利用雙極脈衝電源微弧氧化法探討占空比和頻率對7075-T6鋁合金表面氧化膜層之影響

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.

徐栢榮(Po-jung Hsu)

指導教授 ： 楊木榮

大同大學/工程學院/材料工程學系所/碩士(2009年)

摘要

鋁合金具有質輕、高比強度外，耐磨耗性、耐蝕性與加工性佳，使得鋁被廣泛的應用在民生、工業及汽車、航太甚至是國防用途上，而近來環保意識抬頭和節能減碳的前提下，使得輕金屬及其合金近年來在汽車工業以及自行車上之使用量有增無減。鋁也是化學活性強的金屬，表面會自然生成一層約5nm的氧化鋁保護膜，但此種自然生成的氧化膜厚度太薄、膜層不均，使得此氧化膜層的抗腐蝕和耐磨性有限，進而藉由電化學方法控制氧化層之生成以達到防蝕目的，且增加合金表面機械性質。
微弧氧化技術在輕金屬表面改質近年已經受到廣大的重視，這類的微弧放電現象是利用電漿產生後，在陽極工件表面產生局部高溫熔融態金屬氧化並燒結，沉積在試片的表面，此MAO介電陶瓷氧化膜和基材緊密連結，可阻礙陽極(氧化)之電流通過，保護內部之金屬基材防止腐蝕。
本實驗是藉由雙極脈衝高電壓控制的微弧氧化技術，於鋁酸鈉和氫氧化鉀電解溶液中在7075-T6鋁合金之表面生成一層耐蝕性高且絕緣之陶瓷氧化膜層，在相同之電解液、脈衝頻率和電壓時脈衝比之下時，Ton(+)為主要影響膜層成長速率之參數，隨著參數Ton(+)增加，沉積速率也跟著增加。但頻率提高時易使得氧化膜層發生剝離現象，此與陽極鋁合金工件受到脈衝波之過充(over shoot)現象有關。在低頻且低占空比時，膜層緻密層較明顯者幾乎都有較低之腐電流密度，而在高占空比時只要膜層之表面披覆性佳，也能有效提高外在環境之耐蝕性。膜層外部之膜層(多孔層)大部分是由γ-Al2O3組成，膜層內部(緻密層)靠近基材部分主要是由α-Al2O3組成。陰極脈衝比率增高會使成膜速率提高，其中頻率2000Hz Ton(–) 16%之參數在微弧氧化處理20分鐘膜厚達60μm，平均每分鐘之成膜速率有3 μm/min，Ton(-)之比率提高有助於膜層厚度成長效率提升。Ton(-)比率低時膜層為多孔質，Ton(-)比率提高使成膜作用上升厚度增加，膜層表面出現海棉狀結構，使粗糙度下降或粗糙度(增加)弛緩現象出現之關係，高頻時原本形成之緻密層海棉狀結構被電弧擊穿，膜層剝落現象明顯。鋁合金經MAO技術可在表面形成高溫燒結態之氧化鋁相，且降低腐蝕電流達四個級數(10-10A/cm2)之多，腐蝕電位可提高0.5V左右，成膜速率最高可達3μm/min，硬度最高可接近1800Hv左右。

關鍵字

鋁合金 ； 雙極脈衝電流 ； 微弧氧化 ； 陶瓷鍍層

並列摘要

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.

並列關鍵字

Aluminum alloy ； Pulsed bipolar current ； Microarc Oxidation ； Ceramic coatings

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