Production of very hard surface layers by repetitive use of high energy plasma pulses

At the Fachgebiet Raumfahrttechnik of the Technical University at Munich, electrothermal and electromagnetic launchers are used to accelerate projectiles to velocities of 5-15 km s^-1 to simulate the impact of micrometeorites on structures of spacecraft. The discharge of stored electrical energy (in the order of some kJ) across selected materials generates a plasma armature, which accelerates the projectile. Without a projectile a high pressure (in the order of 10-50 kbar) plasma pulse (duration 10-50 μs, temperature approximately 1-10⁴ K, 10⁶-10⁷ W cm^-2) is accelerated to velocities of up to 20 km s^-1. This pulse, to which selected materials (e.g., Cr, C) can be deliberately added, is directed towards the surface of a target for treatment. Experiments with steel targets show that an extremely hard surface layer of average 30 μm thickness is created by a single pulse [1]. The surface layer thickness could be increased to approximately 400 μm by repetitive impacting. After 10 process steps, where the target is placed with a small offset after each "shot", an adhesive surface layer with a fairly high surface roughness is produced. The surface layer has then to be ground to a thickness of 200 μm in a subsequent step to achieve a smooth surface. The surface layers are investigated by means of metallography, scanning electron microscopy (SEM) and X-ray diffraction. The layer consists of two different zones with a measured microhardness up to 1300 HV_0.01. There is a very hard white etching zone immediately below the surface, and a dark etching zone between the white zone and the core workpiece which has a needle-shaped martensitic microstructure. Preliminary results indicate that the white zone, which is created after every new impact, transforms into the dark zone in dependence on temperature during subsequent treatments. The production of very hard adhesive surface layers with thicknesses of minimum 200 μm is a further step towards industrial application of this method.