Improve the Insulation Performance of Anodic Coating by Plasma Discharge Treatment

Since the plasma discharge treatment has been applied for the insulation enhancement of anodic coatings, we explored how the current density impacted the coating insulation performance. SEM, XRD and electrochemical workstation were utilized for identifying the coating crystallinity, compactness and impedance. As demonstrated by results, the anodic coating is transformable into compact crystalline alumina at a 30-mA/cm2 current density, whose impedance is twice that of original anodic coating.


Introduction
With advantages like high specific strength, preferable decoratability and outstanding heat conductivity, aluminium alloy has been widely applied in electrical industry [1].To ensure normal operation of electronics, their surfaces need to be insulation treated [2].Anodizing is a common treatment to prepare hundreds microns alumina on the aluminium alloy surface [3].Nonetheless, since such coating consists of amorphous alumina [4], and is covered with through micropores [5], the insulation performance of the coating is restricted [6].Liquid phase room temperature plasma discharge can produce in-situ high temperature (10 3 -10 4 K) [7].Upon treatment of pre-anodic aluminium alloy with plasma discharge, part of amorphous alumina is transformable into crystalline structure [8], and part of amorphous alumina forms aluminum hydroxide.As the crystalline alumina has greater dielectric constant compared to amorphous alumina [9], and the formation of aluminum hydroxide can seal through micropores in anodic coating, the coating insulation enhancement is possible [8].In the present study, the plasma discharge was initiated by pulse supply, and the current density effects the plasma discharge intensity significantly.To improve the insulation performance of aluminium alloy, anodizing technique was integrated with plasma discharge for fabricating a compact crystalline alumina coating.Scanning electron microscope (SEM) and X-ray diffractometer (XRD) were utilized to identify the coating microstructure and crystalline composition, while electrochemical workstation was adopted to examine the coating impedance.Results demonstrated that the insulation performance enhancement is possible for the coating.

Materials and methods
Kemiou Chemical Reagent (Tianjin) was the supplier of the entire experimental reagents used herein.Anodizing treatment was undertaken for 80 min at 4 °C with the utilization of commercially-available AA 6061 (20×20×5 mm 3 ) as specimens by setting the current density and stirring rate separately at 50 mA/cm 2 and 60 r/min.Subsequent step was preparation of anodic coatings within the electrolytes comprising 28.5 g/L phosphoric acid and 55 g/L sulphur.When the coating depth was up to 100 µm, every specimen was immersed inside the stirred electrolyte encompassing Na2SiO3 (15 g/L), KOH (5 g/L) and (NaPO3)6 (5 g/L), followed by rinsing with deionized water.Finally, plasma discharge treatment was undertaken for 15 min at current density of 10, 30 and 50 mA/cm 2 , where the pulse frequency and duty cycle were set separately at 20 KHz and 10 %.

Characteristics
Phase compositions of various coatings were assessed via a D/max-rB XRD system (RICOH, Japan) with CuKa source, where the current applied was 30 mA and the accelerating voltage was 40 kV.The coating surface microstructures and cross-sections were examined via a S-4700 SEM system (Hitachi, Japan).A PARSTAT 4000 electrochemical workstation (Princeton Applied Research, USA) was utilized to determine and assess the coating electrochemical impedance.A tri-electrode system was adopted, with micro arc oxidation film serving as the working electrode, and platinum sheet and saturated calomel serving separately as the counter and reference electrodes.The solution used was sodium chloride (3.5 wt %), and the sample-solution contact area was 1 cm 2 .The starting frequency is 100000 Hz and the ending frequency is 0.01 Hz.Following the test, relevant coating details were derived through the zsimpwin fitting.

Compactness
Figure 1 illustrated the cross-sections and micro-surfaces of anodic coating prior to and following the plasma discharge.As is clear from figures 1(a, c, e), microcracks and micropores increased in size and quantity with 10-to-50 mA/cm 2 rise in the current density.The photograph of cross section (figures 1 (b, d, f) confirmed the conclusion.Besides, coating cavitation was carried out progressively from the external surface to the substrate.

Crystalline composition
As is clear from figure 2, the MAO coating consisted of ɑ-, θ-and ḳ-alumina, whereas the anodic coating comprised amorphous alumina.Upon application of plasma discharge technology to the anodic coating, transformation of amorphous alumina into crystalline structure took place.It is also clear that following the plasma discharge, the coating exhibited consistent crystalline composition with the MAO coating.Nonetheless, the peak intensity acquired by plasma discharge was weaker compared to that of MAO, proving transformation of part of amorphous alumina into the crystalline one.Besides, the peak intensity increased with the increasing current density, proving enhancement of anodic coating crystallinity with the intensification of plasma discharge.As is clear, the anodic coating underwent an increase in impedance after treatment by plasma discharge.Besides, the treated coating had its maximum impedance at a 30-mA/cm 2 current density, which was twice that of original one.The main reason is that the coating crystallinity was insufficient at a 10-mA/cm 2 current density, whereas in the 50-mA/cm 2 density setting, there were serious cavitation in the coating [11].

Conclusion
Through integration of anodizing technology with plasma discharge, a compact crystalline alumina coating with thickness reaching hundreds of microns was fabricated.The influence of current density on its impedance was investigated.Results demonstrated that the application of plasma discharge procedure enabled insulation enhancement for the coating.Under a current density setting of 30 mA/cm 2 , the impedance of treated coating was twice of anodic coating.

Figure 1 .
Figure 1.Micro surface (a, c, e) and cross section ( b, d, f) of anodic coating after treated by plasma discharge (a, b) 5 min, (c, d) 15 min and (e, f) 30 min.

Figure 2 .
Figure 2. Crystalline compositions for MAO and anodic coatings prior to and following plasma discharge

Figure 3 .
Figure 3. Nyquist diagram of anodic coatings before and after plasma discharge