Introduction
First we will start with the definition of refractories and ceramics. Refractories and ceramics are non-metallic materials capable of maintaining physical and chemical stability at high temperatures. Refractories in modern practice are usually ceramic in nature, and are used in a wide variety of primary, secondary and tertiary industries.
Wherever an industrial process involves heat in excess of 700 to 800 degrees Fahrenheit (roughly), one will find refractory material in place, either as a lining or forming the process vessel itself. Some common process vessels using refractories are; boiler combustion chambers, furnaces like the one in the foundry, incinerators, many emission control scrubbers, rotary kilns and so on. The list is by know means exhaustive. For example, Launch Pads 39A and 39B at the Kennedy Space Center are refractory lined. The shuttles themselves are lined with ceramic tiles to protect them from the heat of re-entry into earth’s atmosphere, these tiles are! unique to the shuttle, but are non-metallic and heat resistant.
The Making of Refractory Materials
The first step in processing ceramics is crushing of the raw materials. Crushing is usually done in a ball mill, either wet or dry. Wet crushing is more effective because it keeps the particles together and prevents the suspension of fine particles in air. The ground particles are then mixed with additives, the functions of which are one or more of the following:
1. Binder for the ceramic particles
2. Lubrication for mold release and to reduce internal friction between particles
3. Wetting agent to improve mixing
4. Plasticizer to make the mix more plastic and formable.
5. Various agents to control foaming and sintering.
6. De-flocculent to make ceramic-water suspension. De-flocculention changes the electrical charges on the clay particles so that they repel instead of attract each other.
Next, it’s time to begin the casting process. The shaping process for refractories are casting plastic forming and pressing. The most common casting process is slip casting. The slip is poured into a porous mold made usually of plaster of paris. Then inverted and the remaining suspension is poured out for making hollow object much like slush casting. The part is then trimmed the mold opened and the part removed.
The second process of shaping ceramics is plastic forming. We have various methods of plastic forming such as extrusion, injection molding and jiggering. Plastic forming tends to orient the layered structure of clays along the direction of material flow.
This leads to anisotrophic behavior of the material, both in subsequent processing and in the final properties of the ceramic product. In extrusion, the clay mixture is forced through a die opening. The cross section of the extruded product is constant, and there are limitations to wall thickness for hollow extrusions.
Finally the third process in shaping ceramics is pressing. Dry pressing is used for relatively simple shapes. this process has high production rates and close control of tolerances. Dies are usually made of carbides or hardened steel. However the dies can be quite expensive as the must have a high wear resistance from the abrasive ceramic tiles. Wet pressing is used to make very complex shapes. Production rates are high but part size is limited, dimensional control is difficult because shrinking during drying, and tooling can be expensive. The third type of pressing is isostatic pressing mainly used to produce spark plug insulators, silicon nitride vanes for high temperature and so on. Isostatic pressing allows one to obtain uniform density distribution throughout a part. Finally we have hot pressing which combines pressure and temperature. The die life is short as a result of the temperature, and usually protective atmosphere’s are used along with graphite materials used in the punch and die materials. One example of a hot pressing part is the vane for a gas turbine engine in a jet airplane.
Finally, after the part has been cast in anyone of our methods above we begin drying and firing the part to give it strength. Drying is very critical as the part may want to warp and crack from variations in moisture and thickness within the part and the complexity of the shape. Control of atmospheric humidity and temperature is very important to avoid warping and cracking. Next, the part must be fired, this is where the part gains it’s strength and hardness. The improvement in the properties result from (a) development of a strong bond between the complex oxide particles in the ceramic and (b) reduced porosity.
After firing, additional operations may be performed to give the part it’s final shape, remove surface flaws, and improve the surface of the finish and tolerances. Processes used include; grinding, lapping, and ultrasonic, chemical and electrical-discharge machining. The finer the finish the higher the parts strength will be. Most product are finally given a glossy coating with a glaze material the improve appearance, strength once again and to make them impermeable.
Structure of Refractories Materials
A refractory is a type of ceramic so I’ll refer to ceramics once again. the structure of a ceramic crystal is among the most complex of all materials, containing various elements of different sizes. The bonding between these atoms is generally covalent (electron sharing, hence strong bonds). and ionic (primary bonding between oppositly charged ions, thus strong bonds). These bonds are much stronger than metallic bonds. Consequently, the properties of ceramics are significantly higher than those for metals, particularly their hardness and thermal and electrical resistance.
Ceramics are available as a single crystal or in a polycrystalline form, consisting of many grains. Grain size plays a major role in strength and the properties of the part. The finer the grain size the higher the strength and toughness- hence the term fine ceramics.
Among the oldest raw material for ceramics is clay, a fine-grained sheet like structure, the most common being kaolinite. White clay, consists of silicate of aluminum and altering weakly bonded layers of silicon and aluminum ions. When added to kaolinite water attaches itself to the layers, makes them slippery, and gives wet clay it’s well known softness and plastic properties that make it formable.
Another major raw material for ceramics is flint and feldspar. In their general state these materials usually contain impurities and these impurities must be removed prior to further processing of materials into useful products with reliable performance.
Next well move on to some types of refractory materials and refractory metals most commonly used in industry. The types of ceramics I’ll be talking about are carbides. Typical examples of carbides are tungsten and titanium. Tungsten carbide consists of tungsten-carbide particles with cobalt as a binder. The amount of the binder has a major influence on the material’s properties. Toughness increases with cobalt content, whereas hardness, strength, and wear resistance decrease. Titanium carbide has nickel and molybdenum as the binder and is not as tough as tungsten carbide. These metals are typically used as cutting tools and die materials, also abrasives on cutting wheels. Silicon carbide has good wear, thermal shock, and corrosion resistance. It has a low coefficient of friction and retains strength at elevated temperatures. It is suitable for high-temperature components in heat engines and is also used as an abrasive in grinding wheels.
Next, we have Nitrides; Cubic boron nitride, Titanium nitride and Silicon 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 modulous, 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.
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
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.
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.
Courtesy:-
Bibliography
Teledyne Wah Chang. United States Http://TWC.com
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