Optical Materialography Part 1: Sample Preparation
Source: Faisal Alamgir, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA
The imaging of microscopic structure…
- 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.
- 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.
- Fill the remaining volume of the mounting press cavity with Bakelite, a thermosetting resin.
- Press the sample and using the prescribed heat, pressure and duration prescribed for Bakelite or other thermosetting mounting materials you may be using.
- 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.
- 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.
- 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.
- 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.
Materialography is a method for microscopic structure imaging and analysis of solid materials. In particular materialography qualitatively studies the porosity in the material, the size and shape distribution of the grains, and the degree of isotropy of the microstructures.
Such detailed analysis requires specific sample preparation of solid materials. This video will illustrate the four major steps performed to prepare a sample four optical materialographic analysis.
Materialography is used to characterize solid materials. With this method, qualitative analysis, as well as quantitative analysis can be performed. In this video we will focus on the qualitative information obtained for a solid. In materialography the sample can either be probed with light, or with an electron beam. Depending on the choice of the probing tool, the sample needs to be prepared in different ways. We demonstrate here the principles of sample preparation for the optical materialography of solid materials of similar hardness to that of steel. This sample preparation is performed in four major steps, cutting, mounting, polishing, and etching. Let us look in detail at each of these steps.
The very first step is sample cutting. For samples with expected isotropic microstructures, meaning evenly distributed microstructures, the orientation of the cut is arbitrary, but for other cases, said as anisotropic samples, the cutting vector should be oriented according to specific directions or planes of the sample. In the second step the cutting sample is mounted on a support. The solid material is fixed to a hot compression thermosetting material like a resin or an epoxy to form a pressed pellet. The third step is sample polishing. It is performed in multiple subsequent steps, from coarse polishing to finer, and finer polishing. The idea is to reveal micro structural features while removing scratches left on the surface of the sample from the previous polishing sub step.
The sample is then ready for the last step that is etching. This is a chemical exposition of the sample to an acid. Some grain boundaries of the solid material have more atomic defects and are therefore more effected by the acid solution. This will have the effect of carving inside the mounted sample. Consequently, this step enhances the contrast between grains that is revealed by optical microscopy. Now that you understand the principles behind sample preparation for optical materialography, let's see how the main steps of the procedure are performed in the laboratory.
The specimen used in this example is a metal nut. The sample preparation is demonstrated in four main steps as following: First use a linear precision saw to cut the sample normal to the hoop plane. Second, make sure the sample fits the die cavity of the press. Mount the sample in the cavity with the side to be imaged facing down on the mounting press. Then fill the remaining volume of the mounting press cavity with Bakelite.
Find the prescribed heat, pressure, and duration for Bakelite and press the sample accordingly. Note that other thermosetting mounting materials can be used for other types of samples. The third step is polishing of the sample. Start with a coarse 600 grit paper. Employ the rotating polishing wheels for two minutes at a speed of 120 rpm to polish the sample. Then use an optical microscope to check the scratches on the sample surface. Now rotate the sample by 90 degrees from it's first polishing position and repeat the polishing with a 1,200 grit paper. Make sure to keep the pressure and direction of wheel motion constant.
Check the sample surface with the optical microscope. The previously identified scratches should be removed and new ones will be identified. Rotate again the sample by 90 degrees and polish the sample with finer polishing suspensions of one micrometer alumina particles and again verify with microscope the scratches on the sample surface. Repeat the sequence, this time with 0.05 micrometer alumina particles. At the final polishing step, using the highest magnification of the optical microscope.
There should be no observable scratches on the sample surface. The last step is the sample etching. First prepare a 2% Nital solution by mixing 2% volume concentrated nitric acid in ethanol. Dipped the polished face of the sample in the solution for about 20 seconds. Rinse the sample with ethanol, then observe the etched surface on the microscope. Repeat these etching, rinsing steps until sufficient contrast in the granular structure is observed.
Optical materialography is a very useful technique to characterize solid materials for various applications. For instance, toroidal inductor cores are commonly used in electronic applications to regulate electromagnetic interference. These cores are economically manufactured by compacting iron powder. Porosity and grain size of the core material both impact the electromagnetic properties of the inductor and they can be assessed by optical materialography.
Porus materials, due to their permeability, are used for manufacturing of synthetic membranes. Optical materialography is employed to analyze the void structure of the 2D cross section of the membrane material and in consequence to assess the porosity quality of the membrane.
You've just watched Jove's introduction to sample preparation for optical materialography. You should now understand the four steps of sample preparation, cutting, mounting, polishing, and etching and how these are important for a qualitative analysis of material microstructures.
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Q1: What are the four main steps of sample preparation for optical materialography?
Optical materialography requires four sequential preparation steps: cutting, mounting, polishing, and etching. Cutting orients the sample appropriately for analysis. Mounting fixes the sample to a support using thermosetting material like resin or epoxy. Polishing removes scratches progressively using coarser to finer grits. Etching chemically exposes grain boundaries with acid solution to enhance contrast for microscopic observation.
Q2: Why is polishing performed in multiple steps during sample preparation?
Multi-step polishing progressively reveals microstructural features while removing scratches from previous polishing stages. Starting with coarse 600-grit paper and advancing to finer suspensions like 1-micrometer and 0.05-micrometer alumina particles ensures a scratch-free surface. Rotating the sample 90 degrees between steps and checking with an optical microscope confirms that previous scratches are removed before proceeding to finer grits.
Q3: How does etching enhance grain visibility in materialography samples?
Etching exposes the polished sample to an acid solution, typically 2% Nital prepared from nitric acid and ethanol. Grain boundaries contain more atomic defects and are preferentially attacked by the acid, creating surface relief that carves into the material. This differential etching increases contrast between grains, making them clearly visible under optical microscopy for qualitative analysis of microstructure.
Q4: What information can qualitative materialography analysis reveal about solid materials?
Qualitative materialography directly observes porosity presence, grain size and shape distribution, and microstructural isotropy or anisotropy. These observations help characterize material properties without statistical measurement. For example, toroidal inductor cores manufactured from compacted iron powder can be assessed for porosity and grain size, both critical to electromagnetic performance. Porous membrane materials can be analyzed for void structure quality.
Q5: How does sample orientation affect cutting in optical materialography?
For isotropic materials with evenly distributed microstructures, cutting orientation is arbitrary. However, anisotropic samples require the cutting vector to be oriented along specific directions or crystallographic planes to capture representative microstructural features. Proper orientation ensures the resulting cross-section accurately reflects the material's structural characteristics for meaningful qualitative analysis.
Q6: What role does the mounting material play in sample preparation?
Mounting materials, typically thermosetting resins or epoxy, fix the cut sample to a support during preparation and analysis. The sample is placed in a press cavity with the imaging surface facing down, then the cavity is filled with mounting material and pressed under prescribed heat and pressure. This creates a stable, handleable specimen that maintains proper orientation throughout polishing and etching steps.
Q7: Can samples prepared for optical materialography be used for other microscopy techniques?
Yes, samples prepared for optical materialography can be used for scanning electron microscopy with minimal or no additional preparation steps. This versatility makes optical materialography sample preparation an efficient starting point for multiple analytical techniques, allowing researchers to gather complementary microstructural data from a single prepared specimen.