Heterogeneous solid solutioning in carbon nanotube reinforced HfB2-ZrB2-SiC ultra high temperature ceramic composites

Role of carbon nanotubes (CNTs) on solid solutioning in SiC reinforced equi-volume (40:40) composition of HfB2 + ZrB2 is investigated. Incomplete solid solutioning occurred under given sintering conditions in presence of 20 vol% SiC. However, CNTs addition resulted in heterogeneous solid solution formation which is evident from SEM and TEM micrographs and confirmed through EDS analysis and peak de-convolution in XRD. TEM investigation revealed the retention of CNTs and presence of defect structures like stacking faults in the SiC phase is ascribed to the compressive stresses generated during cooling after the sintering process. With CNTs reinforcement, micro-hardness and elastic modulus increased from 10.2 GPa to 11.7 GPa and 129 GPa to 141 GPa, respectively. The reduction in peak oxidation temperature (from 751-773 degrees C to 725-746 degrees C) and weight gain starting temperature (from 731-750 degrees C to 684-707 degrees C) with CNTs reinforcement elicits a concurrent reduction in the oxidation resistance.