Thermal oxidation of high-modulus carbon fibers impregnated with potassium nitrate

Thermal oxidation of ultra-high-modulus Sigrafil UHM-3 carbon fibers (C-fibers) was performed by using potassium nitrate as an oxidizing agent. The impregnating solution consisted of 0.5–10 wt% KNO₃ in a water/methanol (3:1) mixture. Thermal treatment of the impregnated C-fibers was performed at 600 or 800°C in nitrogen or air, respectively. Furthermore, the influence of a subsequent treatment with 60% sulfuric acid was investigated. The thermal treatment of the impregnated C-fibers in nitrogen caused no change in their mechanical properties, whereas in the case of treatments in air, fiber damage was observed as indicated by a decrease in the tensile strength as well as by an increase of the BET surface area. Therefore, further investigations were carried out in a nitrogen atmosphere. An increase of the C-fibers' interlaminar shear strength (ILSS) from 22 to 40 MN/m² without loss of mechanical properties was achieved by impregnation with solutions of <5 wt% KNO₃. Impregnating solutions with higher concentrations damaged the fibers, however. A further increase of the ILSS to 57 MN/m² was obtained by subsequent treatment after the salt decomposition step with 60% sulfuric acid.