Biomedical Engineering Reference
In-Depth Information
sintering driving force when an external pressure is applied includes both the
intrinsic curvature-driven sintering stress and the applied external stress.
The significance of the applied pressure on sintering depends on the relative
magnitudes of the two components. The applied pressure is independent of
the particle size, while the intrinsic sintering pressure increases when the
particle size is reduced, and it can reach a very high value as particle size is in
nanoscale. In order to make the effect of applied pressure on densification
become dominant, the applied pressure has to be larger than the intrinsic
curvature-driven pressure.
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Therefore, a threshold pressure, which is
dependent on the particle size, must be exceeded so that the applied pressure
can have a significant influence on sintering. The existence of the threshold
pressure had been confirmed experimentally by Skandan et al. on sintering
of nanosized zirconia powder.
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The logic for this approach is based on
the belief that densification can be achieved at lower-than-normal sintering
temperatures with the aid of pressure. The lower temperature would, of
course, slow the kinetic rate of grain growth. For example, Hayashi and
Etoh
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studied sintering behavior of nanosized Fe, Co, Ni and Cu metal
powders under pressures ranging from 400 to 500MPa. The nanosized
powders were fabricated by evaporation and condensation method in an
inert gas. The starting average particle size of Fe, Co and Ni were about
20 nm; that of Cu was about 50 nm. The results showed that sintering
temperatures were effectively decreased by increasing pressure. Under
400MPa, the sintering temperatures for Fe, Co, Ni and Cu powders were
about 590, 640, 450 and 450K respectively, which were 380-620 K lower
than those without pressure. The minimum average grain sizes of Fe, Co, Ni
and Cu in the fully densified compacts after sintering were about 80 nm,
210 nm, 120 nm and 400 nm, which were much smaller compared to those
obtained by pressureless sintering.
Pressure-induced phase transformation is a phenomenon that was observed
in several ceramic materials during the consolidation of nanosized powders
under high pressure. It was also characterized as 'transformation-assisted
consolidation' (TAC).
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TAC has also been used to densify several
materials, including Al
2
O
3
and Si
3
N
4
.
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The key criterion for the suitability
of this technique is that the starting material must be a metastable phase,
which transforms into the desired stable phase in a controlled way during
pressing and sintering. The combination of increased nucleation and
controlled grain growth produces fully consolidated material with nanosized
grain structure. As an example, nanosized TiO
2
undergoes phase
transformation from anatase to rutile when pressure is higher than 1 GPa
during a pressure-assisted consolidation process.
128,135
The grain size of the
product phase (rutile) is smaller than that of the parent phase (anatase) after
consolidation. Under such high pressure, sintering temperature is as low as
3
of T
m
. The theory regarding the effect of high pressure on the sintering
