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URN etd-0830109-221849 Statistics This thesis had been viewed 3598 times. Download 1582 times. Author Po-jung Hsu Author's Email Address email@example.com Department Materials Engineering Year 2008 Semester 2 Degree Master Type of Document Master's Thesis Language zh-TW.Big5 Chinese Page Count 187 Title The influence of duty ratio and frequency of pulsed bipolar microarc oxidation on the properties of the oxide ceramic coatings of 7075-T6 Al alloy. Keyword Ceramic coatings Microarc Oxidation Pulsed bipolar current Aluminum alloy Aluminum alloy Pulsed bipolar current Microarc Oxidation Ceramic coatings Abstract Aluminum alloys exhibit low density, high specific strength, good wear resistance, anti-corrosion and great workability, it can be applied in wide application on commodity sector, automobile, aerospace and national defense industry. Due to the concepts of environmental consciousness and energy saving, valve metal and its alloys has increasingly used in automobile and bicycle recently. Meanwhile, aluminum is an active element easily combined with oxygen, leading to a formation of 5 nanometer inert thin film. However, the thin and uneven naturally-formed thin film exhibits poor corrosion and wear resistance. The corrosion resistance can be improved by suitable electrochemical reaction processes through reducing the corrosion current density and raising the reductive electrochemical potential of the oxidation layer.
The microarc oxidation (MAO) technique is a promising method for the valve metal in terms of the improvement of corrosion resistance and wear resistance. The microarc discharge formed on some specific area at high temperature will lead to the oxidation and sintering reaction on the surface of anode workpiece. The insulating MAO thin film can provide a barrier for the passage of current, which will be beneficial to prevent the substrate from corrosion.
The various high voltages and pulsed bipolar waveforms of the MAO experiment are employed on the surface of 7075-T6 Al alloy in this work. The aqueous electrolytes used in MAO process include NaAlO2 and KOH. In the same electrolytic solution, Ton(+) is the most influential parameter on film growth rate under the same pulsed frequency. The deposition rate increased while Ton(+) is increasing. But it will spall off the MAO film when raising the pulsed frequency. It seems that the spallation phenomenon is related to the over-shooting part of the pulsed wave on the anodic workpiece. When both the frequency and duty ratio is low, the more compact layer is formed. The compact layer will lower the corrosion current density. However, the corrosion resistance of samples with compact layer can also be increased even if the duty ratio is high. The improvement can be attributed to the superior coverage at high duty ratio. Moreover, the outer porous layer was composed of γ-Al2O3, and the inner compact layer was composed of α-Al2O3. To increase the cathodic pulse ratio will get higher growth rate. Furthermore, the film thickness is nearly 60μm and the average growth rate approaches 3 μm/min while the electric parameter is 2000Hz, Ton(–) 16% and working time 20 minute. In brief, Ton(-) has positive effect on promoting the growth rate. The lower ratio of Ton(-) produced much more porous layer in contrast to thick MAO films prepared at the high ratio of Ton(-).
The experiment shows: the surface of substrate will form the alumina phase by MAO technique. The MAO coatings can decrease the corrosion current density by 4 order (10-10A/cm2) and raise the corrosion potential by 0.5 volt. The growth rate of MAO film is nearly to 3μm/min and micro-hardness of 1800Hv.
Advisor Committee Mu-Rong Yang - advisor
Hsing-Hsiang Shih - co-chair
Shyan-Kay Jou - co-chair
Files Date of Defense 2009-07-30 Date of Submission 2009-09-01