(197d) Instrumented Uniaxial Compaction Experiments on Silicon Nitride Granulates under Varied Climatic Conditions

When pressing silicate ceramic granulates it is known that, up to an optimum value, higher moisture contents affect the total compression behaviour in a favourable way. There are numerous references from the industrial practice which show that in the area of advanced ceramics the relations are by far more complicated. From one day to the next, problems can arise with the compactibility of the granules which can lead to macroscopic defects such as end - capping defects or laminations in the green body. The term ?press weather? is a common expression.

In the present study an attempt was made to reach a better understanding of the interactions between climatic conditions and compaction behaviour of silicon nitride granules with different binder / lubricant systems.

For the experimental investigations an instrumented pressing tool was used, which was installed in a laboratory in which humidity and temperature can be adjusted in a wide range. Primary measured parameters are the forces at the upper and the lower punches, the radial stress on the die wall and the distance covered by the top punch during compaction. Die and bottom punch are fixed. The instrumented device allows the derivation of numerous important parameters for an estimation of the quality of the compaction process. These comprise a continuous measurement of the pressure - density development and parameters which characterize the friction conditions such as the axial force transmission quotient, wall and powder friction coefficients and the ejection forces in the static and dynamic regions. From the calculation of the total compaction energy and their single parts, conclusions can be drawn about the important parts of plastic or elastic deformation. The measurement of the elastic redeformation in axial and radial directions as well as the calculation of the shear stresses allow statements on acting failure mechanisms. In the end, from the distribution of compressive stresses an overview of the density in the compact can be given.

The starting material was a commercially available silicon nitride powder with an average particle size of 0,9 µm and 11 % of sintering aids. Two types of binders, one of the group of polyvinyl alcohols and one from the acrylic emulsion group were added. A ready for use emulsion of a salt of a fatty acid acted as the lubricant. After a spray granulation process the granulates were stored in a laboratory atmosphere of 15 %, 30 %, 50 %, and 75 % relative humidity at a constant temperature of 22° C until a constant mass was reached.

After storage at 15 % relative humidity the original moisture of granulates stayed still unchanged for both types. At 75 % the moisture content rose from 0,44 % to 0,76 % for the granulates with the acrylic binder, the increase for the PVA -containing granulates was with 0,51 % to 1,02 % higher.

As expected, growing granulate moisture leads in either case to an improvement of the compressibility which can be attributed to a better deformability of the binder component as a result of reducing the glass transition temperature.

The green strength of the compacts was influenced in an opposite way. Apparently the binding force of the binders decreases with higher moisture contents resulting in diminished diametral compression strength of the compacts. After the ejection of compacts pressed from granulates which were stored at 75 % relative humidity, typical macroscopic defects such as laminations and end - capping are indicated; the break pictures after measurement of the diametral compressive strength show no characteristic straight plane breaks as caused in normal cases by exclusively acting tensile stresses.

Serious changes also occur with respect to the friction conditions in dependence on moisture of granulates. No inhomogeneities could be observed in the curves of loading, pressure holding, unloading and ejection for original granulates and those which were stored below a humidity of 50 %. In all other cases typical stick - slip - mechanisms were recorded, which were frequently connected with characteristic noises. These mechanisms must be avoided with respect to the surface quality of the compacts and the wear of the die.

With the increasing moisture of granulates, the wall friction coefficients rise, which is a clear sign of a disturbance of the efficacy of the lubricant. Interestingly, the combination with the acrylic binder proved to be more sensitive. A similar tendency arises from the measured ejection forces. These reduplicate for the static friction and reach a four times higher amount in the sliding region if the original granulates and the ones stored at 75 % are compared. Again, the combination with the acrylic binder must be estimated as the worse version. The radial stress transmission remains nearly unaffected.

Changing moistures influence the deformability of granulates, too. From the system of compaction energies can be derived that with higher moistures the elastic deformations increase up to the specified pressure. This is one of the most important reasons for the development of compaction failures. As it could be expected from the friction coefficients, the part of energy losses enlarges with higher moisture of granulates.

An explanation of the causes for pressing failures can be drawn from the analyses of the proportions of the elastic relaxation during unloading as well as during and after ejection. The results show that the rate of axial relaxation which takes place already inside the die decreases if the moisture content in granulates is on a higher level. This means that the green body contains more elastic stresses when it reaches the top edge of the die. The additional possibility of radial expansion leads to the observed end - capping - defects and laminations if compacts were pressed from granulates stored at higher humidity before.

The calculated stress distributions support all previously made statements on the influence of moisture changes. Pressing of granules with increasing moisture leads to more than a duplication of shear stresses. After the storage of granulates at more than 50 % humidity, the binder is not able to compensate these stresses and defects will develop. In the same way, the differences in the axial distribution of pressure stresses increase, resulting in larger density gradients.

The achieved results show that the compaction behaviour and the quality of uniaxial pressed compacts are significantly influenced by climatic conditions at storage and processing of granulates. Furthermore, it was demonstrated that not every fact known about the compaction of granulates from silicate ceramics can be applied to the group of advanced ceramics. For the examined silicon nitride granulates with the described binder / lubricant combinations a deleterious influence of moistures, after their storage at a humidity of more than 50 %, was verified. Generally speaking, a greater effort is needed for the most important base materials of advanced ceramics to get a better understanding of the complicated correlations and interactions between the binder and lubricant properties of different types under varying humidities, temperatures and pressures. The determination of a system of specific pressing parameters with an instrumented compacting tool is a suitable way to achieve such progresses and to overcome the widespread empiricism.