IV-2 Ceramic and Related Materials [Zbigniew D Jastrzebski]
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CERAMIC AND RELATED MATERIALS
[Adopsi dari: Zbigniew D Jastrzebski, “The Nature And Properties of Engineering Materials” , John Wiley & Sons, ISBN 0-471-63693-2, 1987, CHAPTER 9.]
Ceramics are inorganic, nonmetallic materials that are processed or used at high temperatures. They include a broad range of silicates, metallic oxides, and combinations of silicates and metal oxides. Furthermore, elements such as carbon, boron, and silicon, carbides, borides, and nitrides, various refractory hydrides, sulfides, and selenides are usually considered as ceramics. A common feature of ceramic materials is that they depend on either ionic or covalent bonding or a combination of both. The bonding electrons therefore tend to be localized and ceramics are generally relatively poor conductors of heat and electricity. Because the bonds are strong, ceramics usually have high heat and chemical resistance. Ceramics are often compounds formed between metallic and nonmetallic elements and their crystal structures tend to be more complex than those of pure metals. They can be fairly simple as in the case of magnesium oxide (MgO), beryllium oxide (BeO), silicon carbide
(SiC), or silicon nitride (Si 3 N 4 ), but they are often quite complicated as in various silicates. Furthermore, they may contain both crystalline and glassy phases.
Ceramics can be grouped into three broad divisions—clay products, refractories, and glasses—according to their common characteristic features. Closely related to ceramics in chemical composition are inorganic cements that are used as binding materials to produce concrete, mortars, and similar products.
CLAY PRODUCTS
Clay products include important engineering materials such as bricks, tiles, porcelain, stoneware, and various chemical ware. All these products are made of various clays by manufacturing procedures that are essentially similar.
9-1 PLASTICITY OF CLAYS
The plasticity of clay can be defined as its ability to form a plastic mass (dough) with water. The mass can be molded to shape easily, but it retains sufficient rigidity to prevent deformation on standing. Dry clays are not plastic and a certain amount of added water is always necessary to produce the required plasticity. The function of water is to form a film around the flaky clay particles so that their parallel orientation and movement under pressure are facilitated (Fig. 9-1). The amount of water required to make a clay plastic depends on the size and shape of the clay particles, their surface characteristics, and he presence of electrolytes. There is a certain minimum water content below which a clay ceases to behave as a plastic material and becomes friable or crumbly. This is called the plastic limit of a clay. As the proportion of water increases, the clay becomes more plastic until the point is reached at which the clay begins to flow and becomes wet and sticky. This is called the liquid limit. The difference in the water content between the liquid Limit and the plastic limit is called the plasticity index, which represents the plasticity range of the clay.
Depending on the methods employed in the shaping and forming of ceramic wares, different plasticities of raw mixes are required to produce a product of desired size and properties. Hand molding and machine extrusion require a mass of plastic consistency, classified as stiff mud. Machine pressing is best accomplished on stiff-plastic and semidry mixes, whereas slip casting requires a mix of semiliquid consistency so that it can be easily poured in a mold. The required plasticity is secured by different methods, such as weathering and grinding, blending with other clays or finely pulverized non- plastic ingredients, and addition of alkalies, acids, and salts.
The effect of weathering or grinding is to produce a uniform clay—water paste in which clay particles are well dispersed and surrounded by a water film. This treatment greatly improves the plasticity and homogeneity of the mass.
Blending is the term given to mixing two or more ingredients in such proportions that a mass of required composition and plasticity is obtained. This is a particularly important step in the manufacture of high-grade products such as white-wares, chemical wares, and porcelain. The usual ingredients are clay, non-plastic materials, such as finely ground quartz or flint, and fluxes.
FIGURE 9-1 Plasticity of clay. (a) Random arrangement of clay particles in dry clay. (b) Parallel orientation of clay particles surrounded by water film under shearing stress.
Non-plastic materials reduce the proportion of water in the mass necessary for a desirable plasticity, thereby preventing an excessive drying shrinkage. Fluxes are substances added to the mix to lower its fusion point, thereby making possible a more complete vitrification at not too high temperatures.
9-2 DRYING
Drying removes the water from the formed plastic body before it is subjected to firing. The total water present in the mass consists of the shrinkage water and the pore water. The shrinkage water is the water that is held between the particles of the clay, and it accounts for the plasticity of clay. The pore water is held in the internal pores of the particles. The removal of the shrink age water is accompanied by a contraction of the body, since the water lost comes from the interstices between the particles, thereby causing them to come closer together. This increases the attraction forces between the particles, resulting in a much higher strength of the dry clay as compared to its wet strength. Excessive drying shrinkage may result in cracking and warping of the body if the drying rate is too high. Drying, therefore, must be performed very carefully to prevent damage to the product from a high shrinkage rate. Cheap wares are usually dried in the atmosphere under a roof with open sides for many days. More expensive products are dried in special ovens at 85 to 96°C (185 to 205°F) using air of high humidity. The use of air with a high moisture content prevents excessive drying on the surface, and the elevated temperature in the oven reduces the viscosity of the water within the body, thus increasing its diffusion rate toward the surface. The moisture content of the drying air and the drying temperature are adjusted so that the rate of evaporation of the water from the surface is nearly equal to the rate of diffusion of water from the inside to the surface of the body. This permits relatively high drying rates without the danger of cracking and warping of the material. The amount of drying shrinkage increases with the plasticity of the clay, since this latter requires higher water content to become plastic. Hence, highly plastic clays are always blended with non-plastic ingredients such as fine sand to reduce the water content and subsequently the drying shrinkage.
The removal of pore water does not involve shrinkage, and it can be carried out with dry air at 110°C (230°F) or higher. This can also be made a part of the firing process. The removal of pore water does not involve shrinkage, and it can be carried out with dry air at 110°C (230°F) or higher. This can also be made a part of the firing process.
9-3 FIRING
Firing has the aim ol converting a molded and dried clay article (green) into a permanent product possessing required strength, durability, and very often a better appearance. The firing temperature depends on the character of the clay and the desired properties of the product, and it may vary from 900 to 1400°C. In the initial stage of firing, from 110 to 260°C (230 to 500°F), the last traces of hygroscopic moisture are removed (Fig. 9-2). There is little further change until temperatures of 425 to 650°C (765 to 1200°F) are reached.
Here the clay minerals break down to silica and alumina, liberating the chemically combined water according to the reaction
Al 2 O 3 .2SiO 2 .2H 2 0 → Al 2 O 3 + 2SiO 2 + 2H 2 O (9-1) At this point the clay loses its ability to form a plastic dough with water, and it cannot be
remolded again. There is little change, however, in the strength and porosity of the body. In the temperature range 800 to 900°C (1472 to 1652°F), oxidizing conditions in the furnace should be maintained to secure the burning of any contained organic matter and the oxidation of iron pyrites. At this stage, all other gas-forming reactions should also be completed before vitrification temperatures are reached. At temperatures of 900 to 1000°C (1652 to 1832°F) fusion or vitrification begins, the porosity decreasing as firing shrinkage commences. Vitrification is the result of the gradual formation of liquid that fills up the pore spaces. When cooled, the liquid solidifies to a vitreous or glassy matrix by cementing the inert particles together. This decreases the porosity of the body and greatly increases its strength. With a further rise in temperature more liquid is gradually formed until full vitrify is reached at about 1400°C (1550°F). A still further rise in temperature does not cause any further shrinkage or any decrease in porosity, but it remolded again. There is little change, however, in the strength and porosity of the body. In the temperature range 800 to 900°C (1472 to 1652°F), oxidizing conditions in the furnace should be maintained to secure the burning of any contained organic matter and the oxidation of iron pyrites. At this stage, all other gas-forming reactions should also be completed before vitrification temperatures are reached. At temperatures of 900 to 1000°C (1652 to 1832°F) fusion or vitrification begins, the porosity decreasing as firing shrinkage commences. Vitrification is the result of the gradual formation of liquid that fills up the pore spaces. When cooled, the liquid solidifies to a vitreous or glassy matrix by cementing the inert particles together. This decreases the porosity of the body and greatly increases its strength. With a further rise in temperature more liquid is gradually formed until full vitrify is reached at about 1400°C (1550°F). A still further rise in temperature does not cause any further shrinkage or any decrease in porosity, but it
The degree of vitrification controls properties of ceramics such as cold strength, durability, porosity, and density. Common building blocks are usually fired in the temperature range 800 to 900°C (1472 to 1652°F) and are vitrified to various degrees. For a poorly fired brick the cold compressive strength may be about 7 MPa (1 ksi), while for a well-fired brick it may be 140 MPa (20 ksi) and sometimes higher. Paving bricks, called clinkers, are nearly fully vitrified bodies showing a uniformly dense structure, low porosity, high hardness, high compressive strength, and good abrasion resistance. Highly vitrified ceramics are stoneware and porcelain. Porcelain is such a highly vitrified body that it becomes nonporous and nearly translucent in thin sections. Stoneware is somewhat less vitrified, showing a porosity from 2% to 4%.
Ceramic products undergo considerable shrinkage from molding to the fired stages. This shrinkage is not uniform and can be controlled within a range of plus or minus 2% to 3%. Closer tolerances within 0.05 mm are obtained by grinding with silicon carbide or diamond wheels, but this is an expensive operation. The size of stoneware is limited to pieces of 2 m in diameter and 2.5 to 3 m in height; porcelain wares are restricted to still smaller sizes. Both stoneware and porcelain are used widely in the chemical industry because of their high chemical resistance to all acids except hydrofluoric acid. They are, however, attacked by concentrated alkalies and show low thermal shock resistance and low resistance to tensile stresses.
9-4 POROSITY AND PERMEABILITY
A fired ceramic product shows porosity to a variable degree; porosity is a measure of the volumes of all pores present in a material. The pores may be open or closed. The open pores are generally interconnected with each other by channels or capillaries, thereby making the material permeable to liquids or gases. The closed pores may be enclosed within individual particles or may form isolated spaces within the matrix of the body so that the material is impermeable to liquid or gas, despite its high porosity. Accordingly, two kinds of porosity can be distinguished: apparent porosity and true porosity. Apparent porosity, called also effective porosity. is expressed as the percentage of the volume of the open pores with respect to the exterior volume of the material under consideration. For consolidated masses such as certain ceramics and rocks the percentage of the apparent porosity can be found from the relationship
W −−−− S
(9-2)
where P = the apparent porosity
D = the weight of dry solid S = the weight of the suspended solid in water, after having been soaked in water,
so that all open pores in the body are completely filled with water W = the weight of the soaked body determined by weighing the soaked specimen
from which the excess surface water has been removed by dabbing with a damp cloth
The difference between the weight of the soaked body W and the weight of the dry body
D is equal to the weight of the water that filled all the open pores within the body; hence it represents the volume of pores in the body. The difference between the weight of the soaked body W and the weight of the suspended solid in water S is equal to the weight of D is equal to the weight of the water that filled all the open pores within the body; hence it represents the volume of pores in the body. The difference between the weight of the soaked body W and the weight of the suspended solid in water S is equal to the weight of
The true porosity represents the volume of both open and closed pores in the volume of the body. The percentage true porosity can be found from the relationship
S g −−−− B d
% true porosity ====
(9-3)
where S g = the specific gravity or the density of the solid
B d = the bulk density of the solid The true specific gravity or true density refers only to the solid matter within the body
and does not include any pores. The bulk density is the weight per unit volume of the material, which includes any pore space that may be present. it can be determined from the relationship
B d ====
W −−−− S V
(9-4)
where V is equal to W — S, the volume of the body with the pores, and D, S, and W are the same as in Equation 9-2. The true specific gravity is an inherent property of the material, but the bulk density is affected by the way in which the material has been manufactured, and it varies considerably with many factors. The true specific gravity or true density of a porous ceramic body is determined by crushing the material to a fine powder to eliminate all the internal pores inside the particles. Then the powder is placed in a pycnometer containing a suitable liquid, so that the volume of the fine particles is measured by the volume of the displaced liquid. The weight of the powdered material divided by the displaced volume of the liquid gives the value of the true density.
The presence of pores in a body adversely affects the strength of ceramics. This is because the pores reduce the cross-sectional area exposed to an applied load and also act as stress concentration raisers, which are particularly effective in brittle ceramics. The decrease in the strength of a ceramic body with porosity can be given by’
n σ φ P ==== σ
(9-5) where σ and σ 0 the fracture strengths of porous and nonporous body, respectively
φ P = the volume fraction of pores and n = a constant having a value from 4 to 7
About 10% porosity by volume reduces the rupture strength by half of that for a nonporous material. Similarly, the Young modulus, E , is affected by porosity.
E ==== E 0 (((( 1 −−−− 1 . 9 φ ++++ P 0 . 9
P ))))
(9-6) where E and E 0 are Young moduli of the porous and nonporous body, respectively.
Furthermore, the shape of the pores and their distribution are important factors. Equations 9-2 and 9-3 permit us to determine the total porosity and open porosity;
however, they do not provide us with information regarding the size of the pores and their distribution. An important method with which to determine the size and distribution of pores employs the mercury porosimeter. In this method, the mercury is brought into however, they do not provide us with information regarding the size of the pores and their distribution. An important method with which to determine the size and distribution of pores employs the mercury porosimeter. In this method, the mercury is brought into
where γ = the surface tension of mercury θ = the wetting angle between the mercury and the solid surface of the pore which
is also dependent on the shape of the pore entry P = the applied pressure
Permeability. Permeability of any porous body is the property that permits fluids to flow through its pores under a pressure gradient. It is obvious that only the open pores, which are interconnected, have the capacity to pass a fluid through the porous medium. For the same apparent porosity (effective porosity) the permeability will also be affected by the size of the pores, their uniform distribution, the internal surface area, and capillary effects. Consequently, any general correlation between porosity and permeability cannot exist, and two porous media of the same porosity may have entirely different permeabilities.
The flow rates of incompressible fluids (liquids) through porous masses can be determined from the empirical relation known as Darcy’s law. Darcy’s law states that the pressure gradient of a liquid is proportional to the specific flow rate through a porous
mass and that the proportionality constants are the fluid viscosity η and the permeability α of the porous mass.
∆ (9-8)
==== ×××× η ×××× u
∆ p where
∆ = a pressure gradient across the porous medium of thickness l l
u = the specific flow rate that can be expressed as either volumetric flow rate (u =Q V /A) or mass flow rate (u = Q m /A) per unit area
Equation 9.8 can be written in terms of the volumetric flow rate as
Q v ==== α ×××× A ××××
×××× ∆ (9-9)
The permeability coefficient a is a measure of the volume of fluid flowing through a cross section in a unit time under the action of a unit pressure gradient and having a unit viscosity. If the volumetric flow rate of fluid of viscosity 1 mPa.s is 1000 mm 3 /s across a
100-mm2 area of a 10-mm-thick porous medium under a pressure difference of 101.3 kPa (1 atm), the permeability unit as calculated from Equation 9-9 will be
V ×××× η
1000 mm / s
×××× −−−− 10 Pa.s
A ×××× ∆ p / ∆ l
2 100 3 mm ×××× 101 . 3 ×××× 10 Pa/10mm
6 ==== 2 0 . 9869 10 ×××× −−−− mm
The permeability coefficient must be determined experimentally for each particular porous mass because it varies widely, even for the same effective porosity, according to the size of the pores, their distribution, and internal specific area.
The Darcy equation is applicable to the laminar flow of incompressible fluids through a horizontal bed of finite thickness. It can be applied for compressible fluids provided the changes in fluid properties (density), occurring during flow across the porous medium are accounted for in Equation 9-8. The flow rates through a porous mass may also change owing to possible structural changes in the mass as a result of swelling of the particles or blocking of the pores by solid impurities or corrosion. Furthermore, the effective porosity of the mass may decrease, owing to blocking of the pores by gases, which are usually dissolved in liquid and may escape from liquid as the pressure decreases during flow through the porous mass. Under low gas pressure a slip at wall surfaces may occur, thereby requiring a correction for viscosity. Also, in a free molecular flow, when the pore size is smaller than the mean free path, the flow is independent of the walls and the viscosity and depends only on the partial pressure and on the ratio of the passage diameter to the length. Furthermore, the effect of surface tension of liquid γ necessitates another correction to the total pressure, which can be determined from Equation 9-7 as
REFRACTORIES
Refractories are special materials of construction capable of withstanding high temperatures in various industrial processes and operations. The main bulk of the commercial refractoriies comprises complex solid bodies consisting of high-melting oxides or a combination of oxides of elements such as silicon, aluminum, magnesium, calcium, and zirconium, with small amounts of other elements present as impurities. In recent years intensive work has been con ducted to develop new materials of construction for service at very high temperatures, such as are encountered in gas turbines, ram-jet engines, missiles, nuclear reactors, and various other high-temperature processes and operations. These highly refractory materials are relatively simple crystalline bodies composed of pure metallic oxides, carbides, borides, nitrodes. sialons, and sulfides. Finally, combinations of these refractory compounds with metals yield cermets, which show better thermal shock resistance than ceramics but, at the same time, retain their high refractoriness. Refractoriness is the ability of a material to withstand the action of heat without appreciable deformation or softening under particular service conditions.
9-5 COMMON REFRACTORY MATERIALS
Common refractory materials represent the main bulk of commercial refractories used in high-temperature processes and operations because of their relatively low price and ready availability. They consist of crystalline or partly amorphous constituents held together by a more or less glassy matrix of variable composition. One of the most widely used refractoriies is based on alumina—silica compositions, varying from nearly pure silica, through a wide range of alumina—silicates, to nearly pure alumina.
Silica—Alumina Phase Equilibrium Diagram. The silica—alumina phase equilibrium diagram is of great importance in understanding and predicting the properties and behavior of various clay products and silica—alumina refractories (Fig. 9-3).
The curved line ABCD is the liquidus line separating the liquid phase from the heterogeneous solid—liquid phase. The areas bounded by the curved and horizontal lines The curved line ABCD is the liquidus line separating the liquid phase from the heterogeneous solid—liquid phase. The areas bounded by the curved and horizontal lines
Both silica and alumina can be regarded as refractory oxides with melting points of 1710°C (3110°F) and 2050°C (3722°F), respectively. Small additions of alumina to silica act as a powerful flux for silica, rapidly lowering its melting point. The mixture of 5.5% alumina and 94.5% silica represents the eutectic composition with a melting point of 1545°C (2814°F). If a mixture of composition somewhere between 0% and 5.5% alumina, say 2.5%, is heated, it remains solid until the eutectic temperature of 1545°C (2814°F) is reached. At and just above this temperature, part of the mixture will melt, forming a liquid phase of the eutectic composition (5.5% A1203, 94.5% Si02), whereas the excess of silica remains in the solid state. The amount of the eutectic formed can be calculated from the lever rule = X 100 = 45.5% liquid and the amount of the solid silica left will be correspondingly 100% — 45.5% = 54.5%.
FIGURE 9-3 Equilibrium diagram of the system Al 2 O 3 -SiO 2 . (Journal of the
American Ceramics Society, 7, 2381. 1924.)
Further heating will raise the temperature along line xx, and more solid silica will dissolve in the eutectic liquid whose composition will vary from B to the intersection of line xx with line AB (see Fig. 9-3). At point 2 the composition of the liquid is given by point 2’ on the liquid curve AB. The amounts of liquid and solid can be easily found from the lever rule, as previously. Similar considerations can be applied to a mixture of compositions between 5.5% and 55% alumina. When heated just above the eutectic temperature. a liquid of eutectic composition is formed first in an amount corresponding to 5.5% alumina. Any excess alumina left will react with the remaining silica to form
mullite, a solid of composition 3Al 2 O 3 .2SiO 2 , which corresponds to 71.8% alumina and 28.2% silica by weight. With increasing temperature the amount of liquid formed increases, becoming progressively richer in alumina because of the solution of mullite in the original liquid. Thus the composition of the melt changes, as indicated by the curved liquidus line BC. For compositions lying between 55% and 71.8% alumina not all the mullite will be melted. The excess mullite will dissociate at a temperature of 1810°C (3290°F), forming corundum (crystalline alpha alumina) and liquid containing 55% alumina. This temperature point is known as the incongruent melting point, at which a mullite, a solid of composition 3Al 2 O 3 .2SiO 2 , which corresponds to 71.8% alumina and 28.2% silica by weight. With increasing temperature the amount of liquid formed increases, becoming progressively richer in alumina because of the solution of mullite in the original liquid. Thus the composition of the melt changes, as indicated by the curved liquidus line BC. For compositions lying between 55% and 71.8% alumina not all the mullite will be melted. The excess mullite will dissociate at a temperature of 1810°C (3290°F), forming corundum (crystalline alpha alumina) and liquid containing 55% alumina. This temperature point is known as the incongruent melting point, at which a
At temperatures below 1810°C (3290°F) mixtures of compositions greater than 71.8% alumina consist of the solid phases of corundum and mullite. At 1810°C mullite will dissociate into corundum and liquid (point C) and, with a further temperature increase, the corundum will gradually dissolve into liquid.
The amount of liquid phase and the mineralogical nature of the solid body can be calculated at any temperature for any known chemical composition of a mixture from the phase equilibrium diagram by means of the lever principle. For example, a mixture of 45% alumina (line yy) at a temperature below 1470°C (2680°F) is a solid body composed of tridymite and mullite coexisting in equilibrium. The tridymite present can be calculated from
71 . 8 % −−−− 45 % ×××× 100 % ==== 37 . 3 % tridymite
and the mullite will be 100% — 37.3% = 62.7%. The amount of liquid formed at the eutectic temperature is found from
and the amount of solid phase mullite is 100% — 40.4% = 59.6%. Under equilibrium conditions, a well-fired product containing 45% alumina and 55%
silica will consist of 62.7% mullite and 37.3% cristobalite. At the eutectic temperature, the mixture will melt to a very viscous liquid in the amount of 40.4%. With increasing temperature the amount of liquid will increase until a temperature is reached at which the material will be completely melted. This temperature corresponds to the intersection of line yy with the liquidus curve BC.
The silica—alumina phase diagram data are applicable only to equilibrium conditions between reacting constituents and in pure binary systems. In practice, equilibrium is seldom reached because reactions between solid and solid, and even between solid and liquid, are sluggish, requiring a very long time for completion. Most commercial alumina—silica refractories always contain a certain amount of impurities such as basic oxides of iron, calcium, magnesium, and smaller amounts of alkaline metal oxides. These impurities greatly affect equilibrium relations between the two major components, silica and alumina, and considerably lower the eutectic temperature and alter the eutectic composition. The fluxing action of the basic oxides follows the order MgO < CaO < FeO
< Na 2 O < K 2 O, the latter two being much more effective than the former three. Ferric oxide behaves as part of the refractory portion up to about 1300°C (2372°F). Above this temperature it begins to break down to ferrous oxide (FeO), especially in the presence of
a high proportion of molten silicates. The following practical conclusions can be drawn from the silica—alumina phase
diagram.
1. Refractories of composition between 3% and 8% alumina should be avoided because they are close to the region of low eutectic temperature.
2. Refractoriness increases with an increase in alumina content. This applies particularly to refractoriness under load, which is determined almost en tirely by the amount of liquid formed and its viscosity.
3. Alumina—sillica refractories show a wide softening temperature range extending from the temperature at which the liquid begins to form to the temperature at which the entire body melts. This accounts for the shrink age and deformation of these refractories under load at temperatures well below their fusion points. The compositions from 20% to 40% alumina are known as fireclay refractories, which are classified as super-duty, high-duty, medium-duty, and low-duty bricks, depending on their alumina content and degree of firing.
4. Crystals of mullite and alumina (corundum) are the only stable compounds of silica and alumina at temperature above 1710°C (3110°F). Mullite does not show any abnormality on heating, and its coefficient of expansion is fairly low. It follows that for high refractoriness, “green” bricks should be of a composition that yields the maximum mullite content on firing. Furthermore, in bricks of high mullite content, the glassy bond of the regular brick is replaced by a crystalline bond. This has led to the development of mullite refractories. which are made by fusing alumina and silica materials in any required proportion or by calcining sillimanite, a naturally occurring mineral of the
composition Al 2 O 3. SiO 2 (63% Al 2 O 3 by weight).
5. In the range of composition of 63.5% to 71.8% alumina, corundum appears due to the dissociation of mullite at 1810°C (3290°F). Compositions above 71.8% alumina yield a solid phase consisting of mullite and corundum only. It can be seen from the diagram (Fig. 9-3) that the first Iiquid begins to form at 1810°C. With an increasing amount of alumina the refractory consists mainly of crystalline corundum bonded with a glassy matrix formed from the molten impurities.
9-6 REFRACTORINESS VERSUS THE CERAMIC BOND The refractoriness of refractory materials depends on their chemical and mineralogical
composition. on their dimensional stability on heating, and to some extent on their texture. To obtain high refractoriness, manufacturing methods different from those for porcelain or stoneware need to be used. The main ingredient of the body is a highly refractory, non-plastic material that should have a sufficient dimensional stability at high temperatures. For this reason ingredients that tend to shrink considerably during firing, such as fireclay, diaspore clay, sillimanite, and magnesium oxide, must be well prefired to reduce their subsequent shrinkage on firing of the brick. The material is crushed into fractions of three different sizes that are mixed in suitable proportions to produce a mix of maximum density. The loose refractory aggregate is mixed with a suitable bonding agent to provide a mass with adequate workability for shaping and forming operations and to develop a ceramic bond on firing. Refractories are generally fired at much higher temperatures than ordinary ceramic wares. Firing produces a ceramic bond and insures the necessary dimensional stability of the product when it is used for high-temperature applications. A ceramic bond2 can be defined as a glassy matrix formed on cooling the liquid produced from the more fusible constituents of the mixture at firing temperatures.
The presence of the ceramic bond greatly increases the cold strength of a refractory, but it lowers its refractoriness at high temperatures. For high refractoriness the amount of glassy matrix should be as low as is compatible with the strength requirements of the refractory at room temperature.
The effect of the ceramic bond on refractoriness can be illustrated by referring to a few typical examples. Fireclay brick is made of a non-plastic, refractory material, which is well-fired clay, or old fireclay brick crushed to suitable size fractions, called grog. The grog is mixed with plastic fireclay as a bonding agent, which makes up as much as 50% of the total mixture. Such a mix gives a considerable amount of the ceramic bond on firing, accounting for gradual softening and low refractoriness-under-load of fireclay refractories (Fig. 9-4). When temperature during firing is sufficiently high and the time is long enough, the glassy matrix may be gradually replaced by crystals. This is due to the The effect of the ceramic bond on refractoriness can be illustrated by referring to a few typical examples. Fireclay brick is made of a non-plastic, refractory material, which is well-fired clay, or old fireclay brick crushed to suitable size fractions, called grog. The grog is mixed with plastic fireclay as a bonding agent, which makes up as much as 50% of the total mixture. Such a mix gives a considerable amount of the ceramic bond on firing, accounting for gradual softening and low refractoriness-under-load of fireclay refractories (Fig. 9-4). When temperature during firing is sufficiently high and the time is long enough, the glassy matrix may be gradually replaced by crystals. This is due to the
FIGURE 9-4 Refractoriness-under-load, 345 kPa (50 psi) of fireclay, magnesite, and silica brick.
Silica brick is made of crushed quartz, which is a non-plastic, refractory ingredient. This is mixed with only 2% lime which, on firing reacts with fine particles of silica to form
calcium silicate (CaO.SiO 2 ), functioning as a ceramic bond. This results in a relatively small amount of liquid having high viscosity, thereby explaining the high refractoriness of silica brick, which is close to the melting point of pure silica. In the presence of even small amounts of alumina or alkalies the liquid immiscibility ceases to exist, causing a considerable lowering of refractoriness for the brick. For high refractoriness, therefore, silica brick should be made of very pure quartzite. free from any appreciable amounts of such impurities as alumina and alkalies.
Dimensional Stability. Dimensional stability can be defined as the resistance of a material to any volume changes that may occur on its exposure to high temperatures over
a prolonged time. These dimensional changes can be considered as permanent (irreversible) and reversible.
Irreversible changes may result in either the contraction or the expansion of a refractory. The permanent contraction is due to the formation of increasing amounts of liquid from the low-fusible constituents of the brick when it is subjected to a long period of soaking at high temperatures. The liquid gradually fills the pores in the body, causing a higher degree of vitrification and shrinkage. A typical example of such behavior is fireclay brick. The shrinkage of a refractory can also be caused by the transformation of one crystalline form into another. For example, magnesite brick, an amorphous magnesium oxide that is relatively light (specific gravity 3.05), is converted gradually to a dense Irreversible changes may result in either the contraction or the expansion of a refractory. The permanent contraction is due to the formation of increasing amounts of liquid from the low-fusible constituents of the brick when it is subjected to a long period of soaking at high temperatures. The liquid gradually fills the pores in the body, causing a higher degree of vitrification and shrinkage. A typical example of such behavior is fireclay brick. The shrinkage of a refractory can also be caused by the transformation of one crystalline form into another. For example, magnesite brick, an amorphous magnesium oxide that is relatively light (specific gravity 3.05), is converted gradually to a dense
On the other hand, the transformation of residual quartz in silica brick to tridymite and cristobalite at high service temperatures is accompanied by a decrease in the specific gravity and consequently by a volume increase. The specific gravity of quartz is 2.65, whereas those of tridymite and cristobalite are 2.26 and 2.32, respectively. This transformation accounts for the characteristic permanent expansion of silica brick in service. Reversible volume changes are directly related to the coefficient of thermal expansion.
9-7 CASTABLE AND FUSED REFRACTORIES
Castable refractories are made by mixing refractory aggregate of suitable grading, such as alumina—silicates or high alumina, with a refractory high-alumina cement and water to desired consistency. The mix is then either cast, rammed, gunned, or sprayed into shape and permitted to set until it becomes hydraulically bonded (see Section 9-11). On subsequent heating to high temperature this cementitious bond is dehydrated and replaced by a refractory bond that is developed between the matrix and aggregate particles. Castable refractories have been used in applications where abrasion resistance at elevated temperatures is required and as a protective barrier against corrosive attack by hot gases and liquids that are highly detrimental to other structural materials.
Other types of castable refractories are phosphate-bonded refractory bricks, mortars, ramming mixes, and plastic and cold setting castables. They may contain, as aggregate refractory oxides, carbides such as SiC and other mixed with phosphoric acid or aluminum acid phosphate, or alkali polymetaphosphates and other acid phosphates, and give rise to a phosphate bond between the aggregate particles. The phosphate-bonded alumina materials are highly resistant to thermal shock but have poor resistance to erosion.
To secure castable refractories and, to some extent, brick linings safely in a place, a special anchoring system in the form of either a hexagonal grid and/or studs of various design and shapes is required. These are made of carbon steel or stainless steels or some other heat-resisting alloys depending on the service temperature.
Fusion-cast refractories are produced by mixing suitable refractory ingredients and melting them in an electric-arc furnace at temperatures of 1760—2480°C (3200— 4500°F). The resultant liquid is then poured into a mold made of graphite plates buried in refractory powder where the material solidifies and cools slowly in the mold to room temperature. The refractory ingots are then withdrawn from the mold and sawed into desired shapes and sizes. The fusion-cast process produces a unique refractory having a high density with little porosity due to large isolated voids, high hot strength, improved abrasion resistance, and better resistance to corrosive attack by molten liquids and hot gases. Fusion castings of ceramics has been limited mainly to alumina, mullite, zirconia,
silica, chromia, and AZS (47% A1 2 O 3 , 36.5% Z r O 2 , 16.5% SiO 2 ) refractories.
9-8 SUPERREFRACTORIES
The equilibrium phase diagrams indicate that, in most cases, higher refractoriness can be attained by using pure oxides of high melting points. The presence of even small amounts of impurities considerably lowers the melting point and reduces refractoriness- under-load to a much greater extent than could be expected from the corresponding phase diagrams. Furthermore, the presence of the ceramic bond in a refractory represents an inherent weakness because it reduces their load-bearing characteristics, decreases their chemical resistance to slags and fluxes, and may adversely affect their other properties. Consequently, the development of high-refractory materials has been carried The equilibrium phase diagrams indicate that, in most cases, higher refractoriness can be attained by using pure oxides of high melting points. The presence of even small amounts of impurities considerably lowers the melting point and reduces refractoriness- under-load to a much greater extent than could be expected from the corresponding phase diagrams. Furthermore, the presence of the ceramic bond in a refractory represents an inherent weakness because it reduces their load-bearing characteristics, decreases their chemical resistance to slags and fluxes, and may adversely affect their other properties. Consequently, the development of high-refractory materials has been carried
These techniques may involve compacting of fine powders followed by sintering at a suitable temperature at which gradual crystallization at the grain boundaries occurs, binding the particles into a coherent, strong body. The resulting bond is the crystalline bond, whereas the sintered article is called a self-bonded refractory. Since the bond is composed of crystals of the same material as the particles, self-bonded refractories exhibit high refractoriness, approaching that of the pure material itself.
Hot pressing and liquid phase sintering are frequently used in shaping refractory compounds from powders. These methods increase the rate of densification and lead to strong products. Such methods contributed to the developments of technical ceramics based on alumina, zirconia, silicon carbide, silicon nitride, and various borides that provided materials of high temperature capabilities, excellent wear resistance, and improved brittleness.
Carbides. Carbides are characterized by very high melting points, but they lack oxidation resistance at high temperatures. The most important refractory materials are carbides of silicon and boron and interstitial carbides of the transition elements, such as zirconium and titanium. (See Table 9-1.) The most widely refractory carbide used is silicon carbide (SiC). It is hard but it has excellent resistance to oxidation to 1650°C (3000°F) because
of formation of a protective SiO 2 coating. Low thermal expansion and high thermal
conductivity are factors contributing to its excellent thermal shock resistance. The principal bonds used in silicon carbide ceramics are the following: (1) oxide or silicate bond, (2) silicon nitride and oxynitride bonds, (3) recrystallized or sintered silicon carbide.
For high-temperature usage in excess of 2500°C (4532°F) in vacuo, the carbides and borides are about the only suitable materials available because of their low volatility. Carbides of zirconium, tungsten, molybdenum, tantalum, niobium, and cerium can be used above 2000°C (3635°F) in neutral or reducing atmosphere. Titanium carbides, vanadium, and niobium carbides can be used up to 2500°C (4532°F) in a nitrogen atmosphere. Some of the carbides have the highest known melting temperature of materials, for example, hafnium carbide (HfC) at 3930°C (7100°F). Boron carbide is the hardest and most abrasion-resistant material available in massive form; its melting point is 2430°C (4400°F). It is used as armor because of high strength, high elasticity, and low density. Borides have poor oxidation resistance at elevated temperatures. TiB2 and ZrB2 have the electrical resistivity of the order of copper. They have strong covalent bonds and exists in two modifications: a low-temperature cubic crystal structure
β transforming at about 2100°C (3810°F) to a high-temperature form a having a hexagonal zinc blend structure.
Nitrides. Nitrides are characterized by high melting points, but they have a low resistance to oxidation and poor chemical resistance. The two most industrially important nitrides are silicon nitride and boron nitride.
Boron nitride has the graphite structure and it resembles graphite in its lubricating properties. A cubic crystalline form of boron nitride, known as Borazon. has been produced under high pressure, 145 MPa (106 psi), and temperatures above 1650°C (3000°F) and has a hardness equal to that of diamond. Borazon can withstand temperatures up to 1930°C (3500°F) without becoming appreciably oxidized.
Silicon nitride (Si 3 N 4 ) has a covalently bonded structure resulting from the tetrahedral arrangement of valence orbitals with 4 nitrogen atoms similar to SiO 4 tetrahedra. These tetrahedra form a three-dimensional network by sharing corners such that each N is common to three tetrahedra. Silicon nitride (Si 3 N 4 ) exists in two polymorphic forms: hexagonal β -Si 3 N 4 and hexagonal α -Si 3 N 4 ; the β form is the stable one at high temperatures. Silicon nitride can be made as a powder by a number of methods; the most commercially available one is by nitriding silicon at 1400°C according to the reaction
3Si + 2N 2 = Si 3 N4 ( α + β ) (9-11) Because of its crystal structure and strong covalent bonding Si 3 N 4 shows excellent
intrinsic properties such as low thermal expansion, moderate elastic modulus, high thermal shock resistance, high strength, wear resistance, oxidation resistance, and
thermal stability. Si 3 N 4 powder is relatively easy to produce, but it is not easily converted to high-density products because the bonding is of covalent nature and the structure has only a few intrinsic vacancies.
A possible solution to the fabrication problem is to treat the β -Si 3 N 4 structure with metallic oxides such as Al 2 0 3 ,Y 2 O 3 , MgO, BeO, and others. A simultaneous replacement of silicon and nitrogen by aluminum and oxygen takes place giving the system Si—AI— O—N. Other metal atoms can also be substituted giving rise to new materials called
Sialons with the three-dimensional structure formed by (Si, M)(O, N) 4 tetrahedra. Here M stands for Al, Mg, Be, Y, or others. For example, the reaction sintered mixture of 50 mol% Si 3 N 4 , 25 mol% Al 2 O 3 and 25 mol% AlN gives the sialon Si 4 Al 2 N 6 O 2 with a
97.1% theoretical density of 3.09 g/cm 3 . This material is stronger than reaction bonded Si 3 N 4 and at the same time retains excellent thermal shock resistance. Sialons are of
much scientific and industrial interest because their interatomic bonding may cover a wide spectrum from highly covalent to partial ionic bonding.