Internal protection of C/C composites by boron-based compounds

Boron-based protections were used on carbon fibre preforms in order to protect interfacial zones in C/C composites from oxidation. Wet treatments (H3BO3 in aqueous solution) and B–P CVD-coatings are not stable under low pressures at 1000 °C (the matrix deposition conditions in conventional CVI). Hence, they could not be used as C/C composite internal protections. Boron-ion implantation in carbon fibres improves their tensile strength by healing superficial carbon fibre defects but is not efficient as an internal protection in C/C composites, the boron-ions being implanted too far from the carbon fibre surface. B–C CVD-deposits are also not very efficient as internal protections. They oxidize faster than the carbon and lead to an important volume decrease. Holes are then formed at the interfacial zones at the beginning of the oxidation of the C/B–C/C composite passing from the surface to the core of the material.