Process Monitoring of Plasma Electrolytic Oxidation

Plasma electrolytic oxidation (PEO) is a potential technique to coat a dense ceramic oxide layer on the surface of light metals, e.g. Al, Mg or Ti. In our PEO process, Al-pieces are emerged in aqueous solutions containing dissolved silicate/alkali, e.g. KOH/NaOH and K 2 SiO 3 /Na 2 SiO 3. Using a DC-pulsed system with 20Hz, micro-arcs are induced at the surface of the pieces randomly and plasma is associated to cause oxidation at the surface of metals, where Al 2 O 3 and Ai-Si-O phase coexist. Because the role of the plasma in the physical-chemical process is less clear, an optical emission spectroscopy (OES) technique is used for the in-situ monitoring of the emitting light of the plasma in order to realize the mechanism. During the process monitoring, each whole spectrum (190nm-850nm) is acquired vs. time with a specified time interval, e.g. 1sec. By analyzing these data, we observe several peaks corresponding to the species Al(308.6nm, 394.4nm, 396.2nm), Na(589nm), K(769.9nm), H(486.8nm, 656nm) and a strong continuous spectrum coming from the thermal radiation. These profiles of these peaks vs. time are related to the corresponding profiles of the current and voltage of our system consistently. From the curve fitting of the spectrum of thermal radiation with Planck's law, the temperature of plasma is estimated to be about 5000 o K. Introduction: Plasma electrolytic oxidation (PEO), which utilizes a high voltage to induce micro-arc locally at the surface of metals, is a potential coating technique to deposit a dense ceramic oxide layer at the surface of light metals, e.g. Al, Mg or Ti [1-5]. In the process, metals are as electrodes emerged in basic aqueous solutions containing dissolved silicate/alkali, e.g. KOH/NaOH and K 2 SiO 3 /Na 2 SiO 3. The ceramic oxide layer of PEO coating consists of a porous outer layer and a dense inner layer, where the distribution of Al-Si-O phase gradually changes [1]. The addition of silicate in the electrolyte, e.g. K 2 SiO 3 /Na 2 SiO 3 , leads to accelerate the coating growth, because of the incorporation of Si causing the formation of Al-Si-O phase in the ceramic layer [1]. On the other hand, Si may cause a porous structure, if the amount of Si is too much. Therefore the concentrate of the silicate in the solution is necessary to be optimized. In addition, the pH value of the electrolyte is also critical to …