Abstract
The refractory diborides (HfB2 and ZrB2) 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) HfB2-ZrB2 system. Herein, a novel equimolar composition of HfB2 and ZrB2 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 HfB2-ZrB2 composites. The enhanced fracture toughness of CNT reinforced composite (up to a 196% increment) surpassing that of the parent materials (ZrB2/HfB2-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 HfB2-ZrB2-SiC-CNT composites, mandatory for aerospace applications.
Highlights
IntroductionCarbon-carbon (C–C) composites are attractive materials for hypersonic flight vehicles but they oxidize in air at temperatures >500 ◦ C and needs thermal protection systems (TPS) to survive aero-thermal heating [1]
The enhanced fracture toughness of carbon nanotube (CNT) reinforced composite surpassing that of the parent materials (ZrB2 /HfB2 -SiC) is attributed to the synergy of solid solution formation and enhanced densification (~99.5%)
Carbon-carbon (C–C) composites are attractive materials for hypersonic flight vehicles but they oxidize in air at temperatures >500 ◦ C and needs thermal protection systems (TPS) to survive aero-thermal heating [1]
Summary
Carbon-carbon (C–C) composites are attractive materials for hypersonic flight vehicles but they oxidize in air at temperatures >500 ◦ C and needs thermal protection systems (TPS) to survive aero-thermal heating [1]. Spark plasma sintering (SPS) has emerged as a potential technique for the processing of UHTCs with enhanced densification, fine grain size of a few microns at a much lower sintering temperature ~500 ◦ C less than the conventional techniques and time 10–30 min [7,8,9,10,11] Reinforcements such as SiC and TaSi2 act as a sintering aid to densify the ZrB2 /HfB2 ceramics, and enhances their mechanical, oxidation, and tribological performances [7,8,12,13,14,15]. A study [7] revealed that the addition of CNT in ZrB2 -20SiC ceramic increases the fracture toughness up to ~53% (with 10 vol % of CNT addition) and enhances the oxidation resistance via grain sealing mechanism under 30 s plasma arc jet exposure with a flux of 2.5 MW/m2
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