How to Tame the Aggressiveness of Liquid Silicon in the LSI Process

Liquid Silicon Infiltration (LSI) is a technique to manufacture non-oxide ceramic matrix composites such as C/C-SiC or SiC/SiC. In the beginning of this three-step process, fiber preforms are shaped and impregnated with phenolic resins. After curing, the preforms are pyrolyzed to convert the polymer matrix to a porous carbon matrix. This porosity is then used to infiltrate liquid silicon by capillary forces. Simultaneously, an exothermic reaction of silicon and carbon creates a silicon carbide matrix. Generally the liquid silicon reacts with any carbon and even with SiC present in the form of fibers, fiber coatings or matrix. Therefore, especially the fibers must be protected from Si attack effectively. The morphology of silicon carbide was observed to be heavily driven by Ostwald ripening. This can be suppressed by the addition of boron to the melt. The initially formed SiC crystals in C/C-SiC composites are hereby prevented from grain coarsening, resulting in almost completely preserved C/C blocks. For the manufacture of SiC/SiC composites, the silicon boron alloys allow an effective preservation of the nanocrystalline SiC-fibers. Thus, the use of Si based B containing alloys helps effectively to moderate and control the aggressive reaction during LSI process.