Study On Complex Surface Modification Of Magnesium Alloy Via Closed Field Magnetron Sputtering Ion Platingand Nitriding

In order to improve corrosion resistance and mechanical properties of AZ91D Magnesium alloy, we tried to deposite Al/Ti multi layers with closed field unbalanced magnetron sputtering, followed by plasma nitriding at the temperature ranging from 350℃to 430℃. The Gibbs free energies of intermetallics and nitrides which could form during nitriding were obtained by thermodynamic calculation. The surface and cross-seactional morphologies, diffusion element distributions in surface layer and phase compostions of pre-deposited coatings and duplex modified layers were characterized by optical microscope (OM), scanning electron microscope (SEM), X-ray diffractomer (XRD) and energy dispersive spectrometer (EDS). Additionally, the mechanical properties and corrosion resistance of pre-deposited coatings and duplex modified layers were characterized through microhardness, wear and anodic polarization tests.The results show that the surface of pre-deposited layers seems soomth with distinct particles and some defects existing. Wrinkles and cracks appear on the surface of the deposited layers after nitriding. The SEM results show that the particles get together and the density of layers is increased. The duplex modified layers consist mainly of Mg, Mg17Al12, Al3Ti and some amorphous Al or Ti. The cross-sectional morphologies and element distribution in modified layers indicate the noticeable interdiffusion of elements after nitriding, especially the magnesium atom. As the temperature rises, the diffusion of magnesium and aluminium atoms gets more sufficient, which is in good accordance with the results of diffusion coefficient calculations.The microhardness of modified layers is evidently improved with the temperature. When the temperature is at 425℃, the microhardness of soluted AZ91D coated by 10 layers is as high as 200HV0.01 which is about four times of the untreated sample. The friction coefficients of the samples modified by duplex methods do not decrease without aging treatment. And after aging at 200℃for 10 hours, the sample coated by 10 layers and nitrided at 375℃has the best wear resisitance. The corrosion resistance of soluted AZ91D coated by 10 layers is improved after nitriding. The best one is the soluted AZ91D with 10 Al/Ti layers nitrided at 375℃for 8 hours. Its corrosion potential is about 300mV higher than that of the original sample, and its corrosion speed is also decreased. It can be predicted that the soluted sample coated by 10 layers after nitriding at 375℃for 8 hours displays better performance based on the results of mechanical properties and corrosion resistance tests.