Biomedical Engineering Reference
In-Depth Information
rates are controlled by densification of the metal phase and such FGMs can
be densified by conventional sintering methods (Neubrand and Rodel 1997).
In addition to conventional sintering, reactive powder processing, also
called combustion synthesis or self-propagating high-temperature synthesis
(SHS), can be used if the target compounds can be synthesized from the
starting powder mixture (Stangle and Miyamoto 1995). This process
comprises a rapid and exothermic chemical reaction to simultaneously
synthesize some or all of the constituent phases in the FGM and densify the
component.
More advanced techniques, such as spark plasma sintering (SPS) or
pulsed electric current sintering (PECS) can also be used for FGM
fabrication (Tokita 1999). These are pressure-assisted sintering methods in
which a high current is pulsed through a die/punch/sample set-up, which can
be compared with that of conventional hot-pressing. The large current
pulses generate spark plasmas, a spark impact pressure and Joule heating.
The sintering mechanism and mechanical properties of the sintered
compacts show characteristics different from conventional pressure-assisted
sintering processes and parts. This technique offers significant advantages
for various kinds of new materials and consistently produces a dense
compact in a shorter sintering time and with a finer grain size than
conventional methods. With a spark plasma system, large ceramic/metal
bulk FGMs (
100 nm across) can be homogeneously densified in a short
time with heating and holding times totalling less than one hour. Among the
SPS systems are WC-based materials (WC/Co, WC/Co/steel, WC/Mo),
ZrO
2
-based composites (ZrO
2
/steel, ZrO
2
/TiAl, ZrO
2
/Ni), Al
2
O
3
/TiAl, etc.
(Tokita 1999). To obtain full densification and to overcome the problem of
metal softening in WC-based nanocomposites using the SPS technique,
6 wt% nano-sized ZrO
2
was incorporated in submicrometre WC. This led to
near theoretical densification (
~
>
99%) at a considerably lower sintering
temperature of 1300
C and holding time of 5min P. The role of ZrO
2
in
enhancing the densification kinetics was also critically analyzed by Tokita.
In other investigations, it was observed that WC-based nanocomposites
possess excellent wear resistance (Biswas et al. 2007, Venkateswaran et al.
2005).
Microwave sintering is another promising technique for ceramic/metal
FGMs to avoid the difficulty of inequality of the shrinkage rate. As a newly
developed sintering technique, microwave sintering used microwave
irradiation to heat ceramics or ceramic-based composite compacts (Gerdes
and Willert-Porada 1994, Willert-Porada et al. 1995, Zhao et al. 2000). The
mechanism of microwave heating is based on the dielectric loss of the
ceramic phases involved, resulting in a volumetric heating technique in
which heat is generated by the compact itself.
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