Composite Materials

In addition to the examples already given, examples of composites materials also include: (1) Reinforced and prestressed concrete, which is a composite of steel and concrete. Concrete is itself a composite of rocks (coarse aggregate), sand (fine aggregate), hydrated Portland cement, and usually, voids. (2) Cutters for machining made of fine particles of tungsten carbide, which is extremely hard, are mixed with about 6% cobalt powder and sintered at high temperatures. (3) Ordinary grinding wheels, which are composites of an abrasive with a binder that may be plastic or metallic. (4) Walls for housing, which have been made of thin aluminum sheets epoxied to polyurethane foam. The foam provides excellent thermal insulation. This composite has a higher structural rigidity than aluminum sheets or polyurethane foam alone. The polyurethane foam is itself a composite of air and polyurethane. (5) Underground electrical cables composed of sodium metal enclosed in polyethylene. (6) Superconducting ribbons made of Nb₃Sn deposited on copper. (7) Synthetic hard superconductors made by forcing liquid lead under pressure into porous glass fibers. (8) Micro-electronic circuits made from silicon, which are oxidized to form an insulating layer of SiO₂. This insulating layer is etched away with hydrofluoric acid, and phosphorous is diffused into the silicon to make a junction. Aluminum or another metal can be introduced as a microconductor between points. The microelectronic circuit is thus a tailored composite. (9) Ceramic fiber composites including graphite or pyrolytic carbon reinforced with graphite fibers; and borosilicate glass lithium aluminum silicate glass ceramics reinforced with silicon carbide fibers. It was possible to drive a tungsten carbide spike through such a composition without secondary cracking in much the same way that a nail can be driven through wood.