Preparationandperformance Of TiO₂/Ti Duplex Coatings On Magnesium Alloy

Magnesium and its alloys have excellent properties and wide application prospects. However, the poor corrosion and wear resistance in many environments restrict their widespread application in many fields. Many researches focus on improving the corrosion and wear properties of magnesium alloy by various surface modification technology. In this paper,TiO₂/Ti duplex coatings were prepared on the surface of AZ91 D magnesium alloy by a duplex surface treatment technique combining of close field unbalanced magnetron sputtering ion plating（CFUBMSIP） and micro arc oxidation（MAO）. The effects of the process parameterson the micro-structure of the duplex coating was analyzed using scanning electron microscopy（SEM） 、 energy dispersive spectroscopy（EDS） 、 X-ray diffraction（XRD） 、 friction and wear test 、electrochemical corrosion test、tribocorrosion test and nanoindentation. The formation mechanism of duplex coatings was discussed. The mechanical properties and tribo-corrosion performance of the coating with different structure on magnesium alloy were investigated. It is to supply a reference for improving the comprehensive service performance and expanding the application of magnesium alloy. The obtained main results were shown as follows:1) Auniform and compact titanium coating with a thickness of 4 μm and well bonding to substrate was deposited on AZ91 D magnesium alloy by CFUBMSIP.2) TiO₂/Ti duplex coatings were prepared on magnesium alloyby micro-arc oxidation of the deposited Ti coating. The TiO₂/Tiduplex coatings with a typical pore structure are composed of anatase, rutile phase, Mg2SiO4 and Ti phases.The Ti layer becomes more and more thinner while the thickness of oxidation layer increases gradually with the extension of micro-arc oxidation time. The Ti layer was completely oxidized into ceramic layer after 12 min MAO treatment.3) The electrochemical corrosion test for the duplex coatings indicates that the TiO₂/Ti duplex coating after 6 minMAO shows the best corrosion resistance with the highest self-corrosion potential and the lowest corrosion rate. The corrosion rate is reduced by two orders of magnitude compared with AZ91 D magnesium alloy.4) The tribological test shows that the wear rate of the TiO₂/Ti duplex coating obtained by 6 min MAO is also the lowest which is only 1/7 of that of AZ91 D magnesium alloy.5) The tribo-corrosion properties of magnesium alloy and the modification coating on it by different surface treatment were compared. It indicates that the friction coefficients ofTiO₂/Ti duplex coating、AZ91D magnesiumalloys、Ti coating、magnesium alloy MAO coatingand the complete oxidation ceramic coating in the 3.5% NaCl solution are 0.35, 0.2, 0.32, 0.3 and 0.31, respectively.The corresponding open circuitpotentials（OCP） are-1.31 V,-1.47 V,-1.44 V,-1.42 V and-1.35 V, respectively.The TiO₂/Ti duplex coating shows an improved OCP and a lower wear rate.6) The results of cross-sectional hardness by nano-indentation test for different surface coating on magnesium alloy show that the hardness of the single MAO coating and complete oxidation ceramic coating is changed sharply along the interface while it shows a gradually decrease for the TiO₂/Ti duplex coating. The elastic modulus values display a similar trend.It is beneficial to the improvement of the comprehensive carrying capacity.7) Sealingtreatment was performed by adding TiO₂ colloids to the MAOelectrolyte during the process of MAO. The results show that after sealing treatment, the thickness of modified coating increased, surface roughness and the size of micro-pore reduced and the micro-pore is uniformly distributed. The wear rate was ruduced by 2/3 comparing with the unsealed one. It indicates that sealingtreatment with TiO₂ colloidseffectively reduced the size of the micropores and further improvethe corrosion and wear resistsnce properties of magnisium alloy.