Spark plasma sintering of graphene reinforced zirconium diboride ultra-high temperature ceramic composites

Spark plasma sintering (SPS) of monolithic ZrB2 ultra-high temperature ceramic and 2–6 vol% graphene nanoplates (GNPs) reinforced ZrB2 matrix composites is reported. The SPS at 1900 °C with a uni-axial pressure of 70 MPa and soaking time of 15 min resulted in near-full densification in ZrB2–GNP composites. Systematic investigations on the effect of GNP reinforcement on densification behavior, microstructure, and mechanical properties (microhardness, biaxial flexural strength, and indentation fracture toughness) of the composites are presented. Densification mechanisms, initiated by interfacial reactions, are also proposed based on detailed thermodynamic analysis of possible reactions at the sintering temperature and the analysis of in-process punch displacement profiles. The results show that GNPs can be retained in the ZrB2 matrix composites even with high SPS temperature of 1900 °C and cause toughening of the composites through a range of toughening mechanisms, including GNP pull-out, crack deflection, and crack bridging.