Fabrication Of High Performance Anodic Films On Al-based Metals And The Regularity Of Their Microstructure Evolution

Anodic oxidation and plasma electrolytic oxidation?PEO?are widely used for surface treatment of valve metals.While derived from traditional anodic oxidation,PEO is an improvement and complement to anodic oxidation by the application of higher voltages,forming ceramic coatings on metal surface under the voltages of dielectric breakdown.Anodic oxidation could be utilized to improve the corrosion and wear resistance of aluminum and its alloys as well as their electrical insulation and surface color;therefore it has been broadly applied in fields like aviation,aerospace,electronics,architectural decoration and household products.In recent years,the application of anodic oxidation has been extended to many new fields such as magnetism,optics,photoelectricity,membrane separation and printed circuit boards.In contrast,on account of its high efficiency in fabricating coatings,environmentally-friendly and ease of recycling for electrolyte,PEO has been used to form coatings with supreme wear and corrosion resistance in some important structures like gear and shipping industry.In this thesis,the application of anodic oxidation in the field of electrolytic capacitor and the formation of corrosion and highly wear resistant PEO coating on aluminum alloy were investigated.The main contents of this work are as follows:?1?As an important part of capacitor,the insulativity of aluminum surface determines the performances of aluminum capacitors.To increase the breakdown potential of capacitors up to 1500 V,2 to 3 capacitors with a breakdown potential of around 800 V are usually installed in series.However,the volume of the apparatus is thus increased and the stability is not good.In the third chapter,the breakdown potential of porous anodic coatings formed on the surface of aluminum is discussed.After porous anodic coatings were formed with different processing time in the electrolyte containing 0.3 M H₂SO₄ at 25 V and 293 K,sealing treatment was performed in boiling water.After that,breakdown test was carried out in sodium tungstate electrolyte.It was found that the micro pores in porous anodic coatings were sealed due to the formation of hydrated alumina precipitates within them.The breakdown voltage was raised up to 1500 V after sealing treatment,which might be applied in aluminium electrolytic capacitor.The test result of GDOES showed that the sealing treatment effectively blocked the incorporation of W species into barrier layer and took part in the coating formation process.?2?Wear and corrosion resistance of aluminum alloys are of great importance due to their wide application in many fields including aerospace industry.A systematic investigation of PEO on Al-Li alloy was carried out in aluminate electrolyte and discussed in chapter 4.It was found that a 5,24 g l^-1 electrolyte could directly form a PEO coating,but when the concentration is 32 g l^-1,PEO coatings could not be formed directly,only the traditional galvanoluminescence?GL?phenomenon occurs on the metal surface and typical PEO spark discharge does not appear.Only a thin layer of nano-porous film is observed on the sample surface after SEM examination.This is attributed to high pH of electrolyte which has strong tendency to corrode the substrate.In addition,field-assisted dissolution may also exist during PEO process.Their combination resulted in a higher dissolution rate than the coating formation rate;as a result,thicker coating could not be formed in higher concentration of electrolyte.Subsequent analysis suggests that field-assisted dissolution is only strong when the coating is very thin,if there is a thicker oxide coating on the surface of aluminum,the effect can be overcome and PEO coating can also be formed at higher concentration of electrolyte.Based on those discussions,a two-step method is put forward for PEO at higher concentration of electrolyte:firstly forming a thin layer of around 1?m in low concentration of electrolyte,and then continue the PEO process in higher concentration of electrolyte.Thus PEO coating is successfully formed at higher concentration of electrolyte.?3?Chapter 5 discusses the development of high growth rate and wear resistant coating on the surface of 2A97 Al-Cu-Li alloy.PEO was carried out in 5,32 and 56 g l^-1 NaAlO₂ electrolyte under constant current regime.A double-layered coating with big pores is formed in 5 g l^-1 NaAlO₂ while a dense single layer coating was formed in32 g l^-1 NaAlO₂.The growth rate of the coating increased with increase of electrolytic concentration,from 1.5?m min^-1 in 5 g l^-1 NaAlO₂ to 11.3?m min^-11 56 g l^-1 in NaAlO₂.The coating formed in the 32 g l^-1 NaAlO₂ electrolyte shows the best wear resistance among the coatings formed in the three electrolytes.The coating survived1800 s dry sliding against a Cr steel ball under 100 N of the ball-on-disk configuration,showing a minimal wear rate.The high performance of the coating is attributed to its single layered microstructure,which is free from big pores,and the high content of?-Al₂O₃ in the coating.The high coating growth rates at high concentration of electrolyte reveals that coating growth is not only contributed by the Faraday current,but also by the plasma assisted deposition of coating materials.As the electrolyte concentration increases,the energy consumption also decreases.?4?The microstructure evolution of PEO film is discussed in chapter 6,and a coating growth model is proposed in this chapter.It was found that coating formed in lower concentration of electrolyte(such as 5 g l^-1 NaAlO₂)was double-layered regardless of the current regime,big pores were observed between the inner and outer layer of the coatings.Dense single-layered coating was formed in electrolyte with 32g l^-1 NaAlO₂ under bipolar regime.However,under unipolar regime,the film formed at 32 g l^-11 NaAlO₂ electrolyte was double-layered with big pores between inner and outer layer.When the concentration was increased to 56 g l^-11 NaAlO₂,no matter at bipolar or unipolar regime,the structure of the coatings was compact and single-layered.The formation of big pores in the coating from low concentration aluminate electrolyte was mainly caused by the gas expansion associated with the strong penetrating plasma discharges.The single-layered coating formed in concentrated electrolyte was mainly attributed to the anionic deposition which can restrain the penetrating discharges.?5?The influence of element Mg in the substrate on the phase composition of the coating was studied in chapter 7.Aluminum alloy with 1.5 Wt%?3 Wt%?6 Wt%Mg was prepared.PEO of the fabricated alloy was carried out in electrolyte with 20 g l^-1NaAlO₂,it was found that the content of?-Al₂O₃ was significantly reduced with Mg being added to the alloy,while coating formed on pure aluminum had high proportion of?-Al₂O₃.It is therefore evident that the existence of Mg in the aluminum alloy could restrain the formation of?-Al₂O₃ in the PEO coating.