Adhesion, Thin Films, Friction

In modern corrosion protection systems, the concept of inhibitor release from capsules incorporated into metallic coatings is one important direction.

Mesoporous SiO2 is a well-suited material, however, its incorporation into metallic layers, e.g. Zn, in the electrogalvanisation process, proved to be challenging. By modification of the particles with silane thiols, the metal/oxide interface energy is modified accordingly to enable an incorporation into the growing metal [1]. Incorporation is only observed above a critical radius [2]. Besides, a thorough characterisation of genuine bifunctional silane monolayers for use as adhesion promotors on oxide-covered Si was conducted [3]. This work attracted considerable attention and was the most downloaded article in Appl. Surf. Sci. in summer 2012.

As chromating is already widely banned and now also phosphating is foreseen to be applied to much lesser extent in the near future, the interface between organic or hybrid organic-inorganic coatings and the metal surface become more and more important.  Delamination at such interfaces is one of the key expertises of the department. However, while in the past the focus was mainly on the fundamentals of delamination from plain metals such as zinc and iron, delamination from multiphase zinc alloy coatings, for instance, requires approaches with much higher resolution. For this the Scanning Kelvin Probe Force Microscope (SKP-FM) of the department was modified for dedicated in-situ experiments on delamination. First experiments were successfully performed on filiform corrosion and it could be shown that cathodic delamination at the head of the corrosion filaments can play a crucial role in coupling the anodic head with micron size active cathodic sites at the interface [4,5]. Another approach for better understanding is to simulate polymer/metal interfaces by preparing well defined terminations of the oxide layer and studying the effect on the properties of the resulting interface with polymers [6].

In this respect also molecular forces at electrochemical interfaces play a critical role in understanding and ultimately preventing adhesive bond failures in materials applications. In collaboration with the University of California at Santa Barbara (Prof. J.N. Israelachvili) we developed two newly designed experiments: the Electrochemical Surface Forces Apparatus (EC-SFA) [7] and a novel electrochemical AFM [8] setup, which uniquely provide in-situ control of surface potentials and interfacial electrochemical reactions and a simultaneous measurement of normal interaction forces, friction forces, distances and surface separations as well as contact shapes between dissimilar apposing surfaces. Using the EC-SFA setup we showed that surface morphology and in particular nanoscale roughness significantly altered the effective counterion distribution and measured force profiles at electrified interfaces. Moreover, the EC-SFA allowed for the in-situ structural identification and characterization of electrochemically growing (or modified) oxide thin films. In particular, the thickness of anodically growing oxide films was measured in situ with Å-accuracy on rough and smooth surfaces. Finally, we performed potential dependent friction force measurements [9] at electrode-ceramic contacts that revealed a dramatically increased shear viscosity at electrified interfaces, which is likely due to migration and redistribution of ions at the interface and restructuring of water into solid-like layers. The development of the EC-SFA tribocorrosion and in particular analysis of damage occurring under tribological conditions will further extend our portfolio in the next years. The EC-SFA provides the means to simultaneously correlate thin film rheological properties, interaction forces (adhesion and friction) and interfacial electrochemical reactions at electrified interfaces.

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