Preparation Of Micro-arc Oxidation On Mg Alloy Modified By TiO₂and Its Tribologica! Behaviors

Due to their high specific strength, outstanding castability and excellentmechanical properties, magnesium alloys are widely being used in aircraft, aerospaceand automobile field. However, the poor wear resistance of magnesium alloys hadgreatly limited their range of application in engineering. Therefore, it is necessary formagnesium alloys to improve the wear resistance by corresponding surface treatmenttechnology. For magnesium alloys, there are a lot of surface treatment methods suchas the anodic oxidation and chemical plating. Though these methods can enhance thesurface hardness and improve the wear resistance to a certain extent, it had sacrificedthe toughness and reduced the fatigue property of the magnesium alloys. And parts ofthe treatment fluid are bad for environmental protection, it would pollute theenvironment. For this reason, the search for a feasible modification method toimprove the application of magnesium alloys has certain practical significance.Fortunately, micro-arc oxidation （MAO）, as a relatively new and effective surfacetreatment technology producing in-situ ceramic coating on magnesium alloys, hasaroused great concerns of researchers. Numerous researches showed that MAOtreatment on magnesium alloys was an effective approach to improve its density,hardness and wear resistance. At present, most researchers are focusing on thedevelopment of new electrolytes and optimization of electrical parameters for themicro-arc oxidation coatings on magnesium alloys. There are a lot of intrinsic defectssuch as holes and micro cracks. However, in the friction and wear process, where inthe defects or organization in dense will produce stress concentration phenomenon, ina variety of wear mechanisms of the comprehensive function, the contact surfacefracture and peel off between MAO coatings and dual pieces, eventually lead to thecoatings appear failure behavior. From the above, the microstructure defects has become the major factor which resulting in the MAO coatings failure. And the wearresistance largely depends on the compactness and effective control the defects ofmicrostructure of coatings.In this work, using the MAO technology and adding nanoparticles TiO₂intoaluminates electrolyte to obtain modified ceramic coatings on AZ91magnesium alloy.First of all, the studies focus on the effect of TiO₂addition on the MAO voltage,the morphology, surface hardness, phase composition and the adhesion. Secondly, westudy the micro-tribological properties of modified MAO coatings using the UMTmultifunctional tribometer. And discuss the effect of nano TiO₂, loads and slidingspeed and wear time on the friction coefficient and volume wear rate of the modifiedMAO coatings, respectively. The scanning electron microscopy and laser scanningconfocal microscopy were employed to analyze the morphologies of the worn tracks,and the optimal TiO₂added was determined.The results showed that the growth rate of coatings on AZ91magnesium alloysgradually increased with the content of nano TiO₂changed from0g/L to4.8g/L. Andthe color of the modified coatings gradually changed from gray to dark green, whilethe hardness of the coatings changed from1400Hv to1650Hv. When the content ofTiO₂up to3.2g/L in the electrolyte, the modified MAO coating were consisted of notonly MgAl2O4and MgO, but also TiO₂phase. XRD analysis showed that after MAOprocess, TiO₂particles did not take place the images change. The microstructureobservation showed that compared with not add the TiO₂, the addition of TiO₂particles could reduce the number of holes and improve the compactness of thecoatings. The friction and wear experimental analysis showed that when the load was2N, the nano TiO₂on the friction coefficient of coatings less affected. When the loadswere4,8,15,25N, the wear resistance of modified MAO coatings increased withincreasing the content of TiO₂. Especially when the load was15N, the curve offriction coefficient changed with time of coating which obtained4.8g/L TiO₂morestable, and it had the best wear resistance. In the same experimental conditions, thefriction coefficient increased with the loads and wear time, and it decreased with thesliding speed.