Biomedical Engineering Reference
In-Depth Information
13.20
Coalescence of two platelet-shaped grains of a nanocrystalline
WC-Co compact heated up to 1200
8
C at a heating rate of 10
8
C/min. and
held for 1 min.
discussing the initial grain growth during sintering of nano particles. As
discussed earlier, for single-phase materials at late stages of sintering when
relative density is greater than 90%, grain boundary migration is the most
logical mechanism of grain growth found in bulk single-phase materials.
Grain boundary migration has also been observed during early stages of
sintering nanosized Al
2
O
3
.
59
Figure 13.19 shows that when the nanosized
Al
2
O
3
particles were subjected to high temperatures in a flash sintering set-
up, instant grain growth was observed and grain boundary migration was
believed to be part of the process. This confirms the analysis of the
coarsening of nano particles that when r
1
/r
2
is greater than R
c
, grain
boundary migration will take place.
Coalescence is another grain growth mechanism that is often cited to
qualitatively explain rapid grain growth. Coalescence is a term that is often
loosely used to describe various phenomena. For example, coalescence is
sometimes used interchangeably with the term 'sintering' to describe the
growth of particles during particle synthesis and growth process.
102-105
For
clarity in this chapter, coalescence is used strictly to describe the increase of
grain size due to the merging of two grains by eliminating the common grain
boundaries between them. Differing from other grain growth processes,
which may also be described as the merging of two grains, the two original
grains should not demonstrate significant change from their morphology
prior to coalescence.
The term coalescence, as defined above, describes a unique method of
grain growth, which can be accomplished only through various diffusion
mechanisms. Possible mechanisms for coalescence include grain boundary
diffusion, dislocation climb along grain boundaries, or even grain rotations.
