The influence of lubricants on uniaxial dry pressing of silanised silicon nitride powder

The influence of three surfactants (stearic acid, octadecylamine and octadecyl-trimethoxysilane) on the dry-pressing behaviour of silicon nitride powders was investigated with an instrumented compacting tool. The study reveals that friction processes have a large impact on the compaction behaviour as well as on the properties of green bodies (density, strength, gradients, macroscopic defects). All surfactants reduce friction by boundary lubrication. However, the functional groups cause different interactions of the surfactant molecule with the powder surface, and therefore, different lubrication properties. Octadecyl-trimethoxysilane can form a stable, covalent bond to the powder surface. Hence, it effectively reduces powder–powder friction, but is of little influence on powder–wall friction. This surfactant causes a significant increase of the green density but has only a limited effect on axial density gradients and strength of the green bodies. Stearic acid and octadecylamine have functional groups, which cause only adsorption processes. Therefore, such compounds exhibit an opposite lubrication behaviour with a high efficacy on the die wall. For silicon nitride, stearic acid is more effective than the octadecylamine due to a stronger adsorption of the amine to the silica-like powder surface.