Influences on the carbon preform porosity and microstructure evolution of MiCaSiC® ceramics manufactured by the liquid silicon infiltration (LSI) process

Kurzfassung

In the present study the feasibility of the production of silicon carbide based ceramics via liquid silicon infiltration process (LSI-process) is 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 micro-structure. 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 resulting Mi-CaSiC® ceramics were investigated by means of computer tomography (CT), scanning elec-tron microscopy (SEM) and porosity measurements. It is shown that pore structure formation and pore content in carbon preform stage is significantly influenced by the thermal decompo-sition behavior of the preferred binder agents. Furthermore, the microstructure development after pyrolysis is basically affected by the homogenization rate of the material composition as well as material flow kinematics during shaping process