24. Porous Cast Materials (Gasars). Options of Their Use in Space Rocket Hardware

Annotation: Gasars is a new type of porous cast materials manufactured on the basis of metals and their alloys, some types of ceramics. The basis of the process is gas-eutectic conversion in the system metal-hydrogen. The process of investigation and creation of gasars was commenced in 1979 in the National Metallurgical Academy of Ukraine and is currently continued in Ukraine, the USA, China, Japan, South Korea, Poland and others. The gasars production technological process consists in melting the specified material (metal, alloy, ceramics) in hydrogen (or other active gas) atmosphere at a certain pressure. After the melt is saturated with active gas to a certain concentration, the crystallization process begins at which the pore formation process is launched. As the pores growth occurs perpendicular to crystallization front, the orientation of heat withdrawal influences pores location. So, for example, to obtain radial porosity, radial heat withdrawal is required. To obtain various structures, along with directed crystallization process, the pressure in crystallization chamber is an important factor, which drives the gasar morphology. The porous structure of gasars is diverse, there are the gasars with longitudinal, cylindrical, spherical, conical pores. It is possible to alternate the porosity layers and monolithic metal layers. The dimensions of gasars pores are in the limits from 10 μm to 10 mm at total porosity from 7 to 55 (75%). However, there is a possibility to obtain the pores with smaller diameter. The mechanical properties of gasars have a number of advantages as compared with conventional porous materials produced by different methods. Subsequent processing of the gasars does not differ from analogous non-porous materials, which is also an advantage over conventional porous materials. And in case when the diameter of pores is less than 50 μm, the exceedance of mechanical properties of gasars as compared with monolithic materials of the same chemical composition is observed. This is caused by the fact that the pores were formed during crystallization and at the action of pressure on a gasar, local hardening occurs. At present, the gasars have already found application as light and strong structural materials, filters, heat exchangers, dampers, slide bearings, catalyst elements, friction materials, etc. The use of gasars in space hardware will help to considerably reduce the mass of launch vehicle structural elements without worsening strength properties. The possibility of welding and soldering the gasars allows finding their application in the structure of propellant systems, compressed gas and propellants supply systems, creating filtering elements based on the gasars, including propellant spraying and mixing systems.