Biomedical Engineering Reference
In-Depth Information
the effect of nanoscale size changes and nanoscale position changes on the
strength of nanocomposites. MD analyses can be very useful to reveal this
information, along with information on the effect of interfaces and the
mechanism of deformation for certain specific orientations. Overall, CFEM
analyses demonstrated that, irrespective of the location of second-phase
particles, the final failure mode in all microstructures is brittle fragmentation
with initial microcracks forming and propagating mainly in the Si
3
N
4
matrix. Later parts of brittle fragmentation include GB and SiC
fragmentation. Second-phase particles have two important effects. If they
are present in front of the crack tip they weaken the microstructure because
of the stress concentration caused by them. However, if they are present
near GBs, they cause a crack bridging effect with an increase in strength of
the microstructure.
MD analyses also reveal that the second-phase particles act as significant
stress raisers, resulting in strength reduction of single-crystal and bi-
crystalline Si
3
N
4
blocks by a factor of almost two times. The stress
concentration increases almost 1.5 times with doubling the size of SiC
particles. With smaller SiC particles, the interfacial boundary in the bi-
crystalline Si
3
N
4
block acts as a stress reliever. However, with an increase in
the size of SiC particles and with a decrease in the spacing between adjacent
SiC particles, the presence of an interfacial boundary results in significant
internal stress rise. This indicates that the placement of SiC particles along
interfacial boundaries will not always lead to strengthening of a SiC-Si
3
N
4
nanocomposite. Overall, MD analyses confirm the CFEM findings
concerning the effect of second-phase SiC particles on SiC-Si
3
N
4
nanocomposite strength. In addition, the analyses also indicate that the
strengthening of a nanocomposite by placing second-phase particles along
grain boundaries is only possible for a selective few second-phase particle
sizes with interparticle spacing being another important factor.
5.8 References
1. Niihara, K., New design concept for structural ceramics - Ceramic
nanocomposites. J. Ceram. Soc. Jpn: Centennial memorial issue, 1991, 99(10):
974-982.
2. Weimer, A.W. and Bordia, R.K., Processing and properties of nanophase SiC/
Si3N4 composites. Composites Part B, 1999, 30: 647-655.
3. Gano, S.E., Agarwal, H., Renaud, J.E., and Tovar, A., Reliability based design
using variable fidelity optimization. Struct. Infrastruct. Engg, 2006, 2(3-4): 247-
260.
4. Gano, S.E., Renaud, J.E., and Sanders, B., Variable fidelity optimization using
a kriging based scaling function. 10th AIAA/ISSMO Multidisciplinary Analysis
and Optimization Conference, Albany, New York, 2004.
5. Mejia-Rodriguez, G., Renaud, J.E., and Tomar V., A variable fidelity model
