Fabrication Of The Self-lubricating Complex Coatings On Magnesium Alloy And Their Tribological Property

Because of the poor tribological properties of magnesium alloy, this paper proposes the complex technology of microarc oxidation(MAO) and electrophoresis, The wearable ceramic coating is in situ formed on the surface of AZ91D Mg alloy by MAO and the electrophoretic coating contained PTFE lubricated material is formed on the surface of ceramic layer by electrophoresis. Self-lubricating composite coatings on Magnesium alloy with excellent tribological properties are prepared out.The new electrolyte system is optimized by orthogonal experiment, and the process parameters of MAO and electrophoresis are systematically optimized. Organizational structure and morphology of the film is analyzed by SEM, EDS, XRD and by other means; Film thickness and adhesion is detected by the thickness gauge and paint film scriber; Tribological properties and wear mechanism of the film is analyzed by friction and friction-abrasion testing machine, microscope, electronic analytical balance and SEM.Influence of electrolyte on MAO ceramic coating properties is studied and silicate electrolyte system with good performance is optimized out:Na2SiO3,15g/L; KOH,6g/L;KF, 15g/L; glycerol,15ml/L. The effect of current density, frequency, duty ratio and oxidation time on the MAO coating formed on the Mg alloy substrate is studied. Under optimized silicate electrolyte system, the MAO coating has optimum anti-wear performance and excellent overall performance when the coating is prepared under the current density of 7～9 A/dm2, frequency of 600～800 Hz, duty ratio of 5%～10%, and reaction time of 10～15 min. The friction coefficient is about 0.6, the thickness is about 20 μm.On the basis of the MAO ceramic coating, during electrophoresis process, the influence of electrophoresis voltage, time, temperature, and PTFE content on electrophoretic coating morphology, thickness and tribological properties is systematically analyzed. The results show that the coating friction and adhesion, and overall performance are excellent when electrophoresis voltage is between 60～80V, electrophoresis time is between 90～120s, electrophoresis temperature is 30℃, concentration PTFE is 10%～25%. The friction coefficient of the coating is about 0.1 and the adhesion could reach grade one.The morphology and crystalline phase of MAO ceramic coating is analyzed; the results show that the connection between the MAO ceramic layer and the magnesium alloy matrix is dense and contain no obvious boundaries. Elements from the electrolyte and the metal elements from the magnesium alloy matrix involved in the reaction in the film, Ceramic coating is mainly composed of compounds consisting of Mg, O,Si, F. Sectional morphology and elemental distribution of electrophoretic coating is studied; the results show that the combination between the coating and the ceramic coating is tight, uniform and smooth, and at the same time PTFE lubrication material is evenly distributed in the coating.Wear mechanism of MAO ceramic coating and MAO-electrophoresis composite film is analyzed; The results show that the main wear mechanisms of ceramic coating are adhesive wear, abrasive wear and de lamination wear; wear mechanisms of the composite film are mainly adhesive wear, abrasive wear and fatigue; The Influence of tribological properties of the composite film by different load, speed and wear time is systematically analyzed. The results show that the friction coefficient of coating decreases with the increase of the load, and increases first and then decreases with the increase of the sliding velocity, and decreases first and then increases with the increase of the wear time. The wear weight increases with the increase of the load, sliding velocity and wear time.