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... nitride. Cubic boron nitride is the second hardest known substance, after diamond, and has some special applications, such as abrasives in grinding wheels and cutting tools. It does not exist in nature and is thus made synthetically. Titanium nitride is used widely as a coating on cutting tools. IT improves tool life by virtue of it's low frictional characteristics.
Silicon nitride has a high resistance to creep at elevated temperatures, low thermal expansion, high thermal conductivity, and hence resists thermal shock. It is suitable for high-temperature structural applications, such as automotive engines and gas turbine components. Finally we have cerments. Cerments are combinations of ceramics bonded with a metallic phase.
Introduced in the 1960 's, they combine the high-temperature oxidation resistance of ceramics and the toughness, thermal-shock resistance, and ductility of metals. They have been developed for high temperature applications such as nozzles for jet engines and aircraft brakes. Cerments can be regarded as composite materials and can be used in various combinations of ceramics and metals bonded by powder-metallurgy techniques. General Properties of Refractory Materials Compared to metals, ceramics have the following relative characteristics: brittle, high strength and hardness at elevated temperatures, high elastic modules, low toughness, low density, low thermal expansion, and low thermal and electrical conductivity. However, because of the wide variety of ceramic material composition and grain size, the mechanical and physical properties of ceramics vary significantly. For example, the electrical conductivity of ceramics can be modified from poor to good, which is the principle being semi-conductors.
Because of there sensitivity to flaws, defects, and cracks, the presence of different types of levels of impurities, and different methods of manufacturing, ceramics have a wide range of properties. Some mechanical properties are presented in the back. Strength in tension is approximately one order of magnitude lower than their compressive strength. The reason is their sensitive to cracks, impurities, and porosity.
Such defects lead to the initiation and propagation of cracks under tensile stresses, severely reducing tensile strength. Thus reproducibility and reliability is an important aspect in the service life of ceramic components. Tensile strength of a polycrystalline ceramic parts increases with decreasing grain size. Also, tensile strength and modulus elasticity are both affect by porosity in the ceramic. Although there are exceptions and unlike most metals and thermoplastics, ceramics generally lack impact toughness and thermal shock resistance because of their inherent lack of ductility. Once, initiated, a crack propagates rapidly.
In addition to undergoing fatigue failure under cyclic loading, ceramics exhibit a phenomenon known as static fatigue. When subjected to a static tensile load over a period of time the ceramic will eventually fail. This occurs in environment where water vapor is present. Ceramic components that are to be subjected to tensile stresses may be prestressed, much like prestressed concrete. Some methods include: a) Heat treatment and chemical tempering b) Laser treatment of surfaces c) Coating with ceramics with different thermal expansion coefficients d) Surface finishing operations Significant advances are being made in improving the toughness and the properties of ceramics which include; control of purity and structure, use of reinforcements, emphasis on design of advanced methods of stress analysis in ceramic components, and the processing of raw materials. Physical properties include a relatively low specific gravity, high melting or decomposition temperature, thermal conductivity varies as much as three orders in magnitude, depending on their composition.
Thermal conductivity of ceramics, as well as other materials, decreases with increasing temperature and porosity because air is a poor thermal conductor. Some thermal expansion Characteristics or shown on (Chart# 2). Chart # 2 Thermal expansion and thermal conductivity induce thermal stresses that can lead to thermal shock or thermal fatigue. The tendency for thermal cracking is lower with low thermal expansion and high thermal conductivity. A familiar example for low thermal expansion is the heat resistant ceramics for cookware and stove tops. Ceramics can be made conductive by adding alloys to them, thus making the ceramic act as a semi-conductor or even a super-conductor.
Applications Chart # 3 Chart # 3 shows some common examples of ceramics. Several ceramics are used in the electrical and electronics industry because of there high electrical resistivity, dielectric strength (voltage required for electrical breakdown per unit thickness), and magnetic properties suitable for applications such as magnets for speakers. The ability for ceramics to maintain their strength and stiffness at elevated temperatures makes them very attractive for high temperature applications. Their high resistance to wear makes the very attractive to make cylinder liners, bushings, seals and bearings. Their high operating temperatures made possible by the use of ceramic components means more efficient fuel burning and less emissions in engines. Currently, internal combustion engines are 30 % effective, but with the use of ceramics they can become another 30 % efficient.
Other attractive applications of ceramics lie with their low density and high elastic modulus. Thus engine weight can be reduced, in other applications, the internal forces generated by moving parts can be lowered. Ceramic turbochargers, for example, are about 40 % lighter than conventional ones. The higher elastic modulus of ceramics makes them attractive for improving stiffness, while reducing the weight, of machines. Ceramics are being used successfully in gasoline and diesel engines components and rotors which are made of silicon nitride and silicon carbide. Coating metal with ceramics is another application, may be done to reduce wear, prevent corrosion, and provide a thermal barrier.
The tiles ion the shuttles, for example, are made of silica fibers with an open cellular structure that consist of 5 % silica. The rest of the tile structure is air, thus making the tile not only very lightweight but also an excellent heat barrier (The skin temperature on the shuttle reaches 1400 degrees due to frictional heat with the atmosphere. Ceramics can also be used as coating for high temperature applications. Characteristics such as thermal and electrical insulation, particularly at elevated temperatures, can be imparted on these products by ceramic coatings rather than imparting these properties to the base metals or materials themselves. Ceramic coatings are used in wide variety of purposes as shown on Chart # 4 Chart # 4 Conclusion The subject of ceramics is very broad and I have only given a specialized area some serious consideration. Ceramics are around us everyday with almost all things we use.
When dealing with extreme temperatures ceramics are needed in some way shape or form. We covered many applications for ceramics, their properties, and most of why we need them in the industry. Bibliography Teledyne Wah Chang. United States Internet Address web George's Very Own Refractory Introduction.
Canada Internet Address web Two Minute Course in Ceramics. United States Internet Address Materials Sciences. Sandia National Laboratories. United States Internet Address web
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