Abbas,

Historically, Micro-arc Oxidation (MAO) or Plasma Electrolytic Oxidation (PEO) have been long time conducted with anodic DC (ie workpiece is anode). However it had then been shown that using AC (ie using an anodic half period followed by a cathodic half period) lead to an improvement in the coating quality. One of the explanation is that the cathodic half period prevent from an excessive charge accumulation at the surface of the dielectric coating, and therefore it reduces the size and life time of discharges. Consequently, you prevent from the formation of "destructive" discharges. 

Though cathodic half period has a clear beneficial effect, it has been long time speculated that cathodic discharge do not appear in PEO. We however shown recently the evidence of the possibility to obtain cathodic discharge. Unfortunately those cathodic discharge have a detrimental effect on the coating and they do not take part to the oxidation. 

Nevertheless, it should be noted that plasma electrolysis (not oxidation) cant be conducted with cathodic biasing of the work piece. You could refer to the reviews of Aleksey Yerokhin (Surf. Coat. Tech. 1999) or Gupta (Surf. Coat. Tech. 2007) for more information about that.

I hope i answered your question, if it is not the case, lease do not hesitate to contact me.

All the best

Dear Prof. Nomine,

Really thank you for your good answer. It was very useful to me. Of course the mentioned points are very important in Case of PEO process. I will take care of your recommendation in my work.

Thank you again.

Abbas,

I am happy to help. I actually looked over the literature and I can recommend you a very recent paper of Stevan Stojadinovic "Real-time imaging, spectroscopy, and structural investigation of cathodic plasma electrolytic oxidation of molybdenum" in JAP. They have managed to make Oxidation with only a cathodic polarisation of the sample. The plasma is in fact different that the arcs we usually find in MAO/PEO since it is a plasma envelope rather than arc. This paper could be of interest for you.

Alexandre

Yeah! Exactly ive read that paper in your RG papers yesterday! I found it very interesting. It was very fantastic research.

Thank you for your recommendation Prof. Nomine.

Good Luck