Biomedical Engineering Reference
In-Depth Information
13.25
(a) Increasing grain size of Y
2
O
3
with density in normal sintering
(heating schedule shown in inset). Even with fine starting powders
(30 nm), the final grain size of dense ceramics is well over 200 nm
regardless of whether dopant was used. The shaded area indicates the
grain size regime commonly defined as nanostructured materials. At
lower densities, the mean grain (particle) size was estimated on the
fracture surface. At higher densities, the grain size was obtained by
multiplying by 1.56 the average linear intercept length of at least 500
grains. (b) Grain size of Y
2
O
3
in two-step sintering (heating schedule
shown in inset). Note that the grain size remains constant in the second
sintering step, despite density improvement to 100%.
that the densification can continue, whereas grain growth is limited because
grain boundary migration is suppressed. This significant discovery proves
that it is possible to decouple grain growth from densification and, hence, to
achieve full densification while retaining nanoscaled grain sizes.
13.5.2 Use of grain growth inhibitors
The use of grain growth inhibitors is a common method for controlling
grain growth during sintering. For example, with the addition of SiC to
Al
2
O
3
,
111
ZrO
2
to
β
00
-Al
2
O
3
,
112
and Al
2
O
3
to cubic ZrO
2
,
113
large grain
growth can be effectively prevented. The use of grain growth inhibitor can
also be found during sintering of other materials.
87,114-116
Grain growth inhibitors are widely used in manufacturing fine and
ultrafine grained cemented tungsten carbide (WC-Co) materials.
117-119
When VC, or VC combined with Cr
2
C
3
, is used in liquid-phase sintering of
nanosized WC-Co powder, grain size after sintering is dramatically finer
than if grain growth inhibitors had not been used. Researchers
18,120
found
that the effect of VC on grain growth during sintering of WC-Co
significantly inhibits the rapid grain growth during the solid state as well
as the liquid phase sintering stage. However, the finest grain size that has
been reportedly achieved using grain growth inhibitors in pressureless
liquid-phase sintering processes is approximately 100 to 200 nm or larger,
which is significantly larger than the original grain size (10 nm) of the
