Studies Of Microstructure And Properties Of Oxidized Layer Formed On Aluminium Alloy By Plasma Oxidation

Aluminium and its alloys are some of the basic metals used in automobile and aeronautic applications.They show low density,high strength to weight ratio and good formability.On the other hand,wear and pitting corrosion resistance are not satisfactory for numerous potential application.Though the melting point and hardness of aluminium and its alloys are lower,their oxide have properties of higher melting point, hardness,and chemistry stability,which are used to be protective coating of a varieties of materials.In order to enhance the wear and corrosion resistance of aluminum alloy, the plasma oxidation is one of effective processes.The purpose of this paper was to obtain a thicker oxidized layer formed on aluminium alloy by plasma oxidation,and to investigate the friction and corrosion properties of oxidized layer.Based on the theories of plasma discharge and the principles of the vacuum heat treatment,hollow cathode discharge can be able to enhance ionization.On the one hand, The plasma density increases,the production of ions rises and the ion flux density at the components surface increases.On the other hand,quantity of heat is great for arc discharge,heat the components to the oxidizing temperature by radiation.Since plasma is not formed on the component surface,the arcing damage and the "edge effect" can be eliminated.Investigation in this paper was concentrated on three main parts:(1) All of the oxidized experiments were performed in 99%oxygen treatment atmosphere,at a pressure of 30Pa.The discharge voltage was 650 V and the substrates were positively biased with 150 V.when aluminum alloy samples were oxidized at 450℃-550℃for different times.Plasma oxidation by hollow cathode discharge is a effective way of obtaining an oxidized layer.The modified layer was characterized by X-ray photoelectron spectroscopy(XPS),atomic force microscopy(AFM) and scanning electron microscopy(SEM).The influence of hollow cathodic discharge and positive bias on the microstructure,mechanical properties of oxidized layer by plasma oxidizing was studied,and mechanism of plasma oxidition were studied.(2) Tribological properties of the oxidized layer were investigated on a UMT-2MT tribometer.The oxidized layer provides a smooth low friction surface and a low wear rate.Detailed examination of the wear tracks showed that plasma oxidizing can enhance wear resistance.(3) The immersion experiments in 3.5%NaCl solutions were carried to test the effect of plasma oxidation on the corrosion resistance of Al alloys.It is clear from this study that the corrosion resistance of aluminium alloy in the 3.5%NaCl solution is much enhanced by plasma oxidation.