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 either fiber, fiber coating or matrix. Therefore especially the fibers must be protected from Si attack effectively.The formation of silicon carbide was observed to be heavily driven by Ostwald ripening. This can be suppressed by the addition of boron to the melt. Hereby the initially formed SiC crystals in C/C-SiC composites are 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 nano-crystalline structures such as SiC-fibers or CVD-SiC fiber coatings. In addition silicon-boron alloys show a superior wetting behavior on carbon and silicon carbide in comparison to pure silicon. Thus, the use of Si based B containing alloys help effectively to moderate and control the aggressive reaction during LSI process.