| Ceramic coatings with metallurgical bonding were prepared on the surface of AZ91D magnesium by micro-arc oxidation. The thickness and the adhesion stress of ceramic coatings prepared in different technological parameters were measured. The phase compositions of ceramic coatings were tested by XRD. The interior and exterior morphology of ceramic coatings was observed by SEM. The element distribution of interior and exterior ceramic coatings was detected by EDS. The formation mechanism of AZ91D magnesium ceramic coatings was explored. The ceramic coatings’ friction and wear properties at different load and sliding speed under dry friction or under oil lubrication were studied by a pin-on-disk test rig. The wear mechanism of ceramic coatings was studied via observing ceramic coatings’ wear morphology.The best electrolyte preparing ceramic coatings on AZ91D magnesium was Na2SiO3·9H2O9g/L, KOH6g/L, NaF2g/L, C10H14N2Na2O8·2H2O1g/L by orthogonal experiment with the standard of thickness and adhesion stress. The best electrical parameters were processing time50min, current density7.5A/dm2, frequency300Hz, duty ratio30%by single factor experiment with the same standard. The thickness of ceramic coatings increased with the accretion of single output energy and the shortening of cooling time. The main phases of ceramic coatings were MgO, Mg2SiO4and MgAl2O4. Those phases’ content increased with the accretion of discharge-breakdown strength, and the fastest-increasing phase was MgO.The ceramic coatings’ discharge-breakdown could be divided into two stages during film-forming process on AZ91D magnesium. Firstly, the discharge-breakdown occurred in the gas membrane surrounding magnesium. Then, the position of discharge-breakdown moved to the interior ceramic coatings with the variation of arc-discharge’s colour and sound. Preliminarily, SiO32-entered the ceramic coatings more hardly than O2-due to the gas membrane. So the content of Mg2SiO4was much less than that of MgO in the gas discharge-breakdown stage. The holes in ceramic coatings were not straight holes but made up nested structures that a big hole on the surface contained many small holes. The nested structures were caused by the change of discharge-breakdown position and the mechanism of melt erupting.The growth of ceramic coatings was determined by the exterior growth and the interior growth which happened at the same time. In the process of the exterior growth, the melt which came from the interior ceramic coatings’ discharge-breakdown erupted outside. Then, the melt deposited mechanically and solidified on the surface which leaded to the loose ceramic coatings. In the process of the interior growth, ion implantation, plasma-forming, discharge-breakdown of ceramic coatings, reaction between plasma and metallic matrix, melt eruption and solidification which happened in order leaded forming the internal compact ceramic coatings. The lamellar structure in ceramic coatings was related to the ejection velocity of melt and the pressure imposed on melt during solidification by the gas and formative ceramic coatings. The special pattern on the surface of ceramic coatings maybe arose from non-equilibrium crystallization of melt during rapid cooling.The wear rate increased with the accretion of load but decreased with the accretion of sliding speed under dry friction. The wear rate decreased with the accretion of load and sliding speed under oil lubrication. The oil-storage effect of the ceramic coatings holes played a real important role in enhancing coatings’ abrasive resistance under oil lubrication. The wear rate decreased3/4compared to dry friction. The wear mechanism of ceramic coatings was grain-abrasion and fatigue abrasion combining ceramic coatings’wear morphology and structure features. |
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