Role of graphite nano-flakes on the characteristics of ZrB₂-based composites reinforced with SiC whiskers

Spark plasma sintering technique was utilized to densify the hybrid ZrB₂-based UHTCs co-toughened with SiC whiskers and different weights of graphite nano-flakes. The role of nano-graphite on the structure, hardness, densification, and toughness characteristics of ZrB₂–SiC_w–graphite nanocomposites were inspected. Although all UHTC matrix composites approached to full densification, a 100% dense ceramic was obtained by the incorporation of 5 wt% graphite nano-flakes. The grain coarsening phenomenon stopped significantly in the presence of carbonaceous nano-additive. Phase and microstructural investigations revealed that no in-situ phases were formed during the SPS consolidation, owing to the non-reactivity of ZrB₂–SiC_w–graphite system. The Vickers hardness dropped slightly from 22 to 19.5 GPa with increasing the nano-graphite content. However, the fracture toughness approached 6.2 MPa.m^½ by the incorporation of 5 wt% graphite nano-flakes, remarkably tougher than that of graphitic-carbon-free ceramic with a toughness of 4.7 MPa·m^½.