Formation Mechanism And Tribological Properties Of Micro-arc Oxidated Coating On Aluminium

To improve the antiwear behavior of cast aluminium alloy and prolong the life of aluminum alloy part, ceramic coatings were deposited on the surface of aluminium alloy by micro-arc oxidation in aqueous alkaline electrolytes. The microstructure, phase composition and elements were analysised using SEM, XRD and EDS techniques. The antiwear behaviors of ceramic coatings were tested in the way of sliding friction on abrasive paper. Main research contents, results and conclusions are shown as follows:1. Al₂O₃ ceramic coating(1) Variation trend of discharge energy is decided by instantaneous electrical parameters. And then, the growth rate of ceramic coating is affected by discharge energy relatively.(2) When the thickness of ceramic coating is above 30 m, compact layer appears at bottom and loose layer appears on top. The ratio of compact layer is range from 35% to 86%. Although the thicknesses of compact and loose layer increase with total thickness, ratio of them decreases.(3) Ceramic coating has a porous structure with massive discharge microspore (hundreds nm^ten m). The discharge microspore is surrounded by condensational oxide as a circle. When discharge is intense, oxide is spurted to surface and turned into granule irregularly.(4) Al₂O₃ ceramic coating mainly containsα- Al₂O₃,γ- Al₂O₃ and Mullite. Mullite is a product of sintering happened to electrolyte and alumina. Some Al₂O₃ makes a phase variation fromγphase toαphase. From outer to inner layer,γphase reduces gradually andαphase increases.(5) It is propitious to remove silicic acid colloid by the way of electrophoresis at higher negative voltage that can make coating surface compact and smooth.(6) The roughness concentration (Ra) of ceramic coating is less than 6.8 m after constant current oxidation about 60 min.(7) The hardness of ceramic coating is at the range of 800 HV to 1600 HV. Thiner and thicker coatings have not higher hardness.(8) In constant current oxidation, optimal electrical parameters are as follows: copper current density (12 A/dm²¹5 A/dm²), zinc current density (4 A/dm²⁵ A/dm²), pulse frequency (500 Hz⁶00 Hz), pulse duty factor (0.15⁰.2), time (45 min⁷5 min). In constant voltage oxidation, optimal electrical parameters are as follows: positive voltage (460 V⁵20 V), negative voltage (60 V⁸0 V), pulse frequency (500 Hz 600 Hz), pulse duty factor (0.15⁰.2), time (45 min⁷5 min).(9) A two-stage process—constant current oxidation(16A/dm²)+constant voltage oxidation(510V) is used for the first time. Ceramic coating has an upper growth rate and a lower roughness after treated by this process.2. Al₂O₃/Ti_O2 composite ceramic coatingUsing additive of Ti_O2 nanoparticles, Al₂O₃/Ti_O2 composite ceramic coating can be deposited on the surface of aluminium alloy at the same technics status for the first time. Compare to Al₂O₃ ceramic coating:(1) Al₂O₃/Ti_O2 composite ceramic coating has a growth rate of 1.17 times faster than Al₂O₃ ceramic coating's.(2) It is benefit for surface and section of coating to become compact and smooth.(3) Al₂O₃/Ti_O2 composite ceramic coating not only have the phases ofα-Al₂O₃,γ-Al₂O₃ and Mullite, but also have two new phases of Al2TiO5 and Anatase.(4) The hardness of Al₂O₃/Ti_O2 composite ceramic coating is higher than Al₂O₃ ceramic coating's.3. Antiwear behaviorsTribological properties of ceramic coatings were tested with 1000# waterproof sand paper friction pair for the first time. And results show:(1) The antiwear behavior of ceramic coating i about 5.3 times compared to aluminium alloy at dry friction. While under water-lubricated condition, the antiwear behavior of ceramic coating improves about 94 times compared to aluminium alloy. All the experiments show that ceramic coating has a low wear rate.(2) The proportion relation of wear rate among loose layer, compact layer and composite coating is about 4: 1: 1.86.(3) The wear loss per meter of ceramic coating has little change in different sliding speeds and loads.(4) Ceramic coatings in various roughnesses have different former wear rates and close wear rates at stable stage.(5) Friction coefficient of Al₂O₃/Ti_O2 composite ceramic coating tends to stabilize at a level of 0.21⁰.22. While the Al₂O₃ ceramic coating's is about 0.40⁰.45.