Plasma Electrolytic Oxidation of Pure Titanium at Constant Applied Voltage

Abstract

Plasma Electrolytic Oxidation (PEO) is a method of coating which is used to apply non metal coatings to different metal substrates. This process occurs in an electrochemical cell that consists of an electrolyte, two electrodes submerged in the electrolyte and a power supply supplying voltage to the two electrodes. The goal of this study was to observe how a constant applied voltage would affect the coating thickness at varied applied voltage and varied processing time. This study was a continuation of a constant applied voltage study previously performed in the SaNEL research group. It was continued to add more detailed thickness data, develop a better understanding of the PEO process under constant voltage, and facilitate an undergoing modeling effort in SaNEL by another group member. The electrolyte used in the present study was KOH and K4P2O7, and the substrate pure Titanium (Ti) being the same as previous in experiments for continuity. PEO is an anodic process, which requires the positive polarity to be attached to the Ti electrode and the negative polarity to be attached to the stainless-steel counter electrode. A programmable power supply was used to apply constant voltage to the Ti working electrode. Four different voltages (450, 400, 350 and 300 V) were applied to the Ti electrode at varying processing times for each voltage. For 450 V, the processing times of 2, 10, 20, 30, 50 ,70, 100 and 150 s were selected to get a good range of time across the whole PEO process. For 400, 350 and 300 V the processing times of 50 and 100 s were selected for each. By applying a constant voltage to the Ti substrate, as the coating increases in thickness, the current decreases until it reaches zero. The current density can be calculated from the current decay data and the current density decays are compared across each voltage. As voltage increases the time for current density to decay to zero also increases, as well as the maximum current density value being higher at higher voltages. Surface morphology, composition, and thickness were characterized by SEM, SEM/EDS and XRD. EDS and XRD were used to confirm the presence of Ti, O and P in the coating and offer insight into which possible compounds are formed in the coating. It was determined that multiple Titanium oxides and amorphous Titanium phosphates are present in the coating, with TiO2, Ti2O and TiP2O7 being the most probable compounds found in the coating. SEM was also used to obtain the coating cross section thickness. It was determined that as applied voltage increased the coating thickness also increased and the coating thickness can grow for longer times at higher applied voltage as well. The thickness values obtained for 450 V were compared to a model made to predict oxide growth for the PEO process on Ti. The results of this comparison found that the thickness measurements obtained experimentally matched well with the model. Finally, charge density calculations were conducted for each deposition and correlated to the thickness data. The charge density is found by taking the integral of the current density decay curve with respect to time. A plot of charge density versus thickness was made to show the relationship between the two. It was determined that the relationship between the two is linear and can help be used to predict the thickness based only on experimental data. If the current density decay curve is obtained experimentally and the charge density is calculated from this curve, the thickness can be in turn estimated. This could be a helpful tool to use alongside the oxide prediction model to help be more accurate in trying to predict potential oxide growth at specific PEO parameters.