Treatment of the surface layer of steel with high energy plasma pulses

Abstract

At the Lehrstuhl für Raumfahrttechnik of the Technical University at Munich various types of electrothermal and electromagnetic launchers are used to accelerate projectiles to velocities of 5–15 km s −1 to simulate the impact of micrometeoroids on structures of spacecraft. The discharge of stored electrical energy (of the order of kilojoules) across selected materials generates a plasma armature, which accelerates the projectile. Without a projectile a high pressure (of the order of kilobars) plasma pulse (duration, 10–50 μs; temperature about 10000 K) is accelerated to velocities of the order of 20 km s −1. This pulse, to which selected materials (e.g. Cr, C) can be deliberately added, can be directed towards the surface of a target for treatment. Experiments with Cr-Ni steel showed that an extremely hard surface layer of 20–30 μm thickness was generated. This layer is currently investigated by means of metallography (including scanning electron microscopy), X-ray diffractometry and analytical electron microscopy. Preliminary results indicate that the layer consists of structures with dimensions of the order of several nanometres. Experimental and theoretical results reveal a dependence of the hardness and the dimensions of the structure of the layer on the cooling rate (of the order of 10 6 K s −1). X-ray diffraction and electron diffraction suggest that by addition of graphite (or chromium) to the plasma pulse carbide phases may be formed, which together with the nanocrystallinity can explain the observed hardness of the layers.