Effect of graphite addition on the spark-plasma sinterability of ZrB2 and ZrB2–SiC ultra-high-temperature ceramics

The effect of a 2 wt% addition of particulate graphite on the spark-plasma sintering (SPS) kinetics of both ZrB2 and ZrB2+30 vol% SiC was investigated using to that end two broad sets of powder mixtures prepared by high-energy co-ball-milling. It was observed that the particulate graphite addition enhances the SPS kinetics of both systems, and it was identified that, contrary to the case in conventional sintering, this improvement in sinterability is not due to the carbothermal reduction of oxides in situ during SPS but to the lubrication imposed by the graphite flakes inducing a greater green-body densification. Consequently, the graphite is not consumed during SPS but remains in the microstructure of the resulting ultra-high-temperature ceramics (UHTCs) as flaky particles dispersed homogeneously at grain boundaries. Implications of interest for the UHTC community are discussed.