Biomedical Engineering Reference
In-Depth Information
behavior of these materials has two key points. One is that the pressure
reduces the nucleation barrier for a phase transformation that is
accompanied by volume reduction, as in the case of anatase to rutile of
TiO
2
, and
phase for Al
2
O
3
. The second point is that the high
hydrostatic pressure reduces the diffusion rate and, thus, grain growth rate.
Sinter-forging is another unique technique that has been used to produce
fully dense materials with nanosized grains from nanosized ceramic or metal
powders. First of all, sinter-forging, also termed powder-forging, is a routine
process that has been used to manufacture ferrous powder metallurgy
automobile parts. In this case, green compacts are first sintered via a
standard atmospheric sintering process, and then the sintered compacts are
placed in a forging die and forged at an elevated temperature. The forging
process results in fully dense metal parts.
In recent years, the term 'sinter-forging' has been used to describe the
processing of nanoscaled powder materials in a different way from
conventional sinter-forging. In this process, powders are placed in a die
without lateral constraint. The die and powder are heated via techniques
similar to the hot pressing process and a uniaxial load is applied. During
sinter-forging, the powder compact is allowed to 'upset,' i.e. bulge laterally,
and is densified under load at specific temperature. In most of the published
research regarding sintering of nanosized powders using the sinter-forging
process, a typical hot press is adapted for experiments. Load capacity and
loading rate are thus comparable to a conventional hot pressing process. In
a study on the consolidation of nano 3Y-TZP (3mol% yttria-stabilized
tetragonal zirconia polycrystals) using sinter-forging process, the loading
rate varied from 0.005 to 0.1 kN
-1
, and sintering temperature was around
1000-1100
γ
to
α
C. Maximum load was not allowed to exceed 30 kN.
136
In
another example, Ma et al.
137
applied a 'constrained sinter-forging' or 'upset
hot forging' process scheme to consolidate nanocrystalline Fe and Fe-Cu
alloyed powders. Consolidated nanocrystalline Fe with grain sizes within the
typical range of nanocrystalline (
8
100 nm) was achieved.
Spark plasma sintering (SPS) is a new pressure-assisted sintering process
that quickly gained popularity with researchers looking for ways to
consolidate materials with nanoscale or simply very fine grain sizes, or
with other non-equilibrium microstructures. The history of development,
the principles and applications of SPS are discussed in the literature; readers
can also refer to for reviews.
130,138
<
13.6 Conclusion
Sintering of nanosized particles is a uniquely important topic that is both
scientifically and technologically challenging. From a scientific perspective,
the markedly different sintering behavior of nanosized powders, compared
