Forming Nanoscale Sculptured Surfaces for Advanced Electrical Bonds by Environmentally Friendly Electrochemical Etching

With the modern advances in high performance adhesives, bonded joints are more and more competitive or are even outperforming mechanical joints and welds. Bonded joints exhibit many benefits compared to mechanical joints in respect of thickness, weight, force distribution, application and especially for dissimilar material bonding. The connection of different materials with fundamentally different properties is crucible for applications such as multi-material automotive bodies and composite materials for the aerospace sector. Thereby, the wide range of different favorable materials and the resulting different surface properties create high demands on the adhesives and often require complex surface preparation involving hazardous chemicals.<br/>This work uses an electrochemical nanoscale sculpting process with a seawater-like electrolyte for an eco-friendly surface preparation and nearly perfect bonding on various metals such as aluminum, stainless steel and brass.<br/>The strong adhesion is achieved by creating sub-micron undercuts that facilitate mechanical interlocking of adhesive and material. Chemical surface-to-surface interaction becomes less relevant, resulting in bonds that are limited by intrinsic fracture strength of the used adhesive. The undercuts are produced by a highly selective pulsed electrochemical etching, resulting in three-dimensional formation of chemically stable surfaces.<br/>Additionally, brass can selectively leached by the same process. Upon leaching, the structured surfaces are copper-rich and show extraordinarily good wettability for solder. For common tin-based solders, this work shows a contact angle near zero with several times larger spreading compared to a mechanical cleaned and roughened surface.<br/>Since the chemical surface interaction is no longer decisive for the bonding strength, it also becomes possible to use completely new types of adhesives with superior bulk properties, that would otherwise lack the chemical properties for sufficient surface adhesion. This can not only be used to increase mechanical strength of a joint but is also promising for joining with electrically conductive adhesives as an alternative to soldering. Typical conductive adhesives consist of a conductive filler (, e.g., metal particles) and an adhesive as binder, resulting in a tradeoff between bonding and conductivity properties.<br/>This work will evaluate nano structured surfaces and their use for mechanically strong, conductive-bonded joints.