The high electrical and thermal conductivity of copper make it an irreplaceable material for many applications. Copper possesses adequate aqueous corrosion resistance through the formation of a semi-protective native oxide film, hence its use in residential and commercial plumbing. Still, it is susceptible to elevated corrosion rates and pitting in the presence of contaminants depending on pH and fluid temperature.^1–7 Alloying to enhance corrosion resistance is common, though the corresponding degradation of electrical and thermal conductivity⁸ cannot be tolerated in applications such as high-powered RF systems. In these devices, copper is utilized in cooling channels and/or as electron collectors, where its high conductivities are paramount. Instead, very thin barrier films may provide enhanced corrosion resistance without a marked loss of desirable bulk properties.

In this work, we grow thin films on copper for enhanced corrosion resistance using atomic layer deposition (ALD). ALD is a vapor phase chemical deposition method capable of growing conformal thin films with precise thickness control and no line-of-sight requirement, making it ideal for coating tube interiors and components with intricate geometries⁹.

The films tested here are Al₂O₃, TiO₂, and nanolaminate combinations of these two. The excellent sealing properties of ALD Al₂O₃ complements the high chemical and environmental stability of TiO₂. Total film thickness of single layer and nanolaminate films is approximately 50nm. Corrosion resistance is quantified through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.1M NaCl.

Results show an increase in corrosion resistance for both single layer coatings and nanolaminate films over the bare copper. As expected, the charge transfer resistance of copper coated with a single layer Al₂O₃ film decreases by an order of magnitude over 72 hours, and energy dispersive x-ray spectroscopy (EDS) confirms Al and O depletion in the region exposed to electrolyte. Nanolaminate films exhibited as low as a 10% reduction in charge transfer resistance over that same period. Imaging reveals decreased coating degradation of nanolaminate film stacks compared to single layer films after exposure to saline solution over a 3-day period.

References

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