The aim of this experiment is to introduce metallographic preparations techniques (grinding, polishing and etching) for microscopic investigations to get acquainted with the preparation of metallographic specimens and to gain hands-on experience.

Introduction

Material structure (Metallography, the science of) is one very significant aspect in understanding the behavior of a certain material. Experimentally speaking, there are two ways of carrying out structural investigation: either on a macroscopic level, which deals with investigations done at a low magnification power or using naked eye, or on a microscopic level, requiring magnification power between 100 to 1000 times to identify the existing phases or precipitates, and to get a full account on the chronology of the solidification process and grain formation.

Still, microstructural investigations require certain preparation process so that the specimen is ready for investigation under the optical microscope (OM). These processes are namely:

1. Sectioning
2. Mounting
3. Course (planar) grinding gradually from rough to fine grinding
4. Rough Polishing
5. Fine (final) Polishing
6. Etching

The aim of the stages (3-5) is to remove any oxide layers or deformed layers and scratches that spoil the microscopic view or the first surface layers that might have different microstructure due to excessive heating on the surface under certain working conditions.

Besides these stages, etching comes to be the most important process. Using a certain acidic or alkaline etching solution, a layer of the material surface is removed such that the grain boundaries start to show under the microscope. The etching solution attacks the grain boundaries, yet excessive etching may attack the grains  , destroying the entire view.

Etching leaves the boundaries as bumpy zones in the specimen surface, as compared to the flat polished surface (dashed) in figure 1. These ups and downs, as well as the inclined grain surfaces reflect the light of the optical microscope, showing these details.

SEM can reach magnification powers of 80,000 times. One of the important features of SEM is that the specimen, especially if it is a fracture surface, can be viewed directly under the instrument without preparation. For TEM, it should be very thin so that the electron beam can penetrate. The basic difference between the SEM and TEM is that the TEM shows more a height perception, showing voids or dimples. Whereas the SEM gives a better idea for the surface morphology (Hertzberg: 294). The following table recognizes the basic differences between the SEM, TEM and OM.
 

1. Sectioning
2. Mounting
3. Course (planar) grinding gradually from rough to fine grinding
4. Rough Polishing
5. Fine (final) Polishing
6. Etching

3. Course (planar) grinding: In this process, the specimen surface is ground using silicon carbide powder discs or belts, of different grades. The process has to be done sequentially starting 120, followed by 240, 320, and 400 sand paper. The surface should be lubricated with water, to flush away the remains of the grinding process in order not to stick to the ground clean surface and to cool the surface to avoid any change in the microstructure. This process aims at removing rust and irregularities in the first surface layers. First, the specimen has to be hold in a one direction over the grinding surface, then rotated at  45 degrees to remove the scratches that are formed during the process.

4. Rough Polishing: The specimen is held against a rotating polishing wheel, which is nylon cloth-covered. This nylon cloth is covered by a small amount of diamond abrasive solution, which is made up of the diamond powder as oil-solution. An oil lubricant is used to enhance the polishing effect of the diamond particles. Here the specimen should be herd in a direction opposite to the motion of the wheel.

5. Fine Polishing: This is the final polishing stage, in which the fine scratches are removed. This is done using a polishing Al₂O₃ powder, with particle size of 0.05 microns. This powder is put over the cloth-covered wheel, with distilled water as the lubricant.

6. Chemical Etching: After these stages, it is still impossible to see the microstructure, because right now the specimen surface is very shiny, hence reflecting the lights. In order to make the microstructure visible, the specimen should be etched in an acidic/alkaline solution. The solution attacks the grain boundaries, which are the weakest positions, leaving them a bit darker than the grains themselves. In the experiment , the etching solution was 2 ml HNO₃ + 100mL CH₃OH. It is important to wash the specimen after etching to stop the etching process from deforming the grains.

Optical Microscope
Grinding and Polishing machines
--Make sure to include sketches or images for them in the report.

Results/Discussion

Further References/Images

--The following sites and books contain useful information and images for several articles that we have encountered in this experiment. Make sure to check them.

Lab handouts.

Kalpakjian, Serope. Manufacturing Process for Engineering Materials. Addison Wesley, 3^rd Ed., 1997.

Smith, William F. . Principles of Materials Science and Engineering. McGraw Hill, 3^rd Ed., 1996.

Metallographic Specimen Preparation Basics¶¶¶¶¶ http://www.metallographic.com/Basics.htm

Specimen Preparation Equipment Company¶¶¶¶¶ http://www.metatechind.com/products.htm

Academic Page for a similar experiment¶¶¶ http://compfab.me.lsu.edu/me3701/experiments/metallography/expt3.htm

Specimen Preparation Equipment in NASA¶¶ http://arioch.gsfc.nasa.gov/313/html/1-2-3.html