(680h) Nanoscale Characterization of Water Penetration through Plasma Polymerized Coatings and Water at the Coating/Substrate Interface

Plasma enhanced chemical vapor deposition (PECVD) is a versatile technique to deposit plasma polymerized thin films on various substrates without volatile organic solvents for a wide range of applications including protection of metal against corrosion. Penetration of water to the metal/coating interface is often the first step in the corrosion of coated surfaces. While past research has focused on macroscopic corrosion phenomena, understanding on the nanoscale of some fundamental aspects of the corrosion process at the metal interface is lacking. Gaining nanoscale information on the movement of water and structure of water at the interface is important to understand and design new protective coatings and also predict corrosion failure in a short time. Off specular X-ray measurements have been used to show that the plasma polymerized coatings are conformal to an underlying Al metal substrate with its native oxide, consistent with good coverage of the substrate and minimization of void space at the interface. Neutron reflectivity (NR) and infrared-visible sum frequency generation spectroscopy (SFG) have been used to characterize the water penetration through two plasma polymerized coatings on the nanoscale.

NR can determine the depth profile of a substance near an interface with a resolution of 1-2 nm. To simulate a practical corrosion process, an in situ experiment in which the sample is in the presence of water or water vapor has been used. By replacing H₂O with deuterium oxide, D₂O, the contrast has been enhanced. X-ray reflectometry (XR) measurements provide complementary information about interfacial roughnesses and changes in thickness upon exposure to water. Such measurements have revealed that the hydrophobic plasma polymerized hexamethyldisiloxane (pp-HDMSO) coating can prevent water penetration, while a hydrophilic plasma polymerized maleic anhydride (pp-MA) coating absorbs water and swells to approximately 1.5 times the dry coating thickness.

SFG measurements have directly probed the water between the coating and a sapphire substrate mimicking the native oxide on Al. These measurement have also shown that a pp-HMDSO coating prevents water penetration to the coating/sapphire interface over the time scales probed. They also reveal the hydrogen-bonded water network that forms at the interface when water penetrates through the pp-MA coating.