Investigations of the Deuterium Permeability of As-Deposited and Oxidized Ti2AlN Coatings

Abstract

Aluminum containing M_n+1AX_n (MAX) phase materials have attracted increasing attention due to their corrosion resistance, a pronounced self-healing effect and promising diffusion barrier properties for hydrogen. We synthesized Ti₂AlN coatings on ferritic steel substrates by physical vapor deposition of alternating Ti- and AlN-layers followed by thermal annealing. The microstructure developed a {0001}-texture with platelet-like shaped grains. To investigate the oxidation behavior, the samples were exposed to a temperature of 700 °C in a muffle furnace. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) depth profiles revealed the formation of oxide scales, which consisted mainly of dense and stable α-Al₂O₃. The oxide layer thickness increased with a time dependency of ~t^1/4. Electron probe micro analysis (EPMA) scans revealed a diffusion of Al from the coating into the substrate. Steel membranes with as-deposited Ti₂AlN and partially oxidized Ti₂AlN coatings were used for permeation tests. The permeation of deuterium from the gas phase was measured in an ultra-high vacuum (UHV) permeation cell by mass spectrometry at temperatures of 30-400 °C. We obtained a permeation reduction factor (PRF) of 45 for a pure Ti₂AlN coating and a PRF of ~3700 for the oxidized sample. Thus, protective coatings, which prevent hydrogen-induced corrosion, can be achieved by the proper design of Ti₂AlN coatings with suitable oxide scale thicknesses.

Keywords: MAX phase; PVD coating; Ti2AlN; hydrogen permeation; oxidation.

Figure 1

(a) Schematic illustration of the hydrogen permeation test rig with quadrupole mass spectrometer (QMS), turbomolecular pump (TMP) and ion getter pump (IGP). (b) Schematic illustration of the coated ferritic steel membrane.

Figure 2

XRD diffractograms of as-synthesized and oxidized Ti₂AlN coatings on ferritic steel samples: (a) overview and (b) enlarged region around 2θ° ≈ 40°.

Figure 3

(a) Raman spectra (λ_Nd:YAG = 532 nm) of the Ti₂AlN coatings before and after oxidation. (b) Raman fluorescence spectra (λ_HeNec = 633 nm) of the Ti₂AlN coatings before and after oxidation.

Figure 4

XPS depth profiles after oxidation for 100 h at 700 °C of (a) Ti₂AlN coating, and (b) uncoated ferritic steel substrate.

Figure 5

Measured oxide thicknesses by XPS depth profiles of the Ti₂AlN coatings after oxidation. Underlying fit was performed using (1).

Figure 6

Scanning electron microscope image (a) and electron probe micro analysis (EPMA) scans of the cross-section after oxidation for 100 h at 700 °C for Ti (b), Al (c), N (d), O (e), Ni (f) and Fe (g).

Figure 7

Arrhenius Plot of D₂ ion currents of ferritic membranes with and without Ti₂AlN or Ti₂AlN+thermal grown oxide (TGO) coating. The different symbols on each line refer to results of consecutive measuring cycles. The quasi-linear fit is performed to illustrate the Arrhenius type behavior.