Biomedical Engineering Reference
In-Depth Information
1.8
Test methods for the characterization and
evaluation of thermal shock resistant ceramic
nanocomposites
The performance characteristics and nature of ceramic nanocomposites
depend on well-defined processing routes to achieve a specific microstruc-
ture and detailed characterization of the microstructural features is thus
extremely important. It is particularly critical in identifying the role of the
microstructure in defining the final bulk properties. The microstructural
features in nanocomposites that have been linked to bulk properties include
the matrix grain size, the reinforcing particle size, its distribution and
location (grain boundaries or occluded within the matrix grains), segrega-
tion at the various interfaces and residual stress fields. Various test methods
have been used for the evaluation and characterization of ceramic
nanocomposites, and are discussed in the following subsections.
1.8.1 Scanning electron microscopy (SEM) analysis
The fracture surfaces of composite samples can be analyzed by observing
crack propagation in the matrix phase, interface zones, residual porosity,
grain sizes, etc. from scanning electron micrographs.
1.8.2 X-ray diffraction (XRD) analysis
XRD spectra of ceramic nanocomposites sintered at different temperatures
are extremely useful. Samples sintered at a particular temperature indicate
structural behaviour as either amorphous or crystalline in nature. Such an
indication is of immense help to researchers aiming to improve the
properties of the resultant materials by optimizing the rate of sintering
temperature. XRD spectra can also be used to verify the toughness
characteristics of ceramic nanocomposites. XRD study does not indicate
any phase transformation occurring during a 24 h high-energy ball milling
(HEBM) period even though it is longer than the reported minimal time for
complete transformation (10 h). It is very interesting to note that the width
of XRD for HEBM
￿ ￿ ￿ ￿ ￿ ￿
-Al 2 O 3 nanopowder is much greater than that for the
starting nanopowder without HEBM. The residual stress induced by HEBM
is likely to be responsible for the wider XRD peak. Moreover, HEBM can
lead to high green density due to pore collapse from high compressive and
shear stresses during milling.
γ
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