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 °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 μm 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 °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.