Hydrogen Permeation of Multi-Layered-Coatings

Abstract

Using a substrate of AISI 316L austenitic stainless steel, which is used for components in high-pressure hydrogen systems, the hydrogen barrier properties of samples with single-layer coatings of TiC, TiN, and TiAlN as well as a multi-layered coating of TiAlN and TiMoN were evaluated. The ion plating method was used, and coating thicknesses of 2.0–2.6 μm were obtained. Hydrogen permeation tests were carried out under a differential hydrogen pressure of 400 kPa and at a temperature between 573 and 773 K, and the quantities of hydrogen that permeated the samples were measured. This study aimed at elucidating the relationship between the microstructures of the coatings and the hydrogen permeation properties. Coatings of TiC, TiN, TiAlN, and TiAlN/TiMoN facilitated reductions of the hydrogen permeabilities to 1/100 or less of that of the uncoated substrate. The samples coated with TiN and TiC that developed columnar crystals vertical to the substrate exhibited higher hydrogen permeabilities. The experiment confirmed that the coatings composed of fine crystal grains were highly effective as hydrogen barriers, and that this barrier property became even more efficient if multiple layers of the coatings were applied. The crystal grain boundaries of the coating and interfaces of each film in a multi-layered coating may serve as hydrogen trapping sites. We speculate that fine crystal structures with multiple crystal grain boundaries and multi-layered coating interfaces will contribute to the development of hydrogen barriers.