Material Engineering & Testing: Corrosion Prevention and Control PDF

M E 4 1 3 : 5 M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Chapter Corrosion Prevention and Control Inte...

M E 4 1 3 : 5 M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Chapter Corrosion Prevention and Control Intended Learning Outcomes After studying this chapter, you should be able to do the following: 1. Differentiate the different types of corrosion. 2. Identify the cause of corrosion. 3. Know how to prevent and control the corrosion in a material. CORROSION Corrosion is defined as the destructive and unintentional attack on a metal; it is electrochemical and ordinarily begins at the sur face. The problem of metallic corrosion is significant; in economic terms, it has been estimated that approximately 5% of an industrialized nation’s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions. The consequences of corrosion are all too common. Familiar examples include the rusting of automotive body panels and radiator and exhaust components. Corrosion is a dangerous and extremely costly problem. Because of it, buildings and bridges can collapse, oil pipelines break, chemical plants leak, and bathrooms flood. Corroded electrical contacts can cause fires and other problems, corroded medical implants may lead to blood poisoning, and air pollution has caused corrosion damage to works of art around the world. Corrosion threatens the safe disposal of radioactive waste that must be stored in containers for tens of thousands of years. FORMS OF CORROSION It is convenient to classify corrosion according to the manner in which it is manifest. Metallic corrosion is sometimes classified into eight forms: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion, and stress corrosion. The causes and means of prevention of each of these forms are discussed briefly. UNIFORM ATTACK Uniform attack is a form of electrochemical corrosion that occurs with equivalent intensity over the entire exposed surface and often leaves behind a scale or deposit. In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Familiar examples include general rusting of steel and iron and the tarnishing of silverware. This is probably the most common form of corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease. Module No. 5 – Corrosion Prevention and Control 1 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Uniform Attack Corrosion Source: https://www.nitty-gritty.it/en/morphology-of-corrosion/ How to prevent uniform corrosion? Uniform corrosion or general corrosion can be prevented through a number of methods: Use thicker materials for corrosion allowance Use paints or metallic coatings such as plating, galvanizing or anodizing Use Corrosion inhibitors or modifying the environment Cathodic protection (Sacrificial Anode or Impressed Current -ICCP) and Anodic Protection GALVANIC CORROSION Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The less noble or more reactive metal in the particular environment experiences corrosion; the more inert metal, the cathode, is protected from corrosion. As examples, steel screws corrode when in contact with brass in a marine environment, and if copper and steel tubing are joined in a domestic water heater, the steel corrodes in the vicinity of the junction. Sample of Galvanic Corrosion Source: https://www.nace.org/resources/general-resources/corrosion-basics/group-1/galvanic- corrosion Galvanic Series This represents the relative reactivities of a number of metals and commercial alloys in seawater. The alloys near the top are cathodic and unreactive, whereas those at the bottom are most anodic; no voltages are provided. Comparison of the standard emf and the galvanic series reveals a high degree of correspondence between the relative positions of the pure base Module No. 5 – Corrosion Prevention and Control 2 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G metals. Most metals and alloys are subject to oxidation or corrosion to one degree or another in a wide variety of environments—that is, they are more stable in an ionic state than as metals. Galvanic Series How to prevent galvanic corrosion? Galvanic corrosion can be prevented through a number of methods: Select metals/alloys as close together as possible in the galvanic series. Avoid unfavorable area effect of a small anode and large cathode. Insulate dissimilar metals wherever practical Apply coatings with caution. Paint the cathode (or both) and keep the coatings in good repair on the anode. Avoid threaded joints for materials far apart in the galvanic series. CREVICE CORROSION Electrochemical corrosion may also occur as a consequence of concentration differences of ions or dissolved gases in the electrolyte solution and between two regions of the same metal piece. For such a concentration cell, corrosion occurs in the locale that has the lower concentration. A good example of this type of corrosion occurs in crevices and recesses or under Module No. 5 – Corrosion Prevention and Control 3 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G deposits of dirt or corrosion products where the solution becomes stagnant and there is localized depletion of dissolved oxygen. Corrosion preferentially occurring at these positions is called crevice corrosion. The crevice must be wide enough for the solution to penetrate yet narrow enough for stagnancy; usually the width is several thousandths of an inch. Sample of Crevice Corrosion Source: Chemical Engineering World The major factors influencing crevice corrosion are: crevice type: metal-to-metal, metal-to-non-metal crevice geometry: gap size, depth, surface roughness material: alloy composition (e.g. Cr, Mo), structure environment: pH, temperature, halide ions, oxygen How to prevent crevice corrosion? Crevice corrosion can be designed out of the system Use welded butt joints instead of riveted or bolted joints in new equipment Eliminate crevices in existing lap joints by continuous welding or soldering Avoid creating stagnant conditions and ensure complete drainage in vessels Use solid, non-absorbent gaskets such as Teflon. Use higher alloys (ASTM G48) for increased resistance to crevice corrosion PITTING CORROSION Pitting is another form of much localized corrosion attack in which small pits or holes form. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction. It is an extremely insidious type of corrosion, often going undetected and with very little material loss until failure occurs. The mechanism for pitting is probably the same as for crevice corrosion, in that oxidation occurs within the pit itself, with complementary reduction at the surface. It is supposed that gravity causes the pits to grow downward, the solution at the pit tip becoming more concentrated and denser as pit growth progresses. Module No. 5 – Corrosion Prevention and Control 4 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Causes: lack of homogeneity of the metal surface (high roughness or presence of superficial scales) localized loss of liabilities ferrous or non-metallic surface contamination (e.g. inclusion of sulphides) mechanical disruption or antioxidant coating chemistry. How to prevent pitting corrosion? Pitting corrosion can be prevented through: Proper selection of materials with known resistance to the service environment Control pH, chloride concentration and temperature Cathodic protection and/or Anodic Protection Use higher alloys (ASTM G48) for increased resistance to pitting corrosion INTERGRANULAR CORROSION As the name suggests, intergranular corrosion occurs preferentially along grain boundaries for some alloys and in specific environments. The net result is that a macroscopic specimen disintegrates along its grain boundaries. This type of corrosion is especially prevalent in some stainless steels. When heated to temperatures between 500C and 800C (950F and 1450F) for sufficiently long time periods, these alloys become sensitized to intergranular attack. It is believed that this heat treatment permits the formation of small precipitate particles of chromium carbide (Cr23C6) by reaction between the chromium and carbon in the stainless steel. These particles form along the grain boundaries, as illustrated in Figure 7.12. Figure 7.13 shows this type of intergranular corrosion. Module No. 5 – Corrosion Prevention and Control 5 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Figure 7.12 The pitting of a 304 stainless steel plate by an acid–chloride solution. (Photograph courtesy of Mars G. Fontana. From M. G. Fontana, Corrosion Engineering, 3rd edition. Copyright © 1986 by McGraw-Hill Book Company. Reproduced with permission.) Figure 7.13 Schematic illustration of chromium carbide particles that have precipitated along grain boundaries in stainless steel, and the attendant zones of chromium depletion. Prevention: Stainless steels may be protected from intergranular corrosion by the following measures: (1) subjecting the sensitized material to a high-temperature heat treatment in which all the chromium carbide particles are re dissolved, (2) lowering the carbon content below 0.03 wt% C so that carbide formation is minimal, and (3) alloying the stainless steel with another metal such as niobium or titanium, hich has a greater tendency to form carbides than does chromium so that the Cr remains in solid solution. SELECTIVE LEACHING Selective leaching is found in solid solution alloys and occurs when one element or constituent is preferentially removed as a consequence of corrosion processes. The most common example is the dezincification of brass, in which zinc is selectively leached from a copper–zinc brass alloy. The mechanical properties of the alloy are significantly impaired because only a porous mass of copper remains in the region that has been dezincified. In addition, the material changes from yellow to a red or copper color. Selective leaching may also occur with other alloy systems in which aluminum, iron, cobalt, chromium, and other elements are vulnerable to preferential removal. Sample of Selective Leaching Module No. 5 – Corrosion Prevention and Control 6 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G How to prevent dealloying? Dealloying, selective leaching and graphitic corrosion can be prevented through the following methods: Select metals/alloys that are more resistant to dealloying. For example, inhibited brass is more resistant to dezincification than alpha brass, ductile iron is more resistant to graphitic corrosion than gray cast iron. Control the environment to minimize the selective leaching Use sacrificial anode cathodic protection or impressed current cathodic protection EROSION CORROSION Erosion corrosion arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Virtually all metal alloys, to one degree or another, are susceptible to erosion–corrosion. It is especially harmful to alloys that passivate by forming a protective surface film; the abrasive action may erode away the film, leaving exposed a bare metal surface. Erosion corrosion is commonly found in piping, especially at bends, elbows, and abrupt changes in pipe diameter positions where the fluid changes direction or flow suddenly becomes turbulent. Propellers, turbine blades, valves, and pumps are also susceptible to this form of corrosion Prevention: One of the best ways to reduce erosion corrosion is to change the design to eliminate fluid turbulence and impingement effects. Other materials may also be used that inherently resist erosion. Furthermore, removal of particulates and bubbles from the solution lessens its ability to erode. STRESS CORROSION Stress corrosion, sometimes termed stress corrosion cracking, results from the combined action of an applied tensile stress and a corrosive environment; both influences are necessary. In fact, some materials that are virtually inert in a particular corrosive medium become susceptible to this form of corrosion when a stress is applied. Small cracks form and then propagate in a direction perpendicular to the stress, with the result that failure may eventually occur. Failure behavior is characteristic of that for a brittle material, even though the metal alloy is intrinsically ductile. Furthermore, cracks may form at relatively low stress levels, significantly below the tensile strength. Most alloys are susceptible to stress corrosion in specific environments, especially at moderate stress levels. For example, most stainless steels stress corrode in solutions containing chloride ions, whereas brasses are especially vulnerable when exposed to ammonia. The stress that produces stress corrosion cracking need not be externally applied; it may be a residual one that results from rapid temperature changes and uneven contraction or occur for two-phase alloys in which each phase has a different coefficient of expansion. Also, gaseous and solid corrosion products that are entrapped internally can give rise to internal stresses. Prevention: Probably the best measure to take to reduce or completely eliminate stress corrosion is to lower the magnitude of the stress. This may be accomplished by reducing the external load or increasing the cross-sectional area perpendicular to the applied stress. Furthermore, an appropriate heat treatment may be used to anneal out any residual thermal stresses. Module No. 5 – Corrosion Prevention and Control 7 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G HYDROGEN EMBRITTLEMENT Various metal alloys, specifically some steels, experience a significant reduction in ductility and tensile strength when atomic hydrogen (H) penetrates into the material. Fig. 7.15 Fig. 7.16 Figure 7.15 Impingement failure of an elbow that was part of a steam condensate line. (Photograph courtesy of Mars G. Fontana. From M. G. Fontana, Corrosion Engineering, 3rd edition. Copyright © 1986 by McGrawHill Book Company. Reproduced with permission.) Figure 7.16 A bar of steel bent into a horseshoe shape using a nutand-bolt assembly. While immersed in seawater, stress corrosion cracks formed along the bend at those regions where the tensile stresses are the greatest. (Photograph courtesy of F. L. LaQue. From F. L. LaQue, Marine Corrosion, Causes and Prevention. Copyright © 1975 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.) The mechanism starts with lone hydrogen atoms diffusing through the metal. At high temperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength up to the point where it cracks open (hydrogen induced cracking, or HIC). High-strength and low-alloy steels, nickel and titanium alloys are most susceptible. Tensile stresses, susceptible material, and the presence of hydrogen are necessary to cause hydrogen embrittlement. Residual stresses or externally applied loads resulting in stresses significantly below yield stresses can cause cracking. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. How to prevent hydrogen embrittlement? Hydrogen embrittlement can be prevented through: Control of stress level (residual or load) and hardness. Avoid the hydrogen source. Baking to remove hydrogen. Module No. 5 – Corrosion Prevention and Control 8 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G Video: https://www.youtube.com/watch?v=YF_FhNN5D1w&t=154s https://www.youtube.com/watch?v=M_a5hG9sInY&t=4s https://www.youtube.com/watch?v=UpS1chG2Bas&t=1s https://www.youtube.com/watch?v=K87KvHPwiIU https://www.youtube.com/watch?v=4HCsBMI7nSg&t=269s https://www.youtube.com/watch?v=EYjdCKUMhPM https://www.youtube.com/watch?v=5Sd6TEenwEE https://www.youtube.com/watch?v=Aa8WOKGjm4s Module No. 5 – Corrosion Prevention and Control 9 M E 4 1 3 : M A T E R I A L E N G I N E E R I N G A N D T E S T I N G References: 1. Materials Science and Engineering: An Introduction, 9th Edition, William D. Callister, Jr. Department of Metallurgical Engineering The University of Utah with special contributions by David G. Rethwisch The University of Iowa. 2. TWI Lt (n.d). What is Galvanic Corrosion and How can it be Prevented. Available at: https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-galvanic-corrosion-and-how- can-it-be-avoided 3. Steelfab, 2017. A Guide To Crevice Corrosion & How To Treat It. Available at: https://steelfabservices.com.au/a-guide-to-crevice-corrosion-how-to-treat-it/ 4. Webcorr Corrosion Consulting Services,n.d. Different Types of Corrosion. Available at: https://www.corrosionclinic.com/types_of_corrosion/uniform_corrosion.htm Module No. 5 – Corrosion Prevention and Control 10 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Chapter 6 Ferrous and Non- ferrous Metals Intended Learning Outcomes After studying this chapter, you should be able to do the following: 1. Identify the different types of ferrous and non-ferrous metals 2. Describe the properties and application of ferrous and non-ferrous metals. 3. Name and describe four forming operations that are used to shape metal alloys. 4. State the purposes of and describe procedures for the following heat treatments: process annealing, stress relief annealing, normalizing, full annealing, and spheroidizing. This chapter primarily presents the different types of ferrous and non-ferrous metals. This chapter provides the overview of some of the commercial alloys and their general properties and limitations. Materials selection decisions may also be influenced by the ease with which metal alloys may be formed or manufactured into useful components. Alloy properties can be altered by fabrication processes, and, in addition, further property alterations may be induced by the employment of appropriate heat treatments. This chapter also discussed the details of some of these treatments, including annealing procedures, the heat treating of steels, and precipitation hardening. Metal A metal is a material that is typically hard, opaque, shiny, and has good electrical and thermal conductivity. Metals are generally malleable, that is, they can be hammered or pressed permanently out of shape without breaking or cracking as well as fusible (able to be fused or melted) and ductile (able to be drawn out into a thin wire). Astrophysicists use the term "metal" to collectively describe all elements other than hydrogen and helium. Thus, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Types of Metal Alloys Metal alloys, by virtue of composition, are often grouped into two classes: ferrous and nonferrous. Ferrous alloys, those in which iron is the principal constituent, include steels and cast irons while the nonferrous are alloys that are not iron based. Module No. 6 – Ferrous and Non-ferrous Metals 1 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G FERROUS ALLOYS Ferrous alloys are those of which iron is the prime constituent. They are produced in larger quantities than any other metal type. They are especially important as engineering construction materials. Their widespread use is accounted for by three factors: (1) iron-containing compounds exist in abundant quantities within the earth’s crust; (2) metallic iron and steel alloys may be produced using relatively economical extraction, refining, alloying, and fabrication techniques; and (3) ferrous alloys are extremely versatile; in that they may be tailored to have a wide range of mechanical and physical properties. The principal disadvantage of many ferrous alloys is their susceptibility to corrosion. A taxonomic classification scheme for the various ferrous alloys is presented in Figure 1. Figure 1 Classification scheme for the various ferrous alloys (Callister, 2014) Steels Steels are iron–carbon alloys that may contain appreciable concentrations of other alloying elements; there are thousands of alloys that have different compositions and/or heat treatments. The mechanical properties are sensitive to the content of carbon, which is normally less than 1.0 wt.%. Some of the more common steels are classified according to carbon Module No. 6 – Ferrous and Non-ferrous Metals 2 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G concentration namely: low, medium, and high-carbon types. Subclasses also exist within each group according to the concentration of other alloying elements. Plain carbon steels contain only residual concentrations of impurities other than carbon and a little manganese. For alloy steels, more alloying elements are intentionally added in specific concentrations. Low-carbon steels Low-carbon steels contain less than 0.25%C. it is not very responsive to heat treatments and strengthening is accomplished by cold work. It is soft, weak, tough, ductile, machinable, weldable and not expensive. They typically have a yield strength of 275 MPa (40,000 psi), tensile strengths between 415 and 550 MPa (60,000 and 80,000 psi), and a ductility of 25%EL.Typical applications include automobile body components, structural shapes (I-beams, channel and angle iron), and sheets that are used in pipelines, buildings, bridges, and tin cans. The composition of low carbon steels can be seen in Table 1.0. It can also be seen that the composition of steel is mainly carbon ang manganese. Table 1.0 Composition of Five Plain Low-carbon steels and Three High-Strength, Low-Alloy Steels Source: Callister (2014) In Table 2.0 shows the mechanical properties of hot-rolled material and typical applications for various plain low-carbon and high strength low alloy steels. The mechanical properties include the tensile strength, yield strength and ductility. Module No. 6 – Ferrous and Non-ferrous Metals 3 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Table 2.0 Mechanical Characteristics of Hot-Rolled Material and Typical Applications for Various Plain Low-Carbon and High-Strength, Low-Alloys Steels Source: Callister (2014) High-strength, Low-alloy (HSLA) steels It contains alloying elements such as copper, vanadium, nickel, and molybdenum in combined concentrations of >10 wt%. It is stronger than plain low-C steels. Most may be strengthened by heat treatment, giving tensile strengths in excess of 480 MPa (70,000 psi). They are ductile, formable and machinable. In normal atmospheres, the HSLA steels are more resistant to corrosion than the plain carbon steels Medium-Carbon Steels Medium-carbon steels contain 0.25-0.60 wt.% of carbon. It is stronger than low-carbon steels but less ductile and less tough. These alloys may be heat-treated by austenitizing, quenching, and then tempering to improve their mechanical properties. It can only be heat treated in very thin sections and with very rapid quenching rates but with the addition of chromium, nickel, and molybdenum improve the capacity of these alloys to be heat-treated. Applications include railway wheels and tracks, gears, crankshafts, and other machine parts and high-strength structural components calling for a combination of high strength, wear resistance, and toughness. High-Carbon Steels The high-carbon steels, normally having carbon contents between 0.60 and 1.4 wt%. It is the hardest, strongest, and yet least ductile of the carbon steels. They are almost always used in a hardened and tempered condition, wear resistant and capable of holding a sharp cutting edge. The tool and die steels are high-carbon alloys, usually containing chromium, vanadium, tungsten, and molybdenum. These alloying elements combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr23C6, V4C3, and WC). These steels are used as Module No. 6 – Ferrous and Non-ferrous Metals 4 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G cutting tools and dies for forming and shaping materials, as well as in knives, razors, hacksaw blades, springs, and high-strength wire. Some tool steel compositions and their applications are listed in Table 3.0 Table 3.0 Designation, Composition, and Application of Six Tool Steels Source: Callister, 2014 Stainless steels The stainless steels are highly resistant to corrosion (rusting) in a variety of environments, especially the ambient atmosphere. Their predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required. Corrosion resistance may also be enhanced by nickel and molybdenum additions. Stainless steels are divided into three classes on the basis of the predominant phase constituent of the microstructure—martensitic, ferritic, or austenitic. 1. Martensitic stainless steels are capable of being heat treated in such a way that martensite is the prime microconstituent. Additions of alloying elements in significant concentrations produce dramatic alterations in the iron–iron carbide phase diagram. 2. For austenitic stainless steels, the austenite (or ɣ) phase field is extended to room temperature. The austenitic stainless steels are the most corrosion resistant because of the high chromium contents and also the nickel additions; and they are produced in the largest quantities. 3. Ferritic stainless steels are composed of the α-ferrite (BCC) phase. Austenitic and ferritic stainless steels are hardened and strengthened by cold work because they are not heat treatable. Both martensitic and ferritic stainless steels are magnetic; the austenitic stainlesses are not. Equipment employing these steels includes gas turbines, high-temperature steam boilers, heat-treating furnaces, aircraft, missiles, and nuclear power generating units. Several stainless steels by class, along with composition, typical mechanical properties, and applications can be seen in Table 4.0 Module No. 6 – Ferrous and Non-ferrous Metals 5 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Table 4.0 Designation, Composition, Mechanical Properties, and Typical Application for Austenitic, Ferritic, Martensitic, and Precipitation-Hardenable Stainless Steels Source: Callister, 2014 Cast Irons Theoretically, it contains > 2.14 wt.% of carbon. Usually contains between 3.0-4.5 wt.% C, hence it is very brittle. They become liquid easily between 1150 0C and 1300 0C. They are easily melted and amenable to casting. It is Inexpensive, machinable and wear resistant. The most common cast iron types are gray, nodular, white, malleable, and compacted graphite Gray Iron The carbon and silicon contents of gray cast irons vary between 2.5 and 4.0 wt% and 1.0 and 3.0 wt%, respectively. Mechanically, gray iron is comparatively weak and brittle in tension. Strength and ductility are much higher under compressive loads. They are very effective in damping vibrational energy. In addition, gray irons exhibit a high resistance to wear and the least expensive of all metallic materials Ductile (or Nodular) Iron Adding a small amount of magnesium and/or cerium to the gray iron before casting produces a distinctly different microstructure and set of mechanical properties. Graphite still forms, but as nodules or sphere-like particles instead of flakes. Castings are stronger and much more ductile than gray iron. It has mechanical characteristics approaching those of steel. Module No. 6 – Ferrous and Non-ferrous Metals 6 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Typical applications for this material include valves, pump bodies, crankshafts, gears, and other automotive and machine components White Iron For low-silicon cast irons (containing less than 1.0 wt% Si) and rapid cooling rates, most of the carbon exists as cementite instead of graphite. A fracture surface of this alloy has a white appearance, and thus it is termed white cast iron.Thick sections may have only a surface layer of white iron that was “chilled” during the casting process; gray iron forms at interior regions, which cool more slowly. As a consequence of large amounts of the cementite phase, white iron is extremely hard but also very brittle, to the point of being virtually unmachinable. Its use is limited to applications that necessitate a very hard and wear-resistant surface, without a high degree of ductility—for example, as rollers in rolling mills Malleable Iron Heating white iron at temperatures between 800 and 900 oC for a prolonged time period and in a neutral atmosphere (to prevent oxidation) causes a decomposition of the cementite, forming graphite, which exists in the form of clusters or rosettes surrounded by a ferrite or pearlite matrix, depending on cooling rate. The microstructure is similar to that for nodular iron which accounts for relatively high strength and appreciable ductility or malleability. Representative applications include connecting rods, transmission gears, and differential cases for the automotive industry, and also flanges, pipe fittings, and valve parts for railroad, marine, and other heavy-duty services. Compacted Graphite Iron A relatively recent addition to the family of cast irons. As with gray, ductile, and malleable irons, carbon exists as graphite, which formation is promoted by the presence of silicon. Silicon content ranges between 1.7 and 3.0 wt%, whereas carbon concentration is normally between 3.1 and 4.0 wt%. Microstructurally, the graphite in CGI alloys has a worm-like (or vermicular) shape. Magnesium and/or cerium is also added, but concentrations are lower than for ductile iron. An increase in degree of nodularity of the graphite particles leads to enhancements of both strength and ductility. Tensile and yield strengths for compacted graphite irons are comparable to values for ductile and malleable irons, yet are greater than those observed for the higher strength gray irons. In addition, ductilities for CGIs are intermediate between values for gray and ductile irons; also, moduli of elasticity range between 140 and 165 GPa ( and psi). Compared to the other cast iron types, desirable characteristics of CGIs include the following: higher thermal conductivity, better resistance to thermal shock (i.e., fracture resulting from rapid temperature changes) and lower oxidation at elevated temperatures. Compacted graphite irons are now being used in a number of important applications—these include: diesel engine blocks, exhaust manifolds, gearbox housings, brake discs for high-speed trains, and flywheels. NONFERROUS ALLOYS Non-ferrous alloys are metals that do not have any iron in them at all. It is not attracted to the magnet and do not rust easily when exposed to moisture. Module No. 6 – Ferrous and Non-ferrous Metals 7 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Copper and Its Alloys It is highly resistant to corrosion in diverse environments including the ambient atmosphere, seawater, and some industrial chemicals. Most copper alloys cannot be hardened or strengthened by heat-treating procedures; consequently, cold working and/or solid-solution alloying must be utilized to improve these mechanical properties. The most common copper alloys are the brasses for which zinc, as a substitutional impurity, is the predominant alloying element. The bronzes are alloys of copper and several other elements, including tin, aluminum, silicon, and nickel. The most common heat-treatable copper alloys are the beryllium coppers. They possess a remarkable combination of properties: tensile strengths as high as 1400 MPa (200,000 psi), excellent electrical and corrosion properties, and wear resistance when properly lubricated; they may be cast, hot worked, or cold worked. Applications include jet aircraft landing gear bearings and bushings, springs, and surgical and dental instruments. Table 5.0 shows the composition, mechanical properties and typical application of some copper alloys. Table 5.0 Composition, Mechanical Properties and Typical Applications of Eight Copper Alloys Source: Callister, 2014 Module No. 6 – Ferrous and Non-ferrous Metals 8 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Aluminum and Its Alloys Aluminum and its alloys are characterized by a relatively low density (2.7 g/cm3 as compared to 7.9 g/cm3 for steel), high electrical and thermal conductivities, and a resistance to corrosion in some common environments, including the ambient atmosphere. Many of these alloys are easily formed by virtue of high ductility; this is evidenced by the thin aluminum foil sheet into which the relatively pure material may be rolled. The chief limitation of aluminum is its low melting temperature 660 oC. Principal alloying elements include copper, magnesium, silicon, manganese, and zinc. Generally, aluminum alloys are classified as either cast or wrought. Some of the more common applications of aluminum alloys include aircraft structural parts, beverage cans, bus bodies, and automotive parts (engine blocks, pistons, and manifolds). Magnesium and Its Alloys The most outstanding characteristic of magnesium is its density, 1.7 g/cm3, which is the lowest of all the structural metals. Magnesium has an HCP crystal structure, is relatively soft, and has a low elastic modulus: 45 GPa. Consequently, most fabrication is by casting or hot working at temperatures between 200 and 350 oC. It h has a moderately low melting temperature 651 oC. Chemically, magnesium alloys are relatively unstable and especially susceptible to corrosion in marine environments. Fine magnesium powder ignites easily when heated in air; consequently, care should be exercised when handling it in this state. These alloys are used in aircraft and missile applications, as well as in luggage. For many applications, magnesium alloys have replaced engineering plastics that have comparable densities in as much as the magnesium materials are stiffer, more recyclable, and less costly to produce. Titanium and Its Alloys Titanium and its alloys are relatively new engineering materials that possess an extraordinary combination of properties. The pure metal has a relatively low density (4.5 g/cm3), a high melting point [1668 oC ], and an elastic modulus of 107 GPa ( psi). Titanium alloys are extremely strong; room temperature tensile strengths as high as 1400 MPa (200,000 psi) are attainable, yielding remarkable specific strengths. The major limitation of titanium is its chemical reactivity with other materials at elevated temperatures and quite expensive. In spite of this high temperature reactivity, the corrosion resistance of titanium alloys at normal temperatures is unusually high; they are virtually immune to air, marine, and a variety of industrial environments. They are commonly utilized in airplane structures, space vehicles, surgical implants, and in the petroleum and chemical industries. The Refractory Metals Metals that have extremely high melting temperatures are classified as the refractory metals. Included in this group are niobium (Nb), molybdenum (Mo), tungsten (W), and tantalum (Ta). Melting temperatures range between 2468 0C for niobium and 3410 0C for tungsten. Tantalum and molybdenum are alloyed with stainless steel to improve its corrosion resistance. Molybdenum alloys are utilized for extrusion dies and structural parts in space vehicles; incandescent light filaments, x-ray tubes, and welding electrodes employ tungsten alloys.Tantalum is immune to chemical attack by virtually all environments at temperatures below 150 oC and is frequently used in applications requiring such a corrosion-resistant material. Module No. 6 – Ferrous and Non-ferrous Metals 9 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G The Superalloys The superalloys have superlative combinations of properties. Most are used in aircraft turbine components, which must withstand exposure to severely oxidizing environments and high temperatures for reasonable time periods. These materials are classified according to the predominant metal(s) in the alloy, of which there are three groups—iron–nickel, nickel, and cobalt. Other alloying elements include the refractory metals (Nb, Mo, W, Ta), chromium, and titanium. The several compositions of superalloys can be seen in Table 6.0 Table 6.0 Composition of Several Superalloys Source: Callister, 2014 The Noble Metals The noble or precious metals are a group of eight elements that have some physical characteristics in common. They are expensive (precious) and are superior or notable (noble) in properties, that is, characteristically soft, ductile, and oxidation resistant. The noble metals are silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium; the first three are most common and are used extensively in jewelry. Miscellaneous Nonferrous Alloys Nickel and its alloys are highly resistant to corrosion in many environments, especially those that are basic (alkaline). It is one of the principal alloying elements in stainless steels and one of the major constituents in the superalloys. Lead, tin, and their alloys find some use as engineering materials. Both lead and tin are mechanically soft and weak, have low melting temperatures, are quite resistant to many corrosion environments, and have recrystallization temperatures below room temperature. Module No. 6 – Ferrous and Non-ferrous Metals 10 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Unalloyed zinc also is a relatively soft metal having a low melting temperature and a subambient recrystallization temperature. Chemically, it is reactive in a number of common environments and, therefore, susceptible to corrosion. Zirconium and its alloys are ductile and have other mechanical characteristics that are comparable to those of titanium alloys and the austenitic stainless steels. However, the primary asset of these alloys is their resistance to corrosion in a host of corrosive media, including superheated water. Fabrication of Metals Metal fabrication techniques are normally preceded by refining, alloying, and often heat- treating processes that produce alloys with the desired characteristics. The classifications of fabrication techniques include various metal-forming methods, casting, powder metallurgy, welding, and machining; often two or more of them must be used before a piece is finished. The methods chosen depend on several factors; the most important are the properties of the metal, the size and shape of the finished piece, and, of course, cost. Figure 2. Classifications scheme of metal fabrication techniques Source: Callister, 2014 FORMING OPERATIONS Forming operations are those in which the shape of a metal piece is changed by plastic deformation; for example, forging, rolling, extrusion, and drawing are common forming techniques. Forging Forging is mechanically working or deforming a single piece of a normally hot metal; this may be accomplished by the application of successive blows or by continuous squeezing. Forgings are classified as: 1. closed die - a force is brought to bear on two or more die halves having the finished shape such that the metal is deformed in the cavity between them 2. open die -two dies having simple geometric shapes (e.g., parallel flat, semicircular) are employed, normally on large workpieces. Module No. 6 – Ferrous and Non-ferrous Metals 11 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Figure 3. shows the example illustration of an open and closed-die forging. Figure 3. Open and Closed die forging Source: Canton Drop Forge Rolling Rolling is the most widely used deformation process, consists of passing a piece of metal between two rolls; a reduction in thickness results from compressive stresses exerted by the rolls. Cold rolling may be used in the production of sheet, strip, and foil with high quality surface finish. Circular shapes as well as I-beams and railroad rails are fabricated using grooved rolls. Figure 4 Rolling Source: Callister, 2014 Extrusion For extrusion, a bar of metal is forced through a die orifice by a compressive force that is applied to a ram; the extruded piece that emerges has the desired shape and a reduced cross- sectional area. Extrusion products include rods and tubing that have rather complicated cross- sectional geometries; seamless tubing may also be extruded. Figure 5 Extrusion Source: Callister, 2014 Module No. 6 – Ferrous and Non-ferrous Metals 12 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Drawing Drawing is the pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side. A reduction in cross section results, with a corresponding increase in length. Rod, wire, and tubing products are commonly fabricated in this way. Figure 6 Rolling Source: Callister, 2014 CASTING Casting is a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold but experiences some shrinkage. Casting techniques are employed when: (1) the finished shape is so large or complicated that any other method would be impractical (2) a particular alloy is so low in ductility that forming by either hot or cold working would be difficult, and (3) in comparison to other fabrication processes, casting is the most economical. A number of different casting techniques are commonly employed, including sand, die, investment, lost foam, and continuous casting. Sand Casting With sand casting, probably the most common method, ordinary sand is used as the mold material. A two-piece mold is formed by packing sand around a pattern that has the shape of the intended casting. Furthermore, a gating system is usually incorporated into the mold to expedite the flow of molten metal into the cavity and to minimize internal casting defects. Sand-cast parts include automotive cylinder blocks, fire hydrants, and large pipe fittings. Figure 7 shows the step by step procedure of sand casting from the pattern making up to the sand cast metal. Module No. 6 – Ferrous and Non-ferrous Metals 13 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Figure 7. Step of Sand Casting Source: https://materialrulz.weebly.com/uploads/7/9/5/1/795167/binder_mcm_02.pdf Die Casting In die casting, the liquid metal is forced into a mold under pressure and at a relatively high velocity, and allowed to solidify with the pressure maintained. A two-piece permanent steel mold or die is employed; when clamped together, the two pieces form the desired shape. When complete solidification has been achieved, the die pieces are opened and the cast piece is ejected. However, this technique lends itself only to relatively small pieces and to alloys of zinc, aluminum, and magnesium, which have low melting temperatures. Figure 8 shows the sample of die casting process. Figure 8. Die casting Source: www. substech.com Module No. 6 – Ferrous and Non-ferrous Metals 14 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Investment Casting For investment (sometimes called lost-wax) casting, the pattern is made from a wax or plastic that has a low melting temperature. Around the pattern is poured a fluid slurry, which sets up to form a solid mold or investment; plaster of paris is usually used. The mold is then heated, such that the pattern melts and is burned out, leaving behind a mold cavity having the desired shape. This technique is employed when high dimensional accuracy, reproduction of fine detail, and an excellent finish are require, for example, in jewelry and dental crowns and inlays. Also, blades for gas turbines and jet engine impellers are investment cast. The procedure of investment casting can be seen in Figure 9 Figure 9. Schematic Illustration of Investment Casting Source: Kalpakjian & Schmid Lost Foam Casting A variation of investment casting is lost foam (or expendable pattern) casting. Here the expendable pattern is a foam that can be formed by compressing polystyrene beads into the desired shape and then bonding them together by heating. Alternatively, pattern shapes can be cut from sheets and assembled with glue. Sand is then packed around the pattern to form the mold. As the molten metal is poured into the mold, it replaces the pattern which vaporizes. The compacted sand remains in place, and, upon solidification, the metal assumes the shape of the mold. Metal alloys that most commonly use this technique are cast irons and aluminum alloys; furthermore, applications include automobile engine blocks, cylinder heads, crankshafts, marine engine blocks, and electric motor frames. Figure 10. Lost-foam Casting Source: Tibba, 2014 Module No. 6 – Ferrous and Non-ferrous Metals 15 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G Continuous Casting At the conclusion of extraction processes, many molten metals are solidified by casting into large ingot molds. The ingots are normally subjected to a primary hot-rolling operation, the product of which is a flat sheet or slab; these are more convenient shapes as starting points for subsequent secondary metal-forming operations (i.e., forging, extrusion, drawing). Using this technique, the refined and molten metal is cast directly into a continuous strand that may have either a rectangular or circular cross section; solidification occurs in a water-cooled die having the desired cross-sectional geometry. Furthermore, continuous casting is highly automated and more efficient. Figure 11 shows the schematic illustration of continuous casting. Figure 11. Continuous Casting Source: CALMET, 2017 Miscellaneous Techniques Powder Metallurgy Another fabrication technique involves the compaction of powdered metal, followed by a heat treatment to produce a denser piece. This method is especially suitable for metals having low ductilities, since only small plastic deformation of the powder particles need occur. Metals having high melting temperatures are difficult to melt and cast, and fabrication is expedited using P/M. Furthermore, parts that require very close dimensional tolerances (e.g., bushings and gears) may be economically produced using this technique. Welding In a sense, welding may be considered to be a fabrication technique. In welding, two or more metal parts are joined to form a single piece when one-part fabrication is expensive or inconvenient. Both similar and dissimilar metals may be welded. The joining bond is metallurgical Module No. 6 – Ferrous and Non-ferrous Metals 16 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G (involving some diffusion) rather than just mechanical, as with riveting and bolting. A variety of welding methods exist, including arc and gas welding, as well as brazing and soldering. Heat Treatment Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics. Heat Treatment Methods Annealing Annealing is a heat treatment process in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Typically, annealing is carried out to relieve stresses; to increase softness, ductility, and toughness; and/or to produce a specific microstructure. Annealing process consists of three stages: (1) heating to the desired temperature, (2) holding or “soaking” at that temperature, and (3) cooling, usually to room temperature. Normalizing Normalizing is a heat treatment process used to refine the grains and produce a more uniform and desirable size distribution. It is accomplished by heating at least 55 0C (100 0F) above the upper critical temperature. for making material softer but does not produce the uniform material properties of annealing. Hardening Hardening is the process for making material harder. In this process, the metal is heated to a specific temperature and rapidly cooled (quenched) in a bath of water, brine, oil, or air to increase its hardness. Ageing or Precipitation Hardening Ageing or precipitation hardening is a heat treatment method mostly used to increase the yield strength of malleable metals. The process produces uniformly dispersed particles within a metal’s grain structure which bring about changes in properties. It is usually comes after another heat treatment process that reaches higher temperatures. Ageing, however, only elevates the temperature to medium levels and brings it down quickly again. Stress Relieving Stress relieving is especially common for boiler parts, air bottles, accumulators, etc. This method takes the metal to a temperature just below its lower critical border. The cooling process is Module No. 6 – Ferrous and Non-ferrous Metals 17 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G slow and therefore uniform. This is done to relieve stresses that have built in up in the parts due to earlier processes such as forming, machining, rolling or straightening. Tempering Tempering carried out by preheating previously quenched or normalized steel to a temperature below the lower critical temperature (often from 205 to 595 ˚C), holding, and then cooling to obtain the desired mechanical properties. is used to reduce the brittleness of quenched steel.The temperature chosen for the tempering process directly impacts the hardness of the work piece. The higher the temperature in the tempering process, the lower the hardness. Case hardening Case hardening or Surface hardening is the process of hardening the surface of steel while leaving the interior unchanged. It improves the wear resistance of machine parts without affecting the tough interior of the parts. Many processes are available for surface hardening. This type of process is normally used on a steel with a low carbon content, usually less than 0.2% The principal forms of casehardening are : Carburizing It is process of increasing the carbon content on the surface of steel. It is a heat treatment process in which iron or steel is heated in the presence of another material (in the range of 900 to 950 °C ) which liberates carbon as it decomposes Cyaniding It is a process of producing hard surfaces by immersing low carbon steel in cyanide bath maintained at 800°C – 850°C. The parts are then quenched in water or oil. This process helps to maintain bright finish of the parts. It requires much care and attention in handling the salt because of its poisonous nature. Nitriding It is a process of diffusing the nitrogen in to the surface of steel. The process is carried out by heating of steel in the presence of dissociated ammonia at a temperature 460°C – 570°C. The diffused nitrogen combines with iron & certain alloying elements present in steel and form respective nitrides. Module No. 6 – Ferrous and Non-ferrous Metals 18 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G CHAPTER TEST Directions: Answer the following question comprehensively. Write your answer on the space provided. Questions: 1. What are the four classifications of steels? For each class, briefly describe the properties and typical applications. 2. Cite three reasons why ferrous alloys are used so extensively. Module No. 6 – Ferrous and Non-ferrous Metals 19 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G 3. Give the distinctive features, limitations, and applications of the following alloy groups: titanium alloys, refractory metals, superalloys, and noble metals. 4. Discuss at least four techniques in forming metals. 5. Differentiate the heat treatment method. Module No. 6 – Ferrous and Non-ferrous Metals 20 M E 4 1 3 : M A T E R I A L S E N G I N E E R I N G A N D T E S T I N G References 1. Materials Science and Engineering: An Introduction, 9th Edition, William D. Callister, Jr. Department of Metallurgical Engineering The University of Utah with special contributions by David G. Rethwisch The University of Iowa. 2. Velling, A. (2020), “What Is Heat Treatment? Methods & Benefits”,Retrieved July 29, 2020, Available at: https://fractory.com/heat-treatment-methods/ Module No. 6 – Ferrous and Non-ferrous Metals 21 E N G G 4 1 2 : 7 M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Chapter Ceramics Intended Learning Outcomes After studying this chapter, you should be able to do the following: 1. Differentiate the types of ceramic materials. 2. Determine the different application and properties of the ceramics 3. Explain the processes involved in processing and fabrication of ceramic materials. This chapter will discuss structures and properties of ceramics, as well as its applications and processing. A ceramic is an inorganic non-metallic solid made up of either metal or non-metal compounds that have been shaped and then hardened by heating to high temperature. In general, they are hard, corrosion-resistant, and brittle. Ceramics are all around us. This category of materials includes things like tiles, bricks, plates, glass, and toilets. Depending on their method of formation, ceramics can be dense or lightweight. Typically, they will demonstrate excellent strength and hardness properties, however, they are often brittle in nature. Ceramics can also be formed to serve as electrically conductive materials, objects allowing electricity to pass through their mass, or insulators, materials preventing the flow of electricity. What are Ceramics? A ceramic is an inorganic non-metallic solid made up of either metal or non-metal compounds that have been shaped and then hardened by heating to high temperatures. In general, they are hard, corrosion-resistant and brittle. The term ceramic comes from the Greek word keramikos, which means “burnt stuff,” indicating that desirable properties of these materials are normally achieved through a high-temperature heat treatment process called firing. The clay-based domestic wares, art objects and building products are familiar to us all, but pottery is just one part of the ceramic world. Nowadays the term ‘ceramic’ has a more expansive meaning and includes materials like glass, advanced ceramics and some cement systems as well. STRUCTURES AND PROPERTIES OF CERAMICS Ceramic Bonding In an ionic bond, one of the atoms (the metal) transfers electrons to the other atom (the nonmetal), thus becoming positively charged (cation), whereas the nonmetal becomes Module No. 7 – Ceramics 1 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G negatively charged (anion). The two ions having opposite charges attract each other with a strong electrostatic force. Covalent bonding instead occurs between two nonmetals, in other words two atoms that have similar electronegativity, and involves the sharing of electron pairs between the two atoms. Although both types of bonds occur between atoms in ceramic materials, in most of them (particularly the oxides) the ionic bond is predominant. The ionic and covalent bonds of ceramics are responsible for many unique properties of these materials, such as high hardness, high melting points, low thermal expansion, and good chemical resistance, but also for some undesirable characteristics, foremost being brittleness, which leads to fractures unless the material is toughened by reinforcing agents or by other means. Ceramic Crystal Structure AX-Type Crystal Structures Some of the common ceramic materials are those in which there are equal numbers of cations and anions. These are often referred to as AX compounds, where A denotes the cation and X the anion. There are several different crystal structures for AX compounds; each is typically named after a common material that assumes the particular structure. Rock Salt Structure Perhaps the most common AX crystal structure is the sodium chloride (NaCl), or rock salt, type. The coordination number for both cations and anions is 6, and therefore the cation–anion radius ratio is between approximately 0.414 and 0.732. A unit cell for this crystal structure is generated from an FCC arrangement of anions with one cation situated at the cube center and one at the center of each of the 12 cube edges. An equivalent crystal structure results from a face-centered arrangement of cations. Thus, the rock salt crystal structure may be thought of as two interpenetrating FCC lattices—one composed of the cations, the other of anions. Some common ceramic materials that form with this crystal structure are NaCl, MgO, MnS, LiF, and FeO. Figure 1. A unit cell for the rock salt or sodium chloride (NaCl) crystal structure Adapted from Fig. 12.2, Callister 9e Module No. 7 – Ceramics 2 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Cesium Chloride Structure Figure 2 shows a unit cell for the cesium chloride (CsCl) crystal structure; the coordination number is 8 for both ion types. The anions are located at each of the corners of a cube, whereas the cube center is a single cation. Interchange of anions with cations, and vice versa, produces the same crystal structure. This is not a BCC crystal structure because ions of two different kinds are involved. Figure 2. A unit cellfor the Cesium Chloride (CsCl) crystal structure Adapted from Fig. 12.3, Callister 9e Zinc Blende Structure A third AX structure is one in which the coordination number is 4—that is, all ions are tetrahedrally coordinated. This is called the zinc blende, or sphalerite, structure, after the mineralogical term for zinc sulfide (ZnS). A unit cell is presented in Figure 3, all corner and face positions of the cubic cell are occupied by S atoms, whereas the Zn atoms fill interior tetrahedral positions. An equivalent structure results if Zn and S atom positions are reversed. Thus, each Zn atom is bonded to four S atoms, and vice versa. Most often the atomic bonding is highly covalent in compounds exhibiting this crystal structure, which include ZnS, ZnTe, and SiC. Figure 3. A unit cell for zinc blende (ZnS) crystal structure Adapted from Fig. 12.4, Callister 9e Module No. 7 – Ceramics 3 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G AmXp-Type Crystal Structures If the charges on the cations and anions are not the same, a compound can exist with the chemical formula AmXp, where m and/or p≠1. An example is AX2, for which a common crystal structure is found in fluorite (CaF2). Calcium ions are positioned at the centers of cubes, with fluorine ions at the corners. The chemical formula shows that there are only half as many Ca2 ions as F ions, and therefore the crystal structure is similar to CsCl, except that only half the center cube positions are occupied by Ca2 ions. One unit cell consists of eight cubes, as indicated in Figure 4. Other compounds with this crystal structure include ZrO2 (cubic), UO2, PuO2, and ThO2. Figure 4. A unit cell for the fluorite (CaF2) crystal structures Adapted from Fig. 12.5, Callister 9e AmBnXp-Type Crystal Structures It is also possible for ceramic compounds to have more than one type of cation; for two types of cations (represented by A and B), their chemical formula may be designated as AmBnXp. Barium titanate (BaTiO3), having both Ba2 and Ti4 cations, falls into this classification. This material has a perovskite crystal structure and rather interesting electromechanical properties to be discussed later. At temperatures above 120 0C (248 0F), the crystal structure is cubic. A unit cell of this structure is shown in the figure below. Figure 5. A unit cell for the perovskite crystal structure. Adapted from Fig. 12.6, Callister 9e Module No. 7 – Ceramics 4 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Table 1.0 summarizes the rock salt, cesium chloride, zinc blende, fluorite, and perovskite crystal structures in terms of cation–anion ratios and coordination numbers and gives examples for each. Of course, many other ceramic crystal structures are possible. Table 1.0 Summary of Some Common Ceramic Crystal Structure Mechanical Properties of Ceramics The properties of ceramics, however, also depend on their microstructure. Ceramics are by definition natural or synthetic inorganic, non-metallic, polycrystalline materials. Sometimes, even monocrystalline materials, such as diamond and sapphire, are erroneously included under the term ceramics. Polycrystalline materials are formed by multiple crystal grains joined together during the production process, whereas monocrystalline materials are grown as one three- dimensional crystal. Fabrication processes of polycrystalline materials are relatively inexpensive, when compared to single crystals. Due to these differences (e.g., multiple crystals with various orientations, presence of grain boundaries, fabrication processes), polycrystalline materials should really not be confused with single crystals and should be the only ones included under the definition of ceramics. The properties and the processing of ceramics are largely affected by their grain sizes and shapes, and characteristics such as density, hardness, mechanical strength, and optical properties strongly correlate with the microstructure of the sintered piece. Typical Properties of Ceramics High hardness High elastic modulus Low ductility High dimensional stability Good wear resistance High resistance to corrosion High weather resistance High melting point High working temperature Low thermal expansion Module No. 7 – Ceramics 5 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Low to medium thermal conductivity Good electrical insulation Low to medium tensile strength High compressive strength Classification of Ceramics It is used in completely different kinds of applications and, in this regard, tend to complement each other and also the polymers. Most ceramic materials fall into an application– classification scheme that includes the following groups: glasses, structural clay products, white wares, refractories, abrasives, cements, ceramic biomaterials, carbons, and the newly developed advanced ceramics. Figure 6. Classification of ceramic materials on the basis of application. Adapted from Fig. 13.1, Callister 9e Glass The glasses are a familiar group of ceramics; containers, lenses, and fiberglass represent typical applications. As already mentioned, they are noncrystalline silicates containing other oxides, notably CaO, Na2O, K2O, and Al2O3, which influence the glass properties. A typical soda- lime glass consists of approximately 70 wt% SiO 2, the balance being mainly Na2O (soda) and CaO (lime). Possibly the two prime assets of these materials are their optical transparency and the relative ease with which they may be fabricated. Glass-ceramic Most inorganic glasses can be made to transform from a non-crystalline state into one that is crystalline by the proper high-temperature heat treatment. This process is called crystallization, and the product is a fine-grained polycrystalline material that is often called a glass-ceramic. Glass-ceramic materials have been designed to have the following characteristics: relatively high mechanical strengths; low coefficients of thermal expansion (to avoid thermal Module No. 7 – Ceramics 6 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G shock); good high-temperature capabilities; good dielectric properties (for electronic packaging applications); and good biological compatibility. Some glass- ceramics may be made optically transparent; others are opaque. Possibly the most attractive attribute of this class of materials is the ease with which they may be fabricated; conventional glass-forming techniques may be used conveniently in the mass production of nearly pore-free ware. Glass–ceramics are manufactured commercially under the trade names of Pyroceram, CorningWare, Cercor, and Vision. The most common uses for these materials are as ovenware, tableware, oven windows, and range tops—primarily because of their strength and excellent resistance to thermal shock. They also serve as electrical insulators and as substrates for printed circuit boards and are used for architectural cladding and for heat exchangers and regenerators. Table 2. Composition and Characteristics of Some Common Commercial Glasses Adapted from Table 13.1, Callister 9e CLAY PRODUCTS One of the most widely used ceramic raw materials is clay. This inexpensive ingredient, found naturally in great abundance, often is used as mined without any upgrading of quality. Another reason for its popularity lies in the ease with which clay products may be formed; when mixed in the proper proportions, clay and water form a plastic mass that is very amenable to shaping. The formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength. Most clay-based products fall within two broad classifications: the structural clay products and whitewares. Structural clay products include building bricks, tiles, and sewer pipes—applications in which structural integrity is important. Whiteware ceramics become white after high-temperature firing. Included in this group are porcelain, pottery, tableware, china, and plumbing fixtures (sanitary ware). In addition to clay, Module No. 7 – Ceramics 7 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G many of these products also contain nonplastic ingredients, which influence the changes that take place during the drying and firing processes and the characteristics of the finished piece REFRACTORIES Another important class of ceramics that are used in large tonnages is the refractory ceramics. The salient properties of these materials include the capacity to withstand high temperatures without melting or decomposing and the capacity to remain unreactive and inert when exposed to severe environments. In addition, the ability to provide thermal insulation is often an important consideration. Refractory materials are marketed in a variety of forms, but bricks are the most common. Typical applications include furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, and power generation. On this basis, there are several classifications—fireclay, silica, basic, and special refractories. Compositions for a number of commercial refractories are listed in Table 13.2. For many commercial materials, the raw ingredients consist of both large (or grog) particles and fine particles, which may have different compositions. Upon firing, the fine particles normally are involved in the formation of a bonding phase, which is responsible for the increased strength of the brick; this phase may be predominantly either glassy or crystalline. The service temperature is normally below that at which the refractory piece was fired. Table 3. Composition of Five Common Ceramic Refractory Materials Adapted from Table 13.2, Callister 9e Fireclay Refractories Fireclay bricks are used principally in furnace construction to confine hot atmospheres and to thermally insulate structural members from excessive temperatures. For fireclay brick, strength is not ordinarily an important consideration because support of structural loads is usually not required. Some control is normally maintained over the dimensional accuracy and stability of the finished product. Silica Refractories Module No. 7 – Ceramics 8 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G The prime ingredient for silica refractories, sometimes termed acid refractories, is silica. These materials, well known for their high-temperature load-bearing capacity, are commonly used in the arched roofs of steel- and glass-making furnaces; for these applications, temperatures as high as 1650 0C (30000F) may be realized. Under these conditions, some small portion of the brick actually exists as a liquid. The presence of even small concentrations of alumina has an adverse influence on the performance of these refractories. These refractory materials are also resistant to slags that are rich in silica (called acid slags) and are often used as containment vessels for them. However, they are readily attacked by slags composed of a high proportion of CaO and/or MgO (basic slags), and contact with these oxide materials should be avoided. Basic Refractories The refractories that are rich in periclase, or magnesia (MgO), are termed basic; they may also contain calcium, chromium, and iron compounds. The presence of silica is deleterious to their high-temperature performance. Basic refractories are especially resistant to attack by slags containing high concentrations of MgO and CaO and find extensive use in some steel-making open hearth furnaces. Special Refractories Yet other ceramic materials are used for rather specialized refractory applications. Some of these are relatively high-purity oxide materials, many of which may be produced with very little porosity. Included in this group are alumina, silica, magnesia, beryllia (BeO), zirconia (ZrO2), and mullite (3Al2O3–2SiO2). Others include carbide compounds, in addition to carbon and graphite. Silicon carbide (SiC) has been used for electrical resistance heating elements, as a crucible material, and in internal furnace components. Carbon and graphite are very refractory, but find limited application because they are susceptible to oxidation at temperatures in excess of about 800C (1470F). As would be expected, these specialized refractories are relatively expensive. ABRASIVES Abrasive ceramics are used to wear, grind, or cut away other material, which necessarily is softer. Therefore, the prime requisite for this group of materials is hardness or wear resistance; in addition, a high degree of toughness is essential to ensure that the abrasive particles do not easily fracture. Furthermore, high temperatures may be produced from abrasive frictional forces, so some refractoriness is also desirable. Diamonds, both natural and synthetic, are used as abrasives; however, they are relatively expensive. The more common ceramic abrasives include silicon carbide, tungsten carbide (WC), aluminum oxide (or corundum), and silica sand. Coated abrasives are those in which an abrasive powder is coated on some type of paper or cloth material; sandpaper is probably the most familiar example. Wood, metals, ceramics, and plastics are all frequently ground and polished using this form of abrasive. Grinding, lapping, and polishing wheels often employ loose abrasive grains that are delivered in some type of oil- or water-based vehicle. Diamonds, corundum, silicon carbide, and rouge (an iron oxide) are used in loose form over a variety of grain size ranges. Module No. 7 – Ceramics 9 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G CEMENTS Several familiar ceramic materials are classified as inorganic cements: cement, plaster of Paris, and lime, which, as a group, are produced in extremely large quantities. The characteristic feature of these materials is that when mixed with water, they form a paste that subsequently sets and hardens. Of this group of materials, Portland cement is consumed in the largest tonnages. It is produced by grinding and intimately mixing clay and lime-bearing minerals in the proper proportions and then heating the mixture to about 14000C (25500F) in a rotary kiln; this process, sometimes called calcination, produces physical and chemical changes in the raw materials. The resulting “clinker” product is then ground into a very fine powder, to which is added a small amount of gypsum (CaSO4–2H2O) to retard the setting process. This product is Portland cement. The properties of Portland cement, including setting time and final strength, to a large degree depend on its composition. CARBONS Diamond The physical properties of diamond are extraordinary. Chemically, it is very inert and resistant to attack by a host of corrosive media. Of all known bulk materials, diamond is the hardest—as a result of its extremely strong interatomic sp3 bonds. In addition, of all solids, it has the lowest sliding coefficient of friction. Its thermal conductivity is extremely high, its electrical properties are notable, and, optically, it is transparent in the visible and infrared regions of the electromagnetic spectrum—in fact, it has the widest spectral transmission range of all materials. The high index of refraction and optical brilliance of single crystals makes diamond a most highly valued gemstone. Graphite Graphite is highly anisotropic—property values depend on crystallographic direction along which they are measured. Graphite is very soft and flaky, and has a significantly smaller modulus or elasticity. Its in-plane electrical conductivity is 1016 to 1019 times that of diamond, whereas thermal conductivities are approximately the same. Furthermore, whereas the coefficient of thermal expansion for diamond is relatively small and positive, graphite’s in-plane value is small and negative, and the plane-perpendicular coefficient is positive and relatively large. Furthermore, graphite is optically opaque with a black–silver color. Other desirable properties of graphite include good chemical stability at elevated temperatures and in nonoxidizing atmospheres, high resistance to thermal shock, high adsorption of gases, and good machinability. Applications for graphite are many, varied, and include lubricants, pencils, battery electrodes, friction materials (e.g., brake shoes), heating elements for electric furnaces, welding electrodes, metallurgical crucibles, high-temperature refractories and insulations, rocket nozzles, chemical reactor vessels, electrical contacts (e.g., brushes), and air purification devices. Carbon Fibers Small-diameter, high-strength, and high-modulus fibers composed of carbon are used as reinforcements in polymer-matrix composites. Carbon in these fiber materials is in the form of graphene layers. However, depending on precursor (i.e., material from which the fibers are made) Module No. 7 – Ceramics 10 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G and heat treatment, different structural arrangements of these graphene layers exist. For what are termed graphitic carbon fibers, the graphene layers assume the ordered structure of graphite—planes are parallel to one another having relatively weak van der Waals interplanar bonds. Because most of these fibers are composed of both graphitic and turbostratic forms, the term carbon rather than graphite is used to denote these fibers. Of the three most common reinforcing fiber types used for polymer-reinforced composites (carbon, glass, and aramid), carbon fibers have the highest modulus of elasticity and strength; in addition, they are the most expensive. Table 4. properties of Diamond, Graphite and Carbon (for Fibers) Adapted from Table 13.3, Callister 9e ADVANCED CERAMICS Although the traditional ceramics discussed previously account for the bulk of production, the development of new and what are termed advanced ceramics has begun and will continue to establish a prominent niche in advanced technologies. In particular, electrical, magnetic, and optical properties and property combinations unique to ceramics have been exploited in a host of new products. Advanced ceramics include materials used in microelectromechanical systems as well as the nanocarbons (fullerenes, carbon nanotubes, and graphene) Microelectromechanical Systems (MEMS) Microelectromechanical systems (abbreviated MEMS) are miniature “smart” systems consisting of a multitude of mechanical devices that are integrated with large numbers of electrical elements on a substrate of silicon. The mechanical components are microsensors and micro- actuators. Module No. 7 – Ceramics 11 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G The processing of MEMS is virtually the same as that used for the production of silicon- based integrated circuits; this includes photolithographic, ion implantation, etching, and deposition technologies, which are well established. In addition, some mechanical components are fabricated using micromachining techniques. MEMS components are very sophisticated, reliable, and minuscule in size. Furthermore, because the preceding fabrication techniques involve batch operations, the MEMS technology is very economical and cost effective. There are some limitations to the use of silicon in MEMS. Silicon has a low fracture toughness (0.90 MPa1m) and a relatively low softening temperature (600C) and is highly active to the presence of water and oxygen. Consequently, research is being conducted into using ceramic materials—which are tougher, more refractory, and more inert—for some MEMS components, especially high-speed devices and nanoturbines. One example of a practical MEMS application is an accelerometer (accelerator/ decelerator sensor) that is used in the deployment of air-bag systems in automobile crashes. For this application, the important microelectronic component is a free-standing microbeam. Compared to conventional air-bag systems, the MEMS units are smaller, lighter, and more reliable and are produced at a considerable cost reduction. Potential MEMS applications include electronic displays, data storage units, energy conversion devices, chemical detectors (for hazardous chemical and biological agents and drug screening), and microsystems for DNA amplification and identification. There are undoubtedly many unforeseen uses of this MEMS technology that will have a profound impact on society; these will probably overshadow the effects that microelectronic integrated circuits have had during the past three decades. Nanocarbons A class of recently discovered carbon materials, the nanocarbons, have novel and exceptional properties, are currently being used in some cutting-edge technologies, and will certainly play an important role in future high-tech applications. Three nanocarbons that belong to this class are fullerenes, carbon nanotubes, and graphene. The “nano” prefix denotes that the particle size is less than about 100 nanometers. Fullerenes One type of fullerene, discovered in 1985, consists of a hollow spherical cluster of 60 carbon atoms; a single molecule is denoted by C60. Carbon atoms bond together so as to form both hexagonal (six-carbon atom) and pentagonal (five-carbon atom) geometrical configurations. Material composed of C60 molecules is known as buckminsterfullerene, (or buckyball for short), named in honor of R. Buckminster Fuller, who invented the geodesic dome; each C60 is simply a molecular replica of such a dome. The term fullerene is used to denote the class of materials that are composed of this type of molecule. Uses and potential applications of fullerenes include antioxidants in personal care products, biopharmaceuticals, catalysts, organic solar cells, long-life batteries, high-temperature superconductors, and molecular magnets. Carbon Nanotubes Another molecular form of carbon has recently been discovered that has some unique and technologically promising properties. Its structure consists of a single sheet of graphite (i.e., Module No. 7 – Ceramics 12 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G graphene) that is rolled into a tube. The term single-walled carbon nanotube (abbreviated SWCNT) is used to denote this structure. Each nanotube is a single molecule composed of millions of atoms; the length of this molecule is much greater (on the order of thousands of times greater) than its diameter. Multiple-walled carbon nanotubes (MWCNTs) consisting of concentric cylinders also exist. Nanotubes are extremely strong and stiff and relatively ductile. For single-walled nanotubes, measured tensile strengths range between 13 and 53 GPa (approximately an order of magnitude greater than for carbon fibers—viz. 2 to 6 GPa); this is one of the strongest known materials. Elastic modulus values are on the order of one terapascal [TPa (1 TPa =103 GPa)], with fracture strains between about 5% and 20%. Furthermore, nanotubes have relatively low densities. Carbon nanotubes also have unique and structure-sensitive electrical characteristics. Depending on the orientation of the hexagonal units in the graphene plane (i.e., tube wall) with the tube axis, the nanotube may behave electrically as either a metal or a semiconductor. As a metal, they have the potential for use as wiring for small-scale circuits. In the semiconducting state they may be used for transistors and diodes. Furthermore, nanotubes are excellent electric field emitters. As such, they can be used for flat-screen displays (e.g., television screens and computer monitors). Graphene Graphene, the newest member of the nanocarbons, is a single-atomic-layer of graphite, composed of hexagonally sp2 bonded carbon atoms (Figure 13.9). These bonds are extremely strong, yet flexible, which allows the sheets to bend. Two characteristics of graphene make it an exceptional material. First is the perfect order found in its sheets—no atomic defects such as vacancies exist; also these sheets are extremely pure—only carbon atoms are present. The second characteristic relates to the nature of the unbonded electrons: at room temperature, they move much faster than conducting electrons in ordinary metals and semiconducting materials. In terms of its properties graphene could be labeled the ultimate material. It is the strongest known material (~130 GPa), the best thermal conductor (~5000 W/mK), and has the lowest electrical resistivity (10-8 Ωm)—that is, is the best electrical conductor. Furthermore, it is transparent, chemically inert, and has a modulus of elasticity comparable to the other nanocarbons (~1 TPa). Given this set of properties, the technological potential for graphene is enormous, and it is expected to revolutionize many industries to include electronics, energy, transportation, medicine/biotechnology, and aeronautics. However, before this revolution can begin to be realized, economical and reliable methods for the mass production of graphene must be devised. The following is a short list of some of these potential applications for graphene: electronics— touch-screens, conductive ink for electronic printing, transparent conductors, transistors, heat sinks; energy—polymer solar cells, catalysts in fuel cells, battery electrodes, supercapacitors; medicine/biotechnology—artificial muscle, enzyme and DNA biosensors, photoimaging; aeronautics—chemical sensors (for explosives) and nanocomposites for aircraft structural components. Module No. 7 – Ceramics 13 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Table 5. Properties of Nanocarbon Adapted from Table 13.4, Callister 9e Fabrication and Processing of Ceramics Because ceramic materials have relatively high melting temperatures, casting them is normally impractical. Furthermore, in most instances the brittleness of these materials precludes deformation. Some ceramic pieces are formed from powders (or particulate collections) that must ultimately be dried and fired. Glass shapes are formed at elevated temperatures from a fluid mass that becomes very viscous upon cooling. Cements are shaped by placing into forms a fluid paste that hardens and assumes a permanent set by virtue of chemical reactions. A taxonomical scheme for the several types of ceramic-forming techniques is presented in Figure 13.10. Figure 7. Classification Scheme for the ceramic-forming technique. Adapted from Figure 13.10, Callister 9e Module No. 7 – Ceramics 14 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Glass Forming Glass is produced by heating the raw materials to an elevated temperature above which melting occurs. Most commercial glasses are of the silica-soda-lime variety; the silica is usually supplied as common quartz sand, whereas Na 2O and CaO are added as soda ash (Na2CO3) and limestone (CaCO3). For most applications, especially when optical transparency is important, it is essential that the glass product be homogeneous and pore free. Homogeneity is achieved by complete melting and mixing of the raw ingredients. Porosity results from small gas bubbles that are produced; these must be absorbed into the melt or otherwise eliminated, which requires proper adjustment of the viscosity of the molten material. Different forming methods are used to fabricate glass products: Pressing Pressing is used in the fabrication of relatively thick-walled pieces such as plates and dishes. The glass piece is formed by pressure application in a graphite-coated cast iron mold having the desired shape; the mold is typically heated to ensure an even surface. Blowing Although some glass blowing is done by hand, especially for art objects, the process has been completely automated for the production of glass jars, bottles, and light bulbs. From a raw gob of glass, a parison, or temporary shape, is formed by mechanical pressing in a mold. This piece is inserted into a finishing or blow mold and forced to conform to the mold contours by the pressure created from a blast of air Drawing Drawing is used to form long glass pieces that have a constant cross section, such as sheet, rod, tubing, and fibers Figure 8. The press and blow method for producing a glass bottle. Adapted from C. J. Phillips, Glass: The Miracle Maker. Reproduced by permission of Pitman Publishing Ltd., London Module No. 7 – Ceramics 15 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Figure 9. Schematic diagram showing the float process for making glass. Source: Pilkington Group Limited FABRICATION AND PROCESSING OF CLAY PRODUCTS The as-mined raw materials usually have to go through a milling or grinding operation in which particle size is reduced; this is followed by screening or sizing to yield a powdered product having a desired range of particle sizes. For multicomponent systems, powders must be thoroughly mixed with water and perhaps other ingredients to give flow characteristics that are compatible with the particular forming technique. The formed piece must have sufficient mechanical strength to remain intact during transporting, drying, and firing operations. Two common shaping techniques are used to form clay-based compositions: hydroplastic forming and slip casting. Hydroplastic Forming The most common hydroplastic forming technique is extrusion, in which a stiff plastic ceramic mass is forced through a die orifice having the desired cross-sectional geometry; it is similar to the extrusion of metals. Brick, pipe, ceramic blocks, and tiles are all commonly fabricated using hydroplastic forming. Usually the plastic ceramic is forced through the die by means of a motor-driven auger, and often air is removed in a vacuum chamber to enhance the density. Hollow internal columns in the extruded piece (e.g., building brick) are formed by inserts situated within the die. Slip Casting Another forming process used for clay-based compositions is slip casting. A slip is a suspension of clay and/or other nonplastic materials in water. When poured into a porous mold (commonly made of plaster of Paris), water from the slip is absorbed into the mold, leaving behind a solid layer on the mold wall, the thickness of which depends on the time. This process may be continued until the entire mold cavity becomes solid (solid casting). Alternatively, it may be Module No. 7 – Ceramics 16 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G terminated when the solid shell wall reaches the desired thickness, by inverting the mold and pouring out the excess slip; this is termed drain casting (Figure 13.17b). As the cast piece dries and shrinks, it pulls away (or releases) from the mold wall; at this time, the mold may be disassembled and the cast piece removed. Drying and Firing A ceramic piece that has been formed hydroplastically or by slip casting retains significant porosity and has insufficient strength for most practical applications. In addition, it may still contain some of the liquid (e.g., water) that was added to assist in the forming operation. This liquid is removed in a drying process; density and strength are enhanced as a result of a high-temperature heat treatment or firing procedure. A body that has been formed and dried but not fired is termed green. Drying and firing techniques are critical inasmuch as defects that ordinarily render the ware useless (e.g., warpage, distortion, cracks) may be introduced during the operation. These defects normally result from stresses that are set up from nonuniform shrinkage. Drying As a clay-based ceramic body dries, it also experiences some shrinkage. In the early stages of drying, the clay particles are virtually surrounded by and separated from one another by a thin film of water. As drying progresses and water is removed, the interparticle separation decreases, which is manifested as shrinkage. Module No. 7 – Ceramics 17 E N G G 4 1 2 : M A T E R I A L S S C I E N C E A N D E N G I N E E R I N G Firing After drying, a body is usually fired at a temperature between 900 0C and 14000C (16500F and 25500F); the firing temperature depends on the composition and desired properties of the finished piece. During the firing operation, the density is further increased (with an attendant decrease in porosity) and the mechanical strength is enhanced. POWDER PRESSING Another important and commonly used method that warrants brief treatment is powder pressing. Powder pressing—the ceramic analogue to powder metallurgy—is used to fabricate both clay and nonclay compositions, including electronic and magnetic ceramics, as well as some refractory brick products. In essence, a powdered mass, usually containing a small amount of water or other binder, is compacted into the desired shape by pressure. The degree of compaction is maximized and the fraction of void space is minimized by using coarse and fine particles mixed in appropriate proportions. There is no plastic deformation of the particles during compaction, as there may be with metal powders. There are three basic powder-pressing procedures: uniaxial, isostatic (or hydrostatic), and hot pressing. 1. For uniaxial pressing, the powder is compacted in a metal die by pressure that is applied in a single direction. The formed piece takes on the configuration of the die and platens through which the pressure is applied. This method is confined to shapes that are relatively simple; however, production rates are high and the process is inexpensive. 2. For isostatic pressing, the powde