Suggested mechanism for the MAO ceramic coating on aluminium substrates using bipolar current mode in the alkaline silicate electrolytes

102 samples were treated by the micro arc oxidation at different bipolar pulsing periods in the alkaline silicate electrolyte. The obtained results demonstrated that the duty cycle has no correlation with the layer growth or the surface roughness. The results were analyzed by the multiple linear regression and then the proper diagrams for thickness and roughness were plotted. The growth mechanism of the ceramic coating was influenced by many complicated and interrelated factors. We suggested a new mechanism to describe the resultant coating phenomenon taking into account the different reactions during the four periods of bipolar pulsing mode. The plasma discharge generator, anodic period, affected significantly in the layer growth and the surface roughness. The cathodic period affected the growth by the etching-protection effect, and affected the surface morphology by the production of hydrogen gaseous sheath. The contribution of the anodic neutral period was caused by the internal etching. The cathodic neutral period significantly affected the surface roughness by releasing the cathodic-hydrogen gaseous sheath, and contributed in the layer growth. The phase formation was also described by the suggested mechanism.