Studies On The Process Of Forming Micro-arc Oxidation Ceramic Coatings On Magnesium Alloy And Ceramic Coatings’ Friction And Wear Properties

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 optimal process of coatings formation was confirmed by improving the chemical reagents and electrical parameter. The phase compositions of ceramic coatings were tested by XRD. The interior and exterior morphology of ceramic coatings were observed by SEM. The element distribution of interior and exterior ceramic coatings was detected by EDS. The element transfer process was researched by comparing the element distribution of different samples. The ceramic coatings’ friction and wear properties at different surface porosity 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 Na2SiO3·9H2O 9g/L, Na3PO4·12H2O 5g/L, KOH 6g/L, NaF 2g/L, C10Hi4N2Na2O8·2H20 lg/L by orthogonal experiment with the standard of thickness and adhesion stress. The best electrical parameters were processing time 40min, current density 5A/dm2, frequency 300Hz, duty ratio 20% by single factor experiment with the same standard.The main phases of ceramic coatings were MgO, Mg2SiO4, MgAl2O4 and Mg3(PO4)2, and the fastest-increasing phase was MgO.At the initial stage of MAO, a compact oxidation film was generated at the surface of the sample. As the electric charge gathered at both sides of the film, and the film was discharged to breakdown gradually, the growth mechanism turned into exterior growth at this stage. The SiO32-,PO43-,O2- from the electrilyte and the AlO42- from the matrix would react to form a series of ceramic phases. But as the oxidation continued, the number of pores decreased, making the acid radical ion difficult to move into the coatings, only the oxygen ion with small diameter could enter it. So the main ceramic phase formed at later stage was MgO. The surface of the coatings was composed of many kinds of ceramic phases, and the inside of the coatings was mainly MgO, this may dued to the temperature of internal discharge-breakdown area was extremely high, as the oxidation went on, the ceramic phases with high oxygen atom and low melting point woud get remelted and broke up into plasma under high voltage, the oxygen plasma would react with Mg from the matrix to form MgO phase for their low combination force. The eruption passage was also a cooling passage of the melt, new ceramic phase would be formed at the different point of the passage, making the a series of regular element distribution.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 decreased 9/10 compared to dry friction. The stress relationship and oil thickness will be speculated by analysing Reynolds partial differential equation of emplastic material. The wear mechanism of ceramic coatings was grain-abrasion and fatigue abrasion combining ceramic coatings’wear morphology and structure features.