Optical Materialography Part 1: Sample Preparation

Overview

Source: Faisal Alamgir, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

The imaging of microscopic structures of solid materials, and the analysis of the structural components imaged, is known as materialography. Qualitative information such as, for example, whether or not there is porosity in the material, what the size and shape distribution of the grains look like, or whether there is anisotropy to the microstructure can be directly observed. We will see in Part 2 of the Materialography series, however, that statistical methods allow us to quantitatively measure these microstructural features and translate the analysis from a two-dimensional cross section to the three dimensional structure of a material sample.

This presentation will provide an overview of the techniques and procedures involved in preparing solid material samples for optical microscopy. While materialography can be conducted with optical as well as electron-based microscopy, this presentation will focus on the sample preparation specifically for optical microscopy. It should be noted, however, that a sample prepared for optical materialography can be used for scanning electron microscopy as well with minimum, if any, additional steps.

Procedure

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

Results

From the series of images in Figure 1, particularly from the etched sample (Figure 1e), one can observe that the powder pressing process by which this sample was made rendered the grains to have non-circular, elongated shapes, with non-isotropic grain orientation. There is a significant amount of porosity retained in the material through this processing. Part 2 of the Materialography series will explore the statistics of the grai

Application and Summary

These are the standard methods for preparing cross sections of samples for microscopy. While the procedures detailed here are optimized to provide the best results in optical microscopy, some of the steps are unnecessary for scanning electron microscopy, and are inadequate for transmission electron microscopy. For the latter two, separate sample preparation procedures should be followed.

Materialographic sample preparation described here are the necessary first step towards the analysis of int