A Study Of The Plasma Discharge And Coating Formation Mechanisms For The Plasma Electrolytic Oxidation Of Zirlo Alloy

Plasma electrolytic oxidation(PEO)endows Al,Mg,Zr and their alloys excellent surface properties.The technology works under the breakdown voltage of metal surface oxide film,resulting in plasma discharges on the metal surface and corresponding to a series of complicated physical and chemical process.Plasma spark discharge is related closely with the PEO coating formation mechanism;however,there is lack of a deeper understanding of these relevant processes.In this study,PEO of Zirlo alloy has been carried out in aluminate electrolytes to study the effects of electrolyte concentration and negative pulse on coating formation and plasma spark behavior.The coating morphology,plasma discharges have been characterized by methods such as scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS),Optical emission spectroscopy(OES).(1)When the electrolyte concentration varied from 2 to 32 gl"1,plasma discharge is very intensive.The OES spectra in these electrolytes normally consist of lines from the electrolyte(such as H?,H?,Na I and Al I)and also lines from Zirlo substrate(Zr I,Zr II).However,for PEO in 32 gl-1 electrolyte with bipolar regime,the emission lines from the substrate can hardly be detected.For the coating formed under unipolar regime(without negative pulse),large pores usually exist in the coating,and the surface morphology the coatings is featured by pancake structure.The big pores still exist in the coatings formed in dilute electrolyte when electrical regime is bipolar,however,when the electrolyte concentration increased to 16 gl-1,the large pores in the cross section decreased or disappeared,which is accompanied by the absence of the pancake structure on the coating surface.Moreover,A1 content in the coatings increased greatly under the bipolar mode.(2)The plasma discharge and coating formation characterization are very different in the high concentration electrolyte(56 gl-1).Plasma discharge has turned into weak discharges which may be termed as "soft sparking",however,there are also occasionally "sintering arc" in the late PEO process under the unipolar regime.The OES spectra of the "soft sparking" and "sintering arc" are similar,showing only Na I lines and a continuous spectrum.The lines from the substrate(Zr I,Zr II lines)are absent.In addition,the coatings formed in 56 gl-1 show an absence of both pancake structure on the surface and big pores in the cross-section.(3)The mechanism of coating formation and spark discharge and effect of negative pulse can be explained according to the above phenomena.The PEO process can be divided into two processes,i.e,the oxidation of metal substrate and anion deposition process.When the electrolyte concentration increases,the anion deposition becomes more important.This process leads to higher coating formation rate.The reason for the occurrence of "soft sparking" lies in the fact that anion deposition is associated with lower energy compared to the oxidation of the substrate.The application of negative pulse can promote the anion transportation process since the hydrogen liberation during the negative pulse can cause an additional stirring to the liquid layer near the electrode surface,hence a more homogenous coating and modified plasma discharge mode will be resulted.