Callister - An introduction - 8th edition

452 • Chapter 12 9 / / Phase Structures Diagrams and Properties of Ceramics
WHY STUDY Structures and Properties of Ceramics?
Some of the properties of ceramics may be explained
by their structures. For example: (a) The optical transparency
of inorganic glass materials is due, in part, to
their noncrystallinity; (b) the hydroplasticity of clays
(i.e., development of plasticity upon the addition of
water) is related to interactions between water molecules
and the clay structures (Sections 12.3 and 13.10
and Figure 12.14); and (c) the permanent magnetic
and ferroelectric behaviors of some ceramic materials
are explained by their crystal structures (Sections 20.5
and 18.24).
In the processing/structure/properties/performance
scheme, reasons for studying structures and properties
of ceramics are as follows:
• During the production of a glass-ceramic, a glass is
formed that is subsequently transformed into a crystalline
solid (Chapter 13). We discuss in this chapter
the structure of silica glasses.
• One step in the fabrication of most glass-ceramics
involves forming a semifluid mass into the desired
shape. Thus, it is necessary to specify and quantify a
property (i.e., viscosity) that represents a material’s
resistance of mechanical deformation, which is also
discussed in this chapter.
Learning Objectives
After studying this chapter you should be able to do the following:
1. Sketch/describe unit cells for sodium chloride,
cesium chloride, zinc blende, diamond cubic,
fluorite, and perovskite crystal structures. Do
likewise for the atomic structures of graphite
and a silica glass.
2. Given the chemical formula for a ceramic compound
and the ionic radii of its component ions,
predict the crystal structure.
3. Name and describe eight different ionic point
defects that are found in ceramic compounds.
4. Briefly explain why there is normally significant
scatter in the fracture strength for identical
specimens of the same ceramic material.
5. Compute the flexural strength of ceramic rod
specimens that have been bent to fracture in
three-point loading.
6. On the basis of slip considerations, explain
why crystalline ceramic materials are normally
brittle.
12.1 INTRODUCTION
Ceramic materials were discussed briefly in Chapter 1, which noted that they are
inorganic and nonmetallic materials. Most ceramics are compounds between metallic
and nonmetallic elements for which the interatomic bonds are either totally ionic,
or predominantly ionic but having some covalent character.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.
Up until the past 60 or so years, the most important materials in this class were
termed the “traditional ceramics,” those for which the primary raw material is clay;
products considered to be traditional ceramics are china, porcelain, bricks, tiles, and,
in addition, glasses and high-temperature ceramics. Of late, significant progress has
been made in understanding the fundamental character of these materials and of
the phenomena that occur in them that are responsible for their unique properties.
452 •