Advanced Materials by Design Page: 40
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40. Advanced Materials by Design
in these materials. The high toughness is accom- Transformation Toughening
panied by high strength, both of which result Transformation toughening, a relatively new
from the modified microstructure. approach to achieving high toughness and strength
in ceramics, has great potential for increasing the
use of ceramics in wear-resistance applications.
Figure 2-1 .-Probabiiity of Failure of The key ceramic material is zirconia (zirconium
a Ceramic Component oxide).
oxide).
Zirconia goes through a phase transformation
from the tetragonal to the monoclinic crystal form
while cooling through a temperature of about
21 00 F (1 150 C). This phase transformation is
ril~ accompanied by an increase in volume of 3 per-
cent, similar to the volume increase that occurs
when water freezes. By control of composition,
.. ;: s > I #particle size, and heat treatment cycle, zirconia
can be densified at high temperature and cooled
such that the tetragonal phase is maintained
down to room temperature.
When a load is applied to the zirconia and a
crack starts to propagate, the high stresses in the
vicinity of the crack tip catalyze the transforma
tion of adjacent tetragonal zirconia grains to the
monoclinic form, causing them to expand by 3
percent. This expansion of the grains around the
crack tip compresses the crack opening, thereby
,preventing the crack from propagating.
Ceramic Matrix Composites
A variety of ceramic particulate, whiskers
high-strength single crystals with length/diameter
ratios of 10 or more), and fibers may be added
to the host matrix material to generate a com-
posite with improved fracture toughness.
The presence of these reinforcements appears
to frustrate the propagation of cracks by at least
three mechanisms. First, when the crack tip en-
counters a particle or fiber that it cannot easily
break or get around, it is deflected off in another
of Direction. Thus, the crack is prevented from
propagating cleanly through the structure. Sec-
ond, if the bond between the reinforcement and
The probability of failure of a ceramic component is the over-
lap between the applied stress distribution and the material the matrix is not too strong, crack propagation
strength distribution,as shown in(a).This robability can be energy can be absorbed by pullout of the fiber
reduced by reducing the flaw size (b), or truncation of the from its original location. Third, fibers can bridge
strength distribution through proof testing (c). from its original location. Third, fibers can bridge
a crack, holding the two faces together, and thus
SOURCE: R. Nathan Katz "Applications of High Performance Ceramics In Heat
Engine Design," Materials Science and Engineering 71:227-249,985. prevent further propagation.
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United States. Congress. Office of Technology Assessment. Advanced Materials by Design, report, June 1988; [Washington D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc39896/m1/45/: accessed April 27, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.