Corrosion resistance of 2024 aluminum alloy coated with plasma deposited a-C:H:Si:O films

Abstract

AA 2024 aluminum alloy is widely employed in aeronautic and automobilist industries. Its hardness and low density are attractive properties for such industrial areas. However, since it contains copper, it undergoes severe corrosion in aggressive media as saline or low Earth orbit environments. In this work, it was investigated the properties of films deposited by PECVD on AA 2024 aluminum alloy as well as the corrosion resistance of the film/substrate systems under different corrosive atmospheres. Films were prepared in a plasma atmosphere composed of 50% of oxygen and 50% of hexamethyldisiloxane resulting in a total gas pressure of 4.0 Pa. Plasma ignition was promoted by the application of radiofrequency signal (13.56 MHz) to the sample holder while grounding the topmost electrode. The plasma excitation power, P, was changed from 10 to 80 W in the six different set of experiments. Film thickness, measured by profilometer, increases by 5 times as P was elevated from 10 to 80 W. As demonstrated by the infrared spectra of the samples, films are essentially organosilicons with preservation of functional groups of the precursor molecule and with creation of different ones. The oxide proportion and the structure crosslinking degree are affected by the plasma excitation power. According to the results obtained by sessile drop technique, hydrophilic to moderately hydrophobic films are produced with changing P from 10 to 80 W. The corrosion resistance, evaluated by salt spray and electrochemical impedance spectroscopy, EIS, experiments, in general increases after film deposition. It is demonstrated that film deposition improves, in up to 36 times, the resistance of the alloy to salt spray attack. It is also shown an improvement of about 240 times in the alloy resistance under NaCl solution by the EIS data. Micrographs acquired by Scanning Electron Microscopy after the corrosion tests furnish further information on the importance of the layer physical stability on its barrier properties. Furthermore, films highly protect the alloy against the oxygen attack. Interpretations are proposed based on the modification of the plasma kinetics with P, altering film structure, composition and properties.

Effect of the plasma excitation power on the properties of SiOxCyHz films deposited on AISI 304 steel

“…Plasma Enhanced Chemical Vapor Deposition, PECVD, has emerged as a clean, simple and cost-effective methodology for the production of silicon-based coatings. Many studies, employing plasma deposition and hexamethyldisiloxane, HMDSO, as the starting material, have demonstrated the convenience of this method to prepare nanocomposites for the controlled release of silver [1], ice repellent coatings [2], humidity sensors [3], gas permeation barriers [4], dielectric films for gates in MgZnO transistors [5], corrosion resistant layers for Al 2024 and magnesium [6][7] as well as protective gradient coatings [8]. Thus, a broad range of structures have been prepared using the PECVD of HMDSO.…”

Wettability and surface microstructure of polyamide 6 coated with SiOXCYHZ films

“…While the first material is hydrophobic, flexible and presents low density, the second is hard, hydrophilic and dense [19]. Both structures are reported to provide protection to metallic surfaces in corrosive media [20,21]. Although being softer and less resistant to physical and chemical attacks than silica, organosilicons are sometimes preferred due to the mismatching problems of the rigid silica layer on a flexible polymer surface.…”

Effect of surface modification on the properties of plasma‐polymerized hexamethyldisiloxane thin films

“…Figures and show an important physical reduction of pp‐HMDSO thin film thickness after exposure to SF 6 plasma, which is probably due the high reactivity of fluorine atoms towards silicon and oxygen sites. No remarkable reduction of film thickness is obtained after exposure to O 2 plasma, which may be ascribed to the oxidation of the very first atomic layers of the films with important loss of organic groups as revealed by EDX analysis, resulting in a SiO x top layer, which resists to the atomic oxygen attack and protects the underneath organosilicon bulk from further ablation …”