Densification, microstructure and mechanical properties of hot pressed ZrB₂–SiC ceramic doped with nano-sized carbon black

The effect of nano-sized carbon black on densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB₂) – silicon carbide (SiC) ceramic was studied. A ZrB₂-based ceramic matrix composite, reinforced with 20 vol% SiC and doped with 10 vol% nano-sized carbon black, was hot pressed at 1850 °C for 1 h under 20 MPa. For comparison, a monolithic ZrB₂ ceramic and a ZrB₂–20 vol% SiC composite were also fabricated by the same processing conditions. By adding 20 vol% SiC, the sintered density slightly improved to ~93%, compared to the relative density of ~90% of the monolithic one. However, adding 10 vol% nano-sized carbon black to ZrB₂–20 vol% SiC composite meaningfully increased the sinterability, as a relatively fully dense sample was obtained (RD=~100%). The average grain size of sintered ZrB₂ was significantly affected and controlled by adding carbon black together with SiC acting as effective grain growth inhibitors. The Vickers hardness, flexural strength and fracture toughness of SiC reinforced and carbon black doped composites were found to be remarkably higher than those of monolithic ZrB₂ ceramic. Moreover, unreacted carbon black additives in the composite sample resulted in the activation of some toughening mechanisms such as crack deflections.