Sinter Process Optimization - Si3N4

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.

Fig1. Measured data(symbols) and Kinetic model (solid lines) for sintering of Si3N4
Fig.2 Predicted temperature program for RCS with constant length change rate 0.087%/min

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.

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.

Fig.3 Predicted temperature program for RCS with constant length change rate 0.147%/min

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.