Biomedical Engineering Reference
In-Depth Information
13.14
Grain size vs. temperature during heating up of nanocrystalline
WC-Co powder at a heating rate of 108C/min.
Grain growth during sintering of nano particles is also a strong function
of temperature. Figure 13.13
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shows the relationship between grain size and
temperature during the heat treatment of nanocrystalline cobalt powder. It
is obvious that the grain growth is initially slow at very low temperatures
and that it accelerates dramatically when the temperature is above an
apparent critical temperature range. Figure 13.14 provides another example
of the relationship between grain size and temperature during heating up of
nanocrystalline WC-Co powder at a heating rate of 10
C/min.
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It shows
that the original 20 nm grain size has increased almost 45-fold to 900 nm.
This 'explosive' grain growth occurs almost instantly during heat-up, with
no significant holding time. Similar behavior has also been reported for
sintering of other nanocrystalline ceramics, as well as for metallic
powders.
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It appears that a critical temperature exists, above which
the grain growth accelerates dramatically as a function of temperature.
The issue of grain growth during sintering can be studied by examining
the grain size versus relative density relationship. This approach has been
applied to the study of sintering of nano particles. A typical relationship
between grain size and density during sintering of nano particles is
schematically shown in Fig. 13.15. In one of the earliest studies of the
sintering and grain growth of nanosized ceramic powders in the 1990s,
Owen and Chokshi
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and Averback et al.
88
showed that oxides densify
without significant grain growth until the density reaches approximately
90% of the bulk density. Then the grain growth becomes very rapid. This
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