Plasma-material interaction and electrode degradation in high voltage ignition discharges

Erosion of material caused by electrical discharges takes place in many technical applications. Particularly, in spark plugs, the durability is mainly determined by the electrode erosion caused by ignition discharges. A better understanding of the wear mechanisms will help in developing new electrode materials with enhanced resistance against spark erosion. In this work, different aspects of the complex interaction between the plasma of the ignition discharge and the electrode are investigated based on experimental observations and simulations. The discharge mode behavior is quantitatively analyzed with regard to the arc and glow phase fractions for different electrode materials and conditions of pressure and gas. The influence of these parameters on the discharge is discussed. This work especially focuses on the formation of microscopic erosion craters on the electrode surface. Their morphology and microstructure are characterized by means of FIB/SEM dual beam techniques. The depth of modifications and the extent of the molten region are determined. To complete these experimental observations, thermal analysis of the crater formation is performed using analytical models and FEM simulations. Characteristic values of time, power density and current involved in the crater formation are estimated. These values are related to the electrical characteristic of the spark, and the effects of the discharge phases on the electrode surface degradation are discussed.