DENSIFICATION AND MECHANICAL PROPERTIES of ZrB_2-TiB_2 ULTRA HIGH TEMPERATURE CERAMIC COMPOSITES

摘要

Re-entry vehicles experience extreme high temperatures during the entry into earth's atmosphere. Materials used for thermal protection system in these vehicles should possess structural stability at high temperatures. Ultra high temperature ceramics (UHTCs) are candidate materials for such applications in atmospheric re-entry vehicles, mainly characterised by their high melting point of above 3000℃. ZrB_2 and TiB_2 possess high melting points of about 3245 and 3225 ℃ respectively. High thermal and electrical conductivity and high hardness are also unique to these materials. A composite of ZrB_2 and TiB_2 is expected to have the advantage of being lighter and less expensive than 100% ZrB_2. In this study, composites of ZrB_2-TiB_2 containing 25%, 50% and 75% TiB_2 by volume have been studied. Powder mixtures of ZrB_2 and TiB_2 were milled for 4h followed by spark plasma sintering at 1500 ℃. Microstructure and phase analysis have been carried out using SEM and XRD and the mechanical properties (hardness and indentation fracture toughness) of the composites are evaluated using nanoindentation and micro-hardness testing and compared with ZrB_2 and TiB_2.