Biomedical Engineering Reference
In-Depth Information
13.24
Schematic of pore coordination number distribution of
agglomerated powder indicating three classes of pores, i.e., those
within domains, those between domains, and those between
agglomerates (
R
stands for coordination number).
agglomerate of WC-10%Co when heated to 800
C within a powder
compact, while Fig. 13.22 shows the structure when the same compact is
heated to 1200
8
C. It can be seen that the original agglomerates, within
which the WC grains are visible at 800
8
C. Instead,
the individual grains with sizes similar to those of the agglomerates at lower
temperatures constitute the microstructure. It is thus deduced that the
densification and grain growth processes during sintering of nano particles
progressed via consolidation and grain growth within individual agglom-
erates, and then proceeded to the consolidation and elimination of
porosities between agglomerates. This mechanistic process of sintering was
also observed and discussed by Petersson and A
˚
gren.
72
The process that first
takes place within individual agglomerates was characterized as 'nucleation'
sites.
To explain the effect of agglomerates, Lange
63
classified the structure of a
powder compact as a hierarchical structure of agglomerates, domains and
primary particles, as shown by Fig. 13.23. Defining the coordination
number as the number of particles surrounding the pore, Lange explained
that pores within domains have the lowest coordination number, pores
between domains have higher, and pores between agglomerates have the
highest coordination number. Figure 13.24 shows schematically the volume
distribution of the three classes of pores as a function of coordination
number. When N
8
C, no longer exist at 1200
8
N
c
, a pore is unstable. Otherwise, grain growth, or
coarsening of particles within agglomerates, will be necessary for elimina-
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