Lec. No. (23)
Nanophase ceramic composites
The application of ceramics has infiltrated almost all fields in the last 20
years, because of their advantages over metals due to their strong ionic or
covalent bonding. But it is just this bonding nature of ceramics that
directly results in their inherent brittleness and difficulty in machining. In
other words, ceramics show hardly any macroscopic plasticity at room
temperature or at low temperatures like metals. Hence, super plasticity at
room temperature is a research objective for structural ceramics. In recent
years, many researches have been carried out to investigate nanophase
ceramic composites.
Depending on the matrix grain size, nanophase ceramic composites can
be classified in two fundamental groups. One is composed of micro meter
sized matrices dispersed with a nanometer second phase, In this group,
the second phase plays a crucial role that affects the microstructure and
the properties. it has divided it into three types – intragranular,
intergranular and intra-/intergranular – trying to relate the distribution of
the second nanophase in the matrix and it arises
improvements in
toughness, strength at room temperature and high temperatures, creep
strength and thermal shock resistance by incorporating nanocrystalline
dispersion in a microcrystalline matrix. The other group of nanophase
ceramic composites is nanocrystalline matrix composites, also called
nanoceramics, in which the matrix grain size is below 100 nm. The nano–
nano type microstructure will be formed when the second phase is also
nano-scaled. Nano ceramics exhibit promising properties due to the
changes in deformation mechanisms when the grain size is reduced to the
order of 100 nm. The superplasticity of nanocrystalline CaF2 and
nanocrystalline TiO2 at low temperatures, indicates that ceramics are
learning to ‘bend’ instead of fracture. Furthermore, nanoceramics also
show high toughness, in which a novel toughening mechanism called
Ferroelectric Domain Switching is recognized, different from that in
micro– nano type ceramic composites.
Micro–nano type ceramic composites
In early nanocomposites, hard and strong dispersoids, such as SiC,
Si3N4, TiC, etc., were mainly incorporated into the matrix to improve the
mechanical properties. But in later years, enhancement of fracture
strength was also achieved by addition of even soft and weak dispersoids
like metals, graphite and h-BN . The density, microstructure and
mechanical
properties
of
nano-sized
particulate
dispersion
nanocomposites were strongly dependent on the volume fraction of
particulate dispersion and sintering conditions.
Hard nanoparticles usually have a higher sintering temperature than that
of the matrix, so that the sintering temperature is increased with
increasing hard-particulate content. In Al2O3/SiC systems, only 5 vol%
SiC incorporation can evidently cumber the densification process. The
nearly full densities attained by hot-pressing (HP) were achieved at
1600°C for 5 vol% SiC, at 1700°C for 11 vol% and at 1800°C for up to
33 vol% .
In general, particles disperse according to their grain size and the variety
of the matrix. For Al2O3/SiC, finer particles disperse within the matrix
grains and larger particles at the grain boundaries. The critical grain size
is typically 200 nm. For the MgO/SiC, Al2O3/Si3N4 and natural
mullite/SiC composites, nanoparticles homogeneously disperse within as
well as at the grain boundaries,
The Al2O3/SiC, Al2O3/Si3N4 and MgO/SiC nanocomposites give a
notable improvement in high-temperature strength up to and over
1000°C. In particular, the greatest improvement in high-temperature
strength was observed for the MgO/SiC nanocomposites . It is well
known that grain boundary sliding and/or cavitation are responsible for
the high temperature strength degradation of oxide ceramics. Thus, the
enhancement in strength at high temperatures is mainly due to the
prohibition of the grain boundary sliding or cavitation by the dispersions
within the matrix grains.
Lec. No. (24)
Nano–nano type ceramic composites
Nanocrystalline materials form an exciting area of materials research
because bulk materials with grain size less than 100 nm have properties
not seen in their microcrystalline counterparts. But the brittleness of
nanoceramics has limited their potential for use in structural applications,
namely, research on nanoceramics shows that they are not inherently
tougher than their microcrystalline counterparts. Many strategies have
been proposed to improve the mechanical properties of nanoceramics by
using reinforcement by a second-phase addition and hybridization to
develop nanocrystalline matrix composite materials.
Metal nanoparticle dispersed nanocomposites
The mechanical properties of ceramics were improved by the addition of
nano-sized metal particles that were dispersed in the ceramic matrix.
Ceramic/ metal nanocomposites consisted of an oxide ceramic and either
refractory metal such as in the Al2O3/W, Al2O3/Mo and ZrO2/Mo
systems . These composites were fabricated by hot-pressing fine ceramic
and metal powder mixtures or by reducing and hot-pressing the matrix
and metal oxide powders .
Lec. No. (25)
Fabrication of nanoceramics
Research
on
processing
fully
dense
bulk
nanoceramics
and
nanocomposites is attracting more and more interest. There have been
some low-cost but effective processes to obtain nano-sized ceramic
powder and nanocomposite powders, such as sol-gel, micro emulsion, codeposition and high-energy ball-milling (HEBM). One of the principal
problems is the inability to consolidate nanopowders to high relative
density without grain growth. To obtain the dense bulk nanoceramics, it
is essential to decrease either sintering temperature or retaining time at
the highest point, or both HP, HIP, high-pressure sintering and fast
consolidation techniques such as microwave sintering and spark plasma
sintering (SPS) have been employed.
Among these techniques, high-pressure sintering seems to be the best
way of obtaining fully dense nanoceramics at the present time. The
application of high pressure over 1–8 GPa results in a decrease of the
temperature ,within which fully dense compacts can be obtained without
grain growth or with only very limited grain growth. For Al2O3 based
nanoceramics, success in achieving such fine grain size can be mainly
attributed to two factors;
Firstly, lower sintering temperature no doubt leads to lower coarsening.
Secondly, γ → α transformation before sintering is quite important. This
transformation is known to be nucleation controlled, Also, the presence of
the second phase helps to restrain the grain growth.
Fast consolidation techniques, such as microwave sintering and plasma
activated sintering (PAS), can enhance sintering and reduce the time
available for grain growth. It allows very fast heating and cooling rates,
very short holding times, and the possibility of obtaining fully dense
samples at comparatively low sintering temperatures, typically a few
hundred degrees lower than in normal hot pressing.
Unlike the first-generation spark sintering and the second-generation
PAS, SPS can result in better control of the microstructure and properties
of materials in terms of sintering temperature and time. It is a pressuresintering method based on high-temperature plasma (spark plasma)
momentarily generated in the gaps between powder materials by
electrical discharge at the beginning of on–off DC pulsing. In this
process, powders are loaded into a graphite die and are heated by passing
an electric current through the assembly. These processes have now been
developed beyond the production of small objects with simple shapes, as
continuous production of compacts of complex geometry and of pieces
with diameter larger than 150 mm has been achieved. Despite the fact
that a uniaxial pressure is applied, green bodies of complex geometry can
be exposed to a ‘pseudo-isostatic’ pressure when embedded in freeflowing electrically conducting particulates that act as a pressuretransmitting medium inside the die. By designing the mold, a temperature
gradient along the direct current can be obtained, which is advantageous
to simultaneously consolidating components with different sintering
temperatures .