(Invited) Electroplating of Bright Aluminum on the ABS Resin Substrate Covered with Cu Film in EmImCl-AlCl3 Ionic Liquid
Authors: Ueda Mikito 
Published: December 2020

Al is widely used in daily necessities, automobiles, building materials and plating materials. In addition, since Al formed into a mirror surface has a reflectance of more than 90% in the visible light region, it has been used as a reflection plate of an optical device such as a microscope and an LED light in the optical field. Our group aims to deposit bright aluminum electroplating on a resin that is even lighter than aluminum. This technology can contribute to further weight reduction of mobile devices. However, there is a few reports of formation of bright aluminum electroplating film with a thickness of several 10 μm. Therefore, research was conducted focusing on the bright condition of aluminum electroplating and the uniformity of the thicker plating film. In this study, 1-ethyl-3-methylimidazolium chloride (EmImCl) and AlCl3 a mixture ionic liquid and 1,10-Phenanthroline were selected as the electrolyte for Al plating and additive, respectively.

EmImCl-AlCl3 ionic liquid (molar ratio of 1 : 2) with 20 mM 1,10-Phenanthroline was prepared in the glove box with Ar atmosphere. A Cu plate or a Cu-plated ABS resin was used for the working electrode and an Al plate or Al mesh was used for the counter electrode. Electrodeposition experiments were carried out by current pulse electrolysis with 20 mA cm-2 and duty ratio of 0.83 (ton = 1.0 s, toff = 0.2 s). Charge density was 59.7 Ccm-2 (Theoretical film thickness of 20μm). For the electroplated surface, surface observation and cross-sectional observation were carried out by SEM. The surface roughness and reflectance were measured by AFM and optical reflection probe(reference: Al mirror of 90% reflectance)

In the experiment using Cu plate working and Al plate counter, bright Al formed at only edge of the Cu plate with non-stirring condition of electrolyte. When the stirring rate of the electrolyte was increased, the area of the bright part increased, and the stirring rate of 400 rpm was appropriate rate for formation of bright surface. This may because the additive reaches the whole surface of the Cu surface by stirring. The roughness and the reflectance of the Al electroplated surface formed under the condition of the 400 rpm was 2-3 nm from the AFM measurement and 75%(for the reflectance of the Al mirror) from the optical reflection measurement. However, since the flow of the electrolyte for the Cu substrate was not uniform, bright part was not also uniform, then electroplating in a non-stirring condition was investigated. As a result, it was found that the same effect as stirring of the electrolyte was obtained by using an Al mesh as the counter electrode.

When the cross section of the electroplated specimen was observed by SEM, thickness was thick at the edges and thin at the center of the specimen. Next, in order to reduce a current concentration for the edge part of Cu substrate, support electrodes were placed around the Cu substrate. It was found that the thickness of Al electroplating was almost uniform for the substrate by the supporting electrode. Even if the underlayer was replaced with a Cu-plated ABS resin from the Cu plate, a bright Al-plated film was formed with a uniform thickness. Since the heat resistant temperature of ABS resin is about 100 oC, Al electroplating on ABS resin cannot be achieved in molten salt electrolysis (150 oC). These results were achieved by using an EmImCl-AlCl3 ionic liquid.