Electrical breakdown signal across the oil film

[AY156]

TL;DR

Electrical breakdown of oil film shows a signal resembling arcing. How do I prevent arcing to study electric discharge?

I am a mechanical engineer researching electrical damage in lubricated contacts. In my tests, I apply 18V DC across a steel ball-disc contact with an elastohydrodynamic lubrication film (more simply an oil film between 600-900 nanometers in thickness) between the surfaces. This film, according to recent research, acts as a capacitor and a resistor in parallel. I observe dielectric breakdown of the film (as expected, as the applied voltage exceeds the electric field strength of the film), and the voltage signal across the contact shows the breakdown as a voltage drop.

The voltage drops from 18V to 2V and has a duration between 80-100 microseconds. The current during the breakdown is limited to a maximum of 50 milliamperes using a variable resistor in series to the ball-disc contact. A sample signal of voltage and current captured during the test is attached, showing the nature of the breakdown. I also observe craters on the steel surfaces resembling the electric discharge damage.

Does the signal show an electric discharge or an arc? Because discharges are instantaneous and the signals show a standing wave in some cases, having a 100-microsecond duration. Also, if I want instantaneous discharge of energy and prevent arcing, is adding a capacitor in series the right way to move forward?

 

[Baluncore]

Welcome to PF.

TL;DR: Electrical breakdown of oil film shows a signal resembling arcing. How do I prevent arcing to study electric discharge?

Does the signal show an electric discharge or an arc? Because discharges are instantaneous and the signals show a standing wave in some cases, having a 100-microsecond duration. Also, if I want instantaneous discharge of energy and prevent arcing, is adding a capacitor in series the right way to move forward?

The positive current spike at the instant of contact is the discharge of the electrode and cable capacitance. I see no oscillation that would be present if an arc was being maintained. I would expect an inductive voltage spike as the current stopped flowing.

A series capacitor would upset the DC bias voltage across the gap. It would require more than just a capacitor.

Are you aware of die sinking, or of wire EDM ?
https://en.wikipedia.org/wiki/Electrical_discharge_machining
EDM is carried out, submerged in a dielectric such as kerosene, that is used to flush debris from the deliberate spark erosion site.
You need the opposite response to the EDM contact break.
Any inductance will maintain an arc and keep it running by raising the instantaneous voltage when the current falls. You might minimise the inductance by using larger diameter hollow or cage conductors.

Place fast recovery diodes as close to the spark gap as possible. Connect one to ground, the other to a ceramic bypass capacitor on the 18 volt DC supply. That will keep the switching voltages low, preventing an arc.

Maybe there is a way to use a short transmission line to connect the test gap. That may eliminate the inductance if the line is sensibly terminated. The charge stored in the capacitance of the line will flow through the contact when it is made, but the voltage spike will be limited to twice the supply voltage. To prevent reflections, the line can be series terminated at the current limiting resistor.

 

[AY156]

Thank you for the response. Yes, I am very much aware of wire EDM and same kind of discharge damage is widely studied in lubricated contact in context of electrical damage in rolling-element bearings. Part of my research is to compare the damage in bearing to that observed in EDM. In my setup the wires are shielded and not long (maximum length 20 cm). Placing any component very close ton the contact is not possible since the contact is maintained inside a rig. It is important to note hear that the oil film is transient and dynamic and depends on the surface velocity, surface roughness among other things. DC is not a problem I have similar results with square wave AC voltage and I can use a capacitor with AC voltage to evaluate its effect.