lap 2+3 Report

MIME 1650 Laboratory 3

Microscopy and Sample Preparation

Objective

The student should learn to prepare the sample and to use microscopy to observe material

structures.

Agenda

• Laboratory 2 procedure (Team work)

• Use of microscopes and their nomenclature

• Methods of sample preparation

Procedures and Report Requirement

Read and follow the instruction when you start to work.

1. Cut sample. 1. Mount sample using pneumatic device. 2. Grind sample on coarse and fine grinding surfaces. 3. Polish sample. 4. Etch sample with natal. 5. Observe sample by using microscope. Sketch the microstructure as completely as

possible in your report.

6. Use a polarizing filter on the microscope and observe the grain structure. 7. After sketching the microstructure, measure the sample surface roughness (set

‘cutoff’ 0.010, V-Pilotor).

Sample preparation

(Do Not Forget: You MUST Wear Safety Glasses in Lab!!!)

The sample used in material science microscopy usually has to have a very flat surface. If

accurate structure is to be observed, care must be taken in preparing the surface to avoid

unwanted scratches, heat effects and other forms of damage.

There are five major operations in the preparation of metallographic specimens:

Sectioning, mounting (optional), grinding, polishing and etching.

Sectioning

Sectioning, the removal of a conveniently sized, representative specimen from a

larger sample, is the most important step in preparing specimens for physical or

microscopic analysis. Incorrect preparation may alter the true microstructure and

lead to erroneous conclusions.

Sectioning methods usually include sawing (using hacksaws, band saws and wire

saws), abrasive cutting and electric discharge machining. In our lab we use a

consumable-wheel abrasive cutting machine. It is often performed using a

coolant, ensuring an almost plane surface without serious mechanical or thermal

damage.

Procedure

1. To cut a suitable size, mark your specimen.

2. Use handles to adjust the specimen’s position.

3. Fix specimen rigidly

4. Cover the lid.

5. Push cutting and bump button.

6. Pull handle down slowly and apply proper pressure.

7. Push STOP button to finish cutting.

8. Remove specimen and clean cutting machine.

9. Remove any burrs and clean the sample by washing it.

Mounting

Mounting is often necessary in the preparation of specimens for metallographic

study. Although bulk sample may not require mounting, small or oddly shaped

specimens should be mounted to facilitate handling during preparation and

examination.

Standard mounts usually measure 25 mm (1 inch), 32 mm (1.25 inch), or 38 mm

(1.5 inch) in diameter; mount thickness is often approximately one half the mount

diameter.

How to do?

1. If MOLD BASE is in the bottom of MOLD CYLINDER, rotate RAM CONTROL to ‘UP’ position.

2. Position sample on MOLD BASE. For easy release, spray little lubricant on MOLD BASE in advance.

3. Rotate RAM CONTROL handle to ‘DOWN’ position.

4. Add mounting material – BAKELITE POWDER about 1 spoonful.

5. Insert and lock CLOSURE.

6. Rotate RAM CONTROL to ‘UP’ position. Apply pressure to 4200 psi (left mounting machine: the red point in middle line.

Right one: to Red Point) because the mount size is 1.25 inch.

Don’t exceed the Red point.

7. Put HEATER into MOLD CYLINDER. Turn on.

8. Rotate TIMER to 8-10 minutes.

9. Turn off HEATER. Use cooling cylinders immersed in water

10. Rotate RAM CONTROL to ‘NEUTRAL’ position. Allow pressure to indicate zero.

11. Loosen CLOSURE

12. Raise Handle to vertical position. Keep one hand on Handle. Rotate RAM CONTROL to ‘UP’ position.

13. Remove CLOSURE and mounted specimen with heat-resistant glove. Be careful, it may be ‘Hot’.

14. Immediately mark the sample for identification purposes.

Grinding

The purpose of grinding is to remove any surface damage caused by cutting and

coarse grinding.

1. Coarse grinding: Chamfer the edges of the sample to 45° to prevent tearing of the lapping cloth. Grind the sample surface to remove any

polymer residue. Clean the surface.

2. Fine grinding: • Start with grit 240 abrasive paper which flat working surface is

flooded with water.

• Hold sample face down and apply it proper pressure. Go forward and backward against working surface of paper with

full strokes and without contacting with abrasive paper. Clean

it. Then rotate sample 90°, repeat until the last series of

scratches are removed (shown in Figure).

(This method is used for carbon steel)

1st 2nd 3rd

• Progressively fine grind with smaller grits, of 320, 400 and 600 size, washing the sample between each grinding.

Polishing

Use rotating wheels with a napless wet cloth. Put Al2O3 suspension on the polish

cloth. Turn on with Slow Speed. Be careful to hold sample. Polish the sample

progressively using finer polishing particles. (Each wheel will have on one size

polishing grit, either 0.3 or 0.05) After final polishing, wash the sample under

running water and use a cotton swab and alcohol to dry the sample (prefer to use

blower).

Etching

A polished sample frequently will not exhibit its microstructure, because light is

uniformly reflected. To further reveal the microstructure, the surface may need to

be etched. For ferrous sample, nital is commonly used. Nital is a nitric acid and

alcohol mixture, and is highly toxic and corrosive.

• Put Nital to the polished surface of sample using a cotton swap. • Let Nital stay on surface about 30 – 40 seconds. • Rinse off using running water. • Use Methyl Alcohol (Methanol) to dry it.

Now the sample is ready for microscopy. Take care to protect the surface. Never touch

the surface or carry the sample in your pocket.

Surface Roughness ( µm )

Drawing of sample under microscope: