Improvement of flexure strength and fracture toughness in alumina matrix composites reinforced with carbon nanotubes

In this study, alumina matrix composites reinforced with carbon nanotubes (CNTs) were fabricated by CNT purification, mixing, compaction, and sintering processes, and their relative density, electrical resistance, hardness, flexure strength, and fracture toughness were evaluated. 0-3 vol.% of CNTs was relatively homogeneously distributed in the composites, although some pores existed. The three-point bending test results indicated that the flexure strength increased with increasing volume fraction of CNTs, and reached the maximum when the CNT fraction was 1.5 vol.%. The fracture toughness increased as the CNT fraction increased, and the fracture toughness of the composite containing 3 vol.% of CNTs was higher by 40% than that of the monolithic alumina. According to the observation of the crack propagation path after the indentation fracture test, a new toughening mechanism of grain interface bridging-induced CNT bridging was suggested to explain the improvement of fracture toughness.