Monday, February 28, 2011

MSE 101: Classes of Materials

"What is it made of?" Usually, not a question you need a super-specific answer for, on a daily basis. Steel is usually good enough, but maybe you need even less detail than that. Materials science has several generally agreed upon groups of materials based on certain fundamental properties that come in handy when describing properties.

This includes elemental metals, and alloys like steel and chrome. Metals typically have an ordered crystal structure of protons, with a sort of electron soup. Electrons don't tend to call any particular atom home, but atoms have well established positions relative to one another. This makes metals highly conductive, ductile (easily bent or deformed), and strong.

Most ceramic materials contain multiple elements, like aluminum oxide, of sodium chloride (table salt). They are largely defined by the very strong and stiff interatomic bonds, making them strong, but brittle. Ceramics have great properties at high temperatures, but can be expensive to make, and weak in tension.

Polymers are fundamentally based on large chains of carbon and hydrogen, with other elements sometimes present, such as oxygen, sulfur, and nitrogen. There are two classes of polymers: thermoplastics and thermosets. Thermoplastics can be melted for a solid and formed at high temperature. They are usually cheaper, easier to manufacture, but can be used for a much more limited range of applications. Examples include polyethylene and polystyrene.  Thermosets are usually much strong, but once the polymer chains have formed, that's it. Attempts to melt will result in burning. However, than can be used at higher temperatures than thermoplastics. Examples include polypropylene and epoxies.

Classically considered a subset of ceramics, with the growth of the metallic glass field, I decided to put these off in their own category. Unlike ceramics, which are highly ordered, glasses have no demonstrable long range order. Atom pairs have defined lengths, and small groups of atoms called "network units" (such as SiO4 groups in silica glasses) have a standard geometry. However, beyond that, there is no defined microstructure. As long as there are no defects, glasses can be incredibly strong, due to the interconnected networks of atoms, much like in polymers. However, even the slightest scratch can lead to failure due to the low fracture toughness.

Composites explicitly mix two or more distinct phases (form of a material) of differing properties. Natural precipitates don't count, such as silica in aluminum alloys. Example composites include fiberglass (polymer matrix composite), concrete (ceramic matrix composite) and toughened cylinder heads (metal matrix composites).

My research as a graduate student has mostly been in the last two categories, but I've dabbled in pretty much everything at some point.

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