Hydrogen is an essential element of the energy industry from fuel cells to nuclear reactors and engines. However, its small molecular weight, which makes it attractive for these applications, also allows it to easily diffuse through materials and embrittle them. Hydrogen-induced damage includes increased ductile to brittle transition temperatures, precipitation of hydrides, and decreased interfacial strength between grains due to surface stabilization. Environmental barrier coatings (EBCs) can be used to alleviate hydrogen embrittlement by reducing ingress of the damaging element into susceptible substrates and thereby extend material lifetimes.
Barrier material choice, which determines hydrogen diffusivity, and coating method, which determines microstructure and defect type and concentration, are both essential in forming an effective hydrogen EBC. In this work, density functional theory is used to screen high temperature materials to determine those with the largest activation energy and therefore lowest predicted diffusivity resulting in h-BN, c-BN, HfN, ZrN, and WC predicted to perform better than the legacy material, W, for high temperature hydrogen permeation prevention. Properties including diffusing hydrogen charge state, strain energy, and redistribution of electron density were compared to trends in the magnitude of activation energy. In addition, nitrogen vacancy formation energies with respect to gaseous nitrogen and ammonia formation were used to predict nitrogen retention at elevated temperatures.
Atomic layer deposition (ALD) was used to deposit coatings of WN (decomposes to W) and BN for thermal testing. ALD uses sequential, self-limited surface reactions to form conformal, pinhole-free coatings that are chemically bonded to the substrate. Because BN was predicted to perform well regardless of crystal structure, it was chosen to compare against W. Coatings from WN and BN were conformal and uniform on the substrate surface. The film growth rate for BN was much faster than that of WN allowing for deposition of thicker barrier coatings of BN on the same time scale. Thermal testing in 6% hydrogen atmosphere up to 1770 K confirmed both the inert character of the BN coating compared to the WN/W coating and the computational predictions of BN as an improved material choice over the legacy material, W.</p
