Study On Micro-arc Oxidation Mechanism And Performance Of ZrO₂ Composite Coating On 6061 Aluminum Alloy

MAO coatings were fabricated on 6061 aluminum alloy in electrolyte containing 8g/L Na2SiO₃, 3g/L Na₅P₃O₁₀ and 2g/L CH3 COONa and process parameters were optimized. The sparks were viewed and recorded by using a long working distance microscope, and the dimensions and durations of sparks were measured with image and video analysis software. SEM was used to view the molten pools on the surface of MAO coatings and the dimensions of these molten pools were measured. According to the measurement results, relationships between sparks, molten pools and coatings thickness were established. The space structure of molten pools was obtained by layer analysis. To get more information of MAO coatings, SEM was used to investigate the morphologies of the surface, coating/substrate interface and cross section, and XRD was used to get the phase composition of MAO coatings. In order to improve friction and wear properties of MAO coatings, m-ZrO₂ particles were added into electrolyte, thus a composite coating was fabricated on 6061 aluminum. The effects of ZrO₂ content on coating thickness, surface roughness and morphologies were researched. The content of ZrO₂ in surface and interface of the composite coatings were calculated based on EDS analysis and phase composition was obtained by XRD. Friction and wear tests were conducted in ball on disk method, and the mechanism of improvement of friction and wear properties were discussed. The results obtained from the tests are shown as follows:Coating thickness and surface roughness increase with the improvement of positive current density and positive pulse duty ratio. As pulse frequency increases, thickness and roughness decrease at fist and then rise up at 300 Hz and 400 Hz respectively. The thickness increases as negative current density and duty ratio goes up. The surface roughness decreases at first as the negative current density increases and rises up at 15A/dm². It keeps decreasing with the improvement of negative duty ratio. After comprehensive analysis, a set of processing parameters is obtained and positive/negative current density is 15A/dm², positive/negative pulse duty ratio is 50%, pulse frequency is 500 Hz. Under this condition, a MAO coating with thickness of 54.3?m and surface roughness of 1.927?m is fabricated in 60 min.The dielectric breakdown voltage of the MAO coating is about 400 V, after the broken-down of the coatings, there are many sparks distributed on the surface of the coating. By measuring the dimensions of micro-discharges and molten pools, it is found that they grow larger with increase of oxidation time, and the dimension of micro-discharges keeps equivalent with coating thickness to a certain degree. After oxidized for 90 min, the dimensions of micro-discharges and molten pools reach to 420?m and 69?m respectively. The molten pool looks like a trumpet in cross section view and it gets bigger from surface to the inside of the coating. There are many small pores found in the substrate/coating interface, which are considered to be trails of dielectric breakdown. The proportion of ?-Al₂O₃ phase increases from surface to interface of MAO coating and it is considered to be related to the cooling rate of molten metal and oxides. Based on the outcomes mentioned above, a model for dielectric breakdown of MAO process was proposed.With addition of m-ZrO₂ particles, the breakdown voltage and final voltage get higher and the occurrence of breakdown is delayed. ZrO₂ were found both in surface and interface. Some m-ZrO₂ transform into t-ZrO₂ because of the high temperature produced in MAO process. The ZrO₂ content in surface and interface of composite coating reaches to 26.75% and 16.02% respectively after oxidized for 60 min in electrolyte containing 3g/L ZrO₂. The friction coefficient and wear rate reduces by 47% and 87% respectively compared with that of 6061 aluminum alloy substrate.