Biomedical Engineering Reference
In-Depth Information
4.4.2 Residual stresses
In their seminal work, Niihara et al. (1993) emphasised that of the types
defined in Fig. 4.1, it is the intra-type that is usually expected to improve the
mechanical properties of ceramic nanocomposites, largely due to its ability
to utilise the relatively high residual stresses arising in such structures.
Because of their transgranular fractures, bridging zone mechanisms such
as mechanical or frictional interlocking or pull-out in Al
2
O
3
-SiC
nanocomposites should be excluded a priori. Therefore, only mechanisms
acting ahead of or directly behind the crack tip may be taken as applicable
for these nanocomposites. The spherical shape and small size of the
reinforcing elements means the toughening effect of crack deflection in these
composites is minimal and elastic crack pinning is the only mechanism that
will contribute to toughness.
According to Ohji et al. (1998), even when a dispersed particle consists of
a brittle material that fails at a small crack opening distance, the high
strength and the rigid interface with the matrix create an extremely high
shielding stress and a steep increase in fracture resistance. An R-curve with a
high tearing modulus may be responsible for the high strength in Al
2
O
3
-SiC
nanocomposites. The effects of residual stresses are utilised in this composite
to render the pinning mechanism effective (Fig. 4.15).
Internal stresses are generated during the cooling that follows the
fabrication of ceramic nanocomposites. This is due to the difference in the
thermal expansion coefficients of the matrix (
α
p
),
which influence the grain and grain boundary strength. The thermal
expansion stress,
α
m
) and the nanoparticle (
σ
T
, inside a single spherical particle in an infinite matrix is
4.15
Schematic illustration of (a) crack propagation in Al
2
O
3
-SiC
composite and (b)
-curves of an Al
2
O
3
-SiC nanocomposite and a
monolithic alumina (Dusza and
ˇ
ajgal´k, 2009).
R
