The active development of the aerospace industry requires more and more for aircraft: it must be fast, wear-resistant, and cheaper in production. Many leading space agencies (NASA, ESA, as well as agencies of Japan, China and India) are actively developing such reusable aircraft, spaceplanes, the use of which will significantly reduce the cost of delivering people and cargo to orbit, as well as reduce the time intervals between flights.
“Currently, significant results have been achieved in the development of such devices. For example, reducing the rounding radius of the sharp front edges of the wings to a few centimeters leads to a significant increase in lift and maneuverability, as well as reduces aerodynamic drag. However, when exiting the atmosphere and re-entering it, on the surface of the wings of the spaceplane, temperatures about 2000 °C can be observed, reaching 4000 °C at the very edge. Therefore, when it comes to such aircraft, there is a question associated with the creation and development of new materials that can work at such high temperatures”, says Dmitry Moskovskikh, head of NUST MISIS Center for Constructional Ceramic Materials.
During recent developments, the goal of the scientists was to create a material with the highest melting point and high mechanical properties. The triple hafnium-carbon-nitrogen system, hafnium carbonitride (Hf-C-N), was chosen, since previously scientists from Brown University (USA) predicted that hafnium carbonitride would have a high thermal conductivity and resistance to oxidation, as well as the highest melting point among all known compounds (approximately 4200 °C).
Using the method of self-propagating high-temperature synthesis, NUSTMISIS scientists managed to obtain a material HfC0.5N0.35, (hafnium carbonitride) close to the theoretical composition, with a high hardness of 21.3 GPa, which is even higher than in new promising materials, such as ZrB2/SiC (20.9 GPa) and HfB2/SiC/TaSi2(18.1 GPa).
“It’s hard to measure a material’s melting point when in exceeds 4000 °С. Therefore, we decided to compare the melting temperatures of the synthesized compound and the original ‘champion’, hafnium carbide. To do this, we placed compressed HFC and HfCN samples on a graphite plate shaped like a dumbbell, and covered the top with a similar plate to avoid heat loss”, comments Veronika Buinevich, NUST MISIS post-graduate student.
Next, scientists connected this to a battery using molybdenum electrodes. All tests were performed in a deep vacuum. Since the cross-section of graphite plates differs, the maximum temperature was reached in the narrowest part of it. The results of simultaneous heating of the new material, carbonitride and hafnium carbide showed that the carbonitride has a higher melting point than hafnium carbide.
However, at the moment the specific melting point of the new material is above 4000 °C, and could not be determined precisely in the laboratory. In the future, the team plans to conduct experiments on measuring the melting temperature by high-temperature pyrometry using a laser or electric resistance. It is also planned to study the performance of the resulting hafnium carbonitride in hypersonic conditions, which will be relevant for further application in the aerospace industry.