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Sheng Lu,Weiwei Chen ， Jing Chen,Zexin Wang Jiangsu University of Science and Technology , China

2 nd International Conference and Expo on Ceramics and Composite Materials. Ceramic coatings fabricated by micro-arc oxidation on ZK60 magnesium alloy under step current mode. Sheng Lu,Weiwei Chen ， Jing Chen,Zexin Wang Jiangsu University of Science and Technology , China. Contents. 1.

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Sheng Lu,Weiwei Chen ， Jing Chen,Zexin Wang Jiangsu University of Science and Technology , China

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  1. 2nd International Conference and Expo on Ceramics and Composite Materials Ceramic coatings fabricated by micro-arc oxidation on ZK60 magnesium alloy under step current mode Sheng Lu,Weiwei Chen，Jing Chen,ZexinWang Jiangsu University of Science and Technology, China

  2. Contents 1 Background 2 Experiment 3 Results & Discussion 4 Conclusion

  3. 1.Background • Magnesium alloys • high strength-to-weight ratio, excellent dimensional stability • good damping capacity and electromagnetic shielding property • applications for automobile, cars , aircraft,3C components and so on.

  4. 1.Background The standard electrode potential of magnesium is very low(-2.37V). +12 2 8 2 Mg – 2e- = Mg2+ Difficulties for magnesium alloys to solve corrosion problem • Pretreatment for complex shape workpieces • Treatment for inwall and hole shape workpieces • Environmentally and friendly coating • Low cost in labor-intensive industries A solution for corrosion: Micro-arc oxidation

  5. 2.Experimental methods-step current mode Compositions of ZK60 rolling magnesium alloy (wt%) Size：20mm×20mm×5mm no pollution

  6. 3.Results & Discussion 1 Microstructure and growth characteristics of MAO coatings 2 Phases and elements contribution of MAO coatings 3 Roughness, corrosion and wear resistance of MAO coatings 4 Influence of electrolytes on the growth and quality of MAO coating

  7. 3.1 Microstructure and growth  characteristics of MAO coatings Ⅰ:0-40sAnodic oxidation stage. Ⅱ:41-120sPrimary growth stage of micro-arc oxidation. Ⅲ:121-300sRapid growth stage of micro-arc oxidation. Ⅳ:301-900sDynamic equilibrium stage (melting and solidification) of micro-arc oxidation. Voltage-time curve and thickness under two-step decreasing current mode

  8. 3.1Microstructure and growth  characteristics of MAO coatings b a Scattered micropores Compact and small micropores /sparks d c f h g e Cracks Big dispersive pores The SEM surfaces morphology of coatings at different time ( a) 45s ; ( b) 90s; (c) 180s; (d ) 300s; (e ) 420s; (f) 600s; (g )720s; (h) 900s

  9. 3.1 Microstructure and growth  characteristics of MAO coatings (b) (a) Different micropores on the surface of coating (a)In situ breakdown like “pores formed in pores, stacked layer by layer ”. (b) Self-sealingand repairing.

  10. A model supposed for pores growth substrate (3)Self-sealing and repairing SiO32- PO43- substrate substrate Passive film (1)Anode oxidation (4)In situ breakdown with “pores formed in pores” substrate (2) Passive film breakdown holes or other defects Zn Mg O MgO Silicon compounds

  11. 3.1 Microstructure and growth  characteristics of MAO coatings (b) (a) (c) (a)Continuous and superficial cracks formed by Self-sealing and repairing micropores. (b)Discontinuous and deep cracks formed by in situ breakdown micropores. (c)Discontinuous and superficial cracks formed independently. Differentcracks on the surface of coating

  12. 3.1 Microstructure and growth  characteristics of MAO coatings Nonuniform with max thickness difference of 25μm Uniform and compact (b) (c) (a) resin coating substrate Different pores and cracks with the same characristics-- disconnectd and nonpenerative (d) (e) (f) Cross-section morphology of catings formed at different time (a)90s; (b)180s; (c)300s ; (d)420s ; (e)600s; (f)900s

  13. 3.1Microstructure and growth  characteristics of MAO coatings An experiment designed for the observation of inward and outward coating growth. resin coating Schematic diagram: Outward growth ZK60substrate Inward growth

  14. 3.1Microstructure and growth  characteristics of MAO coatings (a) (b) (c) (d) (e) (f) The cross-section morphology of inward and outward coating (a)50s ;(b)90s;(c)150s;(d)180s ;(e)300s; (f)600s

  15. a=(H1-H0)/2 b=h-a H0-the original thickness before test H1-the final thickness after test h-total thickness a-outward part thickness of the coating b-inward part thickness of the coating 3.1Microstructure and growth  characteristics of MAO coatings Inward and outward growth thickness curve • The inward coating growth dominates at the initial stage. • The part of inward coating is thinker than that of outward.

  16. A model supposedfor inward and outward coating growth Passive film outward growth substrate Spark puncture the weak coating substrate substrate substrate substrate substrate inward growth substrate substrate

  17. 3.2 Phases and elements distribution of MAO coatings More:forsterite,Mg silicate Less :Mg,MgO,ZnO,MgZn2 Time Main phases of coating 90s MgMgZn2 MgO ------ ------ ------ 180sMgMgZn2MgO Mg2SiO4 ------ ------ 300sMg---------MgOMg2SiO4MgSiO3 ZnO 600sMg----- MgOMg2SiO4MgSiO3 ZnO 900sMg ----- MgOMg2SiO4MgSiO3ZnO XRD patterns of MAO coating formed at different time

  18. 3.2 Phases and elements distribution of MAO coatings (a) Mg Si O Zn (b) Mg Si O Zn Elements distribution of MAO coating along the cross-section (a)300s;(b)900s

  19. 3.2 Phases and elements distribution of MAO coatings Surface elements of MAO coating by EDS

  20. 3.2 Phases and elements distribution of MAO coatings P（wt.%）：1.59~4.86 300s 90s The change of elements at MAO coating surface

  21. 3.2 Phases and elements distribution of MAO coatings The amount of P in the surface and hole of coating formed at different time

  22. 3.2 Phases and elements distribution of MAO coatings XPS spectrumof MAO coating Binding energy data of the elements

  23. 3.2 Phases and elements distribution of MAO coatings Zn after fitting P after fitting Peak analysis of XPS spectrum for different elements on the MAO coating

  24. 3.3Roughness ,corrosion and wear resistance of MAO coatings (a) Ra=2.8138 c (b) Ra=2.38 d a b (d) Ra=3.477 (c) Ra=4.4097 Roughness of MAO coatings formed at different time

  25. 3.3 Roughness ,corrosion and wear resistance of MAO coatings Polarization curve of MAO coating Corrosion current and corrosion  potential of coatings formed atdifferent time

  26. 3.3 Roughness ,corrosion and wear resistance of MAO coatings Electrochemical impedance spectroscopy of MAO coatings in 3.5%NaCl solution

  27. 3.3 Roughness ,corrosion and wear resistance of MAO coatings 300s (b)Elements change curves of MAO coating formed at 300s before and after immersion test (a)Elements distribution of MAO coating formed at 300s before electrochemical test(a) and after electrochemical test(b) by EDS The amount of Oand Mg increasedwhile Si decreased.

  28. 3.3 Roughness ,corrosion and wear resistance of MAO coatings 900s (b)Elements change curves of MAO coating formed at 900s before and after immersion test (a)Elements distribution of MAO coating formed at 900s before electrochemical test(a) and after electrochemical test(b) by EDS The amount of Oand Mg increasedwhile Si decreased.

  29. (a) (b) 3.3Roughness ,corrosion and wear resistance of MAO coatings Friction coefficient of MAO coating 3D Morphology of wear scars of MAO coating

  30. 3.4 Influence of electrolytes on the growth and quality of MAO coating （b）Curves of pH vs.time and temperature vs.time for MAO coating （a）Curves of conductivity vs.time and temperature vs.time for MAO coating The conductivity of electrolyte decreased as the MAO time increased while PH without obvious change.

  31. 3.4 Influence of electrolytes on the growth and quality of MAO coating More melt and less micropores and cracks. c b a e d Less melt and more micropores and cracks Surface morphology of MAO coating formed at electrolytes with different conductivity (a)41.3ms/cm ;(b) 39.5 ms/cm;(c) 35.7 ms/cm;(d) 33.3 ms/cm ;(e) 31.2 ms/cm

  32. 3.4Influence of electrolytes on the growth and quality of MAO coating Thicker coating, but nonuniform. c b a resin coating substrate resin coating substrate resin coating substrate d e resin coating substrate Thin coating, but uniform and compact. resin coating substrate Cross-section morpholpgy of MAO coating formed at electrolytes with different conductivity (a)41.3ms/cm ;(b)39.5 ms/cm;(c)35.7 ms/cm;(d)33.3 ms/cm ;(e)31.2 ms/cm

  33. 3.4 Influence of electrolytes on the growth and quality of MAO coating Roughness of MAO coating formed at electrolytes with different conductivity

  34. 5. Conclusion 4 5 2 1 3 The growth process of MAO can be divided into four stages: (1) Anodic oxidation stage, (2) Primary growth of micro-arc oxidation stage, (3) Rapid growth of micro-arc oxidation stage, (4) Dynamic equilibrium stage (melting and solidification) of micro-arc oxidation. There exists two typical growth models for micro-pores formation, one is in situ breakdown ( micro-pores formed in pores and layer upon layer), the other is self-repairing. The inward growth of the coating is dominant at the initial stage, the inward growth coating is thick than that of outward. The inward growth coating has more O elements ( MgO) while the outward growth coating has more Si elements ( MgSiO3 and Mg2SiO4). MgO exhibits better corrosion resistance while MgSiO3 shows poor corrosion resistance. The conductivity of electrolyte declines slowly during MAO process, which has some effects on the coating’s morphology and performance.

  35. Thank you for your attention! Email:lusheng88168@qq.com Jiangsu University of Science and Technology, China

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