FUNDAMENTALS OF MATERIAL SCIENCE AND ENGINEERING BS CHEMICAL ENGINEERING 4 REPORTERS: Maurine Reolo
John Rhen Uy TOPIC: Applications and Processing of Ceramics, Chapter 13 Most Ceramic Materials fall into an applicationclassification scheme that includes the following groups: (1) Glasses a. Glasses b. Glass-Ceramics (2) Clay products a. Structural clay products b. Whitewares (3) Refractories a. Fireclay b. Silica c. Basic d. Special (4) Abrasives (5) Cements (6) Advanced ceramics. GLASSES The glasses are a familiar group of ceramics; containers, lenses, and fiberglass represent typical applications. As already mentioned, they are noncrystalline silicates containing other oxides, notably CaO, Na2O, K2O, and Al2O3, which influence the glass properties. The two prime assets of these materials are their optical transparency and the relative ease with which they may be fabricated. GLASS- CERAMICS Most inorganic glasses can be made to transform from a noncrystalline state to one that is crystalline by the proper high-temperature heat treatment. This process is called crystallization, and the product is a fine-grained polycrystalline material which is often called a glass–ceramic. The formation of these small glass-ceramic grains is, in a sense, a phase transformation, which involves nucleation and growth stages. A nucleating agent (e.g titanium dioxide) is often added to the glass to promote crystallization.
Properties of GLASS-CERAMICS (1) relatively high mechanical strengths; (2) low coefficients of thermal expansion (to avoid thermal shock); (3) relatively high temperature capabilities; (4) good dielectric properties (for electronic packaging applications); (5) good biological compatibility. (6) Some glass–ceramics may be made optically transparent; others are opaque. Applications of GLASS-CERAMICS • Glass–ceramics are manufactured commercially under the trade names of Pyroceram™, Corningware™, Cercor™, and Vision™. The most common uses for these materials are as ovenware, tableware, oven windows, and rangetops — primarily because of their strength and excellent resistance to thermal shock. They also serve as electrical insulators and as substrates for printed circuit boards, and are used for architectural cladding, and for heat exchangers and regenerators. CLAY PRODUCTS • One of the most widely used ceramic raw materials is clay. This inexpensive ingredient, found naturally in great abundance, often is used as mined without any upgrading of quality. Another reason for its popularity lies in the ease with which clay products may be formed; when mixed in the proper proportions, clay and water form a plastic mass that is very amenable to shaping. The formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength. Classification of CLAY PRODUCTS (1) Structural clay products – include building bricks, tiles, and sewer pipes — applications in which structural integrity is important. (2) Whiteware ceramics – become white after the high-temperature firing. Included in this group are porcelain, pottery, tableware, china, and plumbing fixtures (sanitary ware). In addition to clay, many of these products also contain nonplastic ingredients, which influence the changes that take place during
the drying and firing processes, and the characteristics of the finished piece. REFRACTORIES • Refractory ceramics are utilized in large tonnages • Properties: the capacity to withstand high temperatures without melting or decomposing, the capacity to remain unreactive and inert when exposed to severe environments, and the ability to provide thermal insulation. • Marketed in a variety of forms, most common as bricks. Typical applications include furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, and power generation. The performance of a refractory ceramic, to a large degree, depends on its composition. There are several classifications: (1) Fireclay Refractories The primary ingredients for the fireclay refractories are high-purity fireclays, alumina and silica mixtures usually containing between 25 and 45 wt% alumina. Fireclay bricks are used principally in furnace construction, to confine hot atmospheres, and to thermally insulate structural members from excessive temperatures. (2) Silica Refractories Sometimes termed as acid refractories, these materials are well known for their high-temperature loadbearing capacity, are commonly used in the arched roofs of steel- and glass-making furnaces; for these applications, temperatures as high as 1650 degree Celsius may be realized. The alumina content should be held to a minimum, normally to between 0.2 and 1.0 wt%. (3) Basic Refractories The refractories that are rich in periclase, or magnesia (MgO), are termed basic; they may also contain calcium, chromium, and iron compounds. The presence of silica is deleterious to their high-temperature performance. Basic refractories are especially resistant to attack by slags containing high concentrations of
MgO and CaO, and find extensive use in some steel-making open hearth furnaces. (4) Special Refractories There are other ceramic materials that are used for rather specialized refractory applications. Some of these are relatively high-purity oxide materials, many of which may be produced with very little porosity. These specialized refractories are expensive. Included in this group are alumina, silica, magnesia, beryllia (BeO), zirconia (ZrO2), and mullite (3Al2O3–2SiO2). Others include carbide compounds, in addition to carbon and graphite. Silicon carbide (SiC) has been used for electrical resistance heating elements, as a crucible material, and in internal furnace components. Carbon and graphite are very refractory, but find limited application because they are susceptible to oxidation at temperatures in excess of about. ABRASIVES • Abrasive ceramics are used to wear, grind, or cut away other material, which necessarily is softer. The prime requisite for this group of materials is hardness or wear resistance; a high degree of toughness is essential to ensure that the abrasive particles do not easily fracture. • Diamonds, both natural and synthetic, are utilized as abrasives; however, they are relatively expensive. The more common ceramic abrasives include silicon carbide, tungsten carbide (WC), aluminum oxide (or corundum), and silica sand. • Coated abrasives are those in which an abrasive powder is coated on some type of paper or cloth material; sandpaper is probably the most familiar example. Wood, metals, ceramics, and plastics are all frequently ground and polished using this form of abrasive. CEMENT • Several familiar ceramic materials are classified as inorganic cements: cement, plaster of paris, and lime, which, as a group, are produced in extremely large quantities. The characteristic feature of these materials
is that when mixed with water, they form a paste that subsequently sets and hardens. • The cementitious bond develops at room temperature. Portland Cement Of cement group of materials, Portland cement is consumed in the largest tonnages. It is produced by grinding and intimately mixing clay and lime-bearing minerals in the proper proportions, and then heating the mixture to about in a rotary kiln; this process,sometimes called calcination, produces physical and chemical changes in the raw materials. The resulting “clinker” product is then ground into a very fine powder to which is added a small amount of gypsum (CaSO4–2H2O) to retard the setting process. This product is portland cement. • Several different constituents are found in portland cement, the principal ones being tricalcium silicate (3CaO–SiO2) and dicalcium silicate (2CaO–SiO2). The setting and hardening of this material result from relatively complicated hydration reactions that occur among the various cement constituents and the water that is added. • It should be emphasized that the process by which cement hardens is not one of drying but, rather, of hydration in which water actually participates in a chemical bonding reaction. • Portland cement is termed a hydraulic cement because its hardness develops by chemical reactions with water. It is used primarily in mortar and concrete to bind, into a cohesive mass, aggregates of inert particles (sand and/or gravel); these are considered to be composite materials. Other cement materials, such as lime, are nonhydraulic; that is, compounds other than water (e.g., CO2) are involved in the hardening reaction. ADVANCED CERAMICS • Although the traditional ceramics discussed previously account for the bulk of the production, the development of new and what are termed “advanced ceramics” has begun and will continue to establish a prominent niche in our advanced technologies. In particular, electrical,
magnetic, and optical properties and property combinations unique to ceramics have been exploited in a host of new products. • Advanced ceramics are utilized in optical fiber communications systems, in microelectromechanical systems (MEMS), as ball bearings, and in applications that exploit the piezoelectric behavior of a number of ceramic materials. Optical Fibers One new and advanced ceramic material that is a critical component in our modern optical communications systems. The optical fiber is made of extremely high-purity silica, which must be free of even minute levels of contaminants and other defects that absorb, scatter, and attenuate a light beam. Ceramic Ball Bearings • A bearing consists of balls and races that are in contact with and rub against one another when in use. • Over the past decade or so silicon nitride (Si3N4) balls have begun replacing steel balls in a number of applications, since several properties of Si3N4 make it a more desirable material. In most instances races are still made of steel, because its tensile strength is superior to that of silicon nitride. This combination of ceramic balls and steel races is termed a hybrid bearing. • Some of the applications that employ these hybrid bearings include inline skates, bicycles, electric motors, machine tool spindles, precision medical hand tools (e.g., high-speed dental drills and surgical saws), and textile, food processing, and chemical equipment. • Ceramic materials are inherently more corrosion resistant than metal alloys; because Si3N4 is an electrical insulator (bearing steels are much more electrically conductive), the ceramic bearings are immune to arcing damage. FABRICATION AND PROCESSING OF CERAMICS One chief concern in the application of ceramic materials is the method of fabrication. Since ceramic materials have relatively high melting temperatures, casting them is normally
A classification scheme for the ceramic-forming techniques
impractical. Furthermore, in most instances the brittleness of these materials precludes deformation. Some ceramic pieces are formed from powders (or particulate collections) that must ultimately be dried and fired. Glass shapes are formed at elevated temperatures from a fluid mass that becomes very viscous upon cooling. Cements are shaped by placing into forms a fluid paste that hardens and assumes a permanent set by virtue of chemical reactions.
Contrast of Specific Volume versusTemperature behavior of crystalline and noncrystalline materials
Crystalline materials solidify at the melting temperature, Tm. Characteristic of the noncrystalline state is the glass transition temperature, Tg. For glassy materials, volume decreases continuously with temperature reduction; a slight decrease in slope of the curve occurs at what is called the glass transition temperature, or fictive temperature, Tg. Below this temperature, the material is considered to be a glass; above, it is first a supercooled liquid, and finally a liquid.
GLASS PROPERTIES Glassy, or noncrystalline, materials do not solidify in the same sense as do those that are crystalline. Upon cooling, a glass becomes more and more viscous in a continuous manner with decreasing temperature; there is no definite temperature at which the liquid transforms to a solid as with crystalline materials. In fact, one of the distinctions between crystalline and noncrystalline materials lies in the dependence of specific volume (or volume per unit mass, the reciprocal of density) on temperature.
Viscosity-Temperature Characteristics of the Glass: 1. Melting point, the temperature at which the viscosity is 10 Pa-s (100 P); the glass is fluid enough to be considered a liquid. 2. Working point, the temperature at which the viscosity is 10,000Pa-s (10^4 P); the glass is easily deformed at this viscosity. 3. Softening point, the temperature at which the viscosity is 4x10^6Pa-s (4 x 10^7 P), is the maximum temperature at which a glass piece may be handled without causing significant dimensional alterations. 4. Annealing point, the temperature at which the viscosity is 10^12Pa-s (10^13 P); at this temperature, atomic diffusion is sufficiently rapid that any residual stresses may be removed within about 15 min. 5. Strain point, corresponds to the temperature at which the viscosity becomes 3 x 10^13 Pa-s (3 x 10^14 P); for
temperatures below the strain point, fracture will occur before the onset of plastic deformation. The glass transition temperature will be above the strain point. Glass Forming • Most glass-forming operations are carried out within the working range— between the working and softening temperatures. Of course, the temperature at which each of these points occurs depends on glass composition. • Glass is produced by heating the raw materials to an elevated temperature above which melting occurs. Most commercial glasses are of the silica–soda–lime variety. • For most applications, especially when optical transparency is important, it is essential that the glass product be homogeneous and pore free. Four different forming methods are used to fabricate glass products: 1. Pressing - used in the fabrication of relatively thick-walled pieces such as plates and dishes. The glass piece is formed by pressure application in a graphite-coated cast iron mold having the desired shape; the mold is ordinarily heated to ensure an even surface. 2. Blowing - from a raw gob of glass, a parison, or temporary shape, is formed by mechanical pressing in a mold. This piece is inserted into a finishing or blow mold and forced to conform to the mold contours by the pressure created from a blast of air. 3. Drawing - used to form long glass pieces such as sheet, rod, tubing, and fibers, which have a constant cross section. One process by which sheet glass is formed is it may also be fabricated by hot rolling. 4. Fiber Forming - Fibers are formed by drawing the molten glass through many small orifices at the chamber base. The glass viscosity, which is critical, is controlled by chamber and orifice temperatures.
Heat Treating Glasses 1. Annealing Heat Treatment - the glassware is heated to the annealing point, then slowly
cooled to room temperature to reduce the magnitude of thermal stresses. These stresses are important in brittle ceramics, especially glasses, since they may weaken the material or, in extreme cases, lead to fracture, which is termed thermal shock. 2. Glass Tempering - the strength of a glass piece may be enhanced by intentionally inducing compressive residual surface stresses. This can be accomplished by a heat treatment procedure called thermal tempering. FABRICATION AND PROCESSING OF CLAY PRODUCTS Clay Properties: 1. Hydroplasticity of Clay Minerals, when water is added, they become very plastic. 2. Clays are Aluminosilicates, composed of alumina (Al2O3) and silica (SiO2), that contain chemically bound water. Compositions of Clay Products In addition to clay, many of these products (in particular the whitewares) also contain some nonplastic ingredients: 1. Quartz is used primarily as a filler material, being inexpensive, relatively hard, and chemically unreactive. Flint - finely ground quartz 2. When mixed with clay, a flux forms a glass that has a relatively low melting point. The feldspars are some of the more common fluxing agents; they are a group of aluminosilicate materials that contain K+, Na+ and Ca2+ ions. Fabrication Techniques Two common shaping techniques are utilized for forming clay-based compositions: 1. Hydroplastic Forming - Clay minerals, when mixed with water, become highly plastic and pliable and may be molded without cracking; however, they have extremely low yield strengths. 2. Slip Casting - A slip is a suspension of clay and/or other non-plastic materials in water. When poured into a porous mold, water from the slip is absorbed into the mold, leaving behind a solid layer on the mold wall the thickness of which depends on the time. This process may be continued until the entire mold cavity becomes solid (solid
casting), or it may be terminated when the solid shell wall reaches the desired thickness. Drying Liquid is removed from a ceramic piece. As a clay-based ceramic body dries, it also experiences some shrinkage. A body that has been formed and dried but not fired is termed green ceramic body. Firing After drying, a body is usually fired at a high-temperature between 900 and 1400 C; the firing temperature depends on the composition and desired properties of the finished piece. During the firing operation, the density is further increased and the mechanical strength is enhanced. Powder Pressing Powder pressing, the ceramic analogue to powder metallurgy, is used to fabricate both clay and nonclay compositions, including electronic and magnetic ceramics as well as some refractory brick products. There are three basic powder-pressing procedures: 1. Uniaxial Pressing, the powder is compacted in a metal die by pressure that is applied in a single direction. 2. Isostatic Pressing or Hydrostatic Pressing, the powdered material is contained in a rubber envelope and the pressure is applied by a fluid. 3. Hot Pressing, the powder pressing and heat treatment are performed simultaneously the powder aggregate is
compacted at an elevated temperature. The procedure is used for materials that do not form a liquid phase except at very high and impractical temperatures. TAPE CASTING is an important ceramic fabrication technique where thin sheets of a flexible tape are produced by means of a casting process. CEMENTATION is also considered to be a ceramic fabrication process. The cement material, when mixed with water, forms a paste that, after being fashioned into a desired shape, subsequently hardens as a result of complex chemical reactions
