Reinforcing effects of SiC whiskers and carbon nanoparticles in spark plasma sintered ZrB2 matrix composites

Abstract

ZrB2-based ceramics, reinforced with 25 vol% SiC whiskers (SiCw) as well as 0, 2.5, 5 and 7.5 wt% carbon nanoparticles (Cnp), were prepared by spark plasma sintering (SPS) at 1900 ºC under 40 MPa for 7 min in a vacuum environment. The influences of Cnp content on densification behavior, microstructure evolution, hardness and fracture toughness of ZrB2–SiCw ceramics were investigated. Compared to the carbon-free sample, the grain growth of ZrB2 matrix was moderately decreased (~ 20%) after the addition of Cnp. The in-situ formation of B4C and ZrC phases was attributed to the elimination of surface oxide impurities through their chemical reactions with the Cnp additive. All composite samples approached their theoretical densities. A hardness of 21.9 GPa was obtained for ZrB2–SiCw sample, but the hardness values linearly decreased by the addition of soft carbon additives and reached 14.6 GPa for the composite doped with 7.5 wt% Cnp. The fracture toughness showed another trend and increased from 4.7 MPa m½ for the carbon-free sample to 7.1 MPa m½ for 5 wt% Cnp-reinforced composite. The formation of new carbides and the presence of unreacted Cnp resulted in toughness improvement. Various toughening mechanisms such as crack branching, bridging, and deflection were detected and discussed.