Journal of Materials Science and Engineering A 5 (5-6) (2015) 197-201
doi: 10.17265/2161-6213/2015.5-6.002
Hydrogen Permeation Characteristics of TiN-Coated Stainless Steels
Motonori Tamura
Center for Industrial and Governmental Relations, The University of Electro-Communications, Tokyo 182-8585, Japan
Abstract: TiN films consisting of fine crystal grains, with diameters of about 100 nm or less, provided superior hydrogen-permeation barriers. Applying a coating of TiN on a stainless steel substrate reduced the hydrogen permeation by a factor of about 100 to 5,000 compared with uncoated substrates. Effect of microstructure of films on hydrogen-permeation behavior is studied. The test specimens coated with films, with columnar crystals grown vertically on the substrate, tended to exhibit higher hydrogen permeability. The grain boundaries of the films became trap sites for hydrogen and microcrystalline structures with many grain boundaries are expected to provide effective hydrogen-barrier performance.
Key words: Hydrogen permeation, TiN coating, microstructure, crystal grain, stainless steel.
1. Introduction
There have been attempts in recent years to utilize hydrogen gas as an alternative to fossil fuel, leading to the development of numerous hydrogen-energy systems that u high-pressure hydrogen. Molecular hydrogen dissociates relatively easily to form hydrogen atoms on the surface of steel when expod to a hydrogen gas atmosphere, and hydrogen permeation in steel has been reported to cau hydrogen embrittlement [1-8].
Studies concerning hydrogen-barrier films have been conducted in a variety of fields such as nuclear fusion reactors, fuel cells, H2S corrosion components,
or vacuum equipment. Den ceramic films such as
Al2O3, TiC, TiN, and BN have been reported to provide hydrogen-barrier performance. The hydrogen-permeation reduction factors vary significantly from one report to another, however. The variance for Al2O3 is between 10 and 10,000 [9], for example, owing to significant variations in hydrogen permeation behavior with the films’ microstructures. Many rearchers have experimented with the forming
Corresponding author:Motonori Tamura, Ph.D., professor, rearch field: metallurgical coatings. E-mail: *******************.ditions to vary the microstructure of films. For instance, in Thornton’s structure zone model, microstructures are classified according to factors such as the substrate temperature and argon pressure during film formation [10].
This study focud on the impact of differences in thin-film microstructures on hydrogen permeation behavior.
2. Experimental Setup
2.1 Deposition Process and Characterization of Films
Type “316L” austenitic stainless steel was ud for the substrates and TiN films were formed on one side of such substrates by RF sputtering. RF sputtering was performed by the power of 600 to 3,000 W in order to obtain variety of microstructures of TiN films. A film thickness of 1.8 to 2.0 μm was obtained. The film thickness, film cross-ctions, and microstructures were examined with a SEM (Scanning electron microscope); the crystalline pha of was analyzed by XRD (X-ray diffraction); the chemical composition of the films were analyzed by EPMA (Electron probe micro-analyzer); the sizes of the crystal grains were analyzed with a SPM (Scanning probe microscope) and
XTEM (X-ray transmission electron microscope).
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Hydrogen Permeation Characteristics of TiN-Coated Stainless Steels 198
2.2 Hydrogen-Permeation Tests
Hydrogen-permeation tests were performed on the coated stainless steel samples. The tests were bad
on the differential-pressure methods [11]. Fig. 1 shows
a schematic illustration of the apparatus. This part of the standard specifies the u of a pressure nsor
or a gas chromatograph for determining the gas-transmission rate of a single-layered plastic film or s
heet and multi-layered structures under a differential pressure. A gas chromatograph was ud in this study. The samples (diameter: 35 mm; thickness: 0.1 mm) were t on a susceptor, which could be held in place at temperatures up to 773 K. The apparatus was evacuated to 10-6 Pa. After the test temperature stabilized, hydrogen (purity of 99.995%) was introduced into the susceptor side of the chamber at a filling pressure of 400 kPa. The stainless-steel-sample side of the apparatus was continuously evacuated. The permeation area was 6.6 cm2.
The permeability φ , is defined by the expression:
φ = (J d) / (A (Δp)n), (1) where J is the permeation flux of hydrogen through a sample of area A and thickness d, under a partial pressure gradient Δp across the sample called the driving pressure. The exponent n reprents different permeation regimes: diffusion-limited and surface-limited when n = 0.5 and 1 respectively. Hydrogen permeation through a thin-film-coated steel sample is known to be diffusion-limited when the driving pressure is between 104 and 105 kPa [12].
The permeation flux was continuously measured. After 30 min, for example, the standard deviation decread below 10% among the data for permeation flux through the non-coated stainless steel substrate at 773 K under a hydrogen pressure of 400 kPa.
3. Results and Discussion
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3.1 Hydrogen-Permeation Mode
The permeation of hydrogen through solid materials proceeds via adsorption, dissociation, diffusion, and recombination coupled with desorption. Fig. 2 shows a schematic illustration of hydrogen permeation in coated samples.
A hydrogen molecule (H2, ad) is adsorbed on the surface of the film and decompos to hydrogen atoms (H ad). Such adsorbed hydrogen atoms diffu into the film from the film surface (H film, in) and move toward the interface with the substrate (H film, out). Hydrogen atoms diffu into the substrate from the interface of the film (H sub, in) to the non-coated side of the substrate (H sub, out). At the non-coated side of the substrate, the hydrogen atoms (H des) form hydrogen molecules and desorption occurs. In this study, a driving force of hydrogen permeation was the pressure difference
Fig. 1 Apparatus for hydrogen-permeation tests. All Rights Rerved.
Hydrogen Permeation Characteristics of TiN-Coated Stainless Steels 199
Fig. 2 Hydrogen-permeation mechanism.
between the high and low pressure sides. Hydrogen permeation occurred in the film and the substrate becau of decompression in the gas pha at the non-coated side of the substrate.
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Our previous study [13] has demonstrated the dependence of permeation flux J on the driving pressure Δp in BN- and TiN-coated samples. The exponent n had a value of 0.48 ~ 0.53 at 573 ~ 773 K, which indicated that hydrogen pasd through the samples in the diffusion-limited permeation mode. When n = 0.5 (diffusion-limited regime) in Eq. (1), the overall permeation flux of hydrogen through the sample can be given by Fick’ s law. In a steady state, the overall permeation flux J (measured in mol·s-1) through a sample of thickness d film + d sub (d film, thickness of film: 1.5 × 10-6 m; d sub, thickness of substrate: 1.0 × 10-4 m) and area A (6.6 × 10-4 m2) is expresd as: J =φ A ((P H2, in)0.5 - (P H2, out)0.5)/(d film + d sub) (2) where, φ is the permeability of the sample as shown before, and P H2, in (4.0 × 105 Pa) and P H2, out (1.0 × 10-6 Pa) are the hydrogen pressures at the feed side and at the vacuum-pumping side respectively.
3.2 Effect of Microstructure of Coatings on Hydrogen-Permeation Behavior
Fig. 3 shows the XRD pattern of TiN film. EPMA data showed that chemical compositions of Ti in films were 47 to 51 atomic percent.
survive是什么意思高考录取结果查询The test specimens with TiN coatings underwent a significant change in hydrogen permeability as shown in Fig. 4, which depended on the test temperature, and the decline in the hydrogen permeabil
ity was greater at lower temperatures. Since the hydrogen permeability is a reflection of the diffusion of hydrogen in the film, the diffusion coefficient of hydrogen should also decrea with lower temperature in TiN.
The relationship between the crystalline grain size and hydrogen permeability at 773 K is shown in Fig. 5. Test specimens covered with films consisting of smaller crystalline grains tended to have lower hydrogen permeability.
Fig. 3 XRD pattern of TiN film.
Fig. 4 Hydrogen-permeation performance.
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Hydrogen Permeation Characteristics of TiN-Coated Stainless Steels
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Fig. 5 Effect of grain size on hydrogen-permeation.
TiN coatings (TiN-S1, TiN-S2, TiN-S4, TiN-S5), which were deposited by sputtering, compod of microcrystalline grains equivalent to tho in Zone T of Thornton’s structure zone model [10], while the columnar TiN-3 grains, which were deposited by ion plating, were equivalent to tho in Zone II (columnar).
There were differences in the crystalline grain sizes,
with higher hydrogen permeability for the columnar TiN-3 grains with relatively larger size (> 100 nm). The TiN films with columnar structures tended to have relatively high hydrogen permeability. Films consisting of microcrystalline grains equivalent to tho in Zone T of Thornton’s structure zone model, were confirmed to have higher hydrogen-barrier performance. Many grain boundaries existed within the films, which could potentially prent some form of obstruction when hydrogen pasd through.
Studies concerning the state of hydrogen in materials cited lattice defects (atomic vacancy, dislocatio
n, and grain boundaries), impurity atoms, precipitates, inclusion boundaries, voids, etc., as hydrogen trap sites. When the microstructure of examined materials was fine-grained, high density of dislocations and numerous grain boundaries provided longer travel of atomic hydrogen through a crystal lattice of examined materials and slower breakdown of hydrogen at the
opposite side of membrane [12]. Hydrogen solid solubility of a fine-grained crystalline Ni test specimen was known to vary significantly from that of a polycrystalline test specimen of Ni. The hydrogen content in Ni depended on the crystalline grain size [14]. It had been pointed out that the hydrogen diffusion coefficient in some steel depends on the crystalline grain size and the diffusion coefficient decreas when the crystalline grain size was small [15]. The findings led us to believe that the effects of grain boundaries could not be ignored when the diffusion rate of hydrogen was relatively slow.
Hydrogen diffusion in steel has been known to be a few orders of magnitude lower in materials with ion covalent bonds, such as ceramic films [16-18], and the hydrogen trap sites are believed to function sufficiently. The micro-scale of a film’s crystal grains is believed to have caud the grain boundaries to act as effective hydrogen-diffusion barriers.
4. Conclusions and Future Directions
TiN films consisting of fine crystal grains, with diameters of about 100 nm or less, provided superior hydrogen-permeation barriers. Applying a coating of TiN on a stainless steel substrate reduced the hydrogen permeation by a factor of about 100 to 5,000 compared with uncoated substrates. The grain boundaries of the films became trap sites for hydrogen and microcrystalline structures with many grain boundaries are expected to provide effective hydrogen-barrier performance.
Extensive studies of the correlation between film structure and deposition parameters have been carried out over the past ten decades. From an understanding of film formation, follows the possibility for micro-structural and nano-structural engineering in order to design a material for specific technological applications. This has led to the development and refinement of Thornton’s structure zone models that systematically categorize lf-organized structural evolution during physical vapor deposition as a
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Hydrogen Permeation Characteristics of TiN-Coated Stainless Steels
201
function of film growth parameters. The first structure zone models were derived from relatively low-resolution of optical and SEM obrvations. Later, cross-ctional TEM and SPM analys were employed to provide more detailed structural characterization.
In this study, behavior of hydrogen permeability of coated stainless steels was analyzed with a focus on the grain size of TiN films. Thin films usually exhibit a wide variety of microstructures characterized in terms of not only grain size and but also crystallographic orientation, lattice defects, pha composition, and surface morphology. Further study is necessary with a focus on the various microstructures to understand the correlation between the hydrogen permeation behavior and film microstructure, and to develop high-performance hydrogen barrier coatings.
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