Hard-skin development during binder removal from Al2O3-based green ceramic bodies

A paraffin-based binder system was developed for low-pressure injection molding of very fine (0.4 μm) average particle size alumina ceramic bodies. Complex chemical reactions take place, during the process of binder removal, when the ceramic parts are fired in air. The effects of these chemical reactions become more pronounced at about 250°C, giving rise to a hard-skin and promoting cracks in the bodies with large cross-sections. The surface hardening occurs only when the bodies are fired in an oxygen-rich atmosphere. The hardness at the surface of the ceramic bodies fired in air at 250°C varies from about 0.4 to 1.2 GPa, depending on binder composition. Upon increasing temperature, the surface hardness decreases monotonically to more conventional values. Infrared absorption spectroscopy was used to study the debinding process and the chemical mechanism behind the hard-skin development in ceramic bodies fired in air. The experimental evidence suggests that cross-linkage between long carbon chains present in the binder and its interaction with the alumina powder surface is responsible for the high surface hardness observed in parts fired in air at 250°C.