Molecular Characterization of Bonding Interactions at the Buried Steel Oxide–Aminopropyl Triethoxysilane Interface Accessed by Ar Cluster Sputtering

The overall performance of any organic coating system on a metal is largely dependent on the primer coating which has the crucial function to ensure adhesion to the metal substrate. Although performance in terms of adhesion can be empirically measured, the underlying chemical adhesion mechanism is difficult to unravel. A detailed molecular characterization of interfacial chemistry is required for this purpose, but brings up the challenge to reach the buried interface without inducing excessive damage to its molecular structure. In this work, argon gas cluster ions are being applied to sputter through an aminosilane coating on steel, in order to access the steel oxide–silane interface with time-of-flight secondary ion mass spectrometry (ToF-SIMS). In situ atomic force microscopy measurements during the sputter process demonstrate the importance of optimizing the Ar gas cluster ion beam in order to minimize sputter-induced roughness and molecular damage. ToF-SIMS spectra obtained at the buried steel oxide–aminosilane interface accessed by sputtering were compared to spectra from a steel oxide–aminosilane interface that was directly accessible without the need for sputtering. This comparison allowed us to identify contributions from sputter-induced damage in the buried interface spectra. Fragments characteristic for interfacial bonding interactions could be extracted, although there is a significant loss of molecular information because of sputtering. Nevertheless, insights into the role of steel surface hydroxyl groups in the adsorption mechanism of aminosilanes could be obtained through deuteration of the steel substrate.