Micro-arc oxidation (MAO) is an eco-friendly surface modification technology that can in situ fabricate ceramic oxide coatings on surfaces of metals such as aluminum, magnesium and titanium, which is capable of ameliorating the inherent deficiencies of low hardness and insufficient wear resistance of 5052 aluminum alloy. In this work, a composite electrolyte system containing hydrofluoric acid and potassium hexafluorozirconate was innovatively developed. The effects of three positive voltage levels of 400, 450 and 500 V on the properties of MAO coatings on 5052 aluminum alloy were systematically investigated in terms of macroscopic and microscopic morphologies, phase composition, porosity, coating-substrate bonding strength, tribological properties and electrochemical corrosion resistance. The results demonstrate that the coating thickness increases continuously with the rise of oxidation voltage, while the crystallinity of phases, structural compactness, corrosion resistance and wear resistance present a variation trend of first improvement and then deterioration. It possesses stable coating-substrate bonding, excellent corrosion protection and favorable wear resistance, delivering the best comprehensive service performance.
The mechanism of the above phenomena can be explained as follows: In the Zr-F composite modified electrolyte system, K2ZrF6 acts as the zirconium source and is a key substance for generating the lubricating phase (K2ZrO3) and the strengthening phase (ZrO2); KF mainly functions to stabilize the electrolyte system and promote initial film formation, and the two cooperate synergistically to ensure the stability of the coating structure and performance. As the core process parameter that regulates the structure and performance of the film during the micro-arc oxidation process, positive voltage essentially modulates the film formation kinetics, microstructure and phase composition by changing the energy density and breakdown mode of micro-arc discharge, and ultimately determines the service performance of the film. The dense microstructure without connected pores constructs an efficient physical shielding barrier, which can significantly hinder the penetration of corrosive media and achieve the optimal corrosion resistance. The complete and stable hard composite ceramic phase and good coating-substrate interface bonding state jointly endow the coating with extremely a low wear rate, stable friction behaviors and excellent scratch resistance, and ultimately realize the multi-dimensional synergistic matching of interface mechanical properties, corrosion protection performance and wear service performance. The specific conclusions are as follows:
At a low voltage of 400 V, the discharge process is mild, resulting in the highest interface bonding force. However, the coating is thin, and Zr element is only enriched at the interface. XRD results show that the ZrO2 ceramic strengthening phase has extremely poor crystallinity, with a large amount of fluoride impurity phase enriched. The friction test shows that the coating is completely worn through and fails, exhibiting the weakest corrosion resistance among all tested samples.
At a medium voltage of 450 V, the discharge energy is moderate, the coating is dense without connected pores, and the Zr element is uniformly doped in the entire region. XRD results indicate that the ZrO2 hard ceramic phase with the highest crystallinity is obtained, the fluoride impurity phase is the least, and the phase purity is the optimal. The sample has the highest polarization resistance and the best corrosion resistance, while showing the lowest wear rate and the best friction stability, achieving the optimal matching of mechanical, corrosion-resistant and wear-resistant properties.
A high voltage of 500 V induces uncontrolled discharge, leading to a loose and porous coating and segregation of the Zr element. The crystallinity of ZrO2 deteriorates, the fluoride impurity phase rebounds, the interface bonding force decreases, the corrosion resistance is significantly attenuated, and the wear resistance is slightly lower than that at 450 V.
The optimal process voltage is 450 V. Excessively low voltage leads to insufficient modification and phase failure, while excessively high voltage causes structural and impurity phase deterioration. The medium voltage can prepare a long-acting wear-resistant and corrosion-resistant protective coating with pure phase composition and excellent performance.
Key words
micro-arc oxidation /
5052 aluminum alloy /
corrosion resistance /
wear resistance
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