Dielectric Properties and Corrosion Resistance of Ion Plated Thin Film Materials

Abstract

This thesis investigates promising electrically insulating and corrosion-resistant coatings Al2O3, Ta2O5, HfO2, SiO2, and AlN (⁓2 μm thick), deposited by plasma-assisted Reactive Low Voltage Ion Plating (RLVIP). All coating materials have been grown at two different partial pressures of reactive gases, relatively low of 3x10^-4, and comparable high at 8x10^-4 mbar. The overarching goal of the thesis was to characterise the influence of deposition parameters, such as pressure, on the structural, chemical, and mechanical properties, with a particular focus on dielectric and corrosion properties. In more detail, the relation between electrical resistivity and microstructure was correlated with corrosion barrier performance. XRD confirmed that Ta2O5, SiO2, and Al2O3 are fully amorphous, while HfO2 and AlN could be stabilised as crystalline structures. All systems were thermally stable up to 600°C with some minor relaxation effects occurring. TEM of AlN revealed a columnar microstructure (⁓50 nm column width) with improved alignment following annealing. XPS showed that Ta2O5 is the most pressure-sensitive system (O/Ta ratio: 2.05 to 2.53). SiO2 and Al2O3 are pressure-insensitive, while HfO2 and AlN indicate a moderate influence. AlN, Al2O3 and SiO2 are over-stoichiometric in both conditions, while HfO2 is in a nearly ideal composition. Nanoindentation demonstrated superior mechanical performance for crystalline coatings with an E-Modulus and Hardness of 354 GPa, 23.6 GPa (AlN low pressure) and 239-244 GPa, 16.7-17.3 GPa (HfO2 high and low pressure, respectively), with no significant effect of partial pressure on mechanical properties across all systems. Impedance spectroscopy revealed three electrical performance groups: SiO2 and Al2O3 (high resistivity, 10^8-10^11 Ωm), Ta2O5 and AlN (intermediate), and HfO2 (lowest, 10^3-10^5 Ωm). Corrosion testing identified SiO2 (low pressure) as the best active barrier (Rp = 35,145.8 kΩ cm^2) and Al2O3 (low and high pressure) in terms of long-term breakthrough resistance as the most durable coating overall (>168 hours at 1 V vs. Ag/AgCl). A qualitative relationship between electrical resistivity and corrosion protection was established. However, the long-term effectiveness of the barrier is ultimately determined by microstructural integrity, rather than by bulk resistivity alone.