Effect of specimen size on bend properties of doped molybdenum: Y. Hiraoka et al. (Okayama University of Science, Okayama, Japan.) J.Japan Soc. Powder and Powder Metallurgy, vol 42, no 8, 1995, 964–968. (In English.)

Abstract

Dust particles with diameters below 100 μm represent an important part of the space environment. Objects like satellites or spacecrafts, are constantly bombarded with particles of cosmic velocities of 10 km/s and more. These hypervelocity impacts lead to evaporation of a large fraction of these particles and to the formation of craters on the material surfaces which exhibit diameters which are up to one order of magnitude larger than the impinging particles. This results in a remarkable materials degradation of space exposed surfaces which can considerably reduce the life time e.g. of semiconductor panels for the generation of energy, due to the cumulative effect of a multitude of such particles which hit a surface during a certain time.The particle population in space near to earth consists of two distinct groups:(a)Natural particles, micrometeorites: These particles are a natural part of the space environment and cannot be avoided. Most of them have so-called “chondritic” compositions and consist of Si, Al, Mg, Fe, Ca and O, an elemental makeup resembling the bulk composition of the original solar nebula from which our solar system and the planets formed 4.5 billion years ago.(b)Man made debris particles: These stem from various sources, e.g. exhausts from solid rocket fuels, paint chips, fragments from space crafts etc. These particles pose an increasing danger to space flight and their future production must be reduced by strict debris policies.The important question was now:What percentage of impact particles is man-made?Cosmic and man-made particles exhibit different chemical compositions. Hence, a micro-chemical analysis of particle impacts on satellite surfaces can be used to determine the particle ratio. For this purpose, the LDEF-experiment (Long Duration Exposure Facility) was launched by NASA in 1984 to study erosion phenomena on material surfaces (of more than 10,000 test samples) in a Low Earth Orbit (LEO)-environment. LDEF was retrieved only in 1990 because of the Challenger disaster which essentially delayed the retrieval up to the last moment before the experimental setup would have been destroyed upon entrance into the earth atmosphere.The demonstration of pronounced material degradation of space exposed materials logically leads to the question which materials are used in space technology. Plansee SE in Reutte/Tyrol has quite a tradition in the production of smaller rocket nozzles, e.g. satellite propulsion nozzles and general propulsion components made of Mo, W, Nb or Ta-based alloys. The W–Ag evaporation cooled alloy was among the earliest materials for rocket combustion chambers enduring ultra high temperatures due to partial evaporation of the silver which thereby cooled the chamber. Ni- and Fe-based ODS superalloys find application in durable TPS/hot structures of hypersonic vehicles, e.g. as honeycomb panels and fasteners of PM 1000/2000. These are PM-ferritic and nickel based oxide dispersion strengthened alloys. We have studied the significance of Yttrium depletion within the oxide scales of these alloys.