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Syntheses of Al2O3 coatings on aluminum alloys by plasma electrolytic oxidation
|關鍵字:||plasma electrolytic oxidation;電漿電解氧化;alumina;Aluminum nitrate;鋁合金;氧化鋁膜;硝酸鋁||出版社:||材料工程學系所||引用:||(1) 汪建民主編，陶瓷技術手冊 (下)，中華民國粉末冶金協會，第二十章 (黃啟祥、林江財)，第683~715頁，民國83年。 (2) N.M. Yakovleva, L. Anicai, A. N. Yakovlev, L. Dima, E. Y. Khanina, M. Buda and E. A. Chupakhina, “Structural study of anodic films formed on aluminum in nitric acid electrolyte,” Thin Solid Films, 416 (2002) 16-23. (3) P. Vuoristo, T. Mantyla and P. Kettunen, “Stoichiometry and impurities in sputtered alumina films on copper,” Thin Solid Films, 204 (1991) 297-311. (4) Q. Fu, C. B. Cao and H. S.. Zhu, “Preparation of alumina films from a new sol-gel route,” Thin Solid Films, 348 (1999) 99-102. (5) L.P.H. Jeurgens, W.G. Sloof, F.D. Tichelaar and E.J. Mittemeijer, “Composition and chemical state of the ions of aluminium-oxide films formed by thermal oxidation of aluminium,” Surf. Sci., 506 (2002) 313-332. (6) N. M. Yakovleva, L. Anicai, A. N. Yakovlev, L. Dima, E. Y. Khanima, M. Buda and E. A. 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電漿電解氧化法 (Plasma electrolytic oxidation,PEO) 有別於傳統陽極氧化法，其主要差異在於試片表面產生火花放電，可使成膜速率快速且生成結晶性與附著性較佳的陶瓷膜。本研究主要是利用電漿電解氧化法，使用0.1 M 鋁酸鈉中添加不同濃度的硝酸鋁Al(NO3)3作為電解液，於鋁合金上製備氧化鋁膜。首先以單一0.1 M 鋁酸鈉作為電解液，改變電流密度 (1~ 20 A/dm2) 及反應時間 (10~ 120分鐘) 觀察其對生成氧化鋁膜組成之影響；接著在固定電流密度10 A/dm2，於鋁酸鈉溶液中添加少量的硝酸鋁Al(NO3)3，探討硝酸鋁濃度 (1.0 mM~ 5.3 mM) 與反應時間 (10~ 120分鐘) 對 α-Al2O3 相生成之影響，並比較其與未添加結果之差異。
在鋁酸鈉溶液中添加少量硝酸鋁會降低電解液的導電度，當硝酸鋁濃度在1.6 mM ~ 2.1 mM 範圍時，會產生火花放電延遲現象，當濃度大於5.3 mM，則無火花放電產生。以X光繞射儀分析結果顯示所生成之氧化鋁膜的結晶相主要為 α-Al2O3 和 γ-Al2O3，當添加1.0 mM~ 2.1 mM 硝酸鋁後 (10 A/dm2)，氧化膜層中 α-Al2O3 之含量隨硝酸鋁濃度先上升後下降之現象，添加1.6 mM 硝酸鋁可獲得最高 α-Al2O3 含量之氧化膜；另以單一鋁酸鈉為電解液時，氧化鋁膜中之 α-Al2O3 含量隨電流密度及反應時間之增加而升高。在場發射電子顯微鏡表面微結構顯示，在添加硝酸鋁濃度達2.1 mM，表面會產生顯著的孔蝕現象。另外厚度與表面粗糙度一開始皆隨硝酸鋁的添加而有所增加，當硝酸鋁濃度高於1.6 mM，氧化鋁膜粗糙度不變但厚度逐漸下降。不同反應時間，α-Al2O3 皆較未添加有顯著增加，氧化鋁膜之表面硬度亦隨 α-Al2O3 相的含量而變化。然而，鋁基材經PEO製程處理後，大多數熱阻值約 (7.0±1.2℃/W)，雖高於鋁基材 (3.4±0.0 ℃/W)，但已接近氧化鋁塊材 (5.0±1.6 ℃/W)之熱阻，可以做為絕緣之散熱材料。
火花放電延遲現象主要是在反應初期生成膜層會產生孔蝕現象，造成氧化膜層較不緻密，電解液能直接滲進氧化膜層與陽極-鋁合金直接進行反應，導致陽極與電解液界面阻抗無法快速增加，造成電壓值上升趨緩所致。而添加硝酸鋁造成 α-Al2O3 相的增加，主要是在火花放電過程中能提供足夠之能量生成水鋁石，進而再轉變為 α-Al2O3相，且較未添加者可提高約5倍，也高於未添加4.35 (15 A/dm2-60分鐘)。另外，Iα-Al2O3 與 Iγ-Al2O3 的XRD訊號鋒相對強度亦較文獻中最高者多1.1倍，但文獻使用的電流密度卻是本研究的5倍。
In contrast to traditional anodic oxidation, plasma electrolytic oxidation (PEO) can produce better crystalline ceramic coatings with quick deposition rates. The objective of this study is to prepare alumina coatings in an electrolyte with different additive concentrations by PEO on aluminum alloys. It was conducted in 0.1 M NaAlO2 electrolytes at current densities of 1- 20 A/dm2 with reaction time for 10- 120 minutes. Observe the formation of α-Al2O3 phase formed on the influence of differences;and then at fixed the current density of 10 A/dm2 by adding Al(NO3)3 concentration from 1.0 mM-5.3 mM with reaction time for 10- 120 mins, then to compare their results with without adding the difference.
As for voltage characteristics, we found the spark discharge was delayed obviously while adding the concentration of Al(NO3)3 in 1.6 mM- 2.1 mM range. The decreasing of electrolyte conductivity could be observed. When the Al(NO3)3 concentration was above 5.3 mM, the discharge voltage varied dramatically but no spark discharge could be found. Obtained alumina coatings were mainly α-Al2O3 and γ-Al2O3 phases by X-ray diffraction. The relative amount of α-Al2O3 increased with the aluminum nitrate concentration while decreased by adding 1.0 mM~ 2.1 mM aluminum nitrates. Moreover, the highest relative amount of α-Al2O3 was obtained by adding 1.6mM Al(NO3)3. The relative amount of α-Al2O3 gradually increased as the current density and the reaction time increased in 0.1 M NaAlO2. The morphology of PEO coatings revealed hemispherical pits on the surface in 2.1 mM Al(NO3)3 by field emission scanning electron microscopy. In contrast to non-add Al(NO3)3 additives, the thickness and surface roughness of coatings were all increased by adding Al(NO3)3 additives. Once the Al(NO3)3 concentration is above 1.6 mM, the coating surface roughness become slowly increasing, but thickness gradually decreasing. AS for cooling applications for aluminum substrate treated by PEO, the thermal resistance was about 7.0±1.2 ℃/W. It was close to the bulk alumina (5.0±1.6 ℃/W) of the thermal resistance although higher than the aluminum substrate (3.4±0.0 ℃/W). Thus, it can be used as insulation materials and heat sink.
The hemispherical pits on the surface with less dense film might cause electrolyte penetrating into PEO coatings easily. Therefore, it is difficult to increase the interface impedance between the anode and the electrolyte due to the reaction of electrolytes with Al directly, which makes the delayed discharge voltage. Adding aluminum nitrate caused the α-Al2O3 of coatings increasing with the spark discharge process. It is because there was sufficient energy to make other phase transforme to stable α-Al2O3 phase, which was 5 times higher than that without the additive (15 A/dm2-60 minutes). Besides, α-Al2O3 and γ-Al2O3 in the XRD relative intensity of the signal front was 1.1 times than the literature.
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