Influence of Negative Bias and Magnetic Field on Structure of Microarc Oxidation Coating

To reveal effects of three cleaning processes(argon ion,argon ion/negative bias and target/negative bias) and the synergy between negative bias and magnetic field on the structure,thickness,hardness,and roughness of micro arc oxidation(MAO) coatings.The MAO coatings were prepared on the surface of magnesium(Mg) alloy AZ31 B by micro arc oxidation(MAO) technology first,and then followed multi-arc ion plating treatment.Furthermore,the scanning electron microscope,nanoindenter,thickness gauge and roughness meter were used to investigate the structures and properties of the coatings.There is no obvious change on the MAO coating cleaned by argon ion when the distance is 380 mm between the target and the substrate.Under the synergistic effect of argon ions and negative bias,the greater the negative bias,the better the cleaning effect is.At the same time,the numbers of the protrusions on the coating are decreased.The surface of the MAO coating can be destroyed to form deep holes when the negative bias voltage exceeds 500 V with the synergistic effect of argon ions,which fail the MAO coating to protect Mg alloy.Moreover,there is also some deep holes when the negative bias exceeds 200 V with the synergistic effect of Al target.The neutral or charged particles with large size can be filtered by controlling the magnetic current,so that the dense MAO/Al coating without obvious large particle droplets are formed.But the cleaning of negative bias tends to damage the surface structure of the MAO coating.Therefore,the value of the negative bias should be lower than 500 V when synergic cleaning with argon ions,and not higher than 200 V when synergic cleaning with aluminum targets.Compared to the single negative bias or magnetic field,by the synergic effect of negative bias and magnetic field is easier to form an MAO/Al coating with dense and uniform structure.