The crack evolution on the atomistic scale during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

The development of the crack patterns during the pyrolysis of carbon fibre reinforced plastics (CFRP) to carbon/carbon composites as the second manufacturing step in the liquid silicon infiltration (LSI) process was investigated. In the basic examination reported previously, it was discovered that a substantial amount of cracking occurs beyond 650 °C, when the mesoscopic crack pattern has already fully developed. This additional cracking could not be visualized by using standard microscopy. Thus additional investigations were conducted by using conventional and high-resolution transmission electron microscopy to obtain information on the atomistic scale on the assumed cracking activity.It was found that the crack development starts at pores that develop as a compensation for the rough fibre surface. Crack propagation takes place by evolution of new nanoscopic cracks caused by fibre–matrix-debonding in the tensile stress field in front of the crack tip and subsequent connection with the main crack. Thus the interconnection mechanism – cracking as the connection of cracks on a subordinated scale to form a new crack – is the second main cracking mechanism beside transversal cracking (leading to a regular mesoscopic crack pattern) active during the carbonization of CFRP components.