Biomedical Engineering Reference
In-Depth Information
tion of the pore and continuation of the sintering. This explains the
correlation between grain growth and the size of agglomerates.
13.5 Techniques for controlling grain growth while
achieving full densification
With regard to processes for sintering nanosized particles, in principle, all
conventional and advanced sintering techniques can be applied. However, if
maximum densification is to be achieved while retaining the nanoscaled
grain sizes, then special processes and techniques are necessary. This section
will highlight techniques that are particularly useful for sintering nanosized
particles.
13.5.1 Two-step sintering: decoupling of grain growth from
densification
As an example of understanding and controlling normal grain growth
during sintering of nano particles, Chen and Wang developed a clever
approach to decouple grain growth from densificaton of nanosized
particles,
108
using a pressureless sintering process to fully densify
nanocrystalline Y
2
O
3
. In a simple two-step process, the compact is briefly
heated to 1310
C and held at
that temperature for an extended period of time. As a result, the material
can be sintered to full density with minimum grain growth. If the lower
temperature is applied at the onset, complete densification would not be
possible. It is reasoned, then, that suppression of the final-stage grain
growth is achieved by exploiting the difference in kinetics between the grain-
boundary diffusion and the grain boundary migration. Grain growth
requires grain boundary migration, which requires higher activation energy
than grain boundary diffusion. At a temperature that is high enough to
overcome the energy hurdles for grain boundary diffusion, but low enough
to deactivate grain boundary migration, the densification will proceed via
grain boundary diffusion without triggering significant grain growth. This
phenomenon was further studied in multiple publications of Kim et al.
109,110
It is noted, once again, that in this successful work to decouple grain
growth from densification by exploiting differences in grain boundary
mechanisms, the authors explicitly showed that at the beginning of the
second sintering step, the grain size increases to four to six times larger than
the original size of the powder, which is attributed to coarsening during the
first sintering step (Fig. 13.25). It is necessary for the first step to be carried
out at a higher temperature in order to quickly achieve a high relative
density. The second isothermal step at the lower temperature is selected so
8
C; the temperature is then lowered to 1150
8
