A composite is a mixture of two or more materials with different chemical and/or physical properties that attains a combination of properties that is otherwise unattainable. The combination of new properties can include high mechanical strength, light weight, high resistance to heat or chemicals and/or low cost, etc.
Composites differ from compounds in that the mixed materials remain physically distinct, although often on a microscopic level, whereas compounds are homogenous substances that consist of two or more elements that are chemically bonded in fixed proportions. They differ from alloys in that the latter are homogeneous metals that are created by melting a metallic element and mixing in one or more other metallic or non-metallic elements.
Composites exist in great variety and vast quantities, and are both naturally and artificially produced. An example of a natural composite is wood, which consists of long cellulose fibers held together by a much weaker substance lignin. Bone is a composite of a hard but brittle material hydroxyapatite and a soft and flexible material collagen. The most widely manufactured composite by far is concrete, which consists of sand and gravel held together with a binder of cement. Mud bricks, widely used in some countries, are made mainly from mud and straw or rice husks. Other common examples include fiberglass, which is a glass fiber-reinforced plastic, and vinyl asbestos.
One type of composite that has seen particularly rapid growth in recent years is carbon fiber-plastic materials. Such composites can be up to five times stronger than steel but with only one-fifth of the weight, which makes them very useful for some structural purposes, particularly aircraft bodies, as well as for high-performance sporting goods. Major disadvantages are that they are expensive, because of both the cost of the materials and the difficulty of producing and fabricating them with high precision, and that they are difficult to recycle.