251
Let’s turn now to ceramic-matrix nanocomposites. The idea behind
these materials is to improve the fracture toughness, general
strength, and high temperature strength of conventional ceramicmatrix composites. In fact, significant increases in fracture toughness and strength were observed in alumina and zirconia matrices
reinforced with nanoparticles of SiC and/or Si 3 N 4 ranging in size
from 20 nm to 300 nm. For example, a 5 vol% addition of nanoscale particles of SiC and Si 3 N 4 into an alumina matrix resulted in
fracture toughness improvements of more than 50% and strength
of more than 100%. The reason for these enhanced properties is
related to crack-tip bridging, a mechanism by which nanoparticles
cause bridging of cracks at distances close to the crack tip. Therefore, instead of cracks propagating along the grain boundaries, the
residual stresses around the particles drive the cracks to the particles,
leading to crack bridging and higher fracture toughness. In the case
of Si 3 N 4 ceramic-matrix nanocomposites reinforced with SiC, the
beneficial effects provided by reinforcement are less understood.
Some possible mechanisms include Si 3 N 4 grain refinement due to
the presence of SiC at the grain boundaries, leading to larger strains
to failure, and thermal mismatch between the two phases, resulting in improved strength and fracture toughness. In addition to the
improvements in mechanical properties, ceramic matrix nanocomposites can also be used as thermal barriers. A good example is a
nanocomposite composed of SiC nanoparticles dispersed within a
pyrolytic graphite matrix.
This material exhibits outstanding oxidation resistance and thermal
shock. Ceramic-matrix nanocomposites have also been reinforced
with metallic nanoparticles, particularly for improving optical, electrical, and magnetic properties. Examples include alumina-matrix
nanocomposites filled with W, Ni, Fe, Au, and Cu; zirconia-matrix
filled with Ni and Mo; magnesia-matrix filled with Fe, Ni, and Co;
and silica filled with Fe, Au, and Ag. In the optical area, silica-matrix
nanocomposites filled with nanoparticles of Au and Ag exhibit
high optical nonlinearities (which describe the behavior of light
in media in which the dielectric polarization responds nonlinearly
to the electric field of the light), which are essential for photonic
devices. In the case of silica filled with Ag, the nanocomposite shows
changes in behavior from semiconducting to metallic, depending
on the size of the nanoparticles and temperature. This seems to be
the result of electron tunneling between the metallic nanoparticles
and metallic conduction through percolated paths, which can be
controlled by tuning the volume fraction of nanoparticles, their distribution and uniformity, size, and shape. In the field of magnetism,
Special Cases
Let’s turn now to ceramic-matrix nanocomposites. The idea behind
these materials is to improve the fracture toughness, general
strength, and high temperature strength of conventional ceramicmatrix composites. In fact, significant increases in fracture toughness and strength were observed in alumina and zirconia matrices
reinforced with nanoparticles of SiC and/or Si 3 N 4 ranging in size
from 20 nm to 300 nm. For example, a 5 vol% addition of nanoscale particles of SiC and Si 3 N 4 into an alumina matrix resulted in
fracture toughness improvements of more than 50% and strength
of more than 100%. The reason for these enhanced properties is
related to crack-tip bridging, a mechanism by which nanoparticles
cause bridging of cracks at distances close to the crack tip. Therefore, instead of cracks propagating along the grain boundaries, the
residual stresses around the particles drive the cracks to the particles,
leading to crack bridging and higher fracture toughness. In the case
of Si 3 N 4 ceramic-matrix nanocomposites reinforced with SiC, the
beneficial effects provided by reinforcement are less understood.
Some possible mechanisms include Si 3 N 4 grain refinement due to
the presence of SiC at the grain boundaries, leading to larger strains
to failure, and thermal mismatch between the two phases, resulting in improved strength and fracture toughness. In addition to the
improvements in mechanical properties, ceramic matrix nanocomposites can also be used as thermal barriers. A good example is a
nanocomposite composed of SiC nanoparticles dispersed within a
pyrolytic graphite matrix.
This material exhibits outstanding oxidation resistance and thermal
shock. Ceramic-matrix nanocomposites have also been reinforced
with metallic nanoparticles, particularly for improving optical, electrical, and magnetic properties. Examples include alumina-matrix
nanocomposites filled with W, Ni, Fe, Au, and Cu; zirconia-matrix
filled with Ni and Mo; magnesia-matrix filled with Fe, Ni, and Co;
and silica filled with Fe, Au, and Ag. In the optical area, silica-matrix
nanocomposites filled with nanoparticles of Au and Ag exhibit
high optical nonlinearities (which describe the behavior of light
in media in which the dielectric polarization responds nonlinearly
to the electric field of the light), which are essential for photonic
devices. In the case of silica filled with Ag, the nanocomposite shows
changes in behavior from semiconducting to metallic, depending
on the size of the nanoparticles and temperature. This seems to be
the result of electron tunneling between the metallic nanoparticles
and metallic conduction through percolated paths, which can be
controlled by tuning the volume fraction of nanoparticles, their distribution and uniformity, size, and shape. In the field of magnetism,
Special Cases