Influence of CNF content on microstructure and fracture toughness of CNF/alumina composites

Dense 0.4–5.0 wt % carbon nanofiber (CNF)/alumina composites were fabricated by plasma activated sintering. The microstructure—particularly the CNFs distribution—of composites containing different amounts of CNFs was observed in detail, and the influence of the additive amounts of CNF on the microstructure and the fracture toughness of the composites were investigated. The ratio of CNFs distributed individually in the composites decreased with an increase in the addition of CNFs, and the other CNFs formed bundles; notably three-quarters of the CNFs formed bundles in the 5.0 wt % CNF/alumina composite. The alumina grain size distribution of the composites became narrower to smaller grain size side and the average alumina grain size of the composites decreased with an increase in the addition of CNFs from 0.4 to 1.6 wt %. However, the average alumina grain size of the composites did not vary greatly with an increase in the addition of CNFs from 1.6 to 5.0 wt %, because the CNF bundles formed in the 2.5 and 5.0 wt % CNF/alumina composites lowered the grain growth retardation effect of the CNFs. The 1.6 wt % CNF/alumina composite exhibited the highest fracture toughness, because three-fifths of the CNFs distributed individually and uniformly in alumina grain boundaries.