A microstructure is the way a material comes together on a very small scale. An object's microstructure is not visible by the naked eye, although the patterns present at the microscopic level may replicate at a larger level. This larger level is the macroscopic level; it will give an observer a basic impression of the material’s underlying design. The object’s microstructure determines the majority of its physical properties. There are four main categories that materials fall into based on their microstructure: ceramic, metallic, polymeric and composite.
A material’s physical structure will appear to change depending on how closely you look at it. When an object is held at arm’s length, it looks different than if it is a hand’s width away from a person’s face. The same is true when an object is observed under a microscope. In order to create a standard definition of microstructure, the power of the magnification used to look at it is no more than 25x.
When the structure is observed at a higher or lower power, it looks different. These other observable structures, particularly the smaller ones, can have a significant impact on the properties of the object. Instead of expanding the definition of microstructure, the elements that make up the microstructure are changed to accommodate differences in underlying structure.
As an example, it is possible to look at the macroscopic world. If a highway system was the observed item, a road would be the microstructure. Different roads are made of different materials and, therefore, have different properties. Thus, the road is broken down into types of roads.
Generally, a microstructure is observed by taking paper-thin slices of the object and putting them under a microscope. These slices are so thin that light shines through them and highlights the underlying structure. Depending on the material observed, other methods may be employed such as electron microscopy or x-rays.
Through seeing the materials present and by observing the way they interact, it is possible to predict the way a material will act on a macroscopic level. Certain materials have certain properties, so when they are present, they pass those properties onto the material as a whole. Their underlying structure also shows how a material will act. For instance, a material where the structure is arranged in long non-interlocking plates may be prone to breaking or bending.
These properties come together to give the material a broad classification. These classes tell the basic properties of a material without the need to observe its actual structure. Three of them—ceramic, metallic and polymeric—are pure collections of a specific type of structure. The forth, composite, is a mixture of the three basic kinds.