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In the production of
modern high performance technical ceramics a green body is
frequently manufactured by mixing a ceramic powder and a
binder material. The green body is fired afterwards to
achieve the final product. The temperature program during
the firing process, most of all during
binder burnout and
sintering phase, has a paramount influence on the later
product quality.
For decades pushrod
dilatometers have been used for the determination of the
dimensional changes during the sintering process. With these
instruments it is possible to measure the length change of a
sample subjected to a controlled temperature program. By
modification of the temperature program it is possible to
optimize the properties of the sintering products, such as
densification and grain size distribution.
A new method for the
optimization of the sintering process makes use of the
possibilities of NETZSCH Thermokinetics [1]. The
basis are dilatometer measurements at different heating
rates. The result of the kinetic analysis is in most cases a
multiple-step formal kinetic approximation of the
measurement data. On the basis of this description, the
material behavior for different temperature profiles or for
certain sintering processes (e.g. for constant shrinkage
rate) can be calculated.
Experimental
Cold pressed green bodies
of Si3N4 powder (TeCe Technical Ceramics) were examined. The
samples contain 5 m% of Y2O3 and 5 m%
of Al2O3 as sintering additives. A
pushrod Dilatometer (NETZSCH
DIL 402C/7) was
used for the measurements. The tests were carried out
between room temperature and approximately 1800 ¡C with
constant heating rates of 5, 10, and 10 K/min. Within the
same temperature range, measurements were carried out under
the condition of constant shrinkage rates of 0.087 and 0.174
%/min.
Results
Figure 1 depicts the
length change between 1050 and 1850 ¡C. With increasing
heating rate the sintering is shifted to higher
temperatures. Different sintering steps can be seen. In the
step between 1250 and 1450 ¡C a liquid glass phase is
formed, surrounding the Si3N4
particles. and favoring rearrangement of the particles.
Between 1450 and 1700 ¡C the surface of the
alpha-Si3N4 particles dissolve in the
liquid phase and secret
þ-Si3N4.
Figure 1: Length change of
Si3N4-green bodies at heating rates of
5, 10, and 20 K/min.
Also shown is the fit of the measurement by the kinetic
model (solid lines)
For the kinetic description of the measurement results, a
4-step model indicated in figure 1 was employed. Based on
this model, a non-linear regression was carried out
adjusting the different reaction parameters, such as the
pre-exponential factors and the activation energies. The
result of this adjustment is shown in figure 1 as straight
lines. The kinetic model chosen enables a good description
of the experimental data (symbols). With help of this model
it is possible to calculate sintering processes with
different temperature programs or heating rates. It also
provides the possibility to determine temperature programs
necessary for sintering the sample at a constant shrinking
rate.
Figure 2: Comparison of a
measurement on a Si3N4-green body at a
constant shrinking rate of 0.087%/min with the prediction of
Thermokinetics
Temperature programs for constant sintering rates can be
directly measured with the dilatometer using RCS software
[2]. Appropriate measurements were carried out at
different constant sintering rates. The results are compared
with the predictions of the thermokinetic software. Figures
2 and 3 show both the measured and the calculated
temperature profiles for sintering rates of 0.087 and 0.174
%/min. Both results yield a very good
correlation.
Summary
The sintering behavior of
Si3N4-green bodies was examined with a
high temperature dilatometer using three heating rates.
Based on these measurements it was possible to establish a
formal kinetic model with the NETZSCH Thermokinetics
software enabling predictions concerning the behavior of the
material under modified conditions. The comparison between
the predicted and measured temperature profiles proved the
capability of this method.
[1] J. Opfermann, J. Blumm, W.-D. Emmerich:
Thermochimica Acta 318(1998) 213
[2] H. Palmour III, M. L. Kuckabee, T.M. Haye, in
M.M.Ristic (ed.):
Sintering - New Developments,
Elsevier, Amsterdam, 1979, p. 46.
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