Synergistic effects of graphite nano-flakes and submicron SiC particles on the characteristics of spark plasma sintered ZrB₂ nanocomposites

ZrB₂ ceramic (Z), ZrB₂–25 vol% SiC composite (ZS) and ZrB₂–25 vol% SiC composite doped with 5 wt% graphite (ZSG) were fabricated by spark plasma sintering process. The sintered samples were compared to investigate the effects of submicron SiC particles and graphite nano-flakes on sinterability, microstructure and mechanical properties of ZrB₂-based ceramic matrix composites. Spark plasma sintering at 1900 °C for 7 min under 40 MPa resulted in fully dense ZS and ZSG samples but a relative density of 96.1% was achieved for Z sample. The growth of ZrB₂ grains was effectively decelerated by addition of submicron SiC particles and graphite nano-flakes. Hardness values of 13.1, 19.5 and 16.6 GPa were measured for Z, ZS and ZSG samples, respectively, which verify the hardening effect of SiC and softening effect of graphite in ZrB₂-based composites. By the simultaneous addition of SiC and graphite into ZrB₂ matrix, the indentation fracture toughness of ZSG sample reached 6.7 MPa m^½, meaningfully higher than those of Z and ZS samples with toughness values of 3.2 and 4.3 MPa m^½, respectively. Such an improvement in the fracture toughness of ZSG nanocomposite was attributed to the presence of graphite nano-flakes as well as the in-situ formed ZrC and B₄C nano-particles. Flexural strength of Z, ZS and ZSG samples reached 445, 624 and 631 MPa, respectively. Although SiC had a remarkable strengthening effect in the ZrB₂-based ceramic, the addition of graphite together with SiC had not a significant impact on the flexural strength of composite material.