Characteristics of nickel thin films prepared by electroless plating in foam of electrolyte
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Research Letter
Characteristics of nickel thin films prepared by electroless plating in foam of electrolyte Takahiro Furuhashi and Yoshiyasu Yamada, Yamada Co. Ltd., Ryouke, Naka-ku, Hamamatsu, Shizuoka 430-0852, Japan Masato Hayashi, Shoji Ichihara, and Hiroaki Usui, Division of Applied Chemistry, Institute of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan Address all correspondence to Hiroaki Usui at [email protected] (Received 20 June 2017; accepted 24 October 2017)
Abstract Nickel thin films were prepared by electroless plating in a foam of electrolyte that was generated by bubbling nitrogen gas into a hypophosphite-based electroless plating solution to which was added a surfactant of sulfuric acid monododecyl ester sodium salt. Although the film growth rate in the foam was considerably lower than that in the conventional liquid, film growth was enhanced by inducing a flow in the foam. Compared with films deposited in liquid, the films deposited in foam had a smaller number of pinholes, smaller crystallite size, and superior corrosion resistance. The ferroxyl indicator test showed that the area of corrosion can be reduced to less than 1/20 by depositing the film in foam instead of liquid.
Introduction Electroplating is an indispensable film formation process in a wide area of engineering. It has been utilized for enhancing the performance, extending the lifetimes, and improving the appearance of materials. Similarly, electroless plating is useful for forming metal thin films on insulating substrates. In spite of their technical importance, both the electro- and electrolessplating methods involve fundamental problems. The inclusion of pinholes in the films is one of the most persistent problems. A pinhole is formed when a hydrogen bubble, which is generated by a side reaction of reducing metal ions, stays on the substrate surface and blocks the film growth in that spot.[1,2] A conventional method for preventing the formation of pinholes is to agitate the plating solution, thereby removing the hydrogen bubbles from the surface. However, small bubbles tend to adhere strongly to the surface owing to surface tension and are not detached easily by buoyancy or by the drag force of the plating liquid.[3,4] Pinholes cause critical damage to the corrosion resistance of coatings. For example, nickel deposits are known to have a barrier function that protects the substrate without a sacrificial effect. However, a film thickness of at least 10 µm, and preferably 30 µm, is required to avoid the effects of pinholes and achieve practical corrosion resistance.[5] The ability to reduce the minimum thickness while still attaining corrosion resistance would be a significant achievement. Meanwhile, recent studies have revealed the possibility of resolving the pinhole issues by using unconventional plating media. Yoshida et al. proposed a new electroplating technique that performs film deposition in an emulsion consisting of supercritical carbon dioxide and a plating solution.[6,7]
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