Chapter 8
Ceramics, Graphite, and Diamond: Structure, General Properties, and Applications QUALITATIVE PROBLEMS
8.19 Explain why ceramics are weaker in tension than in compression. Ceramics are very sensitive to cracks, impurities, and porosity, and thus generally have low toughness. toughness. In compression, compression, however, however, the flaws in the material material do not cause stress concentrations or crack propagation, as they do in tension. 8.20 What What are the advan advantag tages es of cermets cermets? ? Suggest Suggest applicat application ionss in additi addition on to those given in this chapter. High-temperature oxidation resistance and toughness are the advantageous properties of cermets. They also may be useful for components in furnaces (such as moving belts) and automotive-engine parts (such as pistons and cylinders). 8.21 Explain why the electrical and thermal conductivity of ceramics decreases with increasing porosity. Pores in the ceramic are usually filled with air, and air has much lower thermal and electrical electrical conductivity conductivity than ceramics. ceramics. Increasing Increasing porosity will thus thus reduce reduce the ceramic’s thermal and electrical conductivities. 8.22 Explain why the mechanical property data given in Table 8.2 have such a broad range. What is the significance significance of this in engineering engineering practice? The properties given in Table 8.2 on p. 201 vary greatly because the mechanical properties of ceramics depend greatly on the quality of the raw material, porosity in the product, product, and the manner in which the product is made. Engineering Engineering applications applications that require high mechanical properties must ensure that the material quality and processing of the part are the best available. 8.23 Descr Describe ibe the reasons reasons that that have have encoura encouraged ged the develop developmen mentt of synthet synthetic ic diamond. 92 © 2014 Pearson Education, Inc. Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction ,storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to : Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
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By the student. Synthetic diamonds were first used exclusively in industrial applications; they have superior properties to natural diamonds because of the absence of impurities. The hardness of diamond makes it useful in machining and grinding hard metallic and nonmetallic materials. For widespread and economical use, diamonds are mass produced synthetically. 8.24 Explain why the mechanical properties of ceramics generally differ from those of metals. Metals and ceramics have different types of bonding, metallic and ionic, respectively. Ionic bonds are stronger than metallic bonds, hence more force must be applied to the material to break the bonds. The crystal structure for ceramics is generally more complex than metals, and requires higher forces to cause slip. Also, the high-temperature properties of ceramics are very attractive in many applications. 8.25 Explain how ceramics can be made tougher. Ceramics may be made tougher by using high-purity raw materials, controlled processing techniques, and adding various reinforcements. The presence of tougher second phases and microcracks (less than one µm in size) in the ceramic can also reduce the energy of propagation of an advancing crack tip. (See also partially stabilized zirconia, Section 8.2.2 on p. 196.) 8.26 List and describe situations in which static fatigue can be important. By the student. There are numerous possible answers. Static fatigue occurs in environments where water vapor is present and the part is under a constant load. In applications such sewer piping, if a tensile stress is developed in the pipe by bending or torsion, static fatigue can be a significant problem. Any situation where a tension member is exposed to water vapor (such as mounting brackets or structural members outdoors) are also subject to static fatique. Some MEMS devices see failure of borosilicate glass layers due to static fatigue as well. 8.27 What properties are important in making heat-resistant ceramics for use on oven tops? Why? Heat-resistant ceramics for oven tops should possess hot corrosion resistance to maintain their appearance, hardness for wear and scratch resistance, and toughness for impact loading. Because the heating of oven top is uneven, thermal gradients will be set up across the surface. To minimize the possibility for thermal cracking, the oven top should have low coefficient of thermal expansion and high thermal conductivity. 8.28 A large variety of glasses is now available. Why is this so? By the student. The cost of each type of glass varies, and a high-quality glass (high cost) shouldn’t be used in a low-quality application. However, high quality glasses are required for fiber optics, which need to transmit light with high efficiency. Other applications may require high strength, low cost, chemical resistance, impact resistance, abrasion resistance or resistance to thermal shock, which would be best exhibited by 96% silica or fused-silica type glass. Thus, in summary, there are a large variety of glasses available because of the large variety of applications where glass is used. 8.29 What is the difference between the structure of graphite and that of diamond? Is it important? Explain.
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Ceramics, Graphite, and Diamond: Structure, General Properties, and Applications
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Graphite has a crystalline form of a layered structure of basal planes of close-packed carbon atoms (see Fig. 1.5 on p. 41). Diamond, on the other hand, has a covalently bonded structure. The structure of graphite permits easy slip on basal planes, while the covalent bonding of diamond greatly restricts slip, thus making it very hard. 8.30 List and explain materials that are suitable for use as a coffee cup. By the student. A coffee cup must obviously be made of a material that is safe for food contact, so it should not affect or be affected by coffee. In addition, it should be inexpensive. Furthermore, it must be thermally insulating, have a melting point higher than the boiling point of water, and be easy to clean. Ceramics, polymers, porous polymers (foams), and multi-layered metals (with air or a vacuum between layers) are thus suitable. 8.31 Aluminum oxide and PSZ are described as white in appearance. Can they be colored? If so, how would you accomplish this? Both aluminum oxide and partially stabilized zirconia can be produced in a variety of colors. This can be done by using impurities in small concentrations in the ceramic, which develop a color. There is, of course, also the option of enameling, painting, or coloring the surfaces. 8.32 Why does the strength of a ceramic part depend on its size? Ceramic strength is mainly compromised by the presence of flaws where cracks can initiate. In a small volume, there is less likelihood that a large flaw or a number of flaws can exist, while the reverse is true for larger parts, i.e., in a large volume, it is more likely that a large flaw will exist (see also the discussion regarding Fig. 2.11 on p. 67). 8.33 In old castles and churches in Europe, the glass windows display pronounced ripples and are thicker at the bottom than at the top. Explain. The silica in the glass is not really solid; it is merely a supercooled liquid. Over centuries, the glass has creeps or flows due to the force of gravity. This situation depends greatly on the type of glass, as some will creep faster than others. 8.34 Is a carbide an example of a composite material? Explain. This depends on the extent to which the definition is applied. Cobalt is often the matrix, with tungsten-carbide particles serving as a discontinuous phase. Two-phased metal allows, where the second phase is insoluble in the first, is a similar structure, but alloys are not generally considered to be composite materials. 8.35 Ceramics are hard and strong in b oth compression and shear. Why, then, are they not used as nails or other fasteners? Explain. Ceramics aren’t used as nails or other fasteners because they have poor impact resistance and thus would not be able to be driven into a workpiece with impact forces as applied by a hammer. Other fasteners such as bolts depend on a tensile proof stress (the proof strength of a bolt is a common design specification) for a well-designed joint, and this would be a poor design application for a ceramic which is weak and has a wide range of strengths in tension. 8.36 Perform an Internet search and determine the chemistry of glass used for (a) fiber-optic communication lines, (b) crystal glassware, and (c) high-strength glass fibers.
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By the student. There are a large number of answers possible. For example, fiberoptic communication lines will be graded according to their intended distance for signal transmission, and crystal glassware will have a variety of formulations. 8.37 Investigate and list the ceramics used for high-temperature superconductor applications. By the student. This is an area of significant ongoing development. In recent years, significant progress has been made in understanding high-temperature superconducting materials and their potential use as conductors. Two bismuth based oxides (Bi2 Sr2 CaCu2 O , also known as Bi-2212, and Bi2 Sr2 CaCu3 O or Bi-2223) are superconducting, ceramic materials of choice for various military and commercial applications, such as electrical propulsion for ships and submarines, shallow-water and ground minesweeping systems, transmission cable generators, and superconducting magnetic-energy storage (SMES). However, wide variations in the electrical performance of test magnet coils using monofilament superconducting strips indicate that multifilament conductors are essential in reliably achieving the required critical current densities, so that property variations are averaged over a length. Tape stacking is a new approach to manufacturing multifilament tape. In this method, 7 to 10 monofilament tapes or strips are stacked, heated, and pressed together; the resulting laminate is then rolled to final thickness. The quality and performance of multifilament tapes are directly related to that of the constituents i.e. the monofilament tape. A variety of processes have been explored to produce wires and tapes, using Bi-2212 and Bi-2223. x
x
8.38 Explain why synthetic diamond gemstones are not appreciably less expensive than natural diamond gemstones. This is surprising at first. There is significant labor involved in mining natural diamond, and then an addition cost to ship them to markets. However, there is also significant cost involved in growing artificial diamond. However, gemstones require significant polishing before they are useful, and facets in the stone must be ground and polished with great precision. The vast ma jority of the cost associated with gemstones is associated with the polishing processes involved.
QUANTITATIVE PROBLEMS
8.39 In a fully dense ceramic, UTS = 200 MPa and E = 330 GPa. What are these properties at 15% porosity for values of n = 4, 5, 6, and 7, respectively? Inserting the appropriate quantities into the Eqs. (8.1) and (8.2) on p. 202, we obtain the following: o
n 4 5 6 7
UTS (MPa) 109 94.5 81.3 70.0
o
E (GPa) 242.6 242.6 242.6 242.6
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Ceramics, Graphite, and Diamond: Structure, General Properties, and Applications
Note that the magnitude of n does not affect the magnitude of E . 8.40 Plot the UTS, E , and k values for ceramics as a function of porosity P . Describe and explain the trends that you observe in their behavior. Equations (8.1) through (8.3) on pp. 201-202 are needed to solve this problem. The curves can be obtained using an assumed value at zero porosity, or else they can be non-dimensionalized, as shown below. The plots are as follows: 1
1.0
0.8
n
o
n
S T0.6 U / S 0.4 T U0.2
0
=4
1.0 0.9
0.8
=5 =6
n
o
n
E / E
=7
o
k / k
0.6 0.4
0
10
20
30 40
0.2
50
Porosity (%)
0.8 0.7 0.6
0
10
20
30 40
Porosity (%)
50
0.5
0
10
20
30 40
Porosity (%)
There are several observations that can be made. Most striking is the near-linear behavior of stiffness and thermal conductivity with respect to porosity, and the highly nonlinear behavior of strength. Thus, to produce high-strength ceramics, reduction of porosity is especially important. 8.41 What would be the tensile strength and the modulus of elasticity of the ceramic in Problem 8.39 for porosities of 25% and 50%, for the four n values given? Equations (8.1) and (8.2) on p. 202 are needed to solve this problem. Inserting the appropriate quantities into these equations, we obtain the following: n 4 5 6 7
UTS (MPa) P = 0.25 P = 0.50 73.6 27.0 57.3 16.4 44.6 9.95 34.7 6.09
The modulus of elasticity values are as follows: for P = 0.25, E = 191 GPa, and for P = 0.50, E = 90.7 GPa. Note that the porosities that are examined in this problem are far outside those normally encountered with ceramics. The equations for strength and stiffness are not very applicable to this extreme range. However, students can be encouraged to discuss the validity of the approach and whether or not the values obtained are realistic. 8.42 Calculate the thermal conductivities for ceramics at porosities of 10%, 20%, and 40% for k = 0.7 W/mK. Equation (8.3) on p. 202 is needed to solve this problem. Inserting the values into the equation, we obtain thermal conductivities of: o
P = 10% P = 20% P = 40%
k = 0.63 W/mK k = 0.56 W/mK k = 0.42 W/mK
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50
Ceramics, Graphite, and Diamond: Structure, General Properties, and Applications
97
8.43 A ceramic has k = 0.80 W/mK. If this ceramic is shaped into a cylinder with a porosity distribution given by P = 0.1(x/L)(1 − x/L), where x is the distance from one end of the cylinder and L is the total cylinder length, plot the porosity as a function of distance, evaluate the average porosity, and calculate the average thermal conductivity. The plot of porosity is as follows: o
0.025 0.02 y t i s o r o P
0.015 0.01 0.005 0
0
0.25
0.5 0.75 Position, x/L
1
For the remainder of the problem, use X = x/L. The average porosity is given by ¯= P
0
1
1
0.1X (1 − X )dX =
0
2
−0.1X
+ 0.1X dX = 0.0167
Since the thermal conductivity is linearly related to the porosity, the average porosity can be used, so that the average thermal conductivity is: ¯ k = k
o
1 − P ¯ = (0.80)(1 − 0.0167) = 0.787 W/mK
8.44 It can be shown that the minimum weight of a column which will support a given load depends on the ratio of the material’s stiffness to the square root of its density. Plot this property for a ceramic as a function of porosity. The stiffness of a ceramic is given by Eq. (8.2) on p. 201 as E = E (1 − 1.9P + 0.9P 2 ). The density is given by ρ = ρ (1 − P ). Therefore, the desired quantity is: o
o
E 1 − 1.9P + 0.9P 2 E √ ρ = ρ (1 − P ) o
o
The plot is as shown below in dimensionless form.
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Ceramics, Graphite, and Diamond: Structure, General Properties, and Applications
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1 0.9 0.8 E/Eo
ρ/ρo
0.7 0.6 0.5 0.4 0.3 0
10
20 30 Porosity (%)
40
50
SYNTHESIS, DESIGN AND PROJECTS
8.45 Make a list of the ceramic parts that you can find around your house or in your car. Give reasons why those parts are made of ceramics. By the student. Some examples are: Bathroom fixtures: they will not discolor or corrode in normal use, are hard enough to resist the abrasive action of cleaning powders, and are relatively inexpensive. Coffee mugs: their smooth finish makes them easy to clean, are corrosion resistant, and are inexpensive. Light-fixture components: electric insulators and resistant to heat. Spark plugs: thermal and electrical insulation and corrosion resistance. 8.46 Assume that you are working in technical sales and are fully familiar with all the advantages and limitations of ceramics. Which of the markets traditionally using nonceramic materials do you think ceramics can p enetrate? What would you like to talk about to your potential customers during your sales visits? What questions do you think they may ask you about ceramics? By the student. There are a number of acceptable answers to this question, and students should not be restricted to the answer given here. Applications that require high hot strength and wear and corrosion resistance in components such as car engines. Beneficial features of properties should be pointed out, along with recent trends in the improvement of properties. All the advantages that ceramics have over the current material should also be pointed out. Questions concerning the brittleness and low toughness would also be raised, as well as questions concerning the consistency of the quality of ceramic parts, particularly as their size and shape complexity increases. 8.47 Describe applications in which a ceramic material with a near-zero coefficient of thermal expansion would be desirable. By the student. A ceramic material with a near-zero coefficient of thermal expansion will have a lower tendency of thermal cracking when exposed to temperature gradients. This property would be useful in applications where the ceramic would be cycled through temperature ranges, as with space shuttle tiles and cutting tools in machining, particularly in interrupted cutting operations such as milling.
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8.48 The modulus of elasticity of ceramics is typically maintained at elevated temperatures. What engineering applications could benefit from this characteristic? By the student. Note that by retaining their high stiffness at elevated temperatures, dimensional accuracy can be maintained. Some examples are bearings, cutting tools, turbine blades, machine-tool components, and electronics. The student is encouraged to expand on this answer. 8.49 List and discuss the factors that you would take into account when replacing a metal component with a ceramic component in a specific product. By the student. Note, for example, that the main limitations of ceramics are low tensile strength and low toughness. The application of the metal component that would be replaced should, therefore, not require high tensile strength or high impact resistance. 8.50 Obtain some data from the technical literature in the Bibliography of this chapter, and show quantitatively the effects of temperature on the strength and the modulus of elasticity of several ceramics. Comment on how the shape of these curves differs from those for metals. By the student. The general trend that will be seen is that such properties as strength and stiffness do not change as drastically as in metals (see, for example, Fig. 22.1 on p. 592). 8.51 Conduct a literature search and write a brief paper summarizing the properties and potential applications of graphene. By the student. Graphene is a single sheet of graphite, and it has been suggested that it has many properties, especially in MEMS and microelectronics. 8.52 It was noted in Section 8.4.1 that there are several basic types of glasses available. Make a survey of the technical literature and prepare a table for these glasses, indicating various mechanical, physical, and optical properties. By the student. A large variety of answers are possible. For example, the following table is given in Schey, J., Introduction to Manufacturing Processes , 3d ed., p. 500, based on data from D.C. Boyd and D.A. Thompson:
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Ceramics, Graphite, and Diamond: Structure, General Properties, and Applications
Property 7940 fused silica Composition, wt% SiO2 B2 O3 Al2 O3 Na2 O K2 O Li2 O CaO MgO PbO Viscosity, at C 14 5 10 (hard) 13 10 (anneal pt.) 107 6 (soft) 104 (working) Coefficient of thermal expansion, ×10 7/ C
99.9
Corning glass works code number and type E glass 7740 1720 0800 8871 8830 boro- alumino- soda-lime- potash- borosilicate silicate silica lead silicate 54 10 14
81 13 2 4
17.5 4.5
62 5 17 1
73 1 17
8 7
5 4
42
65 23 5 7
2 6 1
49
◦
.
.
−
956 1084 1580
507 657 846
55
60
510 560 821 1252 33
667 712 915 1202 42
473 514 695 1005 92
350 385 525 785 102
460 501 708 1042 49.5
◦
8.53 Ceramic pistons are being considered for high-speed combustion engines. List the benefits and concerns that you would have regarding this application. By the student. Ceramic pistons would be advantageous in that they would have a high strength and potentially low wear. In addition, the inertial forces associated with a ceramic piston would be much lower than for a metal one, and the need for cooling the piston becomes less imperative. The main drawbacks are that the ceramic could excessively wear the cylinder liner; with three-body wear, any ceramic wear particles could cause severely damage in the engine. Also, the low fracture toughness of the ceramic may cause catastrophic failure of the engine. 8.54 It has been noted that the strength of brittle materials (such as ceramics and glasses) is very sensitive to surface defects, such as scratches (known as notch sensitivity). Obtain several pieces of these materials, scratch them, and test them by carefully clamping them in a vise and bending them. Comment on your observations. By the student. Note that special care must be taken in performing these experiments, and eye protection and the like are necessary. This experiment can be performed using a glass cutter to make a deep and sharp scratch on the glass. It can be demonstrated that glass, with such a scratch, can be easily broken with bare hands (using work gloves). Note also the direction of the bending moment with respect o the direction of the scratch. As
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Ceramics, Graphite, and Diamond: Structure, General Properties, and Applications
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a comparison, even a highly heat-treated aluminum plate will not be nearly as weakened when a similar scratch is made on its surface. 8.55 Electric space heaters for home use commonly utilize a ceramic filament as the heating element. List the required properties for this filament, explain why a ceramic is a suitable material, and perform an Internet search to determine the specific ceramic material actually utilized in this application. This is an open-ended problem, and students should be encouraged to obtain their own solutions with as specific of answers as possible. Some of the mechanical properties required for the filament are:
• High melting point • Sufficient rigidity so that it does not deflect in its retainer. • Sufficient strength to withstand thermal strains. • Low coefficient of thermal expansion so that excessive stresses or deflections do not occur during operation.
There are many types of ceramics used in such heaters, such as aluminisilicates of various types.
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