| Recently, Mg-Gd-Y-Zr magnesium alloys are intensively investigated due to their excellent properties, such as high strength at elevated temperature and good creep resistance. However, they have some drawbacks such as poor corrosion and wear resistance, which greatly limit their wide use in many applications. Surface modification is an effective way to improve the corrosion and wear resistance of magnesium alloys.In this study, carbon coatings are deposited on the GW83 magnesium alloy by magnetron sputtering to improve the corrosion and wear resistance of the substrate. In addition, the effects of the clear bias voltage, the deposition bias voltage and the target current on the surface morphology, hardness and elastic modulus, adhesion strength, corrosion and wear resistance of the carbon coatings have been systematically studied by SEM test, Raman spectra, scratch tests, corrosion and wear tests. The optimized deposition parameters have been obtained.The experimental results show that the proper clear bias voltage prior to deposition can effectively clean the substrate surface thus decrease the defects and improve the density of the coating. During the deposition, the bias voltage can enhance the ion bombardment, which is suitable for the deposition of the dense coating, with low internal stress, good adhesion with the substrate and improved corrosion resistance. The higher the bias voltage, the higher the energy of the bombarding ions, and hence the greater the internal stress in the coating. In addition, the proper target current can form the effective ions bombardment and keep the relatively low deposition rate, which makes the coating particle size decrease and form the coating with few defects and low internal stress. C/Al coating deposited under-40 V bias voltage and the target current of 3.5 A exhibits the dense structure with the hardness of 880.9 HV and the elastic modulus of 68.8 GPa. The Ecorr of the coating in the 3.5 wt% NaCl solution is-1624 mV and the icorr is 198 μA cm-2. The width of the wear track of C/Al coating under the force of 2 N and the wear time of 1800 is 0.93 mm.The C/Cr coating exhibits the higher hardness and better wear resistance compared to C/Al and C/Ti coatings, which is due to the fact that the higher hardness of the Cr as the metal transition layer can effectively improve the bearing capacity of the substrate thus improves the wear resistance of the carbon coating. C/Al coating shows higher adhesion strength and better corrosion resistance. This is because the physical properties of Al(such as hardness, elastic modulus) is close to that of Mg. In addition, the electrode potential of Al is nearer to that of Mg in contrast to Cr so Al will reduce the galvanic corrosion to a relatively great extent.The Ni+C composite coating(PVD carbon coating with electroless plating nickel interlayer) is dense with the typical graphite-like carbon film in the carbon layer. In the Ni+C composite coating, the carbon coating has good adhesion with the nickel interlayer. Moreover, The Ecorr of the Ni+C composite coating in the 3.5 wt% NaCl solution shifts from-1673 mV of the GW83 magnesium alloy to-1372 mV and the icorr decreased from 90 μA cm-2 of the magnesium alloy to 11 μA cm-2. Thus the Ni+C coating can greatly improve the corrosion resistance as well as prolong the wear life of the GW83 magnesium alloy as a result of the mechanical support of the thick nickel interlayer and the dense top carbon layer. |
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