Speaker
Dr
Anton Kokalj
(Jožef Stefan Institute)
Description
Silane based sol-gel coatings are a promising alternative for the replacement of toxic chromate conversion coatings used to protect aluminum and it’s alloys against corrosion. It is well known that sol-gel synthesis occurs via a hydrolysis/condensation mechanism and a similar mechanism is proposed for the reaction of silanols with the surface. In order to test this hypothesis a model silanol molecule, CH3Si(OH)3, was chosen and the bonding of it and its oligomers (up to the trimer) with the aluminum surface was investigated by means of density-functional-theory (DFT) calculations. According to our thermodynamic calculations, oxidized Al surfaces are fully hydroxylated, even in the presence of traces of water vapor and correspondingly a model of the fully hydroxylated surface was used. We find that the formation of a monodentate bonding mode with the hydroxylated surface via the condensation mechanism is exothermic by ≥ 0.5 eV in all considered cases. In contrast, the formation of a bidentate bonding mode is exothermic only for the trimer. These results reveal that after the formation of a single molecule-surface bond for the monomer and dimer, additional molecule-surface bonds do not form via condensation mechanism most likely due to the strained configurations the adsorbates have to adopt. This indicates that the commonly used scheme of silanol-surface bonding where each monomeric silanol subunit in a polymer binds to a surface metal atom may not be generally applicable; according to our calculations there has to be at least one subunit acting as a “molecular spacer”, connecting two monomeric subunits bonded to the surface.
Primary authors
Dr
Anton Kokalj
(Jožef Stefan Institute)
Mr
Matic Poberznik
(Jozef Stefan Institute)
