Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Characterization of less common nitrides as potential permeation barriers Jiří Matějíček a, ⁎ , Jakub Veverka a,b , Vincenc Nemanič c , Ladislav Cvrček b , František Lukáč a , Vladimír Havránek d , Ksenia Illková a a Institute of Plasma Physics, Prague, Czech Republic b Czech Technical University Prague, Czech Republic c Jožef Stefan Institute, Ljubljana, Slovenia d Nuclear Physics Institute, Řež, Czech Republic ARTICLE INFO Keywords: Hydrogen permeation barriers Physical vapor deposition Nitrides Permeation measurement Adhesion Residual stress 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 re- duction factor which was determined at 400 °C and 1 bar ranged from ˜10 2 to ˜5 × 10 3 , 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. 1. 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 re- covery), on the other hand from the effects of hydrogen isotopes on the materials – from radioactivity in case of tritium to degradation of me- chanical properties (hydrogen embrittlement) [1,2]. The metals con- sidered as structural materials for fusion devices, such as reduced ac- tivation 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 ap- plication in a breeding blanket: high thermomechanical integrity, compatibility with the breeder materials/corrosion resistance, applic- ability 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 permea- tion of untreated and treated base metal. In a review by Hollenberg et al. [6], required PRF values in the 10 2 -10 4 range are mentioned, depending on specific design. In [7,8], similar values (˜10 2 ) are pre- sented 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 de- position techniques have been used, including physical vapor deposi- tion (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 environ- ment 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 perme- ability [1,11,12]. More recently, erbia [13], chromia [14], zirconia [15] https://doi.org/10.1016/j.fusengdes.2018.12.056 Received 19 October 2018; Received in revised form 13 December 2018; Accepted 18 December 2018 ⁎ Corresponding author. E-mail address: matejicek@ipp.cas.cz (J. Matějíček). Fusion Engineering and Design 139 (2019) 74–80 Available online 04 January 2019 0920-3796/ © 2019 The Authors. Published by Elsevier B.V. All rights reserved. T