Characterisation of hydrogen diffusion in ceramic thin film materials

Abstract

In this thesis, several ceramic coating materials were investigated in terms of their hydrogen permeation behavior. A strong focus was set on well-established nitrides such as TiN or CrN, but also other ternaries have been screened. In addition, the impact of different physical vapor deposition-based growth techniques, such as cathodic arc evaporation and sputtering, was investigated for selected systems. The hydrogen diffusion characteristics were analysed via electrochemical hydrogen permeation tests performed on a Devanathan-Stachursky cell. While this setup is already well established for investigating bulk materials, its usage in coating technology is still uncommon, and data is rare. We therefore investigated correlations between test results (diffusion coefficient, permeability, and permeation reduction factor) and coating parameters (porosity, coating thickness, deposition temperature, morphology, deposition technique, and material system). In the first step, a clear correlation between diffusion coefficient and deposition temperature for the uncoated steel substrates could be established. With increasing temperature, the initially cold-rolled microstructure recrystallizes, and the diffusivity of hydrogen increases as the initial microstructural state is adapted. This knowledge is important, when estimating the hydrogen permeation characteristics of the bare coatings. The coating materials generally show a correlation between diffusion coefficient and film thickness. For example, cathodic arc evaporated TiN exhibits an apparent decrease in hydrogen permeation with increasing coating thickness. Additional linear sweep voltammetry (LSV) experiments indicate a correlation between the coating porosity and hydrogen permeability. Especially for cathodic arc evaporated coating materials, thicker films appear denser as imperfections are overgrown. Interestingly, sputtered thin films obtained similar or worse permeation characteristics when varying the thickness, even considering the absence of macroparticles compared to their arced counterparts. However, for selected coatings the results obtained by the Devanathan-Stachursky cell are contradictory, with increasing hydrogen permeability after depositing the coating compared to the bare substrate. This phenomenon needs to be investigated further to promote the understanding of hydrogen permeation within PVD-deposited coating materials.