Study on the behavior of atomic layer deposition coatings on a nickel substrate at high temperature

Although many techniques have been applied to protect nickel (Ni) alloys from oxidation at intermediate and high temperatures, the potential of atomic layer deposition (ALD) coatings has not been fully explored. In this paper, the application of ALD coatings (HfO2, Al2O3, SnO2, and ZnO) on Ni foils has been evaluated by electrical characterization and transmission electron microscopy analyses in order to assess their merit to increase Ni oxidation resistance; particular consideration was given to preserving Ni electrical conductivity at high temperatures. The results suggested that as long as the temperature was below 850 °C, the ALD coatings provided a physical barrier between outside oxygen and Ni metal and hindered the oxygen diffusion. It was illustrated that the barrier power of ALD coatings depends on their robustness, thicknesses, and heating rate. Among the tested ALD coatings, Al2O3 showed the maximum protection below 900 °C. However, above that temperature, the ALD coatings dissolved in the Ni substrate. As a result, they could not offer any physical barrier. The dissolution of ALD coatings doped on the NiO film, formed on the top of the Ni foils. As found by the electron energy loss spectroscopy (EELS), this doping affected the electronic transport process, through manipulating the Ni(3+)/Ni(2+) ratio in the NiO films and the chance of polaron hopping. It was demonstrated that by using the ZnO coating, one would be able to decrease the electrical resistance of Ni foils by two orders of magnitude after exposure to 1020 °C for 4 min. In contrast, the Al2O3 coating increased the resistance of the uncoated foil by one order of magnitude, mainly due to the decrease in the ratio of Ni(3+)/Ni(2+).