Optical Materialography Part 1: Sample Preparation

1. The sample examined in this video is a metal nut. First cut the sample normal to the hoop plane using a linear precision saw in order to isolate undamaged microstructural features for later viewing.
2. Mount the specimen with the side to be imaged facing down on a mounting press. You must, of course, make sure that the sample is small enough to fit in the die cavity of the press, both laterally and vertically.
3. Fill the remaining volume of the mounting press cavity with Bakelite, a thermosetting resin.
4. Press the sample and using the prescribed heat, pressure and duration prescribed for Bakelite or other thermosetting mounting materials you may be using.
5. Over the next steps, polish using sequentially finer polishing media, starting with a coarse grit paper (e.g. 600 and 1200 grit paper, where the abrasive particles are 25.8 and 15.3 µm, respectively) followed by finer polishing suspensions (e.g. 1 and 0.01 µm alumina particles). These coarse-to-fine polishing steps should be done on rotating polishing wheels.
6. Between each step, the sample should be rotated by 90⁰ in order to obtain a directionally uniform polishing and such that the scratches from the prior step can be distinguished from the current one.
7. The sample should be checked on an optical microscope that the scratches on the sample surface left by each step in the schedule will be removed by each subsequent step (Figure 1a-d), with the goal of leaving no observable scratches at the highest magnification of the optical microscope after the final polishing step Figure 1d. Table 1 indicates an example of a polishing schedule that can be used.
8. In the final sample preparation step, prepare a 2% nital solution (2% by volume concentrated nitric acid in ethanol), and dip the polished face in the solution for about 20 seconds. Rinse the sample with ethanol before observing the effects of the etch on a microscope. The etching step is repeated until it is judged that there is sufficient contrast in the granular structure observed. The etching time (and the number of iterations), therefore, is somewhat flexible and depends on the subjective judgment of the materialographist.
   NOTE: Care should be taken during abrasion to keep constant pressure and direction of wheel motion constant. When grit sized is changed, the sample is also rotated in order to distinguish the newly introduced scratches from those that were introduced in the previous step.

Step	Media	Grit	Time (min)	Speed (rpm)	Comments
1	SiC	600	2 min*	120	Rotate 90° before step 2
2	SiC	1200	2 min*	120	Rotate 90° before step 3
3	Al2O3	1 µm	2 min*	120	Rotate 90° before step 4
4	Al2O3	0.05 µm	2 min*	120	* or until scratches  from previous step  are removed

Table 1. Polishing schedule for sample.