or
backscattered electrons, and convert them to a signal
that is sent to a viewing screen similar to the one in an
ordinary television, producing an image.
156 electrons!
Image
Detector
Electron gun288 electrons!
The incident electron beam is scattered in the sample, both elastically and inelastically
This gives rise to various signals that we can detect (more on that on next slide)
Interaction volume increases with increasing acceleration voltage and decreases with increasing atomic number
Incoming electrons
Secondary electrons
Backscattered electrons
Auger electrons
X-rays
Cathodo-luminescence (light)
Sample
Where does the signals come from?
• Diameter of the interaction volume is larger than the electron spot resolution is poorer than the size of the electron spot
Generated from the collision between the incoming electrons and the loosely bonded outer electrons
Low energy electrons (~10-50 eV)
Only SE generated close to surface escape (topographic information is obtained)
Number of SE is greater than the number of incoming electrons
We differentiate between SE1 and SE2
The secondary electrons that are generated by the incoming electron beam as they enter the surface
High resolution signal with a resolution which
is only limited by the electron beam diameterSE2
The secondary electrons that
are generated by the
backscattered electrons that
have returned to the surface
after several inelastic
scattering events
SE2 come from a surface area
that is bigger than the spot
from the incoming electrons
resolution is poorer than for
SE1 exclusively
Sample surface
Incoming electronsSE2
A fraction of the incident electrons
is retarded by the electro-
magnetic field of the nucleus and if
the scattering angle is greater
than 180° the electron can escape
from the surface
High energy electrons (elastic
scattering)
Fewer BSE than SE
We differentiate between BSE1
and BSE2
SE produces higher resolution
images than BSE
By placing the secondary
electron detector inside the lens,
mainly SE1 are detected
Resolution of 1 – 2 nm is
possible
Photons not electronsEach element has a
fingerprint X-ray signalPoorer spatial resolution
than BSE and SERelatively few X-ray signals
are emitted and the detector is inefficient
relatively long signal collecting times are needed
Microstructure ranging from crystal structure to Engine components (SiC)
XRD,TEM,STM SEM OM
Grain I
Grain II
atomic
ValveTurbocharge
1
Best resolution that can be obtained: size of the electron spot on the sample surfaceThe introduction of FEG has dramatically
improved the resolution of SEM’sThe volume from which the signal
electrons are formed defines the resolutionSE image has higher resolution than a BSE
imageScanning speed:
a weak signal requires slow speed to improve signal-to-noise ratio
when doing a slow scan drift in the electron beam can affect the accuracy of the analysis
References Fundamentals of materials Science and Engineering – William
D. Callister Physical Metallurgy – Robert W. Cahn Physical Metallurgy and Advanced Materials – R. E. Smallman Physical Metallurgy Principles – Robert E. Reedhill http://en.wikipedia.org/scanning_electron_microscope
Thank You