Ceramics and glasses are solids that are neither metallic nor organic (carbon-chain based) materials. Ceramics thus include clay products, such as porcelain, china, and brick, and also natural stone and concrete. Ceramics used in high-stress applications, called engineering ceramics, are often relatively simple compounds of metals, or the metalloids silicon or boron, with nonmetals such as oxygen, carbon, or nitrogen. Carbon in its graphite or diamond forms is also considered to be a ceramic. Ceramics are predominantly crystalline, whereas glasses are amorphous. Most glass is produced by melting silica (SiO2), which is ordinary sand, along with other metal oxides, such as CaO, Na2O, B2O3, and PbO.
In contrast, ceramics are usually processed not by melting, but by some other means of binding the particles of a fine powder into a solid. Specific examples of ceramics and glasses and some of their properties are given in Table 3.10. The microstructure of a polycrystalline ceramic is shown in Fig. 1.
Properties, Applications of Ceramics and Glasses
Engineering ceramics have a number of important advantages compared with metals. They are highly resistant to corrosion and wear, and melting temperatures are typically quite high. These characteristics all arise from the strong covalent or ionic–covalent chemical bonding of these compounds. Ceramics are also relatively stiff (high E) and light in weight. In addition, they are often inexpensive, as the ingredients for their manufacture are typically abundant in nature.
Slip of crystal planes does not occur readily in ceramics, due to the strength and directional nature of even partially covalent bonding and the relatively complex crystal structures. This results in ceramics being inherently brittle, and glasses are similarly affected by covalent bonding.
In ceramics, the brittleness is further enhanced by the fact that grain boundaries in these crystalline compounds are relatively weaker than in metals. This arises from disrupted chemical bonds, where the lattice planes are discontinuous at grain boundaries, and also from the existence of regions where ions of the same charge are in proximity.
In addition, there is often an appreciable degree of porosity in ceramics, and both ceramics and glasses usually contain microscopic cracks. These discontinuities promote macroscopic cracking and thus also contribute to brittle behavior.
The processing and uses of ceramics are strongly influenced by their brittleness. As a consequence, recent efforts aimed at developing improved ceramics for engineering use involve various means of reducing brittleness.
Noting the advantages of ceramics, as just listed, success in this area would be of major importance, as it would allow increased use of ceramics in applications such as automobile and jet engines, where lighter weights and operation at higher temperatures both result in greater fuel efficiency.
Various classes of ceramics will now be discussed separately as to their processing and uses.
Clay Products, Natural Stone, and Concrete
Clays consist of various silicate minerals that have a sheetlike crystal structure, an important example being kaolin, Al2O3–2SiO2–2H2O. In processing, the clay is first mixed with water to the consistency of a thick paste and then formed into a cup, dish, brick, or other useful shape.
Firing at a temperature in the range 800 to 1200◦C then drives off the water and melts some of the SiO2 to form a glass that binds the Al2O3 and the remaining SiO2 into a solid. The presence or addition of small amounts of minerals containing sodium or potassium enhances formation of the glass by permitting a lower firing temperature.
Natural stone is of course used without processing other than cutting it into useful shapes. The prior processing done by nature varies greatly. For example, limestone is principally crystalline calcium carbonate (CaCO3) that has precipitated out of ocean water, and marble is the same mineral that has been recrystallized (metamorphosed) under the influence of temperature and pressure.
Sandstone consists of particles of silica sand (SiO2) bound together by additional SiO2, or by CaCO3, which is present due to precipitation from water solution. In contrast, igneous rocks such as granite have been melted and are multiphase alloys of various crystalline minerals.
Concrete is a combination of crushed stone, sand, and a cement paste that binds the other components into a solid. The modern cement paste, called Portland cement, is made by firing a mixture of limestone and clay at 1500◦C. This forms a mixture of fine particles involving primarily lime (CaO), silica (SiO2), and alumina (Al2O3), where these are in the form of tricalcium silicate (3CaO–SiO2), dicalcium silicate (2CaO–SiO2), and tricalcium aluminate (3CaO–Al2O3).
When water is added, a hydration reaction starts during which water is chemically bound to these minerals by being incorporated into their crystal structures. During hydration, interlocking needlelike crystals form that bind the cement particles to each other and to the stone and sand. The reaction is rapid at first and slows with time. Even after long times, some residual water remains in small pores, between layers of the crystal structure, and chemically adsorbed to the surface of hydrated paste.
Clay products, natural stone, and concrete are used in great quantities for familiar purposes, including their major use in buildings, bridges, and other large stationary structures. All are quite brittle and have poor strength in tension, but reasonable strength in compression. Concrete is very ranging from 3 to 25%. Other carbides are also used in the same manner, namely, TiC, TaC, and Cr3C2, typically in combination with WC. The most frequent binder metal is cobalt, but nickel and steel are also employed.
The metal matrix of cemented carbides provides useful toughness, but limits resistance to temperature and oxidation. Ordinary ceramics, such as alumina (Al2O3) and boron nitride (BN), are also used for cutting tools and have advantages, compared with cemented carbides, of greater hardness, lighter weight, and greater resistance to temperature and oxidation. But the extra care needed in working with brittle ceramics leads to the prevalence of cemented carbides, except where ceramics cannot be avoided. Some of the advantages of ceramics can be obtained by chemical vapor deposition of a coating of a ceramic onto a cemented carbide tool. Ceramics used in this manner include TiC, Al2O3, and TiN.
Pure silica (SiO2) in crystalline form is a quartz mineral, the crystal structure of one of which is illustrated in Fig. 2.
However, when silica is solidified from a molten state, an amorphous solid results. This occurs because the molten glass has a high viscosity due to a chainlike molecular structure, which limits the molecular mobility to the extent that perfect crystals do not form upon solidification. The three-dimensional crystal structure in Fig. 2 is depicted in a simplified two dimensional form in Fig. 3
A perfect crystal, as formed from solution in nature, is represented by (a). Glass formed from molten silica has a network structure that is similar, but highly imperfect, as in (b).
In processing, glasses are sometimes heated until they melt and are then poured into molds and cast into useful shapes. Alternatively, they may be heated only until soft and then formed by rolling (as for plate glass) or by blowing (as for bottles).
Forming is made easier by the fact that the viscosity of glass varies gradually with temperature, so that the temperature can be adjusted to obtain a consistency that is appropriate to the particular method of forming.
However, for pure silica, the temperatures involved are around 1800◦C, which is inconveniently high. The temperature for forming can be lowered to around 800 to 1000◦C by adding Na2O, K2O, or CaO. These oxides are called network modifiers, because the metal ions involved tend to form non-directional ionic bonds with oxygen atoms, resulting in terminal ends in the structure, as illustrated by Fig. 3 (c) economical to use in construction and has the important advantage that it can be poured as a slurry into forms and hardened in place into complex shapes.
Improved concretes continue to be developed, including some exotic varieties with quite high strength achieved by minimizing the porosity or by adding substances such as metal or glass particles or fibers.
The processing of engineering ceramics composed of simple chemical compounds involves first obtaining the compound. For example, alumina (Al2O3) is made from the mineral bauxite (Al2O3–2H2O) by heating to remove the hydrated water.
Other engineering ceramics, such as ZrO2, are also obtainable directly from naturally available minerals. But some, such as WC, SiC, and Si3N4, must be produced by appropriate chemical reactions, starting from constituents that are available in nature.
After the compound is obtained, it is ground to a fine powder if it is not already in this form. The powder is then compacted into a useful shape, typically by cold or hot pressing. A binding agent, such as a plastic, may be used to prevent the consolidated powder from crumbling. The ceramic at this stage is said to be in a green state and has little strength. Green ceramics are sometimes machined to obtain flat surfaces, holes, threads, etc., that would otherwise be difficult to achieve.
The next and final step in processing is sintering, which involves heating the green ceramic, typically to around 70% of its absolute melting temperature. This causes the particles to fuse and form a solid that contains some degree of porosity. Improved properties result from minimizing the porosity—that is, the volume percentage of voids.
This can be done by using a gradation of particle sizes or by applying pressure during sintering. Small percentages of other ceramics may be added to the powder to improve response to processing. Also, small to medium percentages of other ceramics may be mixed with a given compound to tailor the properties of the final product.
One variation on the sintering process that aids in minimizing voids is hot isostatic pressing. This involves enclosing the ceramic in a sheet metal enclosure and placing this in a vessel that is pressurized with a hot gas. Some additional methods of processing that are sometimes used are chemical vapor deposition and reaction bonding.
The former process involves chemical reactions among hot gases that result in a solid deposit of ceramic material onto the surface of another material. Reaction bonding combines the chemical reaction that forms the ceramic compound with the sintering process.
Engineering ceramics typically have high stiffness, light weight, and very high strength in compression. Although all are relatively brittle, their strength in tension and fracture toughness may be sufficiently high that their use in high-stress structural applications is not precluded if the limitations of the material are considered in the details of the component design. Increased use of ceramics in the future is likely, due to their high-temperature capability.
Cermets; Cemented Carbides
A cermet is made from powders of a ceramic and a metal by sintering them together. The metal surrounds the ceramic particles and binds them together, with the ceramic constituent providing high hardness and wear resistance. Cemented carbides, as made into cutting tools, are the most important cermets. In this case, tungsten carbide (WC) is sintered with cobalt metal in amounts.
This change in the molecular structure also causes the glass to be less brittle than pure silica glass. Commercial glasses contain varying amounts of the network modifiers, as indicated by typical compositions in Table 2.
Other oxides are added to modify the optical or electrical properties, color, or other characteristics of glass. Some oxides, such as B2O3, can form a glass themselves and may result in a two-phase structure. Leaded glass contains PbO, in which the lead participates in the chain structure.
This modifies the glass to increase its resistivity and also gives a high index of refraction, which contributes to the brilliance of fine crystal. The addition of Al2O3 increases the strength and stiffness of the glass fibers used in fiberglass and other composite materials.
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