Investigation On Process, Microstructure And Performances Of Microarc Oxidation Coating On ZK60 Magnesium Alloy

Magnesium alloys, with low density, high special strength and rigidity characteristics and other virtues, have become promising materials for manufacturing light structures and have a bright future in aviation, aerospace and 3C etc. However, the poor corrosion resistance and wear resistance of magnesium alloys has been greatly restricting their further application. A coating on the surface of magnesium alloys can solve these problems. Micro arc oxidation (MAO) processing is an emerging surface technique, which has a wide application future. Although, a lot of researches have been made about the formation mechanisum, microstructrue and performances of MAO coatings, there are many questions such as formation and performance to be answered, which is of great significance theoretically and actually. At present, the MAO researching of magnesium alloys mainly focus on A-Z and A-M alloys, but few about Mg-Zn-Zr alloys such as ZK60. In this pape, based on these problems in MAO researching of magnesium alloys, some valuable researches about ZK60 have been carried out and some innovative results have been obtained.Firstly, in the way of the perparation of the ZK60 alloy MAO coating, a new kind of electrolyte with high efficiency and without pollution has been developed, the effect of electronic parameters and oxidizing time on the thickness, microstructure and corrosion resistance of the MAO coatings and MAO final potential has been systematically inveastigated.1. By means of single variable experiments, the compositions of the MAO electrolyte were obtained with Na2SiO3 (60~80g/L) as the main component, NaOH(10~20g/L) as additives, NaB4O7 (15~25g/L) as accelerant and C6H5Na3O7 (15~25g/L) as stabilizator. By means of orthogonal experiments, an optimized electrolyte with 60g/LNa2SiO3 +20g/LNaOH +15g/L NaB4O7 +20 g / L C6H5Na3O7 was obtained.2. The optimized electronic parameters are 30A/dm2 current density, 600 Hz frequency and 40% duty cycle and 18min MAO time in optimized electrolyte system.Secondly, the microstructure and composition of the coating were analyzed by modern surface analysis methods such as SEM, EDS, XRD, Super depth of field optical microscope, and FTIR. The results are as followed:1. The surface morphology of the ZK60 alloy MAO coating is dense and uniformity. The most diamater of micro-pore is in the range of 3-5μm .The micro-cracks size is also small and most part of cracks are covered by fusion.2. The EDS test results show that the main elements of MAO coatings preapred with the optimized parameters are Mg,Si,O. The XRD test results show that the main phase of the coating is Mg2SiO4 pluse little dissociative MgO pericalse. Neither EDS nor XRD test results show that there is the elements or products of Zn or Zr in coatings.3. It is found that the MAO process can be divided into three stages according to the spark and potential variation, namely the passive film stage, the coating rapid growth stage and the spark ceramic coating stage.Finally, the properties of ZK60 MAO coating such as surface roughness, corrosion, wear resistance,wettability and adhesion were measured by various apparatus and test. The results are follows:1. In terms of surface roughness, the surface roughness of the MAO coating increases with incresing positive duty cycle.2. The corrosion resisitance results show that the impedance of coating is higher than that of substrate about 4 order of magnitude, and the self-corrosion potential of coating is higher than that of substrate by 312 mV, and the self-corrosion current is lower than that of substrate about 3 order of magnitude.3. The microhardness test results show that the microhardness from substrate to coating increases gradually. The coating's microhardness is 1572HV, and much higher than that of substrate. The wear resistance test results show that friction coefficient of optimized MAO coating decreases with increasing of loads under the condition of dry sliding wear. The EDS test results show that rate of iron increses from 41.39 wt% to 55.69 wt% with increasing of loads in scratch. The rate of iron is mainly influnced by surface temperature under different loads.4. The water contact angles test results show that the water contact angles of the optimized MAO coating is 85.6°. The adhesion test results show that adhesion between coating and substrate is level NO.1, and adhesion between coating and paint is level NO.2,according to GB/T-1998eqvISO: 1992.