The carbon black effect on crack formation during pyrolysis step in liquid silicon infiltration process for Cf/C-SiC composites

The formation of crack patterns during the pyrolysis step, which converts the carbon fiber-reinforced plastic composite (CFRP) to a Cf/C composite as one step in the liquid silicon infiltration (LSI) process, was investigated. Generally, cracks occur as a result of variances in thermal shrinkage between the matrix and the fiber during pyrolysis. The addition of carbon black powder as a filler resulted in a reduction in the shrinkage, but more cracks were formed. The transversal crack increased, and the partial delamination was newly occurred as the filler was increased. The properties of the matrices also changed; the fracture toughness decreased, and the void area increased. Consequently, the crack formation occurs not only by the thermal shrinkage difference between fiber and matrix but also the effect of matrix fracture toughness or void content. The flexural strength of LSI-processed samples was also measured, and the filler improved the mechanical strength but reduced its reliability.