Callister - An introduction - 8th edition

488 • Chapter 12 / Structures and Properties of Ceramics
(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,
to the extent that plastic deformation in ceramics is rarely measurable at room temperature.
By way of contrast, in metals, because all atoms are electrically neutral,
considerably more slip systems are operable and, consequently, dislocation motion
is much more facile.
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.
viscosity
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 re-forming 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 12.34.
The characteristic property for viscous flow, viscosity, is a measure of a noncrystalline
material’s resistance to deformation. For viscous flow in a liquid that
originates from shear stresses imposed by two flat and parallel plates, the viscosity
is the ratio of the applied shear stress and the change in velocity dv with distance
dy in a direction perpendicular to and away from the plates, or
h
t
dy/dy F/A
dy/dy
(12.8)
This scheme is represented in Figure 12.34.
The units for viscosity are poise (P) and pascal-seconds (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
Liquids have relatively low viscosities; for example, the viscosity of water at room
temperature is about 10 3 Pa.s. On the other hand, glasses have extremely large
A
F
Figure 12.34 Representation of
the viscous flow of a liquid or fluid
glass in response to an applied
shear force.
y
v