The objective of this work was to elucidate the water incursion at the interface between coating and aluminum oxide with nanoscale depth resolution at the earliest stage. The pathway, rate of the incursion, amount of water at the interface at a given moment, and changes of the interface after water incursion and drying were to be answered. To address these problems, the sensitivity of neutron reflectivity (NR) to the enrichment of isotopically labeled water (D2O) was used to probe water at the interface over a several nm length scale. Neutron scattering length density (NSLD) profiles, which were obtained from nonlinear least squares regressions of NR data, were compared to obtain the information on the location and amount of water in the sample.
With regard to the pathway of the incursion through the face of coatings, we observed that rapid water incursion occurred through the epoxy coating of ca. 15 nm thickness within 30 min. Water was positioned preferentially at the aluminum oxide interface rather than in the coating. The amount of water at the interface was estimated to two monolayers of D2O due to the single pathway. When the stoichiometric ratio was changed in a manner that yields an increase of a factor four in the bulk crosslink density, the amount of water reaching the interface was reduced to be so small as to be unmeasurable. Much later in the exposure, at ca. 2000 min, additional water incursion occurred. We conjecture this occurred together with, or as a result of, hydrolysis of the coating at both the top of the coating and next to the substrate. There was no full recovery in the NSLD of the sample even after a drying time longer than 1 month. Using the liquid-scratch cell, we could successfully follow the water incursion through a scratch and along the interface under humid conditions. Once the coating edge was exposed to liquid water, water appeared at the interface one centimeter away within 40 min, and could travel underneath the coating for several centimeters away from the scratch in several hours.
This work goes beyond earlier studies of the lateral movement of water under coatings, by showing concentrations of the water both in the coating and at the substrate interface with nanometer level depth resolution. In this work, the extremely thin coating provided moderate protection of the oxide-coated metal from water. The amount of water at the interface within 30 min. due to the incursion through the coating, which was only tens of nm thick, was tens of ng/cm2. With incursion along the interface, we had a similar amount of water at the interface within 30 min. regardless of the fact that the length of the path the water had to take was on the order of a few cm. The nominal rate of incursion along the interface was estimated to correspond to a diffusion coefficient of order 10-6 cm 2/s for a simple model of one-dimensional diffusion along the interface. Furthermore, the incursion rate along the interface for a second exposure was of the same order of magnitude, suggesting there was no significant disruption of the interface after the first exposure. The diffusion coefficient of water incursion along the interface was three orders of magnitude higher than the diffusion coefficient for water in the bulk epoxy.
|Commitee:||Hamed, Gary, Soucek, Mark, Tsige, Mesfin|
|School:||The University of Akron|
|School Location:||United States -- Ohio|
|Source:||DAI-B 76/11(E), Dissertation Abstracts International|
|Keywords:||Aluminum oxide, Coatings, Epoxy, Interface, Water diffusion|
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