Pressureless sintering of carbon nanotube–Al2O3 composites

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al2O3 powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ∼99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al2O3. A CNT/Al2O3 composite containing 1 vol.% CNT had a higher flexure strength (∼540 MPa) than pure Al2O3 densified under similar conditions (∼400 MPa). Improved fracture toughness of CNT–Al2O3 composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al2O3 composites can be densified by pressureless sintering without damage to the CNTs.