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The use of allotropic phases of carbon (i.e. nanotubes, graphene or carbon nanofibers) as second phases to design ceramic composites is a hot topic at present. Researchers try to provide a remarkable improvement of the parent ceramic assuming that some of the outstanding mechanical properties of these phases migrate to the resultant composite. This reasonable idea has been questioned severely in the case of nanotubes addition but there is not any analysis for the other two phases cited previously. To elucidate this question, zirconia was selected as a model ceramic. This paper reports the mechanical properties of zirconia composites reinforced either with graphene or carbon nanofibers, with special emphasis on the high-temperature plasticity.

Highlights • Crack-bridging as toughening phenomena revealed by SEM. • Nanoindentation hardness and elastic modulus shows an increasing behavior with CNTs concentration. • Actual elastic modulus for the calculation of fracture toughness of the composites reveals significant fracture improvement. • Sintering parameters have to be tuned for fully dense composite.

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