Biomedical Engineering Reference
In-Depth Information
At room temperature:
.
improving the strength level and reducing its scatter, i.e. enhancing
reliability by reduction of the critical defect size (improved properties of
powders and of powder processing, clean-room manufacturing, etc.) -
the flaw diminution approach (Lange, 1989);
.
promoting energy-absorbing mechanisms occurring in the frontal
process zone ahead of a propagating crack (stress-induced martensitic
transformation, microcracking, etc.) - the flaw tolerance approach
(Evans and Cannon, 1986);
.
promoting localised bridging behind the crack tip (as frictional and
mechanical interlocking, or pull-out), by which the flaw tolerance of the
material can be improved - the flaw tolerance approach (Evans and
Cannon, 1986);
.
improving structural reliability by designing novel laminar composites
that promote crack deflection at the inter-layer boundaries and/or utilise
compressive residual stresses generated during cooling from the
sintering temperature due to the differences in thermal expansion
coefficients between layers of different composition - the laminar
structure approach (Suresh, 1997; Lugovy et al., 1999);
.
improving strength by incorporating nano-sized, second-phase particles
with different expansion coefficients into the matrix - nanoparticle
dispersion strengthening (Sternitzke, 1997).
At high temperature:
.
optimising the grain size and the grain boundary phase geometry,
composition and crystallinity with the aim of
improving high-
temperature
strength,
fatigue and creep resistance
(Dusza and
S
ˇ
ajgalı´ k, 2005).
Over the last decade, a variety of advanced monolithic and composite
ceramics have been fabricated using these approaches and they possess
enhanced toughness, flaw tolerance and improved high-temperature proper-
ties. In addition to the main/matrix phase and sintering additives, a certain
amount of reinforcing phase is present (up to 50 vol.%). These composite
ceramics can be divided into the following groups:
.
ductile phase reinforced: Al
2
O
3
+Al, Al
2
O
3
+Nb, WC+Co etc.
.
particle reinforced: SiC+TiC, Al
2
O
3
+ZrO
2
, etc.
.
whisker/platelet
reinforced:
Si
3
N
4
+Si
3
N
4
-wh.,
Si
3
N
4
+SiC-wh.,
Al
2
O
3
+SiC-wh., etc.
.
in-situ reinforced ceramics: Si
3
N
4
, SiC, etc. (without second reinforcing
phase)
.
nanocomposites: Al
2
O
3
+SiC, Si
3
N
4
+SiC, etc.
