Lloyd H. Hihara

Effect of Hydrogen Bonding Characteristics on the Corrosion of Plasma‐Deposited Silicon Nitride Films

Silicon nitride (SiN[sub x]:H) films were deposited onto Al, Mo, Si, and Ti substrates by RF plasma-enhanced chemical vapor deposition from mixtures of SiH[sub 4] and NH[sub 3] with and without hydrogen dilution. Under conditions yielding the same Si/N ratio in the films (0.76); the addition of H[sub 2] to the gas mix increased total bonded H in the film, resulted in a shift from predominantly Si-H to N-H bonding, and yield higher density films. The anodic polarization behavior of coated and uncoated substrates were characterized in aqueous solutions of HCl and NaCl. Films deposited under conditions of hydrogen dilution exhibited up to three orders of magnitude lower dissolution rates than those deposited in the absence of hydrogen, affording greater protection to the underlying substrates. A high defect density in the SiN[sub x]: H films on aluminum, believed to be due to chemical interaction with the substrate during growth, resulted in high anodic current densities due to rapid dissolution to the underlying metal at the defect sites. The scratch resistance of the nitride films on aluminum was lower than on other substrates, apparently due to the softness of the pure aluminum.

Polarisation behaviour and corrosion initiation mechanisms of Mo coated with amorphous hydrogenated silicon alloy thin ceramic films

Abstract Amorphous hydrogenated silicon, silicon nitride, and silicon–carbon films on molybdenum substrates were anodically and cathodically polarised in NaCl and Na2SO4 solutions at 30°C. The silicon films showed signs of extensive deterioration, whereas the silicon nitride and silicon–carbon films were inert, but led to pitting of the Mo substrate predominantly at pre-existing coating breaches. Analysis of the corrosion morphology, and comparisons of actual polarisation diagrams to those generated by electrochemical polarisation models indicated that submicron cracks accounted for most of the breaches, while larger breaches, such as those caused by coating spallation were less frequent. To a lesser degree, pits also initiated from regions lacking pre-existing breaches during the later stages of anodic polarisation.

Functional Nano-Coating Materials by Michael Addition and Ring-opening Polymerization: Reactivity, Molecular Architecture and Refractive index

Understanding the molecular interaction and morphology of organic-inorganic hybrid materials is an important and fundamental assignment to develop novel high-performance materials. In this work, we developed two types of hybrid coating materials by using different silane coupling agents via Michael addition reaction and ring-opening polymerization. The changes in molecular interaction and morphology of the hybrid coatings due to chemical composition and curing temperature were studied by electron microscopy, spectroscopy and solid state 29Si nuclear magnetic resonance analysis. Fundamental differences were observed in HYBRID I and HYBRID II coatings during the nucleation stage that was dependent on the curing temperature. Higher curing temperature of the hybrid coatings resulted in improved uniformity and greater crystallinity of dispersed phases, and better control of the morphology compared with coatings cured at lower temperatures. The higher curing temperature provided more consistent nucleation sites for the growth of larger nanostructures of desired characteristics (e.g., size and surface features). There is great flexibility in synthesizingg these hybrid materials where different structure and morphology can be achieved to produce materials whose applications can range from adhesives to protective coatings. Refractive index results revealed that HYBRID I (90 °C) coating showed higher refractive index than HYBRID II (90 °C) coating.

Anodic and cathodic models for interpreting polarisation behaviour of ceramic-coated substrates containing pre-existing coating breaches

Abstract Models have been developed to help in the interpretation of polarisation diagrams for ceramic-coated substrates. The models assumed that the coatings contained pre-existing coating breaches from which anodic or cathodic currents of the substrate emanate. The anodic models for non-passivating metals indicated that the slope of the polarisation diagrams should be either equal to that of the bare substrate if coating breaches are relatively large, or equal to one third of the bare substrate value if coating breaches are relatively small and low in density. Pseudopitting potentials usually develop if the ceramic coatings support only very low anodic currents. The cathodic models indicated that the area fraction of coating breaches can only be easily obtained in deaerated solutions if the coatings do not support hydrogen evolution. In aerated solutions, it is more difficult to obtain information such as coating breach area fraction because the cathodic currents are dependent on breach size, breach density, breach distribution, and diffusion layer thickness.