Instrumented uniaxial compaction experiments on silicon nitride granulates under varied climatic conditions

From empirical observations in the industrial practice it is known that compaction behavior and quality of uniaxial pressed ceramic compacts may be significantly influenced by climatic conditions at storage and processing of granulates. At the same time, the kind of basic material, added binders and lubricants can play a major role in most cases. In the present study, an attempt was made to reach a better understanding of the interactions between climatic conditions and compaction behavior of silicon nitride granulates with different binder/lubricant systems by using instrumented compaction experiments. In addition to the continuous measurement of the pressure-density development and the strength of the compacts, parameters are measured which characterize the friction conditions, the total compaction energy and their single parts, the elastic redeformation in axial and radial directions. From the calculation of shear and compressive stresses, statements on acting failure mechanisms as well as predictions on the expected density distribution can be given. For the examined silicon nitride granulates a deleterious influence of moisture on the whole pressing process and the defect rate of the compacts after storage of granulates at a humidity of more than 50 % before compaction, was verified. For this material the changes in compaction behavior are not reversible after storage at lower humidities. The achieved results show that the compaction behavior 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 role of moisture during 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, after their storage at a humidity of more than 50 %, a deleterious influence of moistures was verified. The experiments have shown that the absorption and desorption are indeed reversible processes for that case, but not the compaction behavior. The influence of cyclic changing climatic conditions must be avoided under all circumstances. 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 humidity, 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.