Comparison between wet deposition and plasma deposition of silane coatings on aluminium

Silane coatings are applied to metal surfaces for various purposes, e.g., to form a protective layer against corrosion or to act as a primer for subsequent coating. In this work bis-1,2-(triethoxysilyl) ethane (BTSE) was used as a precursor to deposit coatings on Al 99.99% substrates with three different techniques: dipcoating (water based solution), vacuum plasma and atmospheric plasma. Infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and field emission gun-scanning electron microscope (FE-SEM) were used to characterise the structure, composition and surface morphology of the silane coatings. The aim of this investigation is to compare the surface and bulk characteristics of the films prepared with the three different methods, in order to get information on how the BTSE molecule is modified by the deposition technique. The results show that films could be deposited by both vacuum and atmospheric plasma, besides the more traditional wet dipcoating. The layers deposited by vacuum plasma can be considered as hybrid organic-inorganic, comparable to the silane layers obtained by dipcoating. Atmospheric plasma treatment, however, leads to the formation of more inorganic films richer in Si-O bonds. XPS and FTIR measurements show the presence of Si-O-Si bonds, while Si-O-Si, Si-O-C, Si-O and Si-CH₃ absorption bands are revealed by IRRAS measurements.