Ceramic Materials and Their Processing PDF

Ceramic Materials and Their Processing 677 strength. For that reason, they are very sensitive to the distribution of flaws. However, with the development of higher-purity ceramic materials, with improved high temperature strength, such as...

Ceramic Materials and Their Processing 677 strength. For that reason, they are very sensitive to the distribution of flaws. However, with the development of higher-purity ceramic materials, with improved high temperature strength, such as Si3 N4 and SiC and with the development of processing methods to produce the new materials with low porosity and more consistent properties, the building of high temperature air-craft structural components from ceramic materials is being seriously considered. Ceramics are much stronger under compressive loading. They have low tensile strength. The tensile strength of Alumina is of the order of 190 MPa as compared to its compressive strength which is of the order of 1950 to 3500 MPa. There being virtual absence of ductility in ceramies, so, in general, they can not be machined or built up from stock. However shaping and turning of unfired or slightly fired material is sometimes possible with small articles. 13.4. PROCESSING OF CERAMICS The processing of ceramics, except glass, follows the Powder Metallurgy route, that is, consists of the following steps :- 1. Preparation of powders. 2. Mixing and Blending of powders. 3. Compacting of powders. 4. Firing or Sintering. 1. Preparation of Powders : As discussed above, ceramics are natural or manufactured. Natural ceramics are mined in open-pit mines whenever possible, and reduced to powder in crushers and hammer and ball mills. Undesirable components are removed by screening, magnetic separation, filtering or floating. Among the manufactured ceramics, the two most important ceramics are : Alumina [Aluminium oxide, Al2O3] and Silicon Carbide, SiC. The methods of their preparation have already been discussed in Chapter 8 under Art. 8.6.1. Another ceramic of increasing importance is silicon nitride, Si3N4. It is made by high temperature reaction of silicon metal with N2 gas. Many other varieties of manufactured ceramics (oxides, carbides, nitrides and borides etc.) are also made by this process. The manufactured mass is reduced to powder to controlled sizes and size distributions. 2. Mixing and Blending : The aims of mixing and blending of ceramics are the same, as discussed under the chapter on Powder Metallurgy. A ceramic is mixed and blended with other ceramic/ceramics or with lubricants and binders. Binders may be organic, such as, polymers, waxes, gums, starches etc., or inorganic, such as, clays, silicates, phosphates etc. Lubricant reduces the wall friction of the mould, reduces internal friction between particles during moulding, increases the flowability of the powders and aids in the ejection of compacted parts. A wetting agent (like water) is added to improve mixing. A plasticizer is added to make the mix more plastic and formable. Deflocculents such as Na2CO3 and Na2SiO3 (in amounts of less than 1%) are added to make the ceramic-water suspension. Some other additives are also added to control foaming and sintering. For example, clay-based ceramic products are made by combining several different clay minerals (SiO2, Al2O3 etc.) with certain amount of non-plastic materials, such as crushed and ground quartz, feldspar, talc or “gorg”, a finely ground fire clay material. Non-plastic materials, in general, alter the plasticity of the clay-body, making it more suitable for various processes, act as fluxes causing greater degree of vitrification during firing, and reduce the drying and firing shrinkages of the clay-body. 3. Compacting of Ceramics : All the techniques employed for compacting powders (under Powder Metallurgy porcess) can be used for compacting ceramic powders to the desired shape. 678 A Textbook of Production Technology The techniques can be : pressing into steel dies on mechanical or hydraulic presses, cold isostatic pressing, extrusion, slip casting and injection moulding. (i) Pressing in dies : Most ceramic compacts are made by pressing in dies. (a) Dry pressing : Dry pressing of ceramics requires high tonnage presses and expensive dies. However, the parts can be mass produced to close tolerances. Lubricants and binders are used as required. (b) Wet pressing : Here, the percentage of binders or other additives or liquids (mainly water) is such that the mixture can be processed by plastic forming techniques. This method is frequently used for clay-type ceramics, but can also be used for other ceramics. The main limitation of pressing in dies is that since the granular ceramic material is limited in its plasticity, complex shapes in the lateral plane cannot be formed successfully. (ii) Iso-static pressing : For more uniform density of the compact, “iso-static pressing” can be employed, as discussed in chapter on Powder Metallurgy. Typical product is : Automotive spark plug insulators. HIP is used for forming high-technology ceramics such as SiC, Si3N4 (vanes for high temperature use). (iii) Extrusion : In extruding, the raw material is mixed to a plastic state. It can be a ceramic- binder blend or a ceramic-water blend. It is then extruded in the extrusion press (usually screw type). Variable shapes can be obtained by this method (such as long lengths). Mandrels can be placed in position in the nozzle or die of the extrusion press, so that the extruded rod can have a variety of internal openings. The method is a low cost process, but the binder must be removed afterwards. Also, the orientation of the ceramic particles is fixed by the flow of the blend. (iv) Injection moulding : The method is similar to that used for plastics (see chapter 11). The raw material is ceramic-plastic blend. The method is : fast, can be automated, complex cross- sections can be obtained in high volumes. However, the tooling cost is high and the binder must be removed at the end of the process. The process is used for precision forming of ceramics for high technology applications, such as rocket engine parts. (v) Slip casting : The method of “Slip Casting” or “Drain Casting” has already been discussed in the Chapter on Powder Metallurgy. The raw material is slurry. It is poured in a gypsum mould. The moisture is absorbed by the porous mould and the body is cast against the walls of the mould. To get hollow components, the excess slurry is poured off once the correct wall thickness of the part has been obtained. The advantages of the process are that complex shapes of large sizes can be obtained and the tooling cost is low. However, the process is labour intensive and the cycle time is long. Typical product applications include : Large and complex parts, such as plumbing ware, art objects and dinner ware etc. There are Variations of Slip Casting Method : (a) Doctor-blade method. Here, the slip is cast over a moving plastic belt and its thickness is controlled by a blade. Typical application is : Thin sheets of ceramics less than 1.5 mm thick. (b) Rolling. The slip is rolled between a pair of rolls and the slip is cast over a paper tape. The paper tape is subsequently burned off during firing operation. (vi) Jiggering : Jiggering is usually an automatic forming process. It operates on the pattern of potter's wheel and is mainly used for clay products. The raw material is placed in a heavy mould, made of plaster of paris. The inside shape of the mould is the desired outside shape of the product. The inner shape of the product is obtained by forcing a shaped tool into the material, while the mould is rotated. Ceramic Materials and Their Processing 679 The process is limited to axisymmetric parts and has limited dimensional accuracy. 4. Firing or Sintering : The ceramic product, after compacting, is in the green state. To get the desired strength of the product, it is fired or sintered. However, before it is fired or sintered, it should undergo a drying process, by holding at room temperature and by low temperature heating. This is done to drive away any moisture of organic carriers, thereby minimizing the stresses, distorsion and cracking during the firing process. Then the compact is fired or sintered to obtain the desired level of strength. The complete firing process (Slow heating and cooling) may take days or even weeks. Getting the desired strength of the compact by firing is called “maturing”. If the ceramic product is to be glazed, it can be achieved in two ways. Either the glaze is applied to the product (green compact) and then the product and the glaze are matured in a single firing. Or, first the green compact is matured with a bisque fire and then the glaze is applied, which is matured with a lower temperature firing, known as a “ghost fire”. The matured compact is called “bisque”. Glazing makes the porous ware water tight. Glazes are glassy coatings made of inorganic compounds such as quartz, feldspar, boric oxide and lead oxide. These are finely powdered and mixed with water to form slurry (slip). Feldspar is a group of crystalline minerals consisting of aluminium silicate, potassium, calcium or Sodium, for example, Potash feldspar is K2O.Al2O3.6SiO2; Sodium feldspar is Na2O, Al2O3, 6SiO2 and Limefeldspar is CaO.Al2O3.2SiO2. Machining of Ceramics : Most ceramics are sintered to their finish dimensions. However, sometimes, they are machined to get better dimensional accuracy and surface finish. Machining of ceramics can be done with Diamond abrasives, LBM, EBM and CHM. 13.5. PRODUCT APPLICATIONS 1. Clay products : Clay body ceramics include whitewares and stoneware. Whitewares include such families of products as earthenwares, China and porcelain. Whitewares are largely used as tile, sanitary ware, low and high voltage insulators, and high frequency applications. It is still used extensively in Chemical industry as crucibles, jars and components of chemical reactors. Heat resistant applications include : pyrometer tube, burner tips, and radiant heater supports. (a) Earthenware : Apparent porosity is usually 6 to 8% and may exceed 15%. Firing range is 800 to 950°C and may exceed 1000°C. Typical applications are : Porous drainage pipes, ceramic filter, wall tiles and bricks. (b) Fine China : Apparent porosity is usually less than 1%. Firing range is 1100 to 1200°C. Typical product application : Tableware. (c) Stoneware : Apparent porosity is less than 3%. It is usually 1 to 2%. Firing temperature is above 1250°C. Typical applications are : Glazed pipes, roofing tiles and tableware. (d) Porcelain : Apparent porosity is less than 1% and is usually zero. Firing range is 1300 to 1450°C. Typical applications : Fine tableware, Scientific equipment and spark plug insulators for automobiles. A typical composition of a vitrified porcelain for spark-plug insulators is : Kaolin 30%, Ball Clay 20%, Feldspar 30%, and Silica 20%. “Sintex” ceramic material, developed for spark plugs, is sintered Alumina, with small amounts of silica and some organic material added that provides for better crystal formation to give improved mechanical and electrical properties. “Sintex” is a good conductor of heat, its thermal conductivity is 20 times that of porcelain and slightly more than steel. Its abrasion resistance is remarkable. 2. Refractories : Refractory ceramics are the materials which are capable of withstanding high temperature in various situations. The refractory materials are of three types :- (a) Acidic refractories. 680 A Textbook of Production Technology (b) Basic refractories. (c) Neutral refractories. Acidic refractories are based on alumina-silica composition, varying from pure silica to nearly pure alumina, through a wide range of alumina silicates. The basic constituent of basic refractories is magnesia, MgO. Basic refractories include chrome- magnesite, dolomite, limestone and magnesite. Neutral refractories include substances which do not combine with either acidic or basic oxides. With increasing alumina content, silica-alumina refractories may gradually change from an acidic to neutral type. A typical neutral character is exhibited by such refractories as Carbon, graphite, carbide, chromite, bauxite and forsterite. Refractory powders or porous blocks serve as thermal insulation in high temperature applications. Basic refractories are often used in metal processing applications, to provide compatibility with the metal. Neutral refractories are used to separate the acidic and basic materials, because they tend to attack one another. Acidic refractories are not attacked by acidic medium but are sensitive to basic surroundings. Similarly Basic refractories are attacked by acidic medium. Refractories are used in the construction or lining of furnaces, boilers, flues, regenerators, convertors, crucibles, dryers, pyrometer tubes and in many others, primarly to withstand the high temperature to which it is likely to be subjected without cracking, disintegrating or softening. In recent years, new refractories have been developed for service at very high temperatures produced in gas-turbines, ram jet engines, missiles, nuclear reactors, and similar processes and operations. These refractories are relatively simple crystalline bodies, composed of pure metallic oxides, carbides, boxides, nitrides and sulphides. The most widely used oxide refractory ceramic is alumina, Al2O3. It is sintered into cutting tool bits, spark plug insulators, high temperature tubes, melting crucibles, wear components and substrates for electronic circuits and resistors. Carbides have the highest melting point of all the substances. Silicon carbide, SiC, is difficult to sinter, but pressure sintered or reactive sintered solid bodies of SiC are used as high-temperature resistance-heating elements, rocket nozzels and sand blast nozzles. Ceramics such as UO2, UC and UC2 are used in nuclear applications as fuel elements, fuel containers, moderators, control rods and structural parts. Boron carbide, B4C, is extremely hard and is used as a grinding grit. In the sintered form, it is used for wear-resistant parts and body armour. Other carbides (Tungsten Carbide, Tantalum Carbide and Titanium carbide) are used in the sintered form as cutting tool materials. Nitrides have only slightly lower melting points than carbides. Cubic boron nitride, CBN, is the hardest material after diamond and is used as cutting tool material. Silicon nitride, Si3N4, is used for ceramic engine components, turbine disks and rocket nozzles. Sialon (Si - Al - O - N), that is oxynitrides, have better oxidation resistance and is used for cutting tools and welding pins. Borides (of Chromium, Zirconium and Titanium) are used as turbine- blades, Rocket nozzles and Combustion chamber liners. Lastly, Cermet, a composition of ceramic and metal, has been developed. This material shows better thermal shock resistance than ceramics, but at the same time retains their high refractoriness. It is used as cutting tool material, as crucibles and as jet engine nozzles. 3. Cutting Tool Materials : As discussed above, the various ceramics, which can be used as cutting tool materials are : Carbides, nitrides and oxides and cermets. Cemented carbides are expensive materials, since they contain comparitively rare elements, such as W, Ti, Ta and Co. Cemented oxides (Al2O3) are efficient substitutes for Cemented Carbides in many cases. Their Ceramic Materials and Their Processing 681 manufacturing process is also relatively inexpensive, so that ceramic oxide tips are considerably cheaper than those of Cemented Carbides. 4. Abrasives : An abrasive is a hard material used to wear away a softer material. Ceramics are the hardest materials. Hence their selection as abrasive purposes. Abrasives are used for operations, such as scratching, grinding, cutting, rubbing and polishing. Abrasives have been discussed in detail in chapter, 8 under the article 8.6.1 on Grinding process. 5. Electrical and Magnetic Applications : Ceramics find wide applications in electrical and electronic industries. As insulators, semi-conductors, dielectrics, ferroelectrics, piezoelectric crystals. Ceramics such as glass, porcelain, alumina, quartz and mica, are getting heavy demands. Ceramics, such as SiC, are used as resistors and heating elements for furnaces. Ceramics, having semiconducting properties, are used for thermistors and rectifiers. Barium titantate, for example, is used in capacitors and transducers. High density clay bed ceramics and Al2O3 make excellent high-voltage insulators. 6. Optical Applications : Optical applications of ceramics are not unknown. Ceramics are notably useful as a pigment, because it is exceptionally durable. It is completely oxidised and not subject to chemical attack and variation. Yttralox (a new ceramic material) is useful in optical applications, becuase it is as transparent as window glass and can resist very high temperature. Generally, Ceramics are opaque, because of the presence of tiny pores within them that scatter light. Yttralox is completely free from pores. 7. Phosphorescence. Ceramic phosphors emit light of a characteristic wave-length when excited or pumped by some appropriate energy source (an electric discharge or electron beam). Light tubes, VDT's and colour T.V. rely on this phenomenon. Of increasing interest are Laser materials. The most widely used Laser is ruby (an Al2O3 crystal doped with Cr ions). They are being used for machining, welding and cutting etc. 13.6. ENAMELS Enamels are one of the many types of finishes/coatings applied to metal products. Enamels can be : Organic coatings or Inorganic coatings. In organic finishes, pigment is dispersed in either a varnish or a resin or a combination of both. Enamels may dry by either or both oxidation and polymerization. Both air-drying and baking-type enamels are available. Enamels, belonging to the category of ceramics, are inorganic coatings. They are made up of refractory compounds and have a glasslike finish (glaze) when applied to both ferrous and non- ferrous surfaces. They are better than organic coatings and provide excellent resistance to both corrosion and elevated temperature and good resistance to abrasion. Inorganic coatings include : Porecelain enamels and ceramic coatings which are fused to base metals. Porcelain enamels can be defined as highly durable alkaliborosilicate glass coatings that are bonded by fusion to various metal substrates at temperatures above 425°C. These coatings are widely used for industrial products, household appliances, plumbing fixtures, signs and architectural applications and for jet engine components. The basic material of the porcelain enamel is called “frit”, a special glass of friable particles produced by quenching a molten glass mixture. The frit is ground to a fine powder and is suspended in water with the addition of antiflocculants etc. Clay or organic binders are added to it to impart pseudo-plastic behaviour, so that it can retain the slip on vertical surfaces. Thus, enamels are glassy or partially crystalline coatings, applied in the form of a slurry. Glazes are also glassy coatings made of inorganic compounds. 13.7. Glass : Glasses are, by definition, “Ceramics” because the starting materials needed to produce glass are typical of ceramic materials. However, they are produced by the melt processing 682 A Textbook of Production Technology route, instead of the powder metallurgy route used for other ceramics. The techniques to process glass are closer to those used for thermo-plastic polymers. In ceramic science, the word “glass” signifies any amorphous component of ceramic mixture. However, in general terms, glass is a transparent silica product which may be amorphous or crystalline, depending on heat treatment. Glasses may be either inorganic or organic. Vitreous materials or inorganic glasses are the fusion products which during solidification from a liquid state failed to crystallise. During the cooling process, the glasses exhibit no discontinuous change at any temperature and only a progressive increase in viscosity is noticed. In fact, glass is a hard liquid. Glass is one of the most verstatile of all materials. It is : — woven into cloth. — made into doors, cookware and self de-frosting wind shields. — used as a glazing material for buildings. — made into filters, prisms and other light separating devices, and — made into bottles, jars and many other products. Glass has good corrosion resistance, poor resistance to thermal shock and good electrical resistivity. 13.7.1. Glass Forming Constituents : Silica, which is obtained from high-purity silica sand is the most widely used glass-forming constituent. Other glass forming constituents are the oxides of boron, vanadium, germanium and phosphorous. Some other elements and compounds such as tellurium, selenium and BeF2 can also form glasses. The oxide components added into a glass batch can be grouped on the basis of function they perform within the glass. These are :- Network formers, intermediates, and modifiers. Network Formers : These are indespensible in the formation of glass, since they form the basis of the random three dimensional network of glasses. Silica, SiO2, is the main network forming constituent. Other network formers include oxides such as : B2O3, GeO2, P2O5, V2O5 and As2O3. Intermediates : These are added in high proportions for linking up with the basic glass network to retain structural continuity. These oxides include : Al2O3, Sb3O2, ZrO2, TiO3, PbO, BeO and ZnO. In general, Al2O3 increases hardness and reduces thermal expansion. PbO reduces hardness and increases the refractive index. Modifiers : These oxides are added to modify the properties of glasses. These include: MgO, Li2O, BaO, CaO, SrO, Na2O and K2O. These reduce the melting and working temperatures. Alongwith the above oxides, fluxes are also added to the charge for a glass. Fluxes lower the fusion temperature of the glass and render the molten glass workable at reasonable temperature. However, fluxes may reduce the resistance of glass to chemical attack, render it water soluble or make it subject to partial or complete devitrification (that is, crystallisation) upon cooling. Such a glass is undesirable since the crystalline are extremely weak and brittle. Stabilizers are therefore, added to the glass batch to overcome these problems. The various fluxes used are : arsenic oxide, As2O3, Antimony oxide Sb2O3, boron oxide B2O3, borax Na2B4O7, Calcium fluoride CaF2, NaNO3, KNO3, and ammonium sulphate (NH4)2SO4, 13.7.2. Types of Glasses : There are four principal types of glasses, on the basis of their chemical composition : Silica glass, Borosilicate glass, Lead glass and Sodalime glass. Their chemical compositions are given in Table 13.1. Ceramic Materials and Their Processing 683 Table 13.1. Composition of Glasses Composition, % Silica glass Borosilicate glass Lead glass Sodalime glass SiO2 96 73 – 82 53 – 68 70 – 50 Na2O – 3 – 10 5 – 10 12 – 18 K2O – 0.4 – 1 1 – 10 0–1 CaO – 0–1 0–6 0–4 PbO – 0–1 1540 – B2O 3 5 – 20 – – Al2O3 – 2–3 0–2 0.5 – 2.5 MgO – – – 0–4 Table 13.2. shows a comparison of these types of glasses. Table 13.2. Comparison of Types of Glasses Property Silica Boro Lead Sodalime glass Silicate glass glass Cost Highest Moderate Low Lowest Weight Lightest Medium Heaviest Heavy Electrical Resistance High High Highest Moderate Strength Highest Good Low Low Thermal Shock Resistance Highest Good Low Low Hot workability Poor Fair Best Good Chemical Resistance Highest Good Fair Poor Impact Abrasion Resistance Best Good Poor Fair Heat Strengthening Possibilities None Poor Good Good Ultraviolet light Transmission Good Fair Poor Poor Silica glasses are mainly used where high temperature resistance is required. They can be regularly used at temperatures upto about 900°C. They have a very low co-efficient of thermal expansion and so have a high resistance to thermal shock. Silica glass is also called as “Quartz glass”. In borosilicate glass, a part of silica is replaced by boron oxide to impart desirable properties to glass. Borsoilicate glasses have fair hot workability and still have high strength, high chemical stability, high electrical resistance and low thermal expansion. Because of all this and a lower cost than silica glass, borosilicate glasses have wide industrial applications. Typical applications include : Kitchenware, High tension insulators, telescope mirror and laboratory glassware, sight glasses, gauge glasses. A special glass of this type is manufactured under the trade name “pyrex”. Lead glasses, also called “flint glasses” have low melting point, but exhibit, good hot workability, high electrical resistance and high refractive indices (1.50 to 2.20). Therefore, these glasses are used for products such as : optical purposes, cut glassware (art objects) and jewellery, High quality tableware, thermometer tubing, fluorescent lamps, lamp tubing and television tubes, for windows and shields to protect personnel from X-ray radiation. Soda lime glasses are the cheapest and have good hot workability (low temperature is needed to melt these glasses). They comprise the largest tonnage of glass manufacture. These glasses are 684 A Textbook of Production Technology used as : window glass, bottles, lamp globes, ordinary chemical apparatus like test tubes, beakers and so on. Some Other Types of Glasses 1. Coloured Glasses : Sometimes, various substances are added to the glass fusion to get coloured glasses. For example, the following colours will be obtained by the addition of the substances mentioned against each : Yellow : Ferric salt Green : Ferrous and Chromium salts (Cr2O3) Blue : Cobalt salts (Cobaltous oxide) Purple : MnO2 Red : Nickle salts or Cu2O Lemon Yellow : CdS Fluorscent greenish yellow : Uranuim oxide Opaque milky-white : Cryolite, Na3 AlF6 or Calcium phosphate. Photo-sensitive eye glasses are made from glass that contains AgCl. When this glass is energized, by ultraviolet rays, Ag ions form and impart a deeper colour to the glass. 2. Recrystallised Glass : By adding nucleating agents, such as sodium fluoride, phosphorous pentaoxide, titanium oxide or vanadium oxide to the glass melt, we get recrystallised glass, which is also known as “Polycrystalline glass”. After the glass is formed, it is heat treated to promote crystallisation. Compared to ordinary glasses, such glasses possess : a high hardness and impact strength and better thermal conductivity. Their main application is in the manufacture of the so- called refrigerator-to-oven cooking wares. 3. Fibre Glasses : Fibre glass or glass fibre is glass in fibre form. It is obtained by drawing molten glass through dies into fibres 3 to 20 μm in diameter. Unlike normal glass, this glass possesses high tensile strength and is almost free from surface defects. Glass fibres are ; non-flammable, bad conductors of both heat and electricity, poor conductor of sound and are chemically inactive. They are used for insulating fabric and reinforcing fibre for plastics. ‘‘Glass Plastics’’ are the materials having a synthetic resin as a binder and fibre glass (glass laminate and glass fibre) as a filler. These materials are three or four times lighter than steel but are just as strong. Such materials are widely used to build the hulls of small vessels (boats, yachts), bodies of cars and aircraft parts. 4. Glass Wools : Glass wools are relatively short fibres of about 20 to 30 m in diameter. These are made by forming molten glass through some vents by centrifugal force, in the process known as “Crown process”. Glass wools are suitable for insulation. 5. Foam Glasses : Foam glasses are produced by introducing innumerable air cells or pores into molten glass. On cooling, this glass becomes very light and can even float on water. It is cut into suitable sizes and then used as heat insulating material. 13.7.3 Forms of Glass and Their Manufacture Glass is available in many forms, such as : Sheet, plate, rod, tube, and various finished forms. First of all, molten glass is obtained by fusing together the various glass forming ingredients. These are first finely broken into small particles, blended and then melted in a melting furnace (the temperature may be 1800°C). For example, a typical charge for soft glass (soda-lime glass) is made up of sand, limestone (CaCO3), soda ash (Na2CO3) and cullet (broken glass scrap). A typical charge for borosilicate glass will consist of sand, borax, alumina, soda, pottasium carbonate and cullet. During melting, the carbonates decompose and react with SiO2, Ceramic Materials and Their Processing 685 CaCO3 + SiO 2 CaSiO3 + CO2  Na 2 CO3 + SiO 2 Na 2SiO3 + CO 2  Gas evolution helps to homogenize the melt, but bubbles would remain. In order to promote refining, that is, the removel of small gas bubbles, materials such as sodium sulphate, sodium nitrate, sodium chloride, Arsenic oxide, calcium fluoride and carbon etc. are added to the glass charge. Decolouring agents such as Selenium, ceruim oxide, neodynmium oxide and nitre etc are also added to the melt. Cullet facilitates melting and produces a more uniform product. From the furnace, the melt flows to a “Forehearth” which is an extension of the melting furnace. Here, the melt is kept agitated, by electric current or mechanical stirrers, to main uniformity. The temperature of the melt is also controlled to impart the optimum viscosity for the subsequent forming processes. 1. Sheet Glass : Sheet glass is usually a soda-lime glass. Its thickness ranges from 0.8 to 10 mm. It is produced by drawing or rolling from the forehearth. It can also be produced by extruding vertically from the forehearth. It cools as it descends. Sheet glass is not free of imperfections. So, it is not recommended for automotive or aircraft glazing. However, it is widely used as window glass for domestic and commercial buildings. It is also used as mirrors, table tops and photographic plates. Sheet glass can be heat-treated, which increases its tensile strength by 2 to 5 times. It, then, can be used as fire screens, safety mirrors, gauge shields and office building glazing. A stronger sheet glass can be made by imbedding wiremesh in the glass in its molten state. This glass can withstand penetration of missiles and is less vulnerable to fragmentation. 2. Plate Glass : Plate glass is obtained by rolling the plastic glass upto a thickness of about 31.75 mm. It is then ground and polished to get optically flat surfaces. This glass is used for automotive glazing, storefront windows, tracing tables and surface plates. Plate glass can be heat- treated to get extra strength. Now a days, heavy plate glass is obtained by casting on to the surface of a molten tin bath in a controlled atmosphere. 3. Laminated Glass : Laminated glass is obtained by placing transparent vinyl plastic between two layers, of plate glass. The plastic layer prevents the splintering of the outside glass layers if broken. That is why it is known as “Safety glass”'. Product applications include : automotive and air-craft glazing, protection shields and storefront windows. 4. Glass Tube : Glass tubing and rod come under the group “drawn glassware”. They are used in gauge glasses, chemical pipe and insulation. Molten glass from the furnace is wrapped around a hollow rotating cylindrical mandrel and is mechanically drawn out by a set of rolls. Air is blown through the hollow mandrel to prevent the glass tube from collapsing which gradually stiffens. To produce rods, air is not blown through the hollow mandrel. 5. Pressed Glassware : Pressed glassware is produced by pressing a measured guantity of molten glass (gob) in steel or iron moulds. The process is similar to closed-die forging. Product applications include : eyeglases, household appliances, glass gauges, and decorative and ornamental pieces. 6. Blown Glassware : This method is used for producing hollow pieces of thinner walls and with reentrant sections. The gob is dropped into a mould and a preform (parison) is obtained by pressing with a punch. The parison is reheated and transferred to a split mould. The product is formed by blowing a jet of air, so that it takes the form of the closed mould, upon solidification, Fig. 13.1. Products include : Bottles, jars, vases and bulbs. 686 A Textbook of Production Technology 7. Sagging. In this process, a sheet of glass is placed over a mould and is heated. The glass becomes plastic and sags under its own weight and conforms to the shape of the mould. No pressure or vacuum is applied. Typical product applications include : Shallow disches, sunglass lenses, mirrors for telescopes, and lighting panels. 8. Spinning. This process which is also known as “Centrifugal Casting” is similar to the process used for metals. The molten glass is forced against the walls of the rotating mould, by the centrifugal force, where it cools and solidifies. Typical product applications include : TV picture tubes, Missile nose cones etc. 13.7.4. Strengthening of Glass. Glass can be strengthened by the following methods : 1. Thermal Tempering. This method is also known as “Physical tempering” or “Chill tempering”. In this method the hot glass is cooled rapidly. As the glass begins to cool, it contracts and shrinks. As a result tensile stresses are set up on the surface. 2. Chemical Tempering. In this method, the glass is heated in a bath of molten KNO3, K2SO4, or NaNO3 (depending upon the type of glass). Ion exchange takes place. Larger atoms replace the smaller atoms on the surface. Due to this, residual compressive stresses are set up on the surface of the glass. 13.7.5. Finishing Operations To relieve the glass surface of residual stresses, glass is annealed (similar to the method used for metals). Glass may also be subjected to some finishing operations such as : cutting, drilling, grinding and polishing etc. Sharp edges and corners are smoothened by grinding or by “fire polishing” where a torch is held against the edges which rounds them by localized softening and surface tension. Punch/Plunger Neck ring Parison Blow air Hot Blank gob mould Blow mould Fig. 13.1 Glass Blowing. 13.8 Design Considerations of Ceramic Products : The ceramics are brittle and have low meehanical and thermal shock resistance. Therefore, designing ceramic products needs special considerations, so as to make use of their advantages and avoid their limitations. Since the ceramics are about 10 times stronger in compression than in tension, therefore, every effort should be made to load ceramics in compression and to avoid tensile loading. Ceramics are sensitive to stress concentration, being brittle. Therefore, features like sharp corners, notches and unstrengthened holes should be avoided. Ceramics can be successfully attached to steel by press fits and shrink fits. This allows prestressing the ceramic part in compression, which increases its load carrying capacity. Ceramic Materials and Their Processing 687 Other design considerations are : 1. Avoid large flat surfaces, to eleminate warping of the product. 2. Avoid large changes in thickness, to eliminate non-uniform drying and cracking. 3. Provide generous dimensional tolerances, to avoid the need for machining, which is usually difficult and expensive. PROBLEMS 1. What are ceramic materials ? 2. Classify ceramic materials. 3. Write the names of the various types of ceramics. 4. Write the properties of ceramics. 5. Write the steps for processing ceramics. 6. What is jiggering process ? 7. Write the product applications of ceramics. 8. What is glass ? 9. What are the various glass forming constituents ? 10. Write about the various types of glasses. 11. Write about the various forms of glasses and their manufacture. 12. What are the attractive features of ceramics in comparison with metals ? 13. Why should sharp corners and large changes in thickness be avoided, when designing ceramics ? 14. How glass is strengthened ? 15. Write about the finishing operations done on glass. 16. Write a short note on " Manufactured ceramics". As noted on P. 675, 'manufactured ceramics' also known as : 'High tech. ceramics, 'Fine ceramics', Advanced ceramics', 'Engg. ceramics' or 'technical ceramics' exhibit superior mechanical properties, Corrorion/ oxidation resistance, and thermal, electrical, optical or magnetic properties as compared to 'natural ceramics, Advanced ceramics are classified as : 1. Structural ceramics : Such as industrial wear parts, bioceramics, cutting tools, and engine components. 2. Electrical and Electronic ceramics : Include : capacitors, insulators, substrates, IC packages, magnets, semi conductors and super conductors. The electronic, industry would not exist without ceramics. It is hard to imagine not having cell phones, computers, T.V. and other electronic consumer products. These ceramics have the largest market. 3. Ceramic Coatings : Find application in engine components, cutting tools, and industrial wear parts. 4. Chemical processing and environmental ceramics : include filters, membranes, catalysts and catalyst supports. Ceramics can be defined as inorganic, non-metallic materials that are produced using clays and other minerals from the earth or chemically processed powders. They form one of three large classes of solid materials. The other two being : metals and polymers. The combination of two or more of these materials together to produce a new material whose properties would not be attainable by conventional means in called 'Composite', see chapter 14.  Chapter 14 Composite Materials and Their Processing 14.1. GENERAL Composite materials can be defined as, the structures made up of two or more distinct starting materials. The starting materials can be organic, metals or ceramics. The components of a composite material do not occur naturally as an alloy, but are separately manufactured, before these are combined together mechanically or metallurgically. Due to this, they maintain their identities, even after a composite material is fully formed. However, the starting materials combine to rectify a weakness in one material by a strength in another. Hence, a composite material exhibits properties distinctly different from those of the individual materials used, to make the composite. Thus, a composite material or structure possesses a unique combination of properties, such as stiffness, strength, hardness, weight, conductivity, corrosion resistance and high temperature performance etc. that is not possible by the individual materials. Thus, the search for materials with special properties to suit some specific stringent conditions of use has given rise to the development of materials called “Composite Materials”. Advantages of Composite Materials : 1. High stiffness-to-weight and strength-to-weight ratios. 2. Elimination of corrosion and stress corrosion problems. 3. Significant reduction in fatigue problems. 4. Reduction in structural mass. 5. Improved control of surface contour and smoothness. 6. Improved appearance. 14.2. TYPES OF COMPOSITE MATERIALS Composite materials may roughly be classified as : 1. Agglomerated materials or Particulate Composites 2. Reinforced materials 3. Laminates 4. Surface-Coated materials. The particulate composites and reinforced composites are constituted by just two phases, the matrix phase and the dispersed phase. The matrix phase is continuous and surrounds the dispersed phase. The aim is to improve the strength properties of the matrix material. The matrix material should be : ductile with its modulus of elasticity much lower than that of the dispersed phase. Also, the bonding forces between the two phases must be very strong. 688 Composite Materials and Their Processing 689 In fact, the particulate composites also fall in the category of reinforced composites. Depending upon the nature of the reinforcing materials (shape and size), the reinforced composites can be classified as: 1. Particle reinforced composites or particulate reinforced composites. 2. Fibre reinforced composites. In particle reinforced composites, the dispersed phase is in the form of exi-axed particles, whereas in fibre-reinforced composites, it is in the form of fibres. The particulate composites can be further classified as : Large particle composites and Dispersion-strengthened composites, depending upon the size of the reinforcing particles. The characteristic property of many materials (particularly brittle one) that small sized particles (fibres etc.) are much stronger than the bulk materials, is used in reinforced composites. 14.2.1. Agglomerated Materials : Agglomerated materials or particulate composites consist of discrete particles of one material, surrounded by a matrix of another material. The materials are bonded together into an integrated mass. Two classic examples of such a composite material are : Concrete formed by mixing gravel, sand, cement and water and agglomeration of asphalt and stone particles, that is used for paving the highway surfaces. Other examples of particulate composite materials include :- 1. Grinding and cutting wheels, in which abrasive particles (Al2O3, SiC, CBN or diamond) are held together by a vitreous or a resin bond. 2. Cemented carbides, in which particles of ceramic materials, such as WC, TaC, TiC and of Cobalt and nickle, are bonded together via Powder Metallurgy process, to produce cutting tool materials. Cobalt acts as the binder for ceramic particles. During sintering, the binder melts and forms a continuous matrix between the ceramic particles. This method is called as “Vitreous sintering”, that is, sintering with the formation of liquid phase. Many powdered metal parts and various magnetic and dielectric ceramic materials are produced by solid sintering, which requires diffusion and no liquid phase in the process of sintering. 3. Cermets (Ceramics + metals), see chapter 7. Metals (W, Mo, Ni, Co) act as binders and the product is made by Powder Metallurgy method. The sintering temperature is the melting point of the metal. In the resulting composite material, the metal contributes high toughness and thermal shock resistance, while the ceramic contributes higher refractoriness and creep resistance, superior chemical stability and abrasion resistance. 4. Electrical contact points from powders of tungsten and silver or copper and processed via powder metallurgy method (See chapter 10) 5. Electrical brushes for motors and heavy duty frictional materials for brakes and clutches by combining metallic and non-metallic materials (See chapter 10) 6. Copper infiltrated iron and silver infiltrated tungsten (for nozzles for rockets and missiles) 7. Heavy metal (W + 6% Ni + 4% Cu), See chapter 10 8. Electric resistance welding electrodes from mixtures of copper and tungsten. 9. Dispersion strengthened materials, See Chapter 10 : In these materials, hard, brittle, and fine particles (usually oxides) are dispersed in a softer and more ductile matrix. Examples are : Copper welding electrodes with dispersed alumina, thoriated dispersed nickle with 2% Tho2 (for jet engine components), UO2 dispersed in alumina or stainless steel (for nuclear fuel elements) and sintered aluminium powder consisting of an aluminium matrix strengthened by Al2O3, and so on. The particle size ranges from 10 to 100 nm. The mechanism of strengthening here is similar to that of precipitation hardening (See Art. 2.4). 690 A Textbook of Production Technology 10. Shell moulding sand, using a resin binder, which is polymerized by a hot pattern. 11. Metal-polymer structures (metal bearings infiltrated with nylon or PTFE). 12. Particle board, in which wood chips are held togather by a suitable glue. 13. Elastomers and plastics are also reinforced with suitable particulate materials. The best example is : addition of 15 to 30% of carbon black in the vulcanised rubber for automobile types. It increases : tensile strength, roughness and tear and abrasion resistance of the product. Because of their unique geometry, the properties of particulate composites can be isotropic. This property is very important in many engineering applications. 14.2.2. Reinforced Materials : Reinforced materials form the biggest and most important group of composite materials. The purpose of reinforcing is always to improve the strength properties. Reinforcement may involve the use of a dispersed phase (discussed in the last article) or strong fibre, thread or rod. Fibre-reinforced Materials : In a large number of applications, the material should have high strength, alongwith toughness and resistance to fatigue failure. Fibre-reinforced materials offer the solution. Stronger or higher modulus filler, in the form of thin fibres of one material, is strongly bonded to the matrix of another. The matrix material provides ductility and toughness and supports and binds the fibres together and transmits the loads to the fibres. The fibres carry most of the load. The toughness of the composite material increases, because extra energy will be needed to break or pull out a fibre. Also, when any crack appears on the surface of a fibre, only that fibre will fail and the crack will not propagate catastrophically as in bulk material. Failure is often gradual, and repairs may be possible. Due to the above mentioned desirable properties of the matrix materials, the commonly used matrix materials are : Metals and polymers, such as, Al, Cu, Ni etc. and commercial polymers. Fibre-reinforced materials can be made quite anisotropic through directional control of the strong fibres in the relatively weak matrix. Like this, it is possible to produce parts where strength control is developed in different directions. If the part is loaded parallel to the fibres, the matrix material yields plastically and under equal strain, the stress within the fibres will be much greater than in the matrix. Even if the fibre breaks, the softness of the matrix hinders the propagation of crack. The fibre directions are tailored to the direction of loading. Reinforcing Fibres : A good reinforcing fibre should have : high elastic modulus, high strength, low density, reasonable ductility and should be easily wetted by the matrix. Metallic fibres such as patented steel, stainless steel, tungsten and molybdenum wires are used in a metal matrix such as aluminium and titanium. Carbon fibres and whiskers are also used to produce ultra-high strength composites. Fibres need not be limited to metals. Glass, ceramic and polymer fibres are used to produce variety of composites having wide range of properties. The high modulus of ceramic fibres make them attractive for the reinforcement of metals. The ductile matrix material can be aluminium, magnesium, nickle or titanuim and the reinforcing fibres may be of boron, graphite, alumina or SiC. Forms of Reinforcing Fibres : The fibres used for reinforcing materials are available in different forms : (a) Filaments : These are very long and continuous single fibres. (b) Yarn : This is twisted bundle of filaments. (c) Roving : These are untwisted bundles of gathered filaments. (d) Tows : These are bundles of thousands of filaments. (e) Woven fabrics : These are made from filaments, yarn or roving which have been woven at 90° to each other. Composite Materials and Their Processing 691 (f) Mats : Fibre form is said to be mat form when the continuous fibre is deposited in a swirl pattern or chopped fibre is deposited in a random pattern. (g) Combination mats : Here, one ply of woven roving is bonded to a ply of chopped-strand mat. (h) Surface mats : These are very thin, monofilament fibre mats for better surface appearance. (i) Chopped fibres or roving : These are of 3 to 50 mm in length. (j) Milled fibres : These are of brittle materials, usually 0.5 to 3 mm in length. (k) Whiskers : Whiskers are single crystals in the form of fine filaments, a few microns in diameter (20-50 nm diam) and short in length (a few mm). These single crystal whiskers are the strongest known fibres. Their high strength is due to the high degree of perfection and the absence of dislocation in their structure. Their strength is many times greater than that of the normal metals. For example, the strength of an iron whisker is found to be 13450 MN/m2, compared to about 294 MPa for a piece of pure iron. Besides metal whiskers, long nonmetallic whiskers (Al2O3, SiC, Si3N4) and of graphite are being produced. They are introduced into resin or metallic matrix for the purpose of high strength and high stiffness at high temperatures. The properties of reinforced materials will depend on : (i) The properties of the matrix material. (ii) The properties of the fibre material. (iii) The proportion of the reinforcement in the composite material. It is never less than 20% and may go upto 80% in oriented structures. (iv) The orientation of the fibres, relative to the load application and relative to one another. (v) The degree of bonding between the fibres and the matrix material. (vi) The length-to-diameter ratio (aspect ratio) of the fibres. There has to be some minimum fibre length, known as, critical length, lc, to get the desired strength and stiffness of the composite material. It is given as : f d lc   where,  f = Tensile strength of fibre material d = diameter of fibre and  = shear yield strength of the fibre-martix bond (a) (b) (c) Fig. 14.1. Reinforcing Fibres.