A Study On The Plasma Electrolytic Oxidation Of An Aluminium-lithium Alloy And The Wear Properties Of The Coatings

As an important structural material in modern industry，aluminum alloy has animportant research significance in the corrosion and protection problems. Theplasma electrolytic oxidation is an important method to improve the surfaceperformance of the aluminum alloys. In this thesis，the plasma electrolytic oxidation（PEO） behaviors of2A97aluminum-lithium alloy in silicate electrolyte have beensystematically studied. First，the effects of the electrical parameters，such as currentdensity，time of treatment，on the morphology，and corrosion resistance of PEOcoatings under DC regime were analyzed. Then， the effects of the processingparameters，such as electrolyte concentration，current density，on the surface andcross sectional morphologies，phase compositions，corrosion and wear resistance ofthe PEO coatings under pulsed current regime were studied in detail.The results show that the potential of PEO treatment with DC regime for theimprovement of the corrosion resistance of the aluminum-lithium alloy is limited.Meanwhile, the corrosion resistance of the coatings will be deteriorated by longprocessing time due to the increase of the defects in PEO coatings，and it was alsoless affected by increasing of the current density. In contrast，under the condition ofpulsed current regime, plasma electrolytic oxidation treatment can drasticallyimprove the corrosion resistance of aluminum-lithium alloy. As a result, this articlemainly focuses on the PEO coatings from the pulsed current regime.The PEO coatings formed on the Al-Li alloys in the dilute silicate electrolyteexhibit an appearance of brownish red, which is in contradiction with our previousexperience that the PEO coatings formed on Al alloys in the alkaline silicate arenormally white. By XPS analysis, it is found that cuprite（Cu2O），which has a redcolor， has incorporated into the PEO coatings. There are two types ofmicrostructures， named pancake structure and cauliflower-like structure，existing inthe PEO coatings of aluminum-lithium alloy, which were resulted from the differenttypes of discharge. The pancake structure mainly consists of Al and O elements,which mostly comes from the substrate, while the cauliflower-like structure containslarge amount of Si，which comes from the electrolyte.In dilute silicate electrolyte, bi-layered coatings were formed on Al-Li alloy，with big pores between the outer and inner layer. The coatings formed in dilutesilicate electrolyte mainly consist of α-Al₂O₃. In contrast, the coatings formed in concentrated silicate exhibit a faster growth rate, and the thickness of coatings wasincreased obviously at the same time. The main phase composition of the coatingsformed in concentrated silicate electrolyte is mullite.The friction tests show that the coatings formed in dilute silicate electrolytehave an excellent wear resistances and the outer layer has a strong ability to resistwear，due to its high mechanical strength （the hardness can be up to19GPa）, whichis in contradiction with the thoughts that the outer layer is porous and less wearresistant in many literatures, furthermore, after the removal of the outer layer, a welladherent and hard （～9-12Gpa） inner layer provides better wear resistance. Thehardness of the coatings formed in concentrated silicate electrolyte is lower than thatof the former. And the wear resistance property of the coating decreases, althoughthe coating has an higher thickness.Based on the microstructures of the coatings formed in dilute silicateelectrolyte，a new modified discharge model for the formation mechanism of t hePEO coating formed in dilute silicate electrolyte has been proposed.