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Introduction to Optical MicroscopeIntroduction to Optical Microscope

--Sample Preparation TechniquesSample Preparation Techniques

The initial grit size depends on the

surface roughness and depth of

damage from sectioning.

Surfaces cut by abrasive cutoff

saw start with 120 to 240 grits.

Surfaces cut by EMD or diamond

saw start with 320 to 400 grits.

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I t d ti t O ti l MiI t d ti t O ti l Mi

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Artifact structure from improper grinding

Surface deformation from improper grinding should be avoid, otherwise the

microstructure may be obscured as shown below.

I t d ti t O ti l MiI t d ti t O ti l Mi

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Artifact from improper polishing

Polishing should produce a scratch-free surface. Excessive pressure may cause

artifact of second phase particles as shown below.

20 x 200 x

I t d ti t O ti l MiI t d ti t O ti l Mi

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Introduction to Optical MicroscopeIntroduction to Optical Microscope--Sample Preparation TechniquesSample Preparation Techniques

240 Grit 400 Grit 600 Grit

Polis

hing

direc

tion

Polishing

Ultrasonic washUltrasonic wash Ultrasonic washUltrasonic wash Ultrasonic washUltrasonic wash

Checkun

derO

M

Checkun

derO

M

Checkun

derOM

Checkun

derOM

66 mm 33 mm 11 mm 0.250.25 mm

Polishing

I t d ti t O ti l MiI t d ti t O ti l Mi

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Glass Fiber CompositeGlass Fiber Composite

Microstructure of wood cellMicrostructure of wood cell

PP crystal structurePP crystal structure

A thin section afterA thin section aftergrinding on the machinegrinding on the machine

PatroPatro--thin sectioning machinethin sectioning machine

I t d ti t O ti l MiIntrod ction to Optical Microscope

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(5) Etching

Using chemical to dissolve selectivelythe surface of materials in order to

reveal the inhomogeneous nature in

microscopic scale.

For example the grain boundaries of

polycrystalline metal

Introd ction to Optical MicroscopeIntroduction to Optical Microscope

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Introduction to Optical MicroscopeIntroduction to Optical Microscope--Sample Preparation TechniquesSample Preparation Techniques

Etching is basically a controlled corrosion process resulting from electrolytic action

between surface area of different potential.

Electrolytic activity results from local physical or chemical heterogeneities which

render some features anodic and others cathodic under the specific etching

conditions.

Chemical Etchants produce contrast by

Crystal faceting

Selective phase dissolution.

Common chemical etchants have three components:

A corrosive agent (acids)

A modifier (alcohol, glycerin)

An oxidizer (hydrogen peroxide, Fe3+, Cu2+)

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Common etchants for metals (for example)

Kellers reagent2.5 ml HNO3/1.5 ml HCl

1.0 ml HF, 95 ml water for Al and alloys

Nital1-10 ml HNO3 in 90-99 ml methanol for Fe and steel

Picral

4 -10 g picric acid, 100 ml ethanol for Fe and steel

10 ml HF/5 ml HNO3 85 ml water for Ti and alloys

NH4OH/3% H2O2 for Cu and alloys

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Etchant effects on microstructure images