Preparation And Tribological Properties Of Micro-arc Oxidation Coatings Of 2A12 Aluminum Alloy In Zircon Salt System

Aluminum and its alloys are widely used in aerospace,automobile,ship,decoration and so on because of their advantages of high specific strength,easy processing,high thermal conductivity and electrical conductivity.However,the use of aluminum and its alloys is limited by their low hardness and poor wear resistance.Micro-arc oxidation(MAO)is a kind of technology that in situ grow coating layer on the surface of aluminum alloy,which can significantly improve the wear resistance of the aluminum alloys.In recent years,research on directly adding ZrO₂ particles into the coating to improve the performance of the alloys has become a hot topic.However,this method has a shortcoming of agglomeration.Therefore,the in situ growth of ZrO₂ by micro-arc oxidation which can overcome this shortcoming is of great significance to improve the performance of the coatings.In this paper,zircon salt solution was used as the main MAO salt.The effects of power frequency,termination voltage,oxidation time and the concentration of additive Na₂WO₄ on the micro-arc oxidation coatings were studied.The surface morphology,phase composition and the wear performance of the coatings were characterized by scanning electron microscope(SEM),X-ray diffraction(XRD)and cutting disc friction wear tester.The results show that the frequency of power supply has little effect on the micro-arc oxidation coatings,and the termination voltage and oxidation time have great influence on the coatings.With the increase of the termination voltage and oxidation time,the size of protrusions on the surface increases,the number and size of micro-cracks increase,and the thickness and roughness of the coatings increase.Under the zircon salt system,the growth rate of the coating is relatively fast,the thickness of the film can reach 96?m and the roughness is about 3.6?m when the termination voltage is 480v with a oxidation time of 45min;the main phase components of the coatings without Na₂WO₄ are?-Al₂O₃,?-Al₂O₃,t-ZrO₂ and c-ZrO₂ phase.The presences of WO3 and ZrW2O8 besides those four phases are also detected in the coatings with additive Na₂WO₄.The results show that ZrO₂ is synthesized in situ by micro-arc oxidation.The friction coefficient of the coatings with Na₂WO₄ is larger than that of the coating without Na₂WO₄,because of the larger surface roughness.The wear loss of the coatings decreases with the increase of termination voltage and oxidation time.The optimal oxidation process of the coatings without Na₂WO₄ is 500Hz power frequency,480V termination voltage and 30min oxidation time.The wear loss of the coatings with Na₂WO₄ decreases with the increase of Na₂WO₄ concentration under the optimized oxidation process,and the wear of the coatings with 14g/L is the smallest and show the best wear resistance.Obvious lamellas are founded on the wear surface by looking into the wear morphologies,which is the typical appearance of fatigue wear,and the detached coatings can act as abrasive particles to destroy the coatings.When the concentration of Na₂WO₄ was 12 and 14g/L,there was no obvious lamellar on the wear surface of the coatings.Therefore,the wear mechanism of the coatings is mainly abrasive wear and fatigue wear.