Growth And Mechanism Of Micro-arc Oxidation Coatings On Magnesium Alloys

AZ91D magnesium alloy used widely in industrial application were chosen as the substrate to be processed by micro-arc oxidation(MAO) in silicate-containing electrolyte containing potassium fluoride, sodium hydroxide and sodium silicate. The growth of the MAO coatings and its mechanism were both researched.First, the effect of sodium silicate and potassium fluoride on coating growth was investigated respectively. It was found that when sodium silicate exsits in the electrolyte, the breakdown voltage to the specimen declined, and the sparks generated on the specimen surface tend to be moving instead of being steady, that leads to the ablation phenomenon vanished away. The apparent quality of the coatings was much improved with no more micro-cracks on the coating surface. The compactness of the whole coating is significantly improved, that is benefited from the less micro-pores appearing on the coating cross-section and the decreasing of the micro-pores in the coatings penetrating down to the substrate. After potassium fluoride was added to the electrolyte, the breakdown voltage to the specimen obviously decreased but the breakdown process was more vigorous to lead to formation of larger sparks on the specimen surface during micro arc oxidation. As a result, the grow rate of the coating significantly increased and the obtained coating had a remarkable improvement in terms of coating thickness.Second, the growth of the coating on α-Mg and β-Mg17Al12 phases contained in the substrates was studied. The results show during the MAO process, the oxide coatings without micro pores appeared on the surface of β phase prior to α phase, but the oxide coatings with micro pores which were resulted from the breakdown process occurred first on the surface of α phase, which means MAO coating prefer to generate on α phase. And then MAO coatings gradually covered the entire surface of the substrate. The content difference of magnesium and aluminum elements between α phase and β phase in the substrate led to uneven content distribution of magnesium, aluminum and fluorine elements on the coating surfaces during initial stage of the MAO process. Magnesium and fluorine elements demonstrated the similar distribution patterns while that of aluminum element was the opposite. The content distribution of the above three elements gradually was homogenized with the increase of MAO coating thickness. However, the distribution pattern of silicon and oxygen elements in MAO coatings was kept uniform through whole MAO process.And then, the growth direction of the micro-arc oxidation coating was investigated by three methods, containing directly observing the boundary area, macroscopically calculating the coating and directly measuring the whole substrate. The design idea of the three methods is the key to the research, and the preparation of the samples meeting the demand of research is crucial problem which must be solved. The results show a phenomenon that the MAO coatings growing inward and outward on magnesium alloys was observed. The dimensions of the coatings growing outward were lager than that of inward during the coating growth. Subsequently, According to the growth characteristics of the MAO coating, a calculation formula of the density of the micro arc oxidation coating was derived and the density value of the MAO coatings prepared at different treatment time were obtained. The results show that the coatings fabricated at smaller treatment time had larger density values. The coating density decreased at larger treatment time and it remained constant when treatment time was longer. Meanwhile, the relationship of the density and compactness of the coatings was discussed. The variation trend of the coating density with the treatment time can reflect that of the coating compactness because the coating phase composition was basically unchanged with the increasing of the treatment time,Furthermore, the MAO coatings with same thickness corresponding to different coating growth rate were fabricated by adjusting the experiment parameter and the effect of the growth rate on coating microstructure was discussed. The results show with the increase of coating growth rate, the micro pores on the surface and surface porosity of the coatings both decreased gradually. Meanwhile the amount of bigger micro-pores and the micro-cracks on coating surface increased continually. As a result, the compactness of the coating declined gradually. However, phase composition and key element distribution of the coatings were basically unchanged. In addition, The changes of the characteristics of coating, such as the thickness, microstructure and element composition of the coatings with the treatment time was researched when the samples were respectively treated in two types of power voltage loading ways, namely, constant voltage mode and variable voltage mode. Specifically, the size and amount of the micro pores on coating surface were studied quantitatively. The results show that the change trend of the characteristics of coating fabricated in the two types of power voltage loading ways was same. When the treatment time was improved, the thickness of the coating increased and the amount of total micro pores on coating surface decreased but the amount of the bigger micro pores increased. Meanwhile the surface porosity of the coatings increased gradually. However, the phase composition and the element distribution of the coatings remained unchanged.Finally, based on the above study results, MAO coating growth was discussed An electric field model of breakdown was established and the formulas of electric field intensity and the energy of breakdown were deduced to obtain the further comprehension of the growth mechanism of micro-arc oxidation coating. It was found that the breakdown is a key factor for coating formation and development. During coating formation process the breakdown dielectric in electric field is composed with the oxide film and oxygen generated on substrate surface. When the breakdown occurs, the oxygen is punctured prior to the film, which leads to the breakdown of the film. As a result, whether the breakdown occurs or not mainly depends on the ease of oxygen breakdown. Because the field intensity of oxygen and is proportional to the working voltage and decreases with the increase of coating thickness, the working voltage must be constantly improved in the development stage of the coating to ensure that the breakdown is always able to happen. During the development process of the coating the field intensity of oxygen decreased due to increase of coating thickness, which resulting in a drop in the amount of the breakdown occurring in micro area of specimen surface. As a result, the amount of the migration channel of the substances participating in coating formation reaction is reduced and so the growth rate of coating decreases. Meanwhile, the generated energy of the micro breakdown increased constantly. It makes more coating substance around the breakdown channel is molten and more oxygen is generated. So the micro-pores on coating surface grow.