| The shortage of energy and mineral resources force people to improve the materials and energy's using efficiency. Plasma electrolytic oxidation (PEO) technology provides a new platform for surface modification of light-metallic materials. The films formed by PEO combine the performance of both anodic oxide films and ceramic films, which broadens the application field of light-metallic materials. Many researches related to the PEO films' properties have been down at home and abroad. The breakdown discharge phenomenon, which is the basic problem of PEO, is one of the most important characteristics of the PEO technology. Based on this, the AZ31 magnesium alloy was used as the working electrode. The influence of the dielectric barrier layer's (DBL) characteristics on the breakdown discharge, the influence of the electrolytes' ion nature on the characteristics of the micro-discharge in the plasma field, the mechanism of the breakdown discharge and the contribution of cations and anions to the PEO process were investigated.(1) the oxygen-containing alkaline electrolytes composed by the second and third cycle elements were the objects of the study:the influence of anion nature on the properties of the DBL's composition was investigated. The compactness and insulation of the DBL were evaluated qualitatively. The current densitis before and after the discharge were used to verify the quality of the DBL. The influence of the quality of the DBL on the breakdown discharge and follow-up the film forming ability was studied. (2) fluorozirconate series acidic electrolytes were selected as the research objects; the influence of the electrolyte anions' stability on the discharge characteristics and the films'performance were investigated.It was found that:(1) the anions formed the DBL through their own by forming the oxide or combining with the substrate ions to form the compounds. The more stable the DBL's composition, the more compact the DBL, the better the quality of the DBL, thus the stable equilibrium current density could be got at the stable discharge stage. (2) DBL's formation and the anions's film-forming after breakdown discharge were two necessary conditions for the PEO process been carried out smoothly. (3) if the solubility product of the film composition is smaller than 10-10, the formed DBL exhibits good stability and quality, the discharge characteristics are better with stable micro-discharges and the equilibrium current density is about 15-40 mA/cm2; if the solubility product of the film composition is larger than 10-10, the formed DBL has a poor quality with unstable micro-discharges, the equilibrium current density is about 300-700 mA/cm2; if the film composition is soluble, the discharge can not happen. (4) the additives could improve the stability of the anions in the fluorozirconate series electrolytes, followed by improving the discharge quality and the performance of the films. The corrosion resistance of the films increased 3-4 orders of magnitude compared to the substrate.The optical emission spectroscopy(OES) was used to investigate the different stage characteristics during the PEO process:(1) the energy status on the electrode surface before discharge and the reasons for the formation of species'distribution were studied. The characteristics of micro-discharge in the plasma filed were also analyzed. (2) the contribution of cations and anions to the plasma's composition and the PEO process were analyzed. (3) the source, energy level transition process and attribution of active species in the plasma field were investigated. (4) the breakdown discharge and growth mechanism during the PEO process, the heat and species'transfer mechanism in the plasma field were discussed. (5) the PEO growth process in the particular Na2SiO3 electrolyte was studied to verify the PEO film growth model and the species transfer process model.The results indicated that:(1) the spectra showed different characteristics at different PEO stages. At the anodic oxidation stage, the spectra has blunt peak caused by heat radiation which resulted in the luminescence; at the transition stage, the gas envelop was ionized and emitting light, followed by the breakdown of the gas envelop and DBL; at the discharge stage, the plasma active species were mainly composed of metal cations, the gases and its elements produced by water decomposition. The transfer of the electrolyte ions was mainly due to the ion acceleration, the adsorption and migration of ion bubbles, which resulted from the plasma field and the rupture of bubbles, respectively. (2) the anions in the electrolyte had little influence on the composition of the plasma active species, but they could affect the film composition by forming oxide; the cations could provide high energy environment for the plasma field. (3) the excited order of the plasma active species depended on the energy that the orbit transition of the species needed. It was not related to the ion's concentration. The concentration variation of each special active species with time during the PEO process was related to the energy status on the electrode surface. The active species in the plasma field were found to undergo dissociation, ionization and excitation. The electron temperature of the plasma field was between 6×103 and 3×104 K. (4) the PEO growth process in the Na2SiO3 electrolyte verify the PEO film growth model and the species transfer process model well. |
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