Chemically-modified hafnium diboride for hypersonic applications: synthesis and characterisation

Hypersonic flight at a speed greater than Mach 5 (1715 ms-1) requires materials that can withstand temperatures up to 3000°C, high heat flux, rapid heating and disassociated reactive oxygen in the extreme environment of space and during re-entry. A number of advanced ceramic materials have melting points over 3000°C, of which the refractory metal carbides and borides are of main interest due to their excellent thermal conductivity from room temperature to over 2500°C, good chemical stability and ablation resistance at high temperatures. These materials are classified as ultra-high-temperature ceramics (UHTCs). Among the family of UHTCs, ZrB2 and HfB2 are reported as the most promising candidates to be used as thermal protection systems (TPS) for the nose tip and sharp leading edges. However, the issue of using monolithic ZrB2 and HfB2 is the phase transformation of ZrO2 and HfO2 oxide by-products at elevated temperature, leading to a volume change that results in cracking of the formed oxide scale. Hence, it is necessary to use dopants to stabilize the oxidation products of ZrB2 and HfB2 in-situ and to minimise the transformation induced cracking and thus improving the oxidation resistance. This research is focused on introducing dopants, such as Y and Ta into HfB2 and to understand its effect on the oxidation behaviour of HfB2 based UHT ceramics. The primary objectives were to: (a) Synthesize sub-micron pure and doped HfB2 powders; (b) Sinter the HfB2 based ceramics to achieve relative density >95% (i.e. with close porosity); (c) Assess the effect of dopants on the oxidation resistance of HfB2 ceramics at high temperatures. Sub-micron pure HfB2 powder of ~200 nm was synthesized by a modified sol-gel approach combined with subsequent carbothermal reduction process using hafnium tetrachloride, boric acid, and phenolic resin as the starting materials. HfC and residual carbon were found to be the main impurity phase, owing to the lack of removal of carbon-containing species in the argon atmosphere during the heat treatment. Therefore, a precipitation approach was developed to transfer hafnium tetrachloride into hafnium hydroxide during the mixing stage to get rid of the Cl- and carbon-containing functional groups. Based on the detailed study of the formation mechanism of HfB2, it was found that the particle size of the HfB2 powders was decided by the particle size of the starting Hf source. Although the powders were slightly coarser (~400-800 nm) from the precipitation approach, importantly phase-pure HfB2 was formed at the same furnace heating conditions (1600°C/2 hrs). The precipitation method was also used to prepare doped HfB2 powders as the homogeneity of the dopants (TaB2, Y2O3) could be improved by controlling the pH values at ~8.5 to achieve the simultaneous precipitation of the dopants and HfB2 precursors. As a result, (Hf,Ta)B2 solid solution was prepared successfully at the temperature of 1600°C. Spark plasma sintering (SPS) was used to densify the pure and doped HfB2 powders. The optimized density achieved was around 97% at 2150°C without the use of any sintering aids and the addition of TaB2 slightly improved the sinterability of the HfB2 based powders due to the formation of the (Hf,Ta)B2 solid solution. The sintered density of commercial micron HfB2 powders (Treibacher) was only 94% in the same condition, and the resultant grain size (5-10 µm) is also significantly larger than that from synthesized HfB2-based ceramics (2-6 µm). The oxide impurities, such as HfO2 and B2O3, on the surface of the fine HfB2 based powders were attributed as the main reason for inhibiting further densification. The oxidation behaviours of the HfB2 based ceramics were investigated via both static oven oxidation and oxyacetylene torch testing. In low and intermediate temperature regime ( 1600°C), it was found the oxidation product was mainly tetragonal HfO2, which was stabilized by the Ta-dopants at temperatures well below the HfO2 phase transformation temperature. Therefore, the cracking and volume change due to phase transformation can be avoided and in return, oxidation resistance was improved at high temperature, which should be beneficial for the application of these materials in hypersonic aviation.