UBC Theses and Dissertations
Organosilane adsorption on light metal alloys Kim, Tae Kyong John
This thesis discusses research on the adhesive bonding of organosilanes to alloys of Al and Mg using surface analysis techniques (especially secondary-ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM)). One project involved comparing the adsorption of bis-1,2-(triethoxysilyl)ethane (BTSE) on high-purity Al, and on anodized 7075-Al alloy, for different pre-treatments involving chemical and heating procedures, and exposure to a H₂ plasma. The objectives were to assess the effects of these pre-treatments on the direct Al-O-Si covalent bonding and the strength of the adhesive coating. The separate adsorption of BTSE and γ-glycidoxypropyltrimethoxysilane (γ-GPS) on an air-oxidized mirror-polished 7075-Al surface was studied with an emphasis on the different microstructural regions of the alloy surface with scanning Auger microscopy (SAM). BTSE at its natural pH (4.3) adsorbed on the Al-Cu-Mg and Al-Fe-Zn types of second-phase particles, and on the alloy matrix, while γ-GPS adsorbed on all these regions only after the starting pH was reduced from its natural value (5.7) to 3.2. However, with prior heating of the sample (200°C, 15 min), γ-GPS adsorption occurred on all regions even at pH 5.7. Time-of-flight (TOF)-SIMS was used for the first time to indicate the nature of the metal-O-Si bonding at the different micro-regions. The Mg-based alloy AZ91 was characterized with regard to its different surface micro-regions both from mirror-polished and fractured surfaces, as well as oxide growth and oxide stability on heating. There is a clear tendency for Mg oxidation, which covers the Al component, while other elements (Zn and Mn) are detected only with bulk characterization methods, although localized Mn-rich particles (dimensions ~3 μm) are identified by SEM and energy-dispersive X-ray (EDX) spectroscopy. In a coating environment (by 1% solutions of BTSE, γ-GPS, γ-APS at their natural pH values) the Mg oxide is etched, and adsorption occurs on the different regions of the alloy, but the attachment is weak, especially because of the fragile nature of the underlying substrate. However 4% BTSE (at pH 3.9) formed a thicker and denser coating with better prospects for substrate protection.
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