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ZrC and ZrB2 are both typical ultra-high temperature ceramics, which can be used in the hyperthermal environment. In this study, a method for preparing ultrafine ZrC-ZrB2 composite powder was provided, by using the raw materials of nano ZrO2, carbon black, B4C, and metallic Ca. It is worth pointing out that ZrC-ZrB2 composite powder with any proportion of ZrC to ZrB2 could be synthesized by this method. Firstly, a mixture of ZrC and C was prepared by carbothermal reduction of ZrO2. By adjusting the addition amount of B4C, ZrC was boronized by B4C to generate ZrC-ZrB2 composite powder with different compositions. Using this method, five composite powders with different molar ratios of ZrC and ZrB2 (100ZrC, 75ZrC-25ZrB2, 50ZrC-50ZrB2, 25ZrC-75ZrB2, and 100ZrB2) were prepared. When the temperature of boronization and decarburization process was 1473 K, the particle size of product was only tens of nanometres. Finally, the oxidation characteristics of different composite powders were investigated through oxidation experiments. The oxidation resistance of ZrC-ZrB2 composite powder continued to increase as the content of ZrB2 increased. [ABSTRACT FROM AUTHOR]

Highly disperse and reactive (HfB2–SiC)@(Ta2O5–HfO2–C) composite powders were manufactured by sol–gel technology where Ta2O5–HfO2–C amorphous components were nanostructured and distributed in each other as uniformly as possible. The reactive sintering of the prepared composite powders at a relatively low temperature (1800°С) with an exposure time of 30 min and the pressure 30 MPa yielded (HfB2–30 vol % SiC)–xTa4HfC5 ultra-high temperature ceramics (UHTCs), where x = 5, 10, and 15 vol %, with a relative density of 75–78%. X-ray powder diffraction proved the complete conversion of tantalum and hafnium oxides to complex carbide Ta4HfC5. The average grain size as determined by scanning electron microscopy did not exceed 2–3 µm in HfB2 and 30–60 nm for the Ta4HfC5 phase. Thermal analysis in flowing air showed that, in the temperature range 20–1400°С, the increasing percentage of tantalum–hafnium carbide (the least oxidation resistant phase) leads to a systematic increase in oxidation-induced weight gain; however, a tendency to saturation is observed. The microstructural specifics of the oxidized surface are noticed depending on the composition of HfB2–SiC–Ta4HfC5 ceramics. [ABSTRACT FROM AUTHOR]

The review has been made of recent publications on modification of ZrB2/HfB2-SiC ultra-hightemperature ceramic composite materials (UHTC) by carbon components: amorphous carbon, graphite, graphene, fibers, and nanotubes. Available data have been presented on some aspects of oxidation of such materials at temperatures ≥1500°C and both at the atmospheric pressure and at the reduced oxygen partial pressure; structural features of the formed multilayer oxidized regions have been noted. It has been considered how the type and content of the carbon component and the conditions (first of all, temperature) of UHTC production affect the density, flexural strength, hardness, fracture toughness, and thermal and oxidation resistance of the modified ceramic composites. [ABSTRACT FROM AUTHOR]

ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition (LPCVD) with the Br-Zr-CH-H-Ar system. The effects of deposition time on the microstructures and growth behavior of ZrC coatings were investigated. ZrC coating grew in an island-layer mode. The formation of coating was dominated by the nucleation of ZrC in the initial 20 minutes, and the rapid nucleation generated a fine-grained structure of ZrC coating. When the deposition time was over 30 min, the growth of coating was dominated by that of crystals, giving a column-arranged structure. Energy dispersive X-ray spectroscopy showed that the molar ratio of carbon to zirconium was near 1:1 in ZrC coating, and X-ray photoelectron spectroscopy showed that ZrC was the main phase in coatings, accompanied by about 2.5mol% ZrO minor phase. [ABSTRACT FROM AUTHOR]

To achieve a ZrO-B-C ternary solid-state reaction, a small amount of carbon was introduced into a binary system of ZrO and boron. Amorphous hydrous ZrO precursor was firstly prepared through precipitation method using zirconyl nitrate and ammonia at room temperature. Then, ZrB powder was successfully synthesized using the as-prepared amorphous hydrous nano-ZrO via borothermal and carbothermal reduction (BCTR) by amorphous boron and nanocarbon powders under flowing argon. Plate-like ZrB particles with a uniform distribution and ca. 1.0 μm in two-dimensions were obtained with a B/Zr molar ratio of 4, and 5 wt.% (total weight of dry hydrous ZrO and boron) nanocarbon at 1550 °C for 2 h. [ABSTRACT FROM AUTHOR]