EDM Filter Supplier The first of the four chapters presenting experimental results is dedicated to the prebreakdown phase. The study of the mechanisms leading to breakdown is physically of great interest, but it is also important from the industrial point of view. Since no material removal is performed during this phase, it would be interesting to understand it and to control its duration, in order to reduce this “waste of time” as much as possible.
The first section of this chapter summarizes some observations made about the bubbles created during the pre-breakdown. Current measurements are presented in the second section, and finally a short analysis of the pre-breakdown duration is given in the last section. These results are certainly not exhaustive, but they give a first insight into pre-breakdown phenomena.
4.1 Bubbles
Depending on the conditions, small bubbles can be generated at the electrodes as soon as the gap voltage is applied. One can observe that:
- ² bubbles are created only in water, and not in oil;
- more bubbles are generated when the electric field in the gap is increased. A few
bubbles are already visible with a gap of 15 mm and 80 V, and the emission is strongly increased by diminishing the gap distance and/or increasing the gap voltage (see x 5.1.2 for images). Note that no bubbles are generated in oil even with a gap
of 100 ¹m and 1 kV;
- bubbles are created at the cathode;
- the contamination of the water, and consequently its conductivity, has an effect. For example, very few bubbles are generated in clean conditions (water conductivity < 3 ¹S/cm).
The observed influences of the electric field and of the water conductivity on bubble generation is a direct sign that bubbles are related to the flow of an electric current. Since we observe only bubbles coming from the cathode, they are bubbles containing hydrogen. The oxygen released at the anode is probably directly consumed for the oxidation of the anode surface. One can in fact observe that the anode, in copper or in steel, is rapidly oxidized after a few minutes.
To confirm that electrolysis is really the source of bubbles, a basic analysis of the bubble gas was performed. Bubbles were collected in a test tube as shown in figure 4.1, and the gas was tested with a detector sensitive to explosive gases (hydrogen, methane or propane for example). The detector was quickly saturated already with a small volume of gas, indicating that the bubbles are certainly made of hydrogen. EDM Filter Supplier
