The effect of high-temperature heat-treatment on the strength of C/C-SiC joints

Joining of carbon fiber reinforced C-SiC dual matrix composite (denoted by C/C-SiC) is critical for its aeronautical and astronautical applications. Joining of C/C-SiC has been realized through a reaction joining process using boron-modified phenolic resin with micro-size B4C and nano-size SiO2 powder additives. The effect of the heat-treatment temperature on the retained strength of the joints, calculated by dividing the strength of the heat-treated joints by the strength of the joints before heat-treatment, was studied. The maximum retained strength of the joints is as high as 96.0% after the heat-treatment at 1200 degrees C for 30 min in vacuum, indicating good heat resistance of the joints. The thickness of the interlayer of the joint after the heat-treatment is about 18 mu m and it is uniform and densified. There are no obvious cracks or pores at the interfaces. During the heat-treatment, carbon, oxygen, silicon, and boron diffuse at the interfacial area. The interlayer is composed of B4C, SiO2, glassy carbon, amorphous B2O3, and borosilicate glass. SiC appears in the interlayer of the joint heat-treated at 1400 degrees C for 30 min in vacuum. The addition of B4C and SiO2 powders contributes to the densification of the interlayer, the bonding at the interfaces and the heat resistance of the joints.