Effect of Voltage on Fretting Corrosion Behavior of Micro-Arc Oxidation Coating on N36 Zirconium Alloy

The oxide ceramic coating was prepared on zirconium (Zr) alloy by micro-arc oxidation (MAO) equipment. The electrolyte was a silicate system, which was composed of 15 g/L KOH, 15 g/L Na₂SiO₃, and 3 g/L NaF. The oxidation time and frequency were 15 min and 300 Hz, and the duty ratio was adjusted to 30%. Effects of voltage (220 V, 260 V, 300 V, and 340 V) on the morphology, hardness, roughness, element distribution, and phase structure of MAO coating were studied by scanning electron microscopy combined with energy dispersive spectroscopy, white light interferometer instrument, and X-ray diffraction (XRD). Effect of voltage on the corrosion and fretting corrosion behavior of MAO coating on Zr alloy was analyzed by fretting wear test rig combined with an electrochemical workstation. The test solution was 1 200 mg/L H₃BO₃ and 2.2 mg/L LiOH. The fretting parameters were selected as the displacement amplitude of 100 μm, the load of 20 N, and the frequency of 5 Hz. The test time was 2 000 s for 10000 cycles. Before the fretting corrosion test, the sample was immersed for 500 s to obtain a stable electrochemical state. The results showed that the surface morphology of MAO coating presented typical porous and volcanic melting characteristics. With the increase of voltage, the volcanic melting gradually extruded and the size of pores in coating surface increased. The MAO coating was mainly composed of Zr, m-ZrO₂, and t-ZrO₂, and the high-temperature phase of t-ZrO₂ existed in coating, indicating that high stress in coating stabilized the phase of t-ZrO₂. The MAO coating presented a higher roughness and hardness than substrate, and MAO coating with 260 V had the highest roughness value of 1.36 μm. The increase of hardness in coating was caused by the high hardness of ZrO₂ ceramics. MAO coating can be divided into the inner dense layer and outer porous layer according to the cross-section morphology and EDS line-scan results. The oxide layer had a very obvious distinction from substrate, and no cracks and defects at the interface between coating and substrate were observed. The corrosion resistance and fretting wear resistance of MAO coating were determined by the density of inner layer and the bonding strength with substrate. The thickness of MAO coating increased with the increase of working voltage, and the thickness was about 5~9 μm. The MAO coating with 260 V had the highest bonding strength of 44.3 N. Because MAO coating generated more cracks and defects under high working voltage, resulting in the decrease of bonding strength. Compared with the substrate, MAO coating had better corrosion resistance, and the MAO coating with 260 V had a highest corrosion potential (−0.205 V) and lowest corrosion current density (6.24×10⁻⁹ A/cm²). Because the MAO coating with 260 V had the densest structure, and the inner dense layer prevented the corrosion solution from entering into the surface of substrate. The electrochemical impedance spectroscopy results indicated that the arc radius of MAO coating was larger than that of Zr alloy substrate. According to the phase angle, three time constants were observed. The high frequency regions were corresponding to the properties of solution, and the medium frequency regions was belonging to the outer porous layer, while the low frequency regions represented the inner dense layer. Open circuit potential (OCP) dropped sharply to a lower value when the fretting test began. Because the mechanical wear destroyed the stable electrochemical state of coating. With the progress of fretting test, the formation rate and removal rate of passivation film reached a new dynamic balance. As a result, OCP values for all MAO coatings began to stabilize. After the initial rapid rose, the friction coefficient value quickly reached a stable state. MAO coating with 340 V had the highest friction coefficient value. All the MAO coatings showed obvious furrow traces on the wear surface, and many wear debris with different sizes were non-uniform distributed on the wear surface. The main wear mechanism of MAO coating was abrasive wear and oxidation wear. The 3D profiles of all the wear scar showed obvious cave and the wear damage of Zr alloy was the largest. The wear damage of MAO coating was relatively slight, and the wear depth was shallow, indicating that MAO coating can significantly improve the wear resistance of Zr alloy. The wear rate of Zr alloy was 2.14×10⁵ μm³/(Nm), and the wear rate of MAO coating with 260 V was only 1 / 4 of that Zr alloy substrate.