chaPter 7 nanomaterials: Properties
252
silica nitride matrix nanocomposites containing fine dispersions of
Co nanoparticles (<10 nm) revealed supermagnetism properties, a
large magnetic hysteresis, and high coercive fields.
Ceramic-matrix nanocomposites filled with semiconductor nanoparticles such as GaAs also show interesting optical properties.
When these nanoparticles are embedded in a silica matrix, the
properties of photoluminescence (a process in which a substance
absorbs photons and then radiates back photons) are altered due
to the dimensional confinement of the nanoparticles. Because these
particles need to be separated to maximize the effect, the creation of
a nanocomposite is ideal.
Nanocomposites can also be used as thin films for various applications. These films typically consist of multiple nanoscale layers, each
composed of a different material, or single-layered materials reinforced with a second phase at the nanoscale. The multilayer nanocomposites are normally used in applications where high hardness,
modulus, and wear properties are important. This behavior is due
to the fact that the incoherent interface between the layers provides
a resistance to elastic deformation and plastic deformation. In parallel, multilayered nanocomposites have also been developed for
magnetic recording.
Single-layer nanocomposite materials are used for mechanical,
electrical, and magnetic applications in which at least the matrix
or the filler has particular mechanical, electrical, and/or magnetic
properties. In terms of mechanical properties, nanocomposite thin
films have been widely used for nanocoatings. They provide good
wear resistance, increased toughness, and high thermal stability.
In the case of nanocoating multilayers, which are typically made
of TiC, CrN, TiN, TiAlN, and alumina, the films have many layers
to enable crack deflection at the interfaces and improve toughness. In addition, these multilayer nanocomposite coatings exhibit
increased hardness due to the fact that the interfaces act as barriers
to dislocation motion. However, if the layers are too thin, the strain
field of the dislocations is truncated by the interfaces, leading to a
decrease in hardness. Furthermore, depending on the thickness of
the layers, lattice mismatch can lead to additional residual strains,
with consequences for the mechanical properties. Therefore, for
various applications, the thickness of the layers is carefully tuned
so that high toughness, good thermal stability, and hardness are
present, particularly at high temperatures.
In the case of a single-layer nanocomposite coating, hard nanoparticles are typically embedded in an amorphous matrix. For example,
252
silica nitride matrix nanocomposites containing fine dispersions of
Co nanoparticles (<10 nm) revealed supermagnetism properties, a
large magnetic hysteresis, and high coercive fields.
Ceramic-matrix nanocomposites filled with semiconductor nanoparticles such as GaAs also show interesting optical properties.
When these nanoparticles are embedded in a silica matrix, the
properties of photoluminescence (a process in which a substance
absorbs photons and then radiates back photons) are altered due
to the dimensional confinement of the nanoparticles. Because these
particles need to be separated to maximize the effect, the creation of
a nanocomposite is ideal.
Nanocomposites can also be used as thin films for various applications. These films typically consist of multiple nanoscale layers, each
composed of a different material, or single-layered materials reinforced with a second phase at the nanoscale. The multilayer nanocomposites are normally used in applications where high hardness,
modulus, and wear properties are important. This behavior is due
to the fact that the incoherent interface between the layers provides
a resistance to elastic deformation and plastic deformation. In parallel, multilayered nanocomposites have also been developed for
magnetic recording.
Single-layer nanocomposite materials are used for mechanical,
electrical, and magnetic applications in which at least the matrix
or the filler has particular mechanical, electrical, and/or magnetic
properties. In terms of mechanical properties, nanocomposite thin
films have been widely used for nanocoatings. They provide good
wear resistance, increased toughness, and high thermal stability.
In the case of nanocoating multilayers, which are typically made
of TiC, CrN, TiN, TiAlN, and alumina, the films have many layers
to enable crack deflection at the interfaces and improve toughness. In addition, these multilayer nanocomposite coatings exhibit
increased hardness due to the fact that the interfaces act as barriers
to dislocation motion. However, if the layers are too thin, the strain
field of the dislocations is truncated by the interfaces, leading to a
decrease in hardness. Furthermore, depending on the thickness of
the layers, lattice mismatch can lead to additional residual strains,
with consequences for the mechanical properties. Therefore, for
various applications, the thickness of the layers is carefully tuned
so that high toughness, good thermal stability, and hardness are
present, particularly at high temperatures.
In the case of a single-layer nanocomposite coating, hard nanoparticles are typically embedded in an amorphous matrix. For example,