鋁和鉬/鋁在磷酸-硝酸溶液的蝕刻研究與鎳鈦合金表面陽極處理改質

Abstract

本論文針對鋁在磷酸系統蝕刻液中的蝕刻行為和電化學性質做研究，蝕刻液的主要組成為磷酸和硝酸， XPS分析結果顯示經過蝕刻的鋁表面含有Al2O3、AlO(OH)和PO43-，在利用ICP-OES量測的蝕刻速率中更進一步發現：除了PO43-之外，氫離子對於鋁的溶解速度也有很大的影響。在蝕刻速率的量測中我們發現硝酸的加入可以大幅提升鋁的蝕刻速率，與磷酸互相合作造成”synergetic effect”，根據以上的實驗結果，本論文成功提出能夠解釋鋁蝕刻行為的反應機制，其中，鋁首先氧化成AlO(OH)，接著才與PO43-形成可溶的錯合物，或是與H3O+反應生成鋁離子；而硝酸在磷酸蝕刻液中最大的功效則是帶走吸附於鋁金屬表面的H(ads)，使得蝕刻反應能持續而有效率的進行。在此根據此反應機制推導討論鋁蝕刻的反應動力學，成功解釋鋁蝕刻反應速率的變化。論文中提出了一個簡化的蝕刻速率方程式說明了磷酸、硝酸和磷酸-硝酸三種不同蝕刻液對蝕刻速率的影響。利用交流阻抗的技術建立的等效電路圖可以觀察其表面化學特性及微觀結構之改變。除此之外，因為Mo/Al已經是工業上最主要的製程，所以本論文也用相同的磷酸-硝酸蝕刻液研究Mo/Al的電化學行為與蝕刻過程中的結構變化。結果發現Mo/Al雙層金屬的蝕刻會經歷五個不同階段，開環電位的量測中電位階段性變化的結果與SEM影像都說明Mo和Al在不同階段有不同的相對蝕刻速率。

在NiTi合金改質的研究中，本論文採用陽極處理法使NiTi合金的表面產生氧化物TiO2，比較在兩種不同的電解液NH4F(aq)和CH3COOH(aq)中處理後的性質變化。首先，在NH4F(aq)電解液中，由於氧化物生成的過程中同時會發生TiO2溶解，陽極處理過的表面氧化層呈現孔洞不平整狀，導致表面粗糙度變大，由XRD發現表面產生無晶型的TiO2氧化物；其中以5V電壓處理下的表面鎳含量下降最多，腐蝕電位也表現出最好的抗腐蝕效果，在硬度和耐磨度的測試中也有最高硬度和耐磨度；然而粗糙的表面卻造成鎳金屬在NaCl(aq)的釋放量上升，尤其以5V電壓處理的表面粗糙度最高、孔洞腐蝕最為嚴重、鎳金屬釋放量也最高。接著是CH3COOH(aq)電解液，陽極處理過的NiTi合金呈現平整細緻又均勻的表面，腐蝕電位的大幅提升和陽極電流的明顯下降顯示改質後優良的抗腐蝕性；其中以最高電壓20V處理過的表面粗糙度最小、腐蝕電位最高、鎳金屬釋放量最低，不僅如此，此電壓下處理的表面同時具有最高硬度和極佳的耐磨度；雖然表面鎳含量下降程度不高，但極為平整的表面卻能夠有效防止孔洞腐蝕(pitting corrosion)的發生，達到抗腐蝕和增加機械強度的目的。

This study examined the etching behaviors and electrochemical characteristics of aluminum in phosphoric acid-based etchants. The etchant used in the study was primarily composed of phosphoric acid and nitric acid. The surface analysis of the etched aluminum sample revealed the existence of Al2O3, AlO(OH), and PO43-. Other than the contribution of phosphate ions, the experimental results showed that hydrogen ions in the etchant influenced the etching rate greatly. By including nitric acid in the etchant, a synergetic effect on the etching rate was observed. Hence, this study proposes an etching mechanism in which the passivated film is dissolved by hydronium ions and phosphate ions, respectively. In the etchant, aluminum becomes passivated AlO(OH) and a dissolvable complex simultaneously. The adsorbed H(ads) produced on the Al surface is removed by nitric acid, which leads to acceleration in the etching rate. The kinetics model which was established according to the mechanism proposed in this study illustrated the changes in aluminum etching rates. To simplify the kinetics model, an equation of etching rate was used to explain the contributions of phosphoric acid and nitric acid in aluminum etching. In the study of electrochemical impedance measurements, an equivalent-circuit model with two capacitor elements has been established. The etching behavior of Mo/Al was also investigated in this study. The dramatic change in open circuit potential explained the different aspects for discrete etching durations.

In the research of modification of NiTi alloy, NiTi alloy was anodized in two different electrolytes NH4F(aq) and CH3COOH(aq). The formation of oxidant TiO2 was observed after anodization treatment. For the NiTi samples anodized in NH4F(aq), formation and dissolution of TiO2 took place at the same time. This phenomenon resulted in high roughness and the porous structure on the surface oxide layer. The XRD analysis showed the amorphous TiO2 on the surface. Under the anodizing voltage ranged from 2V to 20 V, the NiTi samples treated at 5 V had the lowest surface nickel content, the best corrosion resist property, the highest hardness, and the best level of wear resist. However, after 14-day immersion in NaCl(aq), the SEM images revealed serious pitting corrosion, the rough surface of the anodized NiTi at 5V caused extremely high nickel release. For the NiTi samples anodized in CH3COOH(aq), the surfaces became even and fine. The obvious increases in corrosion potential and decreases in anodic current all illustrated the excellent corrosion resist. The NiTi samples anodized at 20 V had the lowest roughness, the highest corrosion potential, and the lowest nickel release. Moreover, the modified NiTi samples at 20 V also had the highest hardness and the best ability of wear resist. Despite the high nickel content on the surface, the fine surface effectively prevented pitting corrosion to improve both corrosion resist ability and mechanical properties.