Elsevier

Fusion Engineering and Design

Volume 139, February 2019, Pages 74-80
Fusion Engineering and Design

Characterization of less common nitrides as potential permeation barriers

https://doi.org/10.1016/j.fusengdes.2018.12.056Get rights and content

Abstract

In a fusion reactor, the transport of hydrogen isotopes (primarily tritium) has to be controlled, from the point of view of fuel balance and retention in the reactor components, which can result in material degradation and spreading of radioactivity. To suppress this, tritium permeation barriers are developed. Suitable materials for the permeation barriers are those with low hydrogen isotope permeability - primarily ceramic materials, such as oxides, carbides and nitrides.

In this study, coatings of six less common nitrides prepared by physical vapor deposition – namely AlCrN, CrN, Cr2N, CrWN, WN and ZrN – were investigated. Besides basic characterization (elemental and phase composition, surface morphology and coating thickness), hydrogen permeation, adhesion, residual stress and thermal expansion were evaluated. All coatings were dense, crack-free and well adherent. The permeation reduction factor which was determined at 400 °C and 1 bar ranged from ˜102 to ˜5 × 103, the best performance being achieved by the ZrN coating. As these materials seem not to be investigated as hydrogen permeation barriers, they have a very high potential to be further improved.

Introduction

In nuclear fusion reactors, tight control of the hydrogen isotope transport is indispensable. This stems on one hand from the need to maintain an efficient fuel cycle (especially tritium breeding and recovery), on the other hand from the effects of hydrogen isotopes on the materials – from radioactivity in case of tritium to degradation of mechanical properties (hydrogen embrittlement) [1,2]. The metals considered as structural materials for fusion devices, such as reduced activation ferritic-martensitic steels, have very high permeability of hydrogen isotopes, which increases with temperature [1]. Therefore, permeation barriers have to be applied, while their principal role is to suppress the permeation of hydrogen isotopes into structural materials. The general requirements for permeation barriers are: the capability to prevent or reduce hydrogen adsorption, low hydrogen diffusion rate and the absence (or at least very low density) of pores, cracks and other structural defects [3]. More specific requirements arise from the application in a breeding blanket: high thermomechanical integrity, compatibility with the breeder materials/corrosion resistance, applicability to large engineering components [4,5]. Self-healing capability, i.e. regeneration of the damaged barrier through in situ oxidation is also a benefit. The performance of the permeation barriers is compared through a permeation reduction factor (PRF), i.e. the ratio of permeation of untreated and treated base metal. In a review by Hollenberg et al. [6], required PRF values in the 102-104 range are mentioned, depending on specific design. In [7,8], similar values (˜102) are presented from the point of view of breeder blanket operation.

Prospective candidate materials are ceramics – oxides, nitrides and carbides, which often feature high temperature stability and corrosion resistance, besides low hydrogen permeation [1,9]. A variety of deposition techniques have been used, including physical vapor deposition (PVD), chemical vapor deposition (CVD), hot-dip aluminization (HDA) + oxidation, electro-chemical deposition (ECD), plasma spraying (PS), pack cementation, and others.

Oxides represent the most widely investigated class of materials for permeation barriers. Their general advantage is that, in the environment with oxygen presence, the oxide layer might replenish or even grow [10]. On the other hand, they often have significantly different thermal expansion from the steels, which might result in spallation and/or cracking upon high temperature exposure in service [3]. Among them, the most popular is alumina, thanks to its low inherent permeability [1,11,12]. More recently, erbia [13], chromia [14], zirconia [15] and yttria [16] films have been investigated. The achieved PRF are typically in the 102-103 range, the most successful cases reaching ˜104.

Nitrides have been studied much less frequently [10]. In [17], ion-plasma deposited TiN coatings were studied along with Al + Si + Cr diffusion coatings; PRF of 2–4 was observed, with slight degradation upon exposure. Tamura et al. [18] investigated TiN, BN and SiC coatings prepared by magnetically enhanced plasma ion plating. TiN and BN showed better performance than SiC, reaching PRF values ˜102. Slightly higher permeation was observed after annealing. Forcey et al. [19] have deposited TiC, TiC + TiN and TiC + TiN + Al2O3 coatings by CVD on stainless steel and Mo substrates. About one order of magnitude permeation reduction was achieved, without a notable benefit of the multilayers; this was attributed to the presence of defects within the coatings. Similar bi-layers, but with reversed order (TiC on TiN on MANET steel) were prepared by ion beam assisted deposition [20]; PRF of ˜104 was reported. The permeation barrier was found effective only on the upstream side, indicating a key role of the surface-related processes in hydrogen transport. In PVD multilayers, consisting of Ti, TiC and TiN, Shan et al. [21] reported PRF up to 106. Such widely varying results on similar materials suggest that processing technique plays a crucial role and obtaining a defect-free coating is critical. Racault et al. [22] investigated several oxides, carbides and nitrides on SiC/SiC composites, achieving the best results in rather thick double-layer coatings of TiN produced by CVD and PVD. Magnetron sputtered TiAlN coatings on Eurofer 97 have reached PRF of ˜2 × 104, which reduced to ˜5 × 103 after prolonged exposure to 400 °C [23]. SiN coatings prepared by reactive radio-frequency magnetron sputtering of Si in argon and nitrogen atmosphere have reached PRF of ˜2 × 103; high barrier efficiency was attributed to relatively high content of strongly bound hydrogen in the film [24]. Magnetron-sputtered WN films have shown negligible deuterium diffusion at room temperature after D plasma implantation [25] and sufficient thermal stability up to 600 °C [26]. Native nitrides (e.g. Fe2N) have reduced the permeation by 1–3 orders of magnitude [27], however, may not show sufficient thermal stability [28].

This study is focused on six less common nitrides - AlCrN, CrN, Cr2N, CrWN, WN and ZrN. The choice was motivated by the following criteria:

- applicability in fusion reactor environment, i.e. compounds of low neutron activation elements, stability at foreseen operational temperatures (˜400-500 °C)

- thermal expansion close to steels

- deposition possibility, including previous experience at partner facilities

- lack of prior research in permeation barrier applications [3].

Results of evaluation of several application-relevant characteristics are presented. This is primarily the effectiveness as hydrogen permeation barriers. Moreover, several characteristics are relevant for the coatings’ survivability in high temperature environment, such as adhesion to the substrate, magnitude of residual stress (which should be low and preferably compressive, to prevent spallation and cracking) and thermal expansion (which should be close to that of the substrate, to minimize thermal stresses).

Section snippets

Coating preparation

Six types of coatings were prepared by PVD. CrN, Cr2N and WN coatings were prepared at the Innovation Center for Diagnostics and Application of Materials, Czech Technical University in Prague, by magnetron sputtering in a Flexicoat 850 device (Hauzer, Netherlands), using parameters from previous experience [29]. Prior to deposition, the polished substrates were degreased in acetone. Before the deposition of the nitride layer, additional thin layer of pure Cr or W, respectively, was deposited

Basic characterization

Representative morphologies of the coatings are shown in Fig. 1. The cathodic arc plasma deposited coatings (Fig. 1a–c) are dense and crack free, with relatively smooth surface and occasional spots of the deposited material of various sizes (˜μm or less) and small dimples of slightly reduced thickness. This is a typical appearance of coatings made by this technology. On the other hand, the magnetron-sputtered coatings (CrN shown Fig. 1d) have practically featureless, smooth surface, without any

Conclusions

In this study, physical vapor deposited coatings of AlCrN, CrN, Cr2N, CrWN, WN and ZrN were investigated for their suitability as hydrogen permeation barriers. The permeation reduction factor ranged from ˜102 to ˜5 × 103; the best result being achieved by the ZrN coating. As all these materials were measured for the first time, these results can be considered very promising. The most efficient permeation barrier, ZrN, yielded intermediate values (among the six tested coatings) in other

Acknowledgements

Financial support by Czech Science Foundation grant no. GA14-12837S and Czech Academy of Sciences project Strategy AV21 is gratefully acknowledged. Part of this work was also carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The ERDA analysis was carried out at the

References (64)

  • J. Engels et al.

    Hydrogen saturation and permeation barrier performance of yttrium oxide coatings

    Fusion Eng. Des.

    (2017)
  • I.L. Tazhibaeva et al.

    Hydrogen permeation through steels and alloys with different protective coatings

    Fusion Eng. Des.

    (2000)
  • M. Tamura et al.

    Characteristic change of hydrogen permeation in stainless steel plate by BN coating

    Surf. Coat. Technol.

    (2014)
  • K.S. Forcey et al.

    The formation of tritium permeation barriers by CVD

    J. Nucl. Mater.

    (1993)
  • R. Checchetto et al.

    Analysis of the hydrogen permeation properties of TiN-TiC bilayers deposited on martensitic stainless steel

    Surf. Coat. Technol.

    (1996)
  • C.Q. Shan et al.

    The behavior of diffusion and permeation of tritium through 316L stainless-steel with coating of TiC and TiN + TiC

    J. Nucl. Mater.

    (1992)
  • C. Racault et al.

    Formation of permeation barriers on ceramic SiC/SiC composites

    J. Nucl. Mater.

    (1996)
  • P.J. McGuiness et al.

    Hydrogen permeation through TiAlN-Coated eurofer’ 97 steel

    Surf. Coat. Technol.

    (2011)
  • V. Nemanic et al.

    Hydrogen permeation through silicon nitride films

    J. Alloys. Compd.

    (2012)
  • L. Gao et al.

    Deuterium implantation into tungsten nitride: negligible diffusion at 300 K

    J. Nucl. Mater.

    (2014)
  • Z. Wolarek et al.

    Hydrogen transport in plasma nitrided iron

    Acta Mater.

    (2004)
  • R. Lindau et al.

    Present development status of eurofer and ODS-Eurofer for application in blanket concepts

    Fusion Eng. Des.

    (2005)
  • M. Bartosik et al.

    Thermal expansion of Ti-Al-N and Cr-Al-N coatings

    Scr. Mater.

    (2017)
  • K. Aigner et al.

    Lattice-parameters and thermal-expansion of Ti(CxN1-X), Zr(CxN1-X), Hf(CxN1-X) and TiN1-X from 298-K to 1473-K as investigated by high-temperature X-ray-diffraction

    J. Alloys. Compd.

    (1994)
  • J.A. Sue et al.

    High-temperature erosion behavior of titanium nitride and zirconium nitride coatings

    Surf. Coat. Technol.

    (1991)
  • M. Pettina et al.

    Diffusion-based and creep continuum damage modelling of crack formation during high temperature oxidation of ZrN ceramics

    J. Eur. Ceram. Soc.

    (2016)
  • V. Nemanic et al.

    Hydrogen permeation through disordered nanostructured tungsten films

    J. Nucl. Mater.

    (2012)
  • B.Y. Man et al.

    Microstructure, oxidation and H-2-permeation resistance of TiAlN films deposited by DC magnetron sputtering technique

    Surf. Coat. Technol.

    (2004)
  • E. Spain et al.

    Characterisation and applications of Cr-Al-N coatings

    Surf. Coat. Technol.

    (2005)
  • B.S. Yau et al.

    Tungsten doped chromium nitride coatings

    Thin Solid Films

    (2008)
  • L.C. Chang et al.

    Mechanical properties and oxidation resistance of sputtered Cr-W-N coatings

    Surf. Coat. Technol.

    (2017)
  • T. Polcar et al.

    Tribological characterization of tungsten nitride coatings deposited by reactive magnetron sputtering

    Wear

    (2007)
  • Cited by (13)

    • Ab initio screening of refractory nitrides and carbides for high temperature hydrogen permeation barriers

      2022, Journal of Nuclear Materials
      Citation Excerpt :

      Hydrogen diffusion in and potential embrittlement of pure metals, including molybdenum (Mo) [38–41] and W [11,38,42–45], have been extensively studied both computationally and experimentally. Fewer studies have examined hydrogen diffusion in high melting-point nitrides and carbides [18,46–49], however, promising performance has been observed including for BN [16,28,29], VC [50], and ZrN [51]. In this work, we used density functional theory (DFT) to analyze atomic hydrogen diffusion in refractory nitrides and carbides with melting points greater than 2700 K and compared their computed activation energies to those of Mo and W. Additionally, we have analyzed contributions to the diffusion barrier, Ea, from steric hindrance and the redistribution of electron density during the H hopping event – diffusion from the lowest energy adsorption site to the nearest relaxed site within the first subsurface layer.

    • Permeation barriers for hydrogen embrittlement prevention in metals – A review on mechanisms, materials suitability and efficiency

      2022, International Journal of Hydrogen Energy
      Citation Excerpt :

      Therefore, it has been considered one of the most promising HPBs even if ZrN coating adhesion, residual strain, and thermal expansion are not the best compared to other materials. Further nitrides such as AlCrN, CrN, Cr2N, CrWN, and WN have been investigated as well, more details regarding their performances can be found in Ref. [200]. Although carbides present excellent and desirable properties regarding the mechanical and thermal behavior, there are only a few carbides that have demonstrated excellent properties in terms of hydrogen barrier.

    View all citing articles on Scopus
    View full text