微弧氧化法(Micro arc oxidation)著色相當困難，因為微弧氧化層不同於陽極處理具備規則性的孔洞構造。此研究顯示，透過添加帶負電荷之顏料微粒子到電解液中，可達到均勻著色的目的。我們選擇鉻綠顏料微粒將鋁合金6061表面著色，說明此技術的可行性。結果顯示在定電壓模式下，具負電荷之顏料微粒會使得氧化電流與能量消耗大幅上升。因此我們將著色製程分為兩階段，第一階段是使用定電壓模式在沒有添加顏料的情況下鍍製阻障層；第二階段則是採用定電流模式並添加顏料來生成綠色膜層。但若第二階段的參數設定不好，則正電壓會急遽上升使製程中止。鉻綠顏料與alpha相氧化鋁具有相同的六方晶相，雖然商業鉻綠顏料及自行合成的鉻綠粉末，都屬於高度缺氧相的氧化鉻(defective Cr2O2.4)，但完成著色的綠色膜層則轉變成未缺氧的六方晶相Cr2O3。商業顏料製備之綠色膜層的微觀結構分析下，是由兩層所組成；上層以50%氧化鉻與50%氧化鋁所組成，而下方緻密層幾乎為氧化鋁，總膜厚會隨著佔空比或電流比上升而上升。使用10 g dm-3鉻綠顏料進行鋁合金6061的MAO著色，在電流密度為92mA cm-2、頻率500Hz、佔空比20%及電流比1.0下鍍製40分鐘能得到最佳膜層。

Coloring aluminum alloys with micro arc oxidation (MAO) technique is quite difficult, because, unlike anodizing, the MAO coating does not contain regular pore structure. This study demonstrates that we are capable of achieving uniform coloring via addition of negatively charged pigment nanoparticles in the electrolyte. We select green chromia pigment to color the surface of aluminum alloy 6061 and show the feasibility of MAO coloring. Experimental results indicate the pigment particles, with adsorbed negative charge, drastically raise the oxidation current and the power consumption under the constant-voltage mode. Hence we divide the coloring operation into two steps, first depositing a barrier layer without pigment using the constant-voltage mode, second growing a green layer with pigment under the constant-current mode. Still, the positive voltage of the second step easily runs away if the electrical parameters are not set properly. The phases of chromia oxide and α-alumina are isostructural. Although commercial chromia pigment and in-house chromia powder are highly defective chromium oxide, Cr2O2.4, yet the green layer turns into a much less oxygen deficient crystal with parameters similar to those of hexagonal Cr2O3. Microstructure analysis of commercial powder precursor shows the green coating consists of two layers; the top layer contains approximately 50% chromia and 50% alumina, a dense layer underneath contains ~100% alumina. The total thickness increases with increasing duty ratio, or increasing ratio of positive (I+) divided by negative current (I-). With 10 g dm-3 pigment in the electrolyte for green coating on aluminum alloy 6061 for 40 minute, the optimal parameters are 92 mA cm-2 in current density, 500 Hz in frequency, 20% in duty ratio, and 1.0 in the I+/I- ratio.

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