Ceramic Technology and its Prospects
The word ceramic can be traced back to the Greek term keramos, meaning “a potter” or “pottery.” Keramos in turn is related to an older Sanskrit root meaning “to burn.” Thus the early Greeks used the term to mean “burned stuff” or “burned earth” when referring to products obtained through the action of fire upon earthy materials. The Silicate ceramics include objects made from clay, such as pottery, bricks, and table china. The three major ingredients of common pottery are clay, from weathering of feldspar ash, sand (silica), and feldspar (aluminosilicates).
A ceramic material is often understood as restricted to inorganic crystalline oxide material. It is solid and inert. Ceramic materials are brittle, hard, strong in compression, weak in shearing and tension. They withstand chemical erosion that occurs in other materials subjected to acidic or caustic environment. Ceramics generally can withstand very high temperatures such as temperatures that range from 1,000°C to 1,600°C (1,800°F to 3,000°F). Exceptions include inorganic materials that do not include oxygen such as silicon carbide or silicon nitrite. A glass is often not understood as a ceramic because of its amorphous (non-crystalline) character. However, glass making involves several steps of the ceramic process and its mechanical properties are similar to ceramic materials. Macroscopically, the structure of ceramics contains various atoms of different sizes, and is among the most complex of all material structures. The bonding between these atoms is generally covalent (electron sharing, hence strong bonds) or ionic (primary bonding between oppositely charged ions, thus strong bonds) and these bonds are stronger than metallic bonds. As a result, properties such as hardness, thermal and electric resistance are superior compared with metals. One major influence of ceramic strength and properties is its grain size-the finer the grain size, the higher are the strength and toughness.
Ceramics are favored compared to other materials because: (1) the raw materials for making them are readily available and cheap. (2) In comparison to metals, they are lightweight and can retain their strength at temperatures above 1000?C, where metal parts tend to fail. (3) They also have electrical, optical, and magnetic properties of value in the computer and electronic industries.
The one severely limiting problem in utilizing ceramics is their brittle nature. Since the stress failure of ceramic materials is due to molecular abnormalities resulting from impurities or disorder in the basic atomic arrangements, much attention is now being given to purer starting materials and the control of the processing step.
The art of making pottery by forming and burning clay has been practiced from the earliest civilizations. Indeed, examination of pottery fragments has been one of the best tools of the archeologist. Burnt clayware has been found dating from about 15,000 B.C. and as well developed as an industrial product in Egypt by about 5000 B.C. The greatest advances in pottery making were made in China, where fine white stoneware was first crafted as early as 4000 B.C.
Similarly, the manufacture of silicate glasses is an ancient art. Naturally occurring glasses (obsidian) were used during the Stone Age. Glazes on stone beads have been found dating to about 12,000 B.C. Formed glass dates from the period 7000-5000 B.C. and was a stable industry in Egypt by about 1500 B.C.
In contrast, manufacture of cementations materials has only been practiced for about a hundred years. The Egyptians used burned lime as a mortar; later the Romans combined this with volcanic ash to make a natural hydraulic cement. The art seems then to have disappeared, but hydraulic properties of lightly burned clayey limes were rediscovered in England about 1750, and in the next hundred years the manufacturing process, essentially the same as that used now, was developed.
Classification of Ceramic
Non- crystalline Ceramics:
Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is shaped when either fully molten, by casting or, when in a state of toffee – like viscosity, by methods such as blowing to a mould. If later heat-treatments cause this class to become partly crystalline, the resulting materials are as a glass – ceramic.
Crystalline Ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories- either make the ceramic in the desired shape , by reaction in situ, or by “forming” powders into the desired shape , and then sintering to form a solid body. Ceramic forming techniques include shaping by hand (“sometimes including a rotation process called throwing”), slip casting, tape casting(used for making very thin ceramic capacitors, etc.), injection molding , dry pressing, and other variations. (See also Ceramic forming techniques. Details of these processes are described in the two books listed below.) A few methods use a hybrid between the two approaches.
Porcelain is a ceramic material made by heating selected and refined materials often including clay in the form of kaolinite to high temperatures. Raw materials for porcelain, when mixed with water form a plastic paste that can be worked to a required shape before firing in a kiln at temperatures between about 1200 and 1400 degrees Celsius. The toughness, strength, and translucence of porcelain arise mainly from the formation at high temperatures of glass and the mineral mullite.
Porcelain was named after its resemblance to the white, shiny Venus-shell, called in old Italian porcella. The curved shape of the upper surface of the Venus-shell resembles the curve of a pig’s back (Latin porcella, a little pig, a pig).
Properties associated with porcelain include low permeability, high strength, hardness, glassiness, high durability, whiteness, translucence, resonance, brittleness, high resistance to the passage of thermal shock and high elasticity.
The combined Nomenclature of the European Communities defines porcelain as “completely vitrified , hard, impermeable (even before glazing), white or artificially colored, translucent (except when of considerable thickness ) and resonant”
Porcelain is used to make table, kitchen, sanitary, decorative wares, objects of fine art, and tiles. Its high resistance to the passage of electricity makes porcelain an exceelent insulating material and it is widely used for high-voltage insulators. It is also used in dentistry to make false teeth, caps and crowns.
Porcelain has many uses but this article is concerned mainly with its employment as a material used to make objects of craft and fine art, including decorative and utilitarian household wares. A difficult line to draw is that which divides high – fired stoneware from porcelain because this depends upon how the terms porcelain and stoneware are defined. In this article the term porcelain is taken to encompass a broad range of high-fired ceramic wares, including some that might accord some systems of classification fall into the category of stoneware.
The material used to form the body of porcelain wares is often referred to as clay, even though clay minerals might account for only a small proportion of its whole. The porcelain clay body, unfired or fired , is sometimes spoken of as the paste and porcelain clay is itself sometimes described as the body (for example , when buying materials a potter might order such an amount of porcelain body from a vender).
The composition of porcelain is highly variable, but china clay, comprising mainly or in part the plately clay mineral salinity is often a significant component. Other materials mixed with china clay to make porcelain clay have included feldspar, ball-clay, glass, bone ash, steatite, quartz, petuntse and alabaster.
The clays used by potters are often described as being long or short according to plasticity. Long clays are cohesive and of high plasticity and short clays are less cohesive and are of lower plasticity. In soil mechanics plasticity is determined by measuring the increase in content is of water required to change a clay from a solid state bordering on the plastic, to a plastic state bordering on the liquid, though the term is also used less formally to describe the facility with which a clay may be worked.
Forming: Porcelain wares can be formed by any of the shaping methods listed in the pottery article.
Glazing: It has been speculated that the first glazes were accidental and resulted from the presence in the kiln of lime – rich wood ash.
Decoration: Porcelain wares may be decorated under the glaze, using pigments that include cobalt and copper, or over the glaze using colored enamels.
Categories of Porcelain:
Western porcelain is generally divide into the three main categories of hard-paste, soft-paste and bone china, depending on the composition of the paste (the paste is the material used to form the body of a piece of porcelain).
One of the earliest European porcelains was produced at the Miessien factory and was compounded from china clay kaolin, quartz and alabaster and was fired at temperature in excess of 1350-degrees Celsius to produce porcelain of great hardness and strength. At a later date the composition of Meissen hard paste was changed and the alabaster was replaced by feldspar, lowering the firing temperature required. China clay, feldspar and quartz (or other forms of silica) continue to this day to provide the basic ingredients for most contents European hard paste porcelain.
Its history dates from the early attempt by European potters to replicate Chinese porcelain by using mixtures china clay and ground-up glass or frit; soapstone and lime were known to have also been included in some compositions. As these early formulations suffered from high pyroplastic deformation, or slumping in the kiln at raised temperature, they were uneconomic to produce.
Formations were later developed based on kaolin, quartz, feldspar, nepheline syenite and other feldspathic rocks. These were technically superior and continue in production.
Although originally developed in England to compete with imported porcelain, Bone china is now made worldwide. It has been suggested that a misunderstanding of an account of porcelain manufacture in China given by a Jesults missionary was responsible for the first attempts to use bone-ash as an ingredient of Western porcelain (in China, China clay was sometimes described as forming the bones of the paste, while the Flesh was provided by refined porcelain stone ). For what ever reason, when it was first tried it was found that adding bone-ash to the paste produced white, strong, translucent porcelain. Traditionally English bone china was made from two parts of bone-ash, one part of china clay kaolin and one part of Cornish china stone (a feldspathic rock), although this has largely been replaced by feldspars from non-UK sources.
One significant characteristic of the ceramic industry is that it is vital to other industries. For example, refractories (A refractory is a material that retains its strength at high temperatures) are a basic component of the metallurgical industry. Abrasives (An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away) are essential to the machine tool and automobile industries. As a matter of fact, almost every industrial production line, office, and home is dependent on ceramic materials. Newly designed devices incorporate ceramic materials because of their useful chemical, electrical, mechanical, thermal and structure properties.
The largest segment of the ceramic industry is the manufacture of various glass products. These are manufactured almost entirely as sodium-calcium-silicate glasses. The next largest segment of the ceramic industry is lime and cement products used for building construction. A much more diverse group of products is included in the classification of whitewares which includes pottery, porcelain, and similar fine-grained porcelain-like compositions which comprise a wide variety of specific products and uses. The next classification is porcelain enamels which are mainly silicate glass-like coatings on metals. Another distinct group is the structural clay products which consist mainly of brick and tile but include a variety of similar products such as sewer pipe. A particularly important group for their technical and industrial applications is refractories. About 40% of the refractory industry consists of clay products, and another 40% consists of heavy non-clay refractories such as magnesite, chromite, and similar compositions. In addition there is a sizable demand for various special refractory compositions
That is, ceramics are important, first, because they comprise a large and basic industry and, second, because their properties are critical for many applications.
Traditional ceramics are those comprising the silicate industries which are clay products, cements and silicate glasses. By far, it is the largest portion of the total ceramic industry, and can be considered as the backbone of the field.
New ceramics are based on simpler chemical compounds, such as oxides, carbides, and nitrides. When mixed with water, they do not posses the traditional clay properties of formability and plasticity. Accordingly, other ingredients must be combined with the ceramic powders to achieve plasticity and other desirable properties during forming so that conventional shaping methods can be used. The new ceramics are generally designed for applications that require higher strength, hardness, and other properties that are not found in the traditional ceramic materials. These properties have initiated the introduction of several new processing techniques not previously used for traditional ceramics.
Control of ceramics processes necessarily involves both the raw materials used and the way in which they are formed and heat-treated. The kind and sources of raw materials most commonly used in ceramics are considered. These are mostly natural materials which are available at relatively low cost but contain some impurities. If these impurities are harmful, various chemical or other treatments must be employed. However, these treatments are costly and should not be used unless necessary-depending on their effect on processing or resultant properties.
Similarly, properties, tolerance, cost and even the feasibility of obtaining useful products depends on the ability to fabricate the shapes and sizes needed.
Ceramics Raw Materials
a) Stone materials:
Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth’s crust.
Feldspar is derived from the German Feld, field, and Spat, a rock that does not contain ore. “Feldspathic” refers to materials that contain feldspar. The alternative spelling, felspar, has now largely fallen out of use.
The name “quartz” comes from the German “Quarz”, which is of Slavic origin (Czech miners called it Krem). Other sources insist the name is from the Saxon word “Querkluftertz”, meaning cross-vein ore.
b) Clay materials:
· 2 types of clay are used:
1) ball clay &
2) China clay.
Clay is a term used to describe a group of hydrous aluminum phyllosilicate (phyllosilicates being a subgroup of silicate minerals) minerals), that are typically less than 2 ?m (micrometers) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminum oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate – bearing rocks by carbonic acid but some are formed by hydrothermal activity. Clays are distinguished from other small particles present in soils such as slit by their small size, flake or layered shape, affinity for water and tendency toward high plasticity.
Processing of Ceramics
1. Traditional Ceramics
A. Preparation of Raw Materials
The shaping processes for traditional ceramics require that the starting material be in the form of plastic paste. This paste is comprised of fine ceramic powders mixed with water, and its consistency determines the ease of forming the material and the quality of the final product. The raw ceramic material usually occurs in nature as rocky lumps, and reduction to powder is the purpose of the preparation step in ceramics processing.
Techniques for reducing particle size in ceramics processing deliver mechanical energy in various forms, such as impact, compression, and attrition. The term comminution is used for these techniques, which are most effective on brittle materials, including cement, metallic ores, and brittle metals. Two general types of comminution operations are distinguished: crushing and grinding.
Crushing refers to the reduction of large lumps from the mine to smaller sizes for subsequent further reduction. Several stages may be required (for example, primary crushing and secondary crushing), the reduction ratio in each stage being in the range from 3 to 6. Crushing of materials is accomplished by compression against rigid surfaces or by impact against surfaces in a rigid constrained motion. Equipment used to accomplish crushing is of several types, including (a) jaw crushers, in which a large jaw toggles back and forth to crush lumps against a hard rigid surface; (b) gyratory crushers, which use a gyrating cone to compress lumps against a rigid surface; (c) roll crushers, in which the ceramic lumps are squeezed between rotating drums; and (d) hammer mills, which use rotating hammers impacting the material to break up the lumps.
Grinding, in our context here, refers to the operation of reducing the small pieces after crushing to a fine powder. Grinding is accomplished by abrasion and impact of the crushed mineral by the free motion of unconnected hard media such as balls, pebbles, or rods. Examples of grinding include (a) ball mill, (b) roller mill, and (c) impact grinding.
In a ball mill, hard spheres mixed with the stock to be comminuted are rotated inside a large cylindrical container. The motion causes the ball and stock to be carried up the container wall and then pulled back down by gravity to accomplish a grinding action by a combination of impact and attrition. These operations are often carried out with water added to the mixture so that the ceramic is in the form of slurry.
In a roller mill, stock is compressed against a flat horizontal grinding table by rollers riding over the table surface. The pressure of the grinding rollers against the table is regulated by mechanical springs or hydraulic-pneumatic means.
In impact grinding, which seems to be less frequently used, particles of stock are thrown against a hard flat surface, either in a high velocity airstream or in high speed slurry. The impact fractures the pieces into smaller particles.
The ground particles are then mixed with additives, the functions of which are one or more of the following:
a. Binder – for ceramic particles
b. Lubricant – to aid mold release and to reduce internal friction between particles during molding
c. Wetting agent – to improve mixing
d. Plasticizer – to make the mix more plastic and formable
e. Various agents – to control foaming and sintering
f. Deflocculent – to make the ceramic-water suspension more uniform. Deflocculation changes the electrical charges on the particles of clay, so that they repel rather than attract each other. Water is added to make the mixture more pourable and less viscous. Typical deflocculents are Na2CO3 and Na2SiO3, in amounts less than 1%.
These ingredients must be thoroughly mixed, either wet or dry. The ball mill often serves this purpose in addition to its grinding function.
For processes in which the body is formed by pressing in an essentially dry state, material may be formed by either a wet or dry process. However, it is becoming increasingly common to use the wet process in which the materials are milled together with whatever binders or additives are necessary and are then spray-dried in order to form small spherical granules which are free flowing and readily fill die cavities.
B. Shaping Processes
The optimum proportions of powder and water depend on the shaping process used. Some shaping processes require high fluidity; others act on a composition that contains very low water content. At about 50% water, the mixture is slurry that flows like a liquid. As the water content is reduced, increased pressure is required on the paste to produce a similar flow. Thus the shaping processes can be divided according to the consistency of the mixture: (1) slip casting in which the mixture is a slurry; (2) plastic forming methods, which shape the clay in a plastic condition; (3) semidry pressing, in which the clay is moist but possesses low plasticity; and (4) dry pressing, in which the clay is basically dry, containing less than 5% water. Dry clay has no plasticity.
a. Slip casting
In slip casting, a suspension of ceramic powders in water, called a slip, is poured into a porous plaster of Paris (2CaSO4-H2O) mold so that water from the mix is gradually absorbed into the plaster to form a firm layer of clay at the mold surface. The composition of the slip is typically 25% to 40% water, the remainder being clay often mixed with other ingredients. It must be sufficiently fluid to flow into the crevices of the mold cavity; yet lower water content is desirable for faster production rates. Slip casting has two principal variations:
1. Drain casting – In drain casting, which is the traditional process, the mold is inverted to drain excess slip after the semisolid layer has been formed, thus leaving a hollow part in the mold; the mold is then opened and the part removed. It is used to make teapots, vases, art objects, and other hollow-ware products.
2. Solid casting – In solid casting, used to produce solid products, adequate time is allowed for the entire body to become firm. The mold must be periodically resupplied with additional slip to account for shrinkage due to absorbed water.
The major advantages of the slip casting process are that it allows for the production of complex shapes, such as sanitary ware, artware, and special laboratory ware, and that it allows for the economical production of a relatively small number of an individual shape since the cost of plater molds is low. For this reason it is a technique particularly well adapted to fabrication of test samples and laboratory ware as well as certain artware products.
b. Plastic forming
This category includes a variety of methods, both manual and mechanized. They all require the starting mixture to have a plastic consistency, which is generally achieved with 15% to 25% water. Manual methods generally make use of clay at the upper end of the range because it provides a material that is more easily formed; however, this is accompanied by greater shrinkage in drying. Mechanized methods generally employ a mixture with lower water content so that the starting clay is stiffer.
Although manual forming methods date back thousands of years, they are still used today by skilled artisans, either in production or for artworks.
1. Hand modeling – involves the creation of the ceramic product by manipulating the mass of plastic clay into the desired geometry. In addition to art pieces, patterns for plaster molds in slip casting are often made this way.
2. Hand molding – similar to hand modeling, only a mold or form is used to define portions of the geometry.
3. Hand throwing – Hand throwing on a potter’s wheel is another refinement of the handcraft methods. The potter’s wheel is a round table that rotates on a vertical spindle, powered either by motor or foot-operated treadle.
Strictly speaking, use of a motor-driven potter’s wheel is a mechanized method. However, most mechanized clay forming methods are characterized by much less manual participation than the hand throwing method described. These more mechanized methods include jiggering, plastic pressing, and extrusion.
1. Jiggering – Jiggering is an extension of the potter’s wheel methods, in which hand throwing is replaced by mechanized techniques. It is used to produce large numbers of identical items such as houseware plates and bowls. In this process a lump of soft plastic clay is formed into a flat disk and placed on the surface of a plaster of Paris mold. The mold is set on a chuck and rotated at about 400 rpm. A profile tool is then pulled down which makes contact and forms the upper surface of the ware. During the operation the surface of the clay is coated with a water spray as a lubricant. The major difficulties in the jiggering process are related to the plastic properties of the clay. It is essential to have a clay body with sufficient plasticity to flow readily under the jigger head without excessive spring-back when the jigger tool is lifted. It has been found that high-energy mixing, to completely disperse the fine clay particles, materially improves the jiggering properties.
2. Plastic pressing – plastic pressing is a forming process in which a plastic clay slug is pressed between upper and lower molds contained in metal rings. The molds are made of a porous material such as gypsum so that, when a vacuum is drawn on the backs of the mold halves, moisture is removed from the clay. The mold sections are then opened, using positive air pressure to prevent sticking of the part in the mold. Plastic pressing achieves a higher production rate than jiggering and is not limited to radially symmetric parts.
3. Extrusion – Extrusion is used in ceramics processing to produce long sections of uniform cross section, which are then cut to required piece length. The extrusion equipment utilizes a screw-type action to assist in mixing the clay and pushing the plastic material through the die opening. This production sequence is widely used to make hollow bricks, shaped tiles, drain pipes, tubes, and insulators. It is also used to make the starting clay slugs for other ceramics processing methods such as jiggering and plastic pressing. A major variable in the extrusion process is the pressure used. At low pressures the volume fraction water present in a mixture, or in the more general case the volume fraction fluid, is fairly high. The yield point is relatively low, but the resultant drying shrinkage and ultimate properties are not outstanding. As higher pressures are used, the particles in the mixture are forced closer together, the yield point for extrusion is increased, but the resultant drying shrinkage and final properties are substantially improved.
c. Semidry pressing – In semidry pressing, the proportion of water in the starting clay is typically in the range from 10% to 15%. This results in low plasticity, precluding the use of plastic forming methods, which require very plastic clay. Semidry pressing uses high pressure to overcome the materials low plasticity and force it to flow into a die cavity. Flash is often formed due to excess clay being squeezed between the die sections.
d. Dry pressing – The main distinction between semidry and dry pressing is the moisture content of the starting mix. The moisture content of the starting clay in dry pressing is typically below 5%. Binders are usually added to the dry powder mix to provide sufficient strength in the pressed part for subsequent handling. Lubricants are also added to prevent die sticking during pressing and ejection. The pressing pressure is from 35MPa to 200MPa. Modern presses used for dry pressing are highly automated. Dies, usually made of carbides or of hardened steel, must have high wear resistance, to withstand the abrasive ceramic particles, so they can be expensive. One of the limitations on dry pressing is the length-to-diameter ratio of a sample which can be formed with uniform pressure. Pressure variations arise from uneven filling of a complex die or from wall friction in a relatively long die. This situation can be improved by pressing from both top and bottom as is commonly done; it is also improved by the use of effective lubricants and binders. However, a limit is reached under which initial variations in pressure, and consequently variations in compact density, cause warping and defects during firing.
Water plays an important role in most traditional ceramics shaping processes. Thereafter, it serves no purpose and must be removed from the body of the clay piece before firing. Shrinkage is a problem during this step in the processing sequence because water contributes volume to the piece, and when it is removed, the volume is reduced.
In drying ceramic ware it is found that the initial drying rate is independent of the water content and depends solely on the temperature, humidity, and rate of movement of the air over the surface of the ware.
As water is removed from the wet clay, the volume of the piece shrinks. The drying process occurs in two stages. In the first stage, the rate of drying is rapid and constant, as water is evaporated from the surface of the clay into the surrounding air, and water from the interior migrates by capillary action toward the surface to replace it. It is during this stage that shrinkage occurs, with the associated risk of warping and cracking due to variations in drying in different sections of the piece. In the second stage of drying, the moisture content has been reduced to where the ceramic grains are in contact, and little or no further shrinkage occurs.
Many methods have been developed for the practical drying of ceramic ware. The technique used depends on the quality and size of the ware and the rate of drying required. For automatic forming processes a rapid rate of drying is required in order to minimize space requirements; consequently more controls and elaborate installations are necessary to avoid defects. Ware which is formed at a low rate by hand can be dried on a low schedule without consuming excessive space or requiring excessive capital investment.
The most common drying process in ceramic tiles is the Spray drying. It is characterized by atomization of a solution or suspension into droplets, followed by subsequent drying of these droplets by evaporation of water or other solvents. Moisture from the clay is removed by exposure under great pressure up into the spray dryer from near the bottom, and as it is settling back down to the bottom it is dried by hot air that is flowing in the spray dryer. When the clay has settled to the bottom via gravity it is at desirable moisture content for forming and has formed into a small pellet that is desirable for the forming of the tile.
In production, drying is usually accomplished in drying chambers in which temperature and humidity are controlled to achieve the proper drying schedule. Heating is usually by a combination of convection and radiation using infrared sources.
After shaping but prior to firing, the ceramic piece is said to be green, meaning not fully processed or treated. The green piece lacks hardness and strength; it must be fired to fix the part shape and achieve hardness and strength in the finished ware. Firing is the heat treatment process that sinters the ceramic material; it is performed in a furnace called a kiln.
In sintering, bonds are developed between the ceramic grains, and this is accomplished by densification and reduction of porosity. Therefore, shrinkage occurs in the polycrystalline material in addition to that which has already occurred in drying.
In the firing of traditional ceramics, certain chemical reactions among the components in the mixture may also take place, and a glassy phase also forms among the crystals, which acts as a binder. Both of these phenomena depend on the chemical composition of the ceramic material and the firing temperatures used.
Glazing is the application of glassy coatings on ceramic wares to give them decorative finishes and to make them impervious to moisture.
Materials for glazes and coating are formed in a variety of ways. Most glazes and porcelain enamel batches are composed of mixtures of frit (premelted glass) and other constituents which are wet-milled together with clay just before applying. Frit is employed to increase the glaze uniformity during mixing and firing and to allow the use of materials in an insoluble form. The frit is usually melted in rotating furnaces from which it is poured either continuously or in a batch process into a water quench where it shatters and is subsequently easy to grind.
The usual processing sequence with glazed ware is to (1) fire the ware once before glazing to harden the body of the piece, (2) apply the glaze, and (3) fire the piece a second time to harden the glaze.
2. New Ceramics
1. Preparation of Starting Materials
Since the strength specified for these materials is usually much greater than for traditional ceramics, the starting powders must be more homogeneous in size and composition, and particle size must be smaller (the strength of the resulting ceramic product is inversely related to the grain size). Thus greater control of the starting powders is required. Powder preparation includes mechanical and chemical methods.
The mechanical methods consist of the same ball mill grinding operations used for traditional ceramics. The trouble with these methods is that the ceramic particles become contaminated from the materials used in the balls and walls of the mill. This compromises the purity of the ceramic powders and results in microscopic flaws that reduce the strength of the final product.
Two chemical methods are used to achieve greater homogeneity in the powders of new ceramics: (1) freeze drying and (2) precipitation from solution.
In freeze drying, salts of the appropriate starting chemistry are dissolved in water and the solution is sprayed to form small droplets that are rapidly frozen. The water is then removed from the droplets in a vacuum chamber, and the resulting freeze-dried salt is decomposed by heating to form the ceramic powders. Freeze drying is not applicable to all ceramics, because in some cases a suitable water-soluble salt cannot be identified as a starting material.
Precipitation from solution is another preparation method used for ceramics. In the typical process, the desired ceramic compound is dissolved from the starting mineral, thus permitting impurities to be filtered out. An intermediate compound is then precipitated from solution, which is converted into the desired compound by heating. An example of the precipitation method is the Bayer process for producing high-purity alumina (also used in the production of aluminum). In this process, aluminum oxide is dissolved from the mineral bauxite so that iron compounds and other impurities can be removed. Then, aluminum hydroxide is precipitated from solution and reduced to Al2O3 by heating.
Further preparation of the powders includes classification by size and mixing before shaping. Very fine powders are required for new ceramics applications, and so the grain must be separated and classified according to size. Thorough mixing of the particles, especially when different ceramic powders are combined, is required to avoid segregation.
Various additives are often combined with the starting powders, usually in small amounts. These additives include (1) plasitcizers, to improve plasticity and workability; (2) binders, to bond the ceramic particles into a solid mass in the final product; (3) wetting agents, for better mixing; (4) deflocculants, which help to prevent clumping and premature bonding of the powders; and (5) lubricants, to reduce friction between ceramic grains during forming and to reduce sticking during mold release.
1. Hot pressing – Hot pressing is similar to dry pressing except that the process is carried out at elevated temperatures, so sintering of the product is accomplished simultaneously with pressing. This eliminates the need for a separate firing step in the sequence. Higher densities and finer grain size are obtained, but die life is reduced by the hot abrasive particles against the die surfaces.
2. Isostatic pressing – Isostatic pressing uses hydrostatic pressure to compact the ceramic powders from all directions, thus avoiding the problem of nonuniform density in the final product that is often observed in the traditional uniaxial pressing method. Automotive spark-plug insulators, for example, are made by this method; silicon nitride vanes for high-temperature applications are made by hot isostatic pressing.
3. Doctor-blade process – this process is used for making thin sheets of ceramic. One common application of the sheets is in the electronics industry as a substrate material for integrated circuits.
4. Injection molding – This process is now extensively used for the precision forming of ceramics for high-technology applications, such as in rocket-engine components. The raw material is mixed with a binder, such as a thermoplastic polymer (e.g., polypropylene, low-density polyethylene, or ethylene vinyl acetate) or a wax. The binder usually is removed by pyrolysis; the part is then sintered by firing. This process can produce thin sections, typically less than 10mm-15mm thick, from most engineering ceramics – for example alumina, zirconia, silicon nitride, silicon carbide, and sialon. Thicker sections require careful control of the materials used and of processing parameters, in order to avoid internal voids and cracks, especially those due to shrinkage.
Since the plasticity needed to shape the new ceramics is not normally based on a water mixture, the drying step so commonly required to remove water from the traditional green ceramics can be omitted in the processing of most new ceramic products. The sintering step, however, is still very much required to obtain maximum possible strength and hardness. The functions of sintering are: (1) to bond individual grains into a solid mass, (2) to increase density, and (3) to reduce or eliminate porosity.
Temperatures around 80% to 90% of the melting temperature of the material are commonly used in sintering ceramics. Sintering mechanisms differ somewhat between the new ceramics, which are based predominantly on a single chemical compound (for example, Al2O3), and the clay-based ceramics, which are usually comprised of several compounds having different melting points. In the case of the new ceramics, the sintering mechanism is mass diffusion across the contacting particle surfaces, probably accompanied by some plastic flow. This mechanism causes the centers of the particles to move closer together, resulting in densification of the final material. In the sintering of traditional ceramics, this mechanism is complicated by the melting of some constituents and the formation of a glassy phase, which acts as a binder between the grains.
Parts made of new ceramics sometimes require finishing. In general, these operations have one or more of the following purposes: (1) to increase dimensional accuracy, (2) to improve surface finish, and (3) to make minor changes in part geometry. Finishing operations usually involve grinding and other abrasive processes. Diamond abrasives must be used to cut the hardened ceramic materials.
Uses of Ceramics
|Industry Segment||Common Examples|
|Structural clay products||Brick, sewer pipe, roofing tile, clay floor and wall tile (i.e., quarry tile), flue linings|
|Whitewares||Dinnerware, floor and wall tile, sanitary ware, electrical porcelain, decorative ceramics|
|Refractories||Brick and monolithic products are used in iron and steel, non-ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemicals industries|
|Glasses||Flat glass (windows), container glass (bottles), pressed and blown glass (dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical fibers)|
|Abrasives||Natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused alumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure blasting of materials|
|Cements||Used to produce concrete roads, bridges, buildings, dams, and the like|
|Structural||Wear parts, bioceramics, cutting tools, and engine components|
|Electrical||Capacitors, insulators, substrates, integrated circuit packages, piezoelectrics, magnets and superconductors|
|Coatings||Engine components, cutting tools, and industrial wear parts|
Ceramic Industries in Bangladesh:
At present, there are satisfactory number of ceramic industries in Bangladesh and further new industries is being set up. Out of them a few’s performance is excellent. As for example, Monno Ceramics Industries Ltd is one of the leading company in Bangladesh as well as in the world in the ceramic field which exports its both porcelain and bone china products to many countries of Europe and America. Also there are few local and foreign based companies e.g., Shine Pukur Ceramic, Farr Ceramics, RAK Ceramics etc.,Artisan in our country which keep a vital role to strengthen our economy. As the labour cost in our country is comparably cheaper rather than many other countries, it is a great chance to develop this sector to a supreme position in the world.
From the above description we saw that there are various steps in the production of ceramic wares and the whole process must be maintained at certain condition. The demand of ceramic wares in the western is large. So our government should take proper steps to enhance the growing sector.
1. W.D. Kingery. Ceramic fabrication processes,(Technology Press of Massachusetts Institute of Technology, 1959)
2. Kalpakjian, Serope. Manufacturing engineering