Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51

2nd Revised Pages

8.16 Noncrystalline Ceramics • 271

8.15 CRYSTALLINE CERAMICS

For crystalline ceramics, plastic deformation occurs, as with metals, by the motion

of dislocations. One reason for the hardness and brittleness of these materials is the

difficulty of slip (or dislocation motion). For crystalline ceramic materials for which

the bonding is predominantly ionic, there are very few slip systems (crystallographic

planes and directions within those planes) along which dislocations may move. This

is a consequence of the electrically charged nature of the ions. For slip in some

directions, ions of like charge are brought into close proximity to one another; because

of electrostatic repulsion, this mode of slip is very restricted. This is not a problem

in metals, since all atoms are electrically neutral.

On the other hand, for ceramics in which the bonding is highly covalent, slip

is also difficult and they are brittle for the following reasons: (1) the covalent

bonds are relatively strong; (2) there are also limited numbers of slip systems; and

(3) dislocation structures are complex.

8.16 NONCRYSTALLINE CERAMICS

Plastic deformation does not occur by dislocation motion for noncrystalline ceramics

because there is no regular atomic structure. Rather, these materials deform by

viscous flow,the same manner in which liquids deform; the rate of deformation is

proportional to the applied stress. In response to an applied shear stress, atoms or

ions slide past one another by the breaking and reforming of interatomic bonds.

However, there is no prescribed manner or direction in which this occurs, as with

dislocations. Viscous flow on a macroscopic scale is demonstrated in Figure 8.26.

viscosity The characteristic property for viscous flow,viscosity,is a measure of a noncrys-

talline material’s resistance to deformation. For viscous flow in a liquid that origi-

nates from shear stresses imposed by two flat and parallel plates, the viscosityηis the

ratio of the applied shear stressτand the change in velocitydvwith distancedyin a

direction perpendicular to and away from the plates, or

η=

τ

dv/dy

=

F/A

dv/dy

(8.10)

This scheme is represented in Figure 8.26.

The units for viscosity are poise (P) and pascal-second (Pa-s); 1 P=1 dyne-

s/cm^2 , and 1 Pa-s=1 N-s/m^2. Conversion from one system of units to the other is

according to

10 P=1 Pa-s

A

v

F

y

Figure 8.26 Representation of the

viscous flow of a liquid or fluid glass

in response to an applied shear

force.