Materials are types of matter in solid form from which people can make objects. They belong to two groups: natural materials and extracted materials. Natural materials, which include stone, wood, and wool, are used much as they occur in nature. Extracted materials, such as plastics, alloys (metal mixtures), and ceramics, are created by processing various natural substances.
Manufacturers determine which material to use for a given product by evaluating the properties (qualities) of materials. For example, stainless steel, an alloy of steel and chromium, resists heat and corrosion (rust and other chemical damage). It thus makes a good material for saucepans, prosthetic (artificial) body parts, and parts for missiles.
Some properties can be linked with a material's macrostructure (structure visible to the unaided eye). For example, the long, parallel fibers of wood give wood relatively strong resistance to a force applied in the same direction as the grain (direction of the fibers), but relatively weak resistance to a force applied at a right angle to the grain. Other properties are explained by a material's microstructure (structure that can be seen only through a microscope). The low density of wood, for example, is due to the open structure of its microscopic cells. At the most basic level, the properties of materials are determined by chemical bonds, forces that attract atoms to one another and hold them together.
Materials scientists study how the structure of materials relates to their properties. A large part of the scientists' work involves experimentation. For example, they alter the microstructure of a material, then determine how these changes affect the properties. Materials engineers, who work in much the same way, develop new materials for use in commercial products.
Materials scientists and engineers have developed a wide variety of engineered materials. These materials include strong but lightweight composite materials made up of reinforcing materials embedded in a continuous solid body called a matrix. For example, carbon fibers in a matrix of adhesive resin are used to make tennis rackets and parts for aircraft. Embedding sensors and actuators (devices that produce movement in response to a signal) in composite materials results in sophisticated smart materials. These materials can alter their shape or operation in response to changes in conditions in their environment.
