Phase and microstructural correlation of spark plasma sintered HfB 2 -ZrB 2 based ultra-high temperature ceramic composites

Abstract

The refractory diborides (HfB 2 and ZrB 2 ) are considered as promising ultra-high temperature ceramic (UHTCs) where low damage tolerance limits their application for the thermal protection system in re-entry vehicles. In this regard, SiC and CNT have been synergistically added as the sintering aids and toughening agents in the spark plasma sintered (SPS) HfB 2 -ZrB 2 system. Herein, a novel equimolar composition of HfB 2 and ZrB 2 has shown to form a solid-solution which then allows compositional tailoring of mechanical properties (such as hardness, elastic modulus, and fracture toughness). The hardness of the processed composite is higher than the individual phase hardness up to 1.5 times, insinuating the synergy of SiC and CNT reinforcement in HfB 2 -ZrB 2 composites. The enhanced fracture toughness of CNT reinforced composite (up to a 196% increment) surpassing that of the parent materials (ZrB 2 /HfB 2 -SiC) is attributed to the synergy of solid solution formation and enhanced densification (~99.5%). In addition, the reduction in the analytically quantified interfacial residual tensile stress with SiC and CNT reinforcements contribute to the enhancement in the fracture toughness of HfB 2 -ZrB 2 -SiC-CNT composites, mandatory for aerospace applications.