The influence of thermal treatment of carbon fibers on the interfacial shear strength during the pyrolysis of CFRP to C/C composites

C/SiC composites created by the LSI (liquid silicon infiltration) process are potential materials for the application of high-performance structures. The microstructure and phase composition of C/SiC composites are closely associated with the formation and development of cracks during pyrolysis converting the CFRP (carbon fiber reinforced plastic) green-body material into the C/C intermediate state. The fiber/matrix interface bonding, which can be changed by modifying the functional groups on the surface of the carbon fiber, is a dominant factor for the evolution of the microstructure during the pyrolysis step. The surface conditions of the carbon fibers can be modified by thermal treatment, chemical treatment, sizing, etc. In the present work, carbon fibers are thermally treated at high temperatures in an inert atmosphere to modify the fiber/matrix bonding properties. The interfacial bonding properties were investigated by measuring the ISS (interfacial shear strength) with a micro-indentation method. The main results are summarized as follows: I. Thermal treatment of carbon fibers can reduce the fiber/matrix bonding. The interfacial shear strength of composites reinforced with thermally treated fibers at 1500 deg C is 3 times lower than that of composites reinforced with as-received fibers. II. The composites in their polymeric state show a high interfacial shear strength. After pyrolysis at elevated temperatures ISS decreases. This is due to the evolution of volatile products and resin matrix degradation. This result is in agreement with the TG data analysis. III. A slight increase of ISS was observed at about 500 deg C. Such an increase might be attributed to the physical changes and chemical reactions of the resin. IV. Further research of the role of interface bonding on the controlling of the overall performance of final product C/SiC composite is necessary.