Durable nanostructured superhydrophobic coatings on aluminum alloy substrates

Superhydrophobic thin films based on metal substrateshas attracted great attention due to a wide range of applications in the industry including anti-corrosion, anti-icing and self-cleaning surfaces. Recently, numerous methods have been reported to prepare superhydrophobic thin films but most of them will be destroyed or degrade quickly when applied outdoor or in harsh environments. Therefore, it is extremely important to develop durable superhydrophobic thin films, especially considering the properties of corrosion resistance, Ultra-Violet (UV) durabilityand mechanical durability. In this project, superhydrophobic cobalt stearate thin films with excellent anti-corrosion properties were successfully fabricated on aluminum substrates via a simple, electrodeposition process. The correlation between the surface morphology, composition as well as wetting properties and the molar ratio of inorganic cobalt salt/organic stearic acid in the electrolyte were studied carefully.The optimum superhydrophobic surface formed on the cathodic aluminum substrate was found to have a maximum contact angle of 161o and the largest polarization resistance of 1591 kΩ cm2, indicating an excellent barrier against chemical corrosion. Electrical equivalent circuits have been suggested to better understand the corrosion principals on the interface based on the data from the corresponding electrochemical impedance spectroscopy (EIS). In the aspect of UV durable superhydrophobic thin films, the electrodeposited superhydrophobic cobalt stearate thin films as mentioned above were found to be UV durable as the roll-off properties sustain in the period of two months’ accelerated UV degradation test. Furthermore, inspired from this finding, another novel superhydrophobic nanocomposite thin films have been successfully fabricated on aluminum substrates by embedding cobalt stearate (CoSA)-covered TiO2 nanoparticles in a hydrophobic polymethyl-hydrosiloxane (PMHS) matrix (PMHS/TiO2@CoSA) utilizing the sol-gel process. Compared to sharp decreases of water contact angle on the superhydrophobic PMHS/TiO2 thin films (without CoSA), the PMHS/TiO2@CoSA superhydrophobic thin films exhibited a nearly constant water contact angle of 160o under continuous UV irradiation for more than one month. The designed scheme of TiO2@CoSA core-shell structure not only increases the hydrophobic properties TiO2 nanoparticles surface,but also confined the photocatalytic efficiency of TiO2 nanoparticles. A plausible model has been suggested to explain the UV durable mechanism. In addition, to improve the adhesion strength of superhydrophobic nanocomposite PMHS/TiO2@CoSA thin films, the thin self-assembled monolayers (SAM) of GPTS were deposited on the surface of aluminum substrates. The presence of SAM was confirmed by ATR-FTIR, EDX as well as EIS studies. The adhesion strength was expected to be improved by the formation of chemical bondings due to the presence of SAM. But both of the results were found to be 2B between the substrates of as-received aluminum and SAM modified aluminum following the (ASTM) D 3359-02 adhesion test. However, a few improvements on adhesion strength could be detected by comparing the surfaces on as-received aluminum and SAM modified aluminum substrates after the removal of adhesive tape. It is believed that this project will contribute to a better understanding of superhydrophobic phenomena as well as realizing the application of superhydrophobic surfaces in the industrial world. Les couches minces superhydrophobes deposees sur des substrats metalliques ont suscite beaucoup d’attention vu leurs nombreuses applications industrielles comme les revetements anticorrosifs et antigivrants ainsi que les surfaces autonettoyantes. Ces derniers temps, de nombreuses methodes d’elaboration de couches minces ont ete proposees. Cependant, la plupart des couches obtenues se detruisent ou se degradent assez rapidement lorsqu’elles sont utilisees a l’exterieur ou exposees a des conditions severes. Par consequent, il est tres important de developper des couches minces superhydrophobes resistantes ayant de bonnes proprietes anticorrosion et anti ultraviolet ainsi qu’une bonne resistance mecanique. Dans ce projet, des couches minces superhydrophobes de stearate de cobalt, ayant de tres bonnes proprietes anticorrosion, ont ete elaborees avec succes par electrodeposition sur un substrat d’aluminium. Ensuite, les correlations entre les proprietes de la surface (morphologie, composition et mouillabilite) et le rapport molaire sel de cobalt inorganique/acide stearique organique dans l’electrolyte ont ete etudiees minutieusement. Il a ete observe que la surface superhydrophobe optimale, formee sur un substrat d’aluminium cathodique, possede l’angle de contact maximal de 161° et la resistance de polarisation la plus elevee de 1591kΩ cm2, offrant ainsi une excellente barriere contre la corrosion. Afin de mieux comprendre les principes de la corrosion a l’interface, un circuit electrique equivalent a ete propose en se basant sur les donnees obtenues par spectroscopie d’impedance electrochimique (EIS). Par ailleurs, il a ete observe que le depot electrolytique superhydrophobe de stearate de cobalt possede une bonne resistance aux rayonnements ultraviolets (UV). En effet, apres un test de degradation acceleree par UV de deux mois, les proprietes hydrophobes des couches minces ne se sont pas alterees. En se basant sur ces resultats, d’autres couches minces innovantes, faites a partir depots nanocomposites superhydrophobes, ont ete fabriquees avec succes sur des substrats d’aluminium. Pour cela, des nanoparticules de TiO2 revetues de stearate de cobalt (CoSA) ont ete incorporees dans une matrice de polymethyle-hydrosiloxane (PMHS) superhydrophobe (PMHS/TiO2@CoSA) en utilisant le procede sol-gel. Alors que les couches minces superhydrophobes de PMHS/TiO2 (sans CoSA) montrent une diminution rapide de l’angle de contact avec l’eau apres irradiation continue par rayonnement UV pendant plus d’un mois, les couches de PMHS/TiO2@CoSA conservent un angle quasi-constant de 160°. La structure en noyau-enveloppe de TiO2@CoSA permet non seulement d’augmenter les proprietes hydrophobiques de la surface des nanoparticules TiO2, mais aussi de limiter leur activite photocatalytique. Un modele plausible a ete propose pour expliquer les mecanismes de resistance aux UV. En outre, afin d’ameliorer l’adherence des couches minces nanocomposites superhydrophobes PMHS/TiO2@CoSA, des monocouches auto-assemblees (SAM) de GPTS ont ete deposees sur des substrats d’aluminium. La presence des SAM a ensuite ete confirmee par des analyses ATR-FTIR, EDX ainsi que EIS. Il a ete espere que les SAM ameliorent l’adherence au substrat par la formation de liaisons chimiques. Cependant, Les tests sur des substrats non-traites et des substrats traites par SAM montrent, dans les deux cas, une adherence de type 2B selon la norme ASTM D 3359-02. Neanmoins, une legere amelioration peut etre observee en comparant les surfaces non traitees a celle traitees par SAM apres l’enlevement du ruban adhesif. Ce projet contribuera a une meilleure comprehension des phenomenes superhydrophobiques ainsi qu’a la realisation des applications des surfaces superhydrophobes dans le monde industriel.