Microstructure and the Origins of Metallography

Metallography is the preparation of materials in order to observe the internal microscopic features of the material structure.  The name derives from the inspection of the internal microscopic features of metals, but similar processes are used to inspect the internal structure of any material, including circuit boards, ceramics, composites, and minerals.  In fact, the process was initiated by geologists who polished rock specimens to better observe the individual crystals of minerals.  In 1863 a geologist named Henry Clifton Sorby was the first to polish metal for microscopic inspection.  His father had left him a cutting tool manufacturing shop in Sheffield, England and he was suffering financially because the factors controlling the hardness and toughness of the tools were not understood.  Henry observed that cutting tools sometimes broke with a crystalline appearance similar to rocks.  He suspected that, just like rocks, the microscopic features of metals might be revealed by polishing.  He also suspected that, just like rocks, the properties of the metal might be correlated to the features revealed under the microscope.  This turned out to be the most important discovery of modern metallurgy.

The internal microscopic features of materials are now called the “microstructure” of the material, and the microstructure correlates with properties better than any other material characteristic.  This surprises some people who assume chemistry or heat treatment or manufacturing process has the most significant influence on properties, but steel (for example) can be made very hard despite enormous variation in composition and heat treatment.  The only requirement is that the combination of composition and heat treatment produce a particular microstructure (martensite).

Examining the microstructure of a material is therefore a great way to interpret the properties or the behavior of the material.  Furthermore, as a material undergoes changes due to deformation, heating, or fatigue in service, examining the microstructure is a great way to interpret what has happened or to diagnose a problem.  The microstructure records the history of the material more faithfully than any other characteristic.

In order to see the microstructure, we take a sample through an aptly named process called "metallographic preparation." The intent of metallographic preparation is to accurately reveal the microstructure of the material.  This is achieved by grinding the surface of a sample through successively finer grits. Each grit must remove the coarse scratches as well as the subsurface damage left by the proceeding grit.  Through successive grits the damage to the microstructure becomes finer and finer and the revealed microstructure more and more closely resembles the true microstructure of the material.  The final polishing stages are performed with sub-micrometer size abrasives held in a slurry on a soft cloth so that there is absolutely minimal damage to the microstructure.  The pH of the slurry may be adjusted to chemically remove the final layer of disturbed metal.

While conceptually simple, the process is difficult in practice.  The microstructures of most materials are not uniformly hard and therefore the microstructural constituents polish at different rates.  This produces an uneven polished surface that does not present a single focal plane, and the magnified image of the microstructure will be blurred.  Circuit boards are an extreme example of this because they frequently contain ceramics, such as alumina (aluminum oxide), with a hardness of about 2700 HV adjacent to tin solder with a hardness of about 27 HV.