Steffen Weber

Thermoplastic Forming as an Alternative ShapingProcess for Near-Net-Shape Production of C-SiSiCCeramics via Liquid Silicon Infiltration (LSI) Process

The present study demonstrated the feasibility of producing carbon-based green bodies by means of extrusion, pyrolysis and subsequent conversion to C-SiSiC ceramics according to the LSI process.Athermoplastic binder system was used for the development of feedstocks that differ from conventional duroplastic binder systems as these would have created difficulties during processing owing to their curing behavior. Activated carbon, carbon fibers and semicoke were used as fillers and a source of a sufficiently stable carbon backbone in the carbonized intermediate. Special attentionwaspaid to the characterizationandquantification of material behavior at the water debinding step as well as at thermal treatment during pyrolysis. Porosity and microstructural characteristics of carbon bodies and subsequent C-SiSiC microstructure after Si-infiltration were investigated by means of scanning electron microscopy and porosity measurements.Thefeasibility of producing carbon-based extrusion feedstocks with high homogeneityandflowability which can be converted into SiC by means of LSI shows that the concept may be applied to production of near-netshape C-SiSiC components by means of thermoplastic injection molding

Influences on the Carbon Preform Porosity and Microstructure Evolution of MiCaSiC Ceramics Manufactured by the Liquid Silicon Infiltration (LSI) Process

In the present study, the feasibility of the production of silicon carbide based ceramics via liquid silicon infiltration process (LSI-process) was demonstrated. The manufactured MiCaSiC ceramics (metal infiltrated carbon based silicon carbide) were based on novel raw material compositions. For these material compositions, activated carbon, carbon fibers, and a sinterable pitch based semi-coke were used as fillers and source of a sufficiently stable carbon backbone in the carbonized intermediate state. Different binder agents were used for the compound and feedstock development and basically compared. This involves thermoplastic and duroplastic binders, which promote the use of two different shaping methods (warm pressing and extrusion process). This allows the formation of highly porous and complex shaped carbon preforms for further silicon infiltration. The main focus of the present study was to evaluate the impact of debinding and pyrolysis process step on the preform microstructure. That includes the determination and quantification of matrix and porosity evolution in carbon preforms. The defined formation of porosity during pyrolysis is a key parameter for subsequent silicon infiltration. In this context, pore structure formation of carbon preforms as well as porosity and microstructural characteristics of carbon preforms, and the resulting MiCaSiC ceramics were investigated by means of computer tomography (CT), scanning electron microscopy (SEM) and porosity measurements. It was shown that pore structure formation and pore content in carbon preform stage was significantly influenced by the thermal decomposition behavior of the preferred binder agents. Furthermore, the microstructure development after pyrolysis was basically affected by the homogenization rate of the material composition as well as material flow kinematics during shaping process.