Thermomechanical applications of ceramics are the most interesting of all but the most sensitive. They must be a substitute for metals and metal alloys that lose all or part of their properties when the temperature exceeds a few hundred degrees Celsius. The mechanical components exposed to very high temperatures, between 1 000 ° C and 1 400 ° C, are the domain of economic and technical excellence of these ceramics.
The best metal materials currently used (based super alloys nickel) lose a significant part of their properties when the temperature exceeds 500 ° C, and their use stops at about 1 100 ° C. This temperature limit is 1 200 ° C for the family ceramics alumina and zirconia and 1 400 ° C for the carbide and silicon nitride. This gain in operating temperature of internal combustion engines is not just looking for a technical achievement, it will directly improve their performance and efficiency. Thus, experts believe that General Motors engine explosion that operate at a temperature of about 1 300 ° C to yield 30% higher than a conventional engine, which operates at a temperature of about 900 ° C and, despite the cooling is limited by current materials. This engine would power equal to a consumer economy in the order of 30%. Similarly, aerospace experts believe a gain of 100 ° C on a turbojet would advance by about 10% in engine performance. All major automakers have begun a program to develop engines or less ceramists: piston heads, cylinders, combustion chambers lined with ceramics. In Japan, Nissan sold a sport with a turbo charger nitride silicon. More modestly, exhaust pipes are used in series by some European manufacturers.
These thermal added advantage of a lower density. The majority of ceramic thermal are at least two times lighter than the ferrous alloys. A ceramic automobile engine would have the double advantage of being both lighter and more efficient. In addition, it could, in principle, do without cooling (fan, water pump, expansion tank) and thus become adiabatic or semi-adiabatic, i.e. work without heat exchange with the outside world, what is the dream of every engine with a sense of the economy. But all the difficulties are far from resolved, especially the connection between the ceramic coating and metals, which retain their role as support. It is an assembly particularly difficult because both materials have coefficients of expansion very different.
Apart from automotive applications, technical ceramics applications are already at room temperature. This is the case for military armor, reinforced fiber-based boron carbide. They are also used for reinforcing components subjected to the wear high: elements pump in the mining industry, spray nozzles. But in the space sector that we saw of the most ambitious. U.S. shuttles have shown the way with their insulation coating silica tiles, reported on the body of the ship. The European Hermes space plane was to call for more original solutions. Its thermal protection was provided by ceramic panels that were used as structural elements. In total, Hermes would include nearly two tons of ceramic products.
Tags: nitride silicon
, super alloys nickel
, thermal protection
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