First report of fumed alumina incorporation in carbon–carbon composite and the consequent improvement of the oxidation resistance and mechanical properties

The first report of fumed alumina incorporation in carbon–carbon (C/C) composite is provided. Fumed alumina (0.5 vol.% solid alumina in the C/C, 15-nm average pore size, 100–200-nm squishable/conformable aggregates of nanoparticles) as a filler in unidirectional C/C (44 vol.% fibers, fabricated by 1000 °C 21-MPa hot-press mesophase-pitch carbonization) is as effective as one-cycle densification for decreasing the porosity (from 16% to 14%) and increasing the flexural modulus (from 52 to 68 GPa) and longitudinal hardness (from 2.7 to 5.5 GPa), and more effective than one-cycle densification for improving the flexural strength (to 330 vs. 280 MPa), longitudinal compressive modulus (to 38 vs. 30 GPa) and oxidation resistance. The positive mechanical/thermal effects of fumed alumina incorporation suggest pore structure refinement, due to the nanostructure and conformability of fumed alumina. Thus, a quality C/C is obtained at a low density (1.54 g/cm3). Fumed alumina is more effective than organobentonite (3.3. vol.%, previously shown to serve unusually as both filler and binder) for improving the oxidation resistance and as effective as organobentonite for increasing the flexural strength/modulus, in spite of the higher porosity of its C/C (14% versus 12%). However, fumed alumina is inferior to organobentonite for increasing the toughness or longitudinal compressive modulus.