Pulsed electrophoretic deposition of hybrid coatings from aqueous suspensions as surface functionalization and sealing technique of anodized AA2024. Part I: Morphological characterization, analysis of the interfacial interactions, and evaluation of pore impregnation of the anodic layer

Abstract

In this two-part series, pulsed direct current for cathodic electrophoretic deposition (pulsed-CEPD) was studied as sealing technique of anodized AA2024 by depositing hybrid sol-gel films from aqueous suspensions. The coating was obtained using tetraethyl orthosilicate and hexadecyltrimethoxysilane as precursors and two pulse widths were used: 10 microseconds and 100 milliseconds. For comparison, continuous direct current for cathodic electrophoretic deposition was also performed. In this first article, it was developed a low-cost automated apparatus capable of creating pulsed fields using a commercial power supply. This was necessary since the automation system provided by the power supply manufacturer did not have a short enough response time able to create pulse widths in the order of microseconds. Field emission scanning electron microscopy (FESEM) and focused ion beam milling techniques were utilized to evaluate the surface morphology and interaction mechanisms in the interface anodic layer/film. Low voltage scanning transmission electron microscopy (STEM)-in-FESEM, and transmission electron microscopy (TEM) were used to evaluate the pore impregnation provided by pulsed-CEPD. Results demonstrated that the developed apparatus successfully created pulsed fields. FESEM micrographs of the anodic layer sealed by pulsed-CEPD showed that the shortest pulse provided the best films, likely due to the greater control of hydrogen bubbles generation and the promotion of a more regular deposition. Also, they revealed two interfacial interactions between the anodic layer and the film. STEM-in-FESEM images showed that the shortest pulse width provided greater pore impregnation by particles. This observation was confirmed by TEM, which demonstrated an incomplete pore impregnation for the longest pulse. Discussion regarding result interpretations is presented based on the generation and dissipation of H2(g) (considering the current interruption); on an alternative to the colloidal stability theory due to the presence of a pulsed field; and on the electrokinetic effects at the electrodes during pulsed-CEPD, that discusses how electroosmotic flow influences the deposition.