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Spark Plasma Heat Treated ZrB2-SiC and HfB2-SiC Composites

Ultra-high-temperature ceramics (UHTC) such as ZrB2 and HfB2 with SiC nanofiller are useful for propulsion and thermal protection systems. ZrB2 and HfB2 with 10-20 wt% SiC were prepared using ultra-sonication, rotary evaporation, and spark plasma heat treatment to high temperatures (~2,000°C) and pressures (50-60 MPa). We used positron annihilation lifetime spectroscopy (PALS) to study the nanoporosity, SEM for particle size distribution, and microhardness tester for Vickers hardness. The PALS studies were performed using a 22Na positron source and the positron lifetime spectra were analyzed to three components using POSFIT program. The first and second components are related to positrons annihilating in bulk and in vacancy clusters, respectively; and the third component to positronium annihilation in nanopores within the granules. The PALS results indicate that HfB2 has larger vacancy clusters and nanopores with lesser concentrations compared to ZrB2 and SiC. The SEM observations showed that HfB2 has larger particles compared to ZrB2 and SiC showed wide range of size distribution. The Vickers-Hardness Number (VHN) is measured for spark plasma heat treated composites using a microhardness tester and the results indicate that 10wt%SiC composite has higher hardness compared to 20wt%SiC in both ZrB2-SiC and HfB2-SiC composites. HfB2-SiC composites seem to be more brittle compared to ZrB2-SiC composites. This may be due to larger size and smoother surface of HfB2 particles (600 nm) compared to ZrB2 particles (240 nm).