Advanced Process Model for Polymer Derived Ceramic Processing

Abstract

Pyrolysis of preceramic polymers allows a new type of ceramic materials to be processed at a relatively low temperature. The ceramics via polymer pyrolysis display a number of exceptional mechanical, thermal and chemical properties, including high thermal stability, high oxidation/creep resistance, etc. Moreover, they offer better geometrical accuracy compared to conventional ceramics. In addition, thermal induced pyrolysis of organometallic polymer precursors offers the possibility of net shape manufacturing at a lower temperature compared to traditional powder sintering process. The pyrolysis of polymer precursors involves curing of polymer precursors in which the polymer undergoes cross-linking to form a green body, followed by a pyrolysis stage that involves the formation of amorphous SiC and crystallization of SiC at a higher temperature. The source material changes phase and composition continuously during polymer pyrolysis based ceramic process. Chemical reactions and transport phenomena vary accordingly. To obtain ceramics with high uniformity of microstructure and species without crack, transport phenomena in material processing needs to be better understood and a process model needs to be developed to optimize the fabrication process. In this paper, a numerical model is developed, including heat and mass transfer, polymer pyrolysis, species transport, chemical reactions and crystallization. The model is capable of accurately predicting the polymer pyrolysis and chemical reactions of the source material. Pyrolysis of a sample with certain geometry is simulated. The effects of heating rate, particle size and initial porosity on porosity evolution, mass loss and reaction rate are investigated. Optimal conditions for the manufacturing are also proposed.