Effects of graphite flake diameter on mechanical properties and thermal shock behavior of ZrB2–nanoSiC–graphite ceramics

• With the increase of graphite diameter, flexural strength increased and then decreased.
• Relative density changed in the same trend as flexural strength.
• Strength depends on graphite dispersion, microcrack length, and intragranular particle.
• The graphite with finer diameter can block crack initiation.
• The graphite with bigger diameter can restrain crack propagation.

ZrB2–20 vol.% nanoSiC–20 vol.% graphite (ZSnpG) ceramics that contained graphite flakes with different diameters were investigated to determine the effects of graphite flake diameter on the microstructure and mechanical properties, as well as thermal shock resistance. The results showed that the flexural strength and relative density of ZSnpG ceramics firstly increased and then decreased with increasing the diameter of graphite, whereas the fracture toughness did not change significantly. The improvement in the flexural strength of ZSnpG ceramics was strongly dependent on the controlling of graphite distribution, the reduction of the length of the microcrack that resulted from thermal residual stress and the presence of intragranular nano-sized SiC particles. The calculated thermal shock parameters revealed that the graphite with finer starting diameter had a beneficial effect on the blocking of the crack initiation and the graphite with larger diameter restrained the crack propagation of ZSnpG ceramics.