On the Erosion of Material Surfaces caused by Electrical Plasma Discharging
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On the Erosion of Material Surfaces caused by Electrical Plasma Discharging Flavio A. Soldera and Frank Mücklich Department for Materials Science - Functional Materials, Saarland University, P.O. Box 151150, D-66041 Saarbrücken, Germany. ABSTRACT The erosion of material surfaces produced by electrical discharges plays an important role on the degradation of many electrical devices, such as electrical contacts, switches or spark plugs. A discharge produces an extreme and concentrated flow of energy into the material that heats it and can even cause melting or vaporization. The plasma pressure may cause an even greater removal of material by the emission of droplets of molten material, producing craters in the surface of the material. In this contribution the microscopic erosion mechanisms on RuAl basis intermetallic compounds are compared with those for pure metals. Single discharge experiments at high pressure were done and the erosion structures were characterized with white light interferometry and scanning electron microscopy. The effects of microstructure on the surface erosion are discussed on the basis of different samples of RuAl. In certain high temperature applications, formation of oxide scales is an important process that may influence the discharge characteristics and erosion mechanisms. These effects are discussed on results in pre-oxidized samples. It was shown, that the surface can be additionally stabilized by controlling the protecting oxide coatings. INTRODUCTION The current transfer between the plasma of high-pressure electrical discharges and the cathode occurs in most cases in a constricted regime. The current is collected by a small area, which is called the cathode spot. The complex physical processes taking place in this area are associated with energy transfers between the plasma and the cathode. The cathode surface is primarily heated by the impact of ions accelerated in the cathode sheath, which deliver their kinetic, thermal, and recombination energy. In addition, resistive heating (also called Joule heating) by the discharge current within the cathode contributes to the enhancement of the surface temperature [1,2]. On the other side, emission of electrons, evaporation of cathode material and thermal conduction into the electrode produces a cooling effect on the hot spot [1,2]. Due to the extremely intense interaction between the cathode spot plasma and the solid surface, a crater is formed on this. Both very rapid, intense heating and strong mechanical forces are necessary for melting and excavation of craters and other surface deformations [ 1]. Previous investigations on material erosion by ignition discharges [3] showed that there is a strong relation between the eroded volume of material and the energy necessary for heating the material up to the melting point and to melt it. These results together with the presence of particles of metal on the cathode surface suggested that the electrode erosion occurs following the model of particle ejections proposed by Gray et al. [4] for
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