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