Mechanical engineers design and build machines that enable people to live and work in space, in the air, on the ground, and under water. Much of what engineers can or can't do depends on the materials they have available to work with. Materials can be categorized not only with the mechanical, but also chemical and electrical properties and their bonds that create into four groups: ceramics and glasses, composite materials, polymers and metal alloys. The first type, ceramics and glasses, use covalent and ionic-covalent types with silicon dioxide as a building block. Ceramics are as soft as clay and as hard as stone and concrete. Usually, they are crystalline in form. Engineering ceramics are known for their inflexibility and constancy under high temperatures. The second type, composite materials, they are structured materials composed of two or more macroscopic phases. Applications range from structural elements to the thermally insulative tiles. The next type, polymer, which are widely used and most popular. Polymers are the raw materials used to make what we usually call plastics. Plastics are created after one or more polymers. They are materials with unique characteristics, such as ultra-high strength, electrical conductivity, electro-fluorescence and high thermal stability. Finally, the last type is metal alloys. The alloys of iron constitute the majority of both the quantity and the commercial value. For steels, the hardness and tensile strength of the steel is related to the amount of carbon present. Heat treatment processes such as quenching and tempering can change these properties.
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