Effects of fiber/matrix interface and matrix microstructure on the mode I interlaminar fracture toughness of C/C composite materials were investigated by coating bismaleimide-triazine co-polymer (BT-resin) on the surface of carbon fiber and changing the heat-treatment temperature (HTT). For the case of laminates with HTT of 1600°C (carbonized C/C composites), the initial fracture toughness, G
IC, was insensitive to BT-resin coating. Moreover, the fracture toughness during crack propagation, G
IR, increased by coating BT-resin. On the other hand, both G
IC and G
IR decreased with BT-resin coating for the laminates with HTT of 2500°C. While both G
IC and G
IR are insensitive to HTT for laminates without BT-resin coating, they both decreased by increasing HTT for laminates with BT-resin coating. The difference of the effects of interface control and HTT was discussed on the basis of microscopic mechanism consideration. Comparison between in-plane and interlaminar strength indicated the possibility to optimize the interface control.
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