| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 102 === Employing a bipolar pulsed high voltage power supply, we overlay an oxynitride film on top of the 6061-T6 aluminum alloy substrate through micro arc technology with current control. The electrolyte generally contains urea, sodium hydroxide, sodium hexametaphosphate, lithium hydroxide. And we study the influences of electrolyte composition on the film, which is targeted for electrical insulation.
We compare surface microstructure and phase differences of two oxynitride films prepared in the electrolytes of high and low urea concentration, namely, 500 and 20 g/l. The electrolyte of high urea concentration raises the nitrogen content of the oxynitride film, but alters the morphology and phase drastically. The nitrogen-rich film contains a cubic phase of Al3O3N oxynitride and the gamma phase of aluminum oxide. However, the nitrogen-poor film contains the Al3O3N oxynitride and the alpha and gamma phases of aluminum oxide. It is speculated that the alpha aluminum phase has been formed by melting and recrystallization at expense of gamma phase through local high energy micro arc. Influences of other ingredients in the electrolyte are described as follows. Addition of sodium hexametaphosphate, 1 g/l, stabilizes the surface appearance of micro arc. Addition of lithium hydroxide, 1 g/l, enhances the nitrogen content, and the breakdown strength as well. In addition, the electrolyte requires sufficient ion conductivity, ~30 mS/cm, and high pH value ~12.
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