Brook Concise Encyclopedia of Advanced Ceramic Materials

1. Auflage 2012
ISBN: 978-0-08-098370-7
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 605 Seiten

Reihe: Advances in Materials Sciences and Engineering

Advanced ceramics cover a wide range of materials which are ceramic by nature but have been developed in response to specific requirements. This encyclopedia collects together 137 articles in order to provide an up-to-date account of the advanced ceramic field. Some articles are drawn from the acclaimed Encyclopedia of Materials Science and Engineering, often revised, and others have been newly commissioned. The Concise Encyclopedia of Advanced Ceramic Materials aims to provide a comprehensive selection of accessible articles which act as an authoritative guide to the subject. The format is designed to help the readers form opinions on a particular subject. Arranged alphabetically, with a broad subject range, the articles are diverse in character and style, thereby stimulating further discussion. Topics covered include survey articles on glass, hot pressing, insulators, powders, and many are concerned with specific chemical systems and their origins, processing and applications. The Concise Encyclopedia of Advanced Ceramic Materials will be invaluable to materials scientists, researchers, educators and industrialists working in technical ceramics.

Autoren/Hrsg.

Brook, R. J.

Leseproben

Advanced Ceramic Materials: An Overview

R.J. Brook,     Max-Planck-Institut für Metallforschung, Stuttgart, Germany

Publisher Summary

This chapter discusses the fabrication and applications of advanced ceramic materials. Ceramics make up one part of the portfolio of materials available for use and development in civilization and are in this respect complementary to the metals and polymers. Mechanical applications at ambient temperature are largely based on the combination of wear resistance, hardness, and corrosion resistance that many ceramics provide. Alongside the many advantages that ceramics bring to these uses, the principal disadvantage and impediment to more rapid exploitation has been that, while the materials can display great strength, the mode of fracture is most commonly brittle resulting in sudden and complete failure of the component. In the most idealized form, solid-state sintering, the powder consists of individual crystal grains that remain solid throughout the heat treatment. The system is heated to temperatures where diffusion of the atoms in the solid state can occur. The pattern of development of advanced ceramics has been characterized by a steadily increasing awareness of each of the linking factors between processing and microstructure and between microstructure and properties.

Ceramics make up one part of the portfolio of materials available for use and development in civilization and are in this respect complementary to the metals and polymers. The fabrication and application of ceramics are among the oldest technological skills, and modern products which bear a close and direct relationship to these ceramics include pottery, structural clay products and clay-based, heat-resistant (refractory) materials, all of which exploit the availability and properties of a particular group of nonmetallic, inorganic raw materials, the clays. These products constitute the traditional ceramics.

At a much later stage in historical development, there appeared a range of products which were similarly formed from nonmetallic, inorganic solids but which relied on very considerable raw material modification and refinement or even on the synthesis of entirely new compositional systems in order to provide properties matched to more specific and exacting requirements. Examples of these materials are the simple binary oxides such as alumina (Al2O3), magnesia (MgO) and zirconia (ZrO2), or the ternary oxides such as the spinels (compounds based on MgAl2O4) and the perovskites (those based on CaTiO3) (see ) There are also more complex oxide systems synthesized to satisfy an exact property requirement, as for example the ionically conducting material Na3.1Zr1.55Si2.3P0.7 O11. In addition, there are groups of nonoxide ceramics such as the carbides (SiC, silicon carbide or carborundum, is an important example), the nitrides (Si3N4, silicon nitride), the borides, the silicides, the halides and other such categories of solid compound (see ) With the possibility of mixtures between these types (the sialons, a set of materials based on combinations of Si3N4 with Al2O3 and other oxides, are the prime example (see )), it is apparent that the variety of such ceramic systems becomes very wide. As recognition of the differences that lie between these systems and the classical or traditional clay-based systems, a number of attempts have been made to establish a distinctive term to describe them. No usage has won general acceptance and the forms “technical,” “special,” “engineering,” “fine” and, as here, “advanced” will all be encountered. The distinction, despite its occasional convenience, should not be overemphasized; there is much to be gained from transfer of technology and experience from the “traditional” to the “advanced” sectors and vice versa.

1 Applications

The applications for which advanced ceramics have been developed and proposed are many. A first division can, however, be made into categories of materials with electrical and electronic functions, those with mechanical function at ambient temperatures and those with mechanical function at elevated temperatures.

The first application of ceramics in the electrical context was as an insulator, and electrical porcelains and aluminas sustain an important role in this connection. In the period since 1940, however, a great diversification in function has occurred with innovations in materials development and in the associated solid-state theory taking place in concert. Thus, the range of magnetic materials based on the ferrimagnetism of the spinel ferrites and the range of dielectric phenomena (notably ferroelectricity and piezoelectricity) encountered in the perovskite titanates have sprung from a close linking of solid-state scientists and engineers with the materials design of ceramics. The products stemming from this dramatic development now form an established industry characterized by rapid innovation and an increasing sophistication of materials specification.

Mechanical applications at ambient temperature are largely based on the combination of wear resistance, hardness and corrosion resistance which many ceramics (notably alumina and silicon carbide) provide. Such applications include wear parts in medical engineering (total prostheses for hip joints), in process plant (pump components and valve faces, lining for pipework) and in mechanical engineering (bearings and valves). Alongside the many advantages which ceramics bring to these uses, the principal disadvantage and impediment to more rapid exploitation has been that, while the materials can display great strength (the attainment of 1 GPa associated with high-strength steels is no longer seen as exceptional), the mode of fracture is most commonly brittle, resulting in sudden and complete failure of the component. The reluctance of engineers to welcome ceramics more enthusiastically is mainly caused by the severity of the design problems posed by this attribute.

Despite the great advances being made elsewhere, mechanical applications at elevated temperatures are seen as perhaps the key target for the realization of the promise of advanced ceramics. The properties that lie behind this promise include wear resistance, hardness, stiffness, corrosion resistance and relatively low density. The main attraction, however, is the refractoriness of ceramics; that is, the high melting point and the retention of mechanical strength to high temperature. Some materials of interest for engineering applications are listed, together with their melting (or sublimation) temperatures, in Table 1.

Table 1

Melting or sublimation temperatures of some materials used in engineering applications

Material

Melting point (°C)

Aluminum oxide (alumina)

Al2O3

2054

Zirconium oxide (zirconia)

ZrO2

2770

Silicon carbide

SiC

2650

Silicon nitride

Si3N4

1900a

Nickel

Ni

1453

Aluminum

A1

660

The significance of these properties can already be seen in one successful application, namely in the use of ceramic tool tips (aluminas and sialons) for the high-speed machining of metals. The conditions of operation of the tip are arduous (high tip temperature, high mechanical loads, severe impact conditions in intermittent machining) and the success of ceramics in this sector is one of the most convincing demonstrations of their eventual suitability for a wider range of applications in high-temperature engineering. Of these, undoubtedly the most significant in terms of eventual scale is that of components for heat engines (diesels and turbines), the objective being to allow a raising of the operating temperature and, with it, engine efficiency and fuel economy.

The great attraction of this development has long been recognized, and from earlier experience it is known that ceramics pose severe problems in such applications, the main ones being susceptibility to thermal shock (the tendency to sudden fracture on the part of ceramics exposed to sudden cooling is familiar even in the domestic environment) (see ) and to sudden brittle fracture when subjected to impact. The reasons for thinking that successful development of engine components will eventually result are the large levels of current financial support, the pressing requirement for the resulting economies, the dramatic advances in materials quality and, particularly, the fact that the origins of the associated problems are now better recognized. A key feature will be the quality and refinement that it is possible to bring to the processing involved in the fabrication of the ceramics.

In summary, therefore, the advanced ceramics have won a place in everyday use through their successful exploitation ih a wide variety of electrical and magnetic applications and in growing range of mechanical applications of which tool tips are perhaps the most established. A large question remains, however,...