Microstructure, Properties And Growth Behavior Of Microarc Oxidation Coatings On Magnesium-Based Materials

Protective coatings were prepared on magnesium-based materials including AZ91 alloy, Al18B4O33w/AZ91 composite and SiCw/AZ91 composite by microarc oxidation (abbreviated as MAO) surface treatment technique in order to enhance corrosion resistance of these Mg-based materials. Microstructure of MAO coatings was observed and analyzed using SEM, XRD, XPS, etc. Corrosion resistance and corrosion behaviors of MAO coatings were evaluated by electrochemical test and salt immersion test. Elastic modulus, hardness and tribological behaviors of MAO coatings were measured and investigated, and that the effects of microarc oxidation surface treatment on mechanical properties of Mg-based materials were researched by tensile test. Growth process of MAO coating on different Mg-based materials were observed and effects of different second phases on microarc oxidation behaviors of Mg alloys and Mg matrix composites were investigated systematically.The results show that electrical parameters influence growth rate and microstructure of MAO coatings, and different Mg-based materials show different microarc oxidation behaviors and the MAO coatings formed on these substrates have different microstructure. Al18B4O33w/AZ91 composite has almost the same cell voltage evolution trend and breakdown time with AZ91 alloy, while SiCw/AZ91 composite shows a nonideal cell voltage evolution trend, a long breakdown time and rough coating when compared with AZ91 alloy and Al18B4O33w/AZ91 composite. Crystalline MgO is a main constituent phase in MAO coatings obtained on Mg-based materials, but the chemical composition of electrolyte and microstructure of substrate also influence the phase composition of MAO coatings to some extend.Microarc oxidation treatment can improve corrosion resistance of Mg-based materials. Adjustment of electrical parameters and electrolyte will change the thickness and microstructure of MAO coatings and then affect corrosion resistance of the coated Mg-based materials. The MAO coatings obtained on AZ91 alloy, Al18B4O33w/AZ91 composite and SiCw/AZ91 composite show different corrosion behaviors under electrochemical accelerated corrosion condition and immersion corrosion condition, respectively.Elastic modulus and hardness of MAO coatings are higher than those of Mg-based materials. The wear resistance of Mg-based materials is improved by microarc oxidation treatment. The MAO coatings show abrasive wear and adhesive wear when sliding against GCr15 steel counterpart under low load and low sliding speed conditions, yet the coatings formed on AZ91 alloy, Al18B4O33w/AZ91 composite and SiCw/AZ91 composite exhibit different wear mechanism since these coatings have different microstructure. When MAO technique is adopted to protect Mg-based materials against corrosion or wear attacks, appropriate electrical parameters should be selected to avoid or decrease the damage to their mechanical properties as far as possible.From microarc oxidation behaviors of Mg-based materials, it can be found that different precipitated phases or reinforced phases in substrates have different effect mechanisms on formation process of MAO coatings. Although selective sparking discharge occurs at the early stage of microarc oxidation of AZ91 alloy and Mg-Si alloy, Mg17Al12 phase in AZ91 alloy and Mg2Si phase in Mg-Si alloy don't hinder the growth of MAO coating since barrier films can form on the two precipitated phases. As an elecinsulating reinforced phase, Al18B4O33 whisker also will not hinder the growth of MAO coating on Al18B4O33w/AZ91 composite, but exists in the coating as a heterogeneous phase. SiC whisker delays the growth of MAO coating on SiCw/AZ91 composite because it destroys the integrity of barrier film. Electroconductive carbon fibre seriously hinders growth of MAO coating on Mg matrix composite.