Biomedical Engineering Reference
In-Depth Information
studies, both the reinforcement size and volume fraction significantly affect
the fracture toughness and strength [20]. A fundamental understanding of
the effect of nanosized SiC reinforcements on the mechanical behavior of
Si
3
N
4
matrix composites is required before further attempting to improve
the properties of these composites by varied morphological alterations in
experiments.
Tomar [21, 22] reported on the effect of morphological variations in
second-phase SiC particle placement and GB strength on the dynamic
fracture strength of SiC-Si
3
N
4
nanocomposites using continuum analyses
based on a mesoscale cohesive finite element method (CFEM). It was found
that high-strength and relatively small sized SiC particles act as stress
concentration sites in the Si
3
N
4
matrix, leading to intergranular Si
3
N
4
matrix cracking as a dominant failure mode. However, as a result of a
significant number of nanosized SiC particles being present in microsized
Si
3
N
4
matrix, the SiC particles invariantly fall in wake regions of
microcracks, leading to significant structural strength. This mechanism
was further examined using 3D molecular dynamics (MD) simulations of
crack propagation in SiC-Si
3
N
4
nanocomposites with cylindrical SiC
inclusions.
In the case of SiC-Si
3
N
4
nanocomposites, MD analyses have also
revealed that the second-phase particles act as significant stress raisers in the
case of single crystalline Si
3
N
4
phase matrix, affecting the strength
significantly. However, the particles' presence does not have a significant
effect on the mechanical strength of bicrystalline or nanocrystalline Si3N4
phase matrices. The strength of the SiC-Si
3
N
4
nanocomposite structures
showed an uncharacteristic correlation between GB thickness and tempera-
ture. The strength showed a decrease with increase in temperature for
structures having thick GBs having diffusion of C, N, or Si atoms. However,
for structures with no appreciable GB thickness (no diffusion of C, N, or Si
atoms), due to particle clustering and increase in SiC-Si
3
N
4
interfacial
strength with temperature, the strength improved with an increase in
temperature. Figure 5.6 shows snapshots of fracture propagation analyses in
such nanocomposites obtained using the CFEM. Current research work
focuses on obtaining experimental images of the ceramic nanocomposites
developed by collaborators, developing nanoscale CFEM meshes on such
images, and performing failure analyses using the combination of MD and
CFEM techniques.
As noted earlier, high-strength and relatively small sized SiC particles act
as stress concentration sites in the matrix, leading to intergranular Si
3
N
4
matrix cracking as a dominant failure mode. CFEM analyses also revealed
that the SiC nanosized particles invariantly fall in wake regions of
microcracks, leading to significant mechanical strength. This finding was
confirmed in the MD analyses that revealed that particle clustering along the
