Nano-carbon fiber composite materials generally begin to phase change at 2400°C, changing from a disordered layer structure to a three-dimensional ordered structure. Because the felt-based carbon/carbon composite material is composed of CVD carbon, resin carbon, and fiber carbon, it is in phase change Mutual influence is inevitable. CVD carbon is easy to graphitize, while the other two are non-graphitized carbons, but the graphitization degree can reach 60%-80% at a temperature above 2700℃. The reason is believed to be resin carbon at high temperature. The stress between shrinkage and fiber carbon expansion promotes the phase change of resin carbon and improves the graphitization ability of felt-based carbon/carbon composites.

After the graphitization process, the main problems of the product are cracks and delamination. The delamination is mainly caused by the low longitudinal strength of the felt, which is a congenital deficiency. The causes of longitudinal cracking are more complicated. In summary, there are three reasons:

1. In the CVD process, due to the internal thermal gradient method, the density difference is caused, which causes stress in the thermal environment, and the stress is released during graphitization, and its value exceeds the strength value, causing cracks.

2. During graphitization, the temperature difference between the inside and outside is large, causing enough thermal stress to cause cracks.

3. The original defects in the carbon felt matrix, such as slag inclusion and coking, become the origin of cracks, and the discontinuity of the felt fibers cannot effectively prevent the development of cracks, thereby cracking.Of course, the size effect is also very large, and the cracking rate of products exceeding φ400 is higher than that of products below φ400. This can be explained in the thermal stress formula.