Over the past half century ceramics have received significant attention as
candidate materials for use as structural materials under conditions of high
loading rates, high temperature, wear, and chemical attack that are too severe
for metals. However, inherent brittleness of the ceramics has prevented their
wide use in different applications.
Significant scientific effort has been directed towards making ceramics more
flaw-tolerant through design of their microstructures by incorporation of fibers
or whiskers which bridge the crack faces just behind the crack tip; by designing
microstructures with elongated grains which act as bridges between crack faces
just behind the crack tip; by incorporating second phase particles which deflect
the crack making it travel a more tortuous path; and by incorporating secondary
phases which undergo stress induced volume expansion that forces the crack faces
together. However, one of the most recent development has been the distribution
of multiple phases in a ceramic composite at the nanoscopic length scale. Owing
to prevalence of nanoscopic features, such composites are referred to as ceramic
The definition of nanocomposite material has broadened significantly to
encompass a large variety of systems such as one-dimensional, two-dimensional,
three-dimensional and amorphous materials, made of distinctly dissimilar
components and mixed at the nanometer scale. The general class of nanocomposite
organic/inorganic materials is a fast growing area of research. Reducing the
sizes of structural features in materials leads to a significant increase in the
portion of surface/interface atoms.
The surface/interface energies essentially control the properties of a solid.
Interfaces provide a means to introduce non-homogeneity in the material. This
non-homogeneity acts as a significant modification of both thermal and
mechanical properties of the composites. Selective mixing of materials in a
highly tailored morphology with high percentage of interface area, leads to
materials with enhanced properties.
The properties of nano-composite materials depend not only on the properties
of their individual parents but also on their morphology and interfacial
characteristics. The nanocomposites find their use in various applications
because of the improvements in the properties over the simpler structures. Few
of such advantages can be summarized as:
- Improved Mechanical properties e.g. strength, modulus and dimensional
- Decreased permeability to gases, water and hydrocarbons
- Higher Thermal stability and heat distortion temperature
- Higher Flame retardancy and reduced smoke emissions
- Higher Chemical resistance
- Smoother Surface appearance
- Higher Electrical conductivity
For components used in a gas turbine engine, a lifetime upto 10000 h and a
retained strength of ~300 MPa at a temperature of 1400 °C have been postulated,
together with negligible creep rate. Furthermore, at elevated temperatures, the
material must exhibit high resistance to thermal shock, oxidation, and
subcritical crack growth. Ceramic nanocomposites have been shown to be extremely
important for such future applications.
Advanced bulk ceramic composite materials that can withstand high
temperatures (>1500 °C) without degradation or oxidation can also be used for
applications such as structural parts of motor engines, catalytic heat
exchangers, nuclear power plants, and combustion systems, besides their use in
fossil energy conversion power plants. These hard, high-temperature 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/Si3N4) composites, have been shown to perform
very well under high temperature oxidizing conditions. Interest in such
nanocomposites started with experiments of Niihara2
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. 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 (Si3N4) nanocomposites, Figure 1, contains
multiple length scales with grain boundary (GB) thickness of the order of 50 nm,
SiC particle sizes of the order of 200-300 nm and Si3N4
grain sizes of the order of 0.8 to 1.5 µm1.
Designing the microstructure of such a composite (and similar others such as
TiN-Si3N4, SiC-Al2O3, 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.
|Figure 1. Actual
microstructure of a SiC-Si3N4 nanocomposite1
The ceramic nanocomposite work in Multiphysics Lab
at Purdue focuses on (1) Understanding Performance of Carbide and Nitride
Based High Temperature Ceramic Nanocomposites for Extreme Environments found in
power generation cycles Including Nuclear Applications, (2) Multiscale Modeling
and Characterization in Oxide Ceramic Materials and (3) Understanding thermal
conduction and thermal issues in materials for thermoelectric power generation.
A description of major areas of interest and contributions is as follows:
- Understanding thermal conduction and thermal issues to develop materials
with low thermal conductivity3-5: This work
focuses on understanding atomistic mechanisms of operation of nanocomposites for
thermoelectric power generation such that materials with low thermal
conductivity could be developed. Explicit molecular simulations using molecular
dynamics (MD) are performed to understand how morphology alterations can be used
to reduce thermal conductivity in nanocomposites. We have found certain
biomimetic arrangements that could achieve significant reduction in thermal
conduction. We are in the process of making and testing such materials.
- Understanding Performance of Carbide and Nitride Based High Temperature
Ceramic Nanocomposites for Extreme Environments Including Nuclear
Applications6-12: This research work focuses
on understanding mechanisms of room temperature and high temperature operations
of advanced nanocomposite ceramic materials that can enable power plant
operation at temperatures in excess of 1750 K leading to efficiencies of almost
70% and significant reduction in the plant emissions. As an off-shoot, this
project also focuses on thermal properties of these materials for possible use
as high temperature multifunctional materials, high temperature structural
materials in nuclear applications or heat sensors in nuclear applications.
- Multiscale Modeling and Characterization in Oxide Ceramic
Materials13-18: Focus during this work has
been on understanding multiscale thermomechanical behavior of advanced composite
materials such as multifunctional Al+Fe2O3 nanocrystalline
composites and high-strength Al2O3/TiB2 ceramic
armor composites. This research on atomistic deformation analyses of
Al+Fe2O3 multifunctional nanocomposites using MD is one of
the first in the area of atomistic deformation analyses of advanced ceramic
composite nanomaterials. In this work large scale MD simulations of
nanocrystalline Al+Fe2O3 multifunctional composites, of
single crystalline Al, of single crystalline Fe2O3, and of
various interfacial configurations of single crystalline Al and
Fe2O3 are performed. In the case of
Al2O3/TiB2 ceramic armor composites, we have
developed and used a new cohesive finite element method (CFEM) for quantitative
characterization of dynamic fracture.
The above contribution is strongly based on a collaborative multiscale
modeling-material design-experimental processing approach. A snapshot of the
overall collaborative research approach on modeling, design, and fabrication
highlights is provided below.
Multiscale Modeling of Ceramic Nanocomposites: An Example of Work in
SiC-Si3N4 Ceramic Nanocomposites
multiscale analyses (at nanometer and micrometer length and time scales) based
on a combination of CFEM and MD based techniques have revealed that high
strength and relatively small sized SiC particles act as stress concentration
sites in Si3N4 matrix leading to inter-granular
Si3N4 matrix cracking as a dominant failure mode. CFEM
analyses have also revealed that due to a significant number of nano-sized SiC
particles being present in micro-sized Si3N4 matrix, the
SiC particles invariantly fall in wake regions of micro-cracks leading to
significant mechanical strength. This finding was confirmed in the MD analyses
that revealed that particle clustering along the GBs significantly increases the
strength of these nanocomposites. While some nanocomposite morphologies have
sharply defined SiC-Si3N4 interfaces19, other nanocomposite morphologies have diffusion of C,
N, or Si atoms at the interfaces20.
In the case of SiC-Si3N4 nanocomposites, MD analyses
have also revealed that the second phase particles act as significant stress
raisers in the case of single crystalline Si3N4 phase
matrix affecting the strength significantly. However, the particle's presence
does not have a significant effect on the mechanical strength of bicrystalline
or nanocrystalline Si3N4 phase matrices. The strength of
the SiC-Si3N4 nanocomposite structures showed an
uncharacteristic correlation between the grain boundary (GB) thickness and
The strength showed 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 the
particle clustering and increase in SiC-Si3N4 interfacial
strength with temperature, the strength improved with increase in temperature.
Figure 2 shows snapshots of fracture propagation analyses in such nanocomposites
obtained using the CFEM.
Figure 2. Snapshots of
mesoscale crack propagation and damage propagation in the
Figure 3 displays snapshots obtained using MD. 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.
Figure 3. Snapshots of
atomistic damage and failure propagation in two different SiC (particle) and
Si3N4 (matrix) nanocomposites at two different
Petascale Computing Based Material Design
Atomistic analyses at the nanoscale can impart important information about
the effect of critical features such as a GB, an interface, or a triple
junction, etc. on mechanical deformation behavior of a small nanoscale (~ few
nm) sample. In multiscale modeling such information is used to formulate
macroscale (>few µm) material models for understanding microstructure
dependent deformation behavior of a material sample such as the one shown in
Appropriate mathematical models of microstructure property relations allow to
relate performances like 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. 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. We have pioneered the
development of a variable fidelity model management framework that can
incorporate material behavior analyses at multiple length scales in a design
optimization framework21-24, (Collaboration with
Prof. John Renaud's group at the University of Notre Dame).
Figure 4 details the process flow of a petascale multi-physics model
management tool for multiscale material design. Deployed on a petascale machine,
the design tool developed in this research, that integrates atomistic and
mesoscale analyses using a variable fidelity model management framework, will
facilitate a significant reduction in nanomaterials' development cost and time
with a simultaneous increase in the possible different combinations of
individual composite material phases to achieve desired material
The model management framework21,22, besides managing the models and scales, is also well
suited to control hierarchical parallelism. The natural hierarchy is MD within
CFEM within design under uncertainty, using a mixed programming model
SHMEMTM by SGI for CFEM and MPI for MD and the
uncertainty modeling. Both MD and the uncertainty quantification (via
quasi-Monte Carlo integration) can use 1000 processors, and CFEM 10, so 1000
uncertainty quantification groups of 10 CFEM groups of 1000 HMC processors is
107 processors, nearing exascale.
Figure 4. Schematic
Petascale Material Design Framework
Preliminary material design analyses of the model system have been performed
to understand the morphology related parameters that must be controlled for
optimal targeted set of properties. The application of design tool is focusing
on the continuous fiber ceramic composite (CFCCs) models of
SiC-Si3N4 nanocomposites, Fig. 5. The second phase
(circles and cylinders) are the SiC fibers that have higher elastic modulus and
higher creep resistance (E) but lower yield stress and fracture
toughness, than that of the primary Si3N4 phase. The
problem is to design the most suitable CFCC, with maximum strength and creep
resistance for a set of external temperatures T, where the number of
design variables will depend on whether the simulation tests are run on the
2-dimensional (2-D) or 3-dimensional (3-D) model. The design variables to be
considered in the nanocomposite design optimization problem, for the 2-D model,
are the fibers diameter (d) and the external temperature (T). And
for the 3-D model the design variables to be considered are the fibers diameter
(d), the length of fibers (l) and the external temperature
(T). The problem definition in standard form is given below:
Figure 5. High and low
fidelity models for the CFCC
Figure 6 illustrates normalized (0-100) function values for the strength and
creep strain rate as a function of design variables for the high fidelity model
(3-D) and low fidelity model (2-D). Figure 6 (left) shows an increase in the
CFCC strength and a corresponding decrease in the creep strain rate as the
design variable d increases. Similarly for the high fidelity model, Fig.
6 (right) shows an increase in the CFCC strength and a corresponding decrease in
the creep strain rate as the design variables d and l
Figure 6. (left)
Strength and creep strain rate at 1500oC as a function of the design variable
width-height (d) for the 2-D low fidelity model. (right) Strength and
creep strain rate at 1500°C as a function of the design variables width-height
(d) and length of fibers (l) for the 3-D high fidelity
Focus during this activity is on forming a collaborative
modeling-deign-processing framework where complex ceramic nanocomposites for
targeted set of mechanical and non-mechanical properties could be produced
without wasting significant trial-and-error time and money. We are collaborating
with Prof. Rajendra K Bordia's group at the University of Washington-Seattle.
Polymer derived ceramics (PDCs) are an attractive approach to make material
design predicted morphology of ceramic nanocomposites. First Niihara and his
coworkers and then others used this approach to make high performance nanoscale
Continued research in this area has led to the development of a range of
nanostructures. One particularly interesting class of materials have
predominantly amorphous Si-O-C nanodomains containing nanoscale SiC and C
reinforcements. These materials have the desired characteristics for a broad
range of high-temperature applications while offering greater control over
processing, compositions and nanostructure. PDCs are produced by pyrolyzing
preceramic polymers and are typically amorphous up to very high temperature but
provide very intriguing ceramic-like properties, such as good creep and
Some of their unique properties are associated with in-situ formation of
nanodomains and lack of grain boundaries in their microstructures. Due to the
polymeric nature (thermoset) of the precursors, this family of materials is
easily processable as fibers, matrices for composites, porous structures and
coatings30,31. Most studied PDCs
can be categorized into three main groups: (i) silicon carbide (SiC) (ii)
silicon oxycarbides (SiOC) and (iii) silicon carbonitrides (SiCN). SiOCs and
SiCNs are distinctive due to their hybrid molecular composition between
SiO2 and SiC and between SiC and Si3N4,
respectively with additional level of "free" carbon as schematically illustrated
in Fig 7 for Si-O-C system.
Figure 7. Schematic of
Phase Relations in the Si-O-C System
A unique nanostructural feature of these materials is that the controlled
excess carbon is dispersed as graphene layers with domain size of a few nms.
Control of, and understanding of development of such nanostructural features,
using an integrated experimental and atomistic simulation approach, is the focus
of our collaborative research.
The related research work in our lab has been made possible by support from
the US-Air Force Office of Scientific Research (Program manager: Dr. Joan
Fuller), the US-Department of Energy, and the US-National Science Foundation
1. Weimer, A.W. and Bordia, R.K., Processing and properties of
nanophase SiC/Si3N4 composites. Composites Part B: Engg, 1999. 30: p.
2. Niihara, K., New design concept for structural
ceramics-Ceamic nanocomposites. J. Ceram. Soc. Jpn: The centennial memorial
issue, 1991. 99(10): p. 974-982.
Samvedi, V. and Tomar, V., Analyses of interface thermal boundary resistance of
Si-Ge superlattice system as a function of film thickness and periodicity.
Nanotechnology 20 (2009) 365701 (11pp) (Special Mention by Editors and
4. Samvedi, V. and
Tomar, V., Role of heat flow direction, monolayer film thickness, and
periodicity in controlling thermal conductivity of a Si-Ge superlattice system.
J. Appl. Phys. (also featured in Virtual Journal of Nanoscale Science and
Technology), 2008. 105: p. 013541.
Samvedi, V. and Tomar, V., Role of straining and morphology in thermal
conductivity of a set of Si-Ge superlattices and biomimetic Si-Ge
nanocomposites. Submitted to Journal of Physics-D, Applied Physics, 2009.
6. Tomar, V. Multiscale simulation of dynamic
fracture in polycrystalline SiC-Si3N4 using a molecularly motivated cohesive
finite element method. in 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural
Dynamics, and Materials Conference (April 23-26, 2007) Honolulu, Hawaii. 2007:
Paper No. AIAA-2007-2345.
7. Tomar, V.,
Analyses of the role of the second phase SiC particles in microstructure
dependent fracture resistance variation of SiC-Si3N4 nanocomposites. Modelling
Simul. Mater. Sci. Eng. , 2008. 16: p. 035001.
8. Tomar, V., Analyses of the role of grain boundaries in
mesoscale dynamic fracture resistance of SiC-Si3N4 intergranular nanocomposites.
Eng. Fract. Mech., 2008. 75: p. 4501-4512.
Tomar, V. and Gan, M., Temperature dependent nanomechanics of si-c-n
nanocomposites with an account of particle clustering and grain boundaries.
submitted to Int. J. Hydrogen Energy, 2009.
10. Tomar, V., Gan, M., and Kim, H., Effect of temperature and
morphology on mechanical strength of Si-C-O and Si-C-N nanocomposites. Submitted
to the Journal of European Ceramic Society, 2009.
11. Tomar, V. and Samvedi, V., Atomistic simulations based
understanding of the mechanism behind the role of second phase SiC particles in
fracture resistance of SiC-Si3N4 nanocomposites. International Journal of
Multiscale Computational Engineering, 2009(DOI:
10.1615/IntJMultCompEng.v7.i4.40, 277-294 pages).
12. Tomar, V., Samvedi, V., and Kim, H., Atomistic understanding
of the particle clustering and particle size effect on the room temperature
strength of SiC-Si3N4 nanocomposites. to appear in Int. J. Multiscale Comp.
Engg. special issue on Advances In Computational Materials Science, 2009.
13. Tomar, V., Molecular Modeling of Al-Fe2O3
Nanomaterial System. 2009: VDM Verlag Dr. Müller Aktiengesellschaft & Co.
KG, ISBN 978-3-639-15858-8.
14. Tomar, V.
and Zhou, M. An empirical molecular dynamics potential for an Al+Fe2O3 reactive
metal powder mixture. in 45th AIAA/ASME/ASCE/AHS/ASC structures, structural
dynamics and materials conference. 2004. Palm Springs, CA USA: AIAA.
15. Tomar, V. and Zhou, M., Deterministic and
stochastic analyses of dynamic fracture in two-phase ceramic microstructures
with random material properties. Eng. Fract. Mech., 2005. 72: p.
16. Tomar, V. and Zhou, M.,
Classical molecular-dynamics potential for the mechanical strength of
nanocrystalline composite fcc-Al+a-Fe2O3. Phys Rev B, 2006. 73: p. 174116
17. Tomar, V. and Zhou, M.,
Tension-compression strength asymmetry of nanocrystalline a-Fe2O3+fcc-Al
ceramic-metal composites. Appl. Phys. Lett., 2006. 88: p. 233107 (1-3).
18. Tomar, V. and Zhou, M., Analyses of tensile
deformation of nanocrystalline a-Fe2O3+fcc-Al composites using classical
molecular dynamics. Journal of the Mechanics and Physics of Solids, 2007. 55: p.
19. Bill, J., Kamphowe, T.W., Mueller, A.,
Wichmann, T., Zern, A., Jalowieki, A., Mayer, J., Weinmann, M., Schuhmacher, J.,
Mueller, K., Peng, J., Seifert, H.J., and Aldinger, F., Precursor-derived
Si-(B-)C-N ceramics: thermolysis, amorphus state, and crystallization. Appl.
Organometallic Chemistry, 2001. 2001(15): p. 777-793.
Jalowiecki, A., Bill, J., Aldinger, F., and Mayer, J., Interface
characterization of nanosized B-doped Si3N4/SiC ceramics. Composites Part A,
1996. 27A: p. 721.
21. Gano, S.E.,
Agarwal, H., Renaud, J.E., and Tovar, A., Reliability based design using
variable fidelity optimization. Structure and Infrastructure Engineering, 2006.
2(3-4): p. 247-260.
22. Gano, S.E.,
Renaud, J.E., and Sanders, B. Variable fidelity optimization using a krigin
based scaling function. in 10th AIAA/ISSMO Multidisciplinary analysis and
optimization conference. 2004. Albany, New York.
23. Mejia-Rodriguez, G., Renaud, J.E., and Tomar V., A variable
fidelity model management framework for designing multiphase materials. ASME
Journal of Mechanical Design, 2007. 130: p. 091702-1 to 13.
24. Mejia-Rodriguez, G., Renaud, J.E., and Tomar V. A
methodology for multiscale computational design of continuous fiber SiC-Si3N4
ceramic composites based on the variable fidelity model management framework. in
3rd AIAA Multidisciplinary Design Optimization Specialist Conference (April
23-26, 2007) Honolulu, Hawaii. 2007: Paper No. AIAA-2007-1908.
25. Kroke, E., Li, Y.-L., Konetschny, C.,
Lecomte, E., Fasel, C., and Riedel, R., Silazane derived ceramics and related
materials. Mat. Sci. and Engr.: R: Reports, 2000. 26(4-6.): p. 197-199.
26. Niihara, K., Izaki, K., and Kawakami,
T., Hot-Pressed Si3N4-32%SiC nanocomposites from amorphous Si-C-N powder with
improved strength above 1200O C. J of Materials Science Letters, 1990. 10: p.
27. Wan, J., Duan, R.-G.,
Gasch, M.J., and Mukherjee, A.K., Highly creep-resistant silicon nitride/silicon
carbide nano-nano composites. J. Am. Ceram. Soc., 2006. 89(1): p. pp.
28. Raj, R., An, L., Shah, S.R., Riedel, R., Fasel,
C., and Kleebe, H.-J., Oxidation kinetics of an amorphous silicon carbonitride
ceramic. J. Am. Ceram. Soc., 2001. 84(7): p. 1803-10.
Rouxel, T., Soraru, G.D., and Vicens, J., Creep viscosity and stress relaxation
of gel-derived silicon oxycarbide glasses. J. Am. Ceram. Soc., 2001. 84(5): p.
30. Riedel, R., Mera, G., Hausner, R., and
Klonczynski, A., Silicon-based polymer-derived ceramics: Synthesis properties
and applications-a review. J. Ceram. Soc of. Japan, 2006. 114(6): p.
31. Torrey, J.D., Bordia, R.K., Henager Jr., C.H.,
Blum, Y., Shin, Y., and Samuels, W.D., Composite polymer derived ceramic system
for oxidizing environments. Journal of Materials Science, 2006. 41: p.
Copyright AZoNano.com, Professor Vikas Tomar (Purdue