Biomedical Engineering Reference
In-Depth Information
stable, oxidation-resistant ceramic composites and coatings are also in
demand for aircraft and spacecraft applications.
One such material system in this class of composites, silicon carbide/
silicon nitride (SiC/Si
3
N
4
) composites, has been shown to perform very well
under high-temperature oxidizing conditions. Interest in such nanocompo-
sites started with the experiments of Niihara [1] who reported large
improvements in both the fracture toughness and the strength of materials
by embedding nanometer range (20-300 nm) particles within a matrix of
larger grains and at the grain boundaries (GBs). A 200% improvement in
both strength and fracture toughness, better retention of strength at high
temperatures, and better creep properties were observed. An advanced
nanocomposite microstructure such as that of polycrystalline silicon carbide
(SiC)-silicon nitride (Si
3
N
4
) nanocomposite contains multiple length scales
with GB thickness of the order of 50 nm, SiC particle sizes of the order of
200-300 nm and Si
3
N
4
grain sizes of the order of 0.8 to 1.5
m [2]. Designing
the microstructure of such a composite (and similar others such as TiN-
Si
3
N
4
, SiC-Al
2
O
3
, SiC-SiC, graphene/CNT+SiC, and carbon fiber+SiC
nanocomposites) for a targeted set of material properties is, therefore, a
daunting task. Since the microstructure involves multiple length scales,
multiscale analyses based material design is an appropriate approach for
such a task.
μ
5.2 Multiscale modeling and material design
A multiscale modeling paradigm is shown in Fig. 5.1. Atomistic analyses at
the nanoscale can impart important information about the effect of critical
features such as a grain boundary (GB), an interface, or a triple junction,
etc. on mechanical deformation behavior of a small nanoscale (
few
nanometers) sample. In multiscale modeling such information is used to
formulate macroscale (
~
a few micrometers) material models for under-
standing microstructure-dependent deformation behavior of a material
sample such as the one shown in Fig. 5.1. Appropriate mathematical models
of microstructure property relations allow one to relate performances such
as fracture toughness, ultimate strength, fatigue lifetime, etc. to key material
microstructure parameters like volume fraction, particle size, and phase
composition.
Since a typical nanoscale test sample is much smaller and is subjected to
varied surroundings in a typical microstructure (e.g. Fig. 5.1), the
incorporation of nanoscale information in macroscale models is subjected
to statistical uncertainty. If a complex microstructure is to be designed for a
targeted set of properties, it is important that such uncertainties be correctly
quantified and incorporated within a robust material design framework. The
development of a variable fidelity model management framework that can
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