Nb 4 Fe 4 Si 7 coatings to protect niobium and niobium silicide composites against high temperature oxidation S. Knittel a , S. Mathieu b, ⁎, M. Vilasi b a SNECMA- Site Evry-Corbeil, BP81, 91003 Evry Cedex, France b Université de Lorraine, Institut Jean Lamour, UMR7198, BP70239, 54506 Vandoeuvre lès Nancy, France abstract article info Article history: Received 18 March 2013 Accepted in revised form 8 July 2013 Available online 15 July 2013 Keywords: Silicides Diffusion coating Pack cementation Niobium silicide composites Oxidation The aims of this study were to develop Nb 4 Fe 4 Si 7 coating for Nb-silicide composite and to assess their oxidation resistance. The Nb 4 Fe 4 Si 7 was first manufactured as single phase and its oxidation behaviour was characterised from 1100 to 1300 °C. Its oxidation resistance at 1300 °C in air was high because of the formation of a duplex pro- tective oxide layer of Fe 2 O 3 and SiO 2 . Halide activated pack-cementation was successful to deposit the Nb 4 Fe 4 Si 7 compound as outer layer of the coating both on pure niobium and Nb-silicide composite. In both cases, the mas- ter alloy 10%FeSi 2 + 10%NbSi 2 + 80%Nb 4 Fe 4 Si 7 can be employed. Oxidation tests performed both cyclically and isothermally at 1100 °C demonstrated that the oxidation resistance of coated Nb-silicide composite was not as good as that observed on the single phase Nb 4 Fe 4 Si 7 compound. Nevertheless lifetime as long as 500 1 h-cycle at 1100 °C was obtained both on pure niobium and Nb-silicide composite. © 2013 Elsevier B.V. All rights reserved. 1. Introduction High temperature applications require materials combining both low creep rate and oxidation rate at high temperature with satisfactory fracture toughness at room temperature. Current nickel-based superal- loys approach their operating temperature limit in most of high- temperature structural applications due to the vicinity of their melting point. So other types of materials which are able to withstand simulta- neously at very high level of temperature and oxidative conditions have to be developed. Refractory metal intermetallic composites (RMIC) are of interest to replace nickel-based superalloys in the hottest sections of turbine engines [1–3]. Among RMIC, lightweight Nb-silicide compos- ites are considered to be the most promising candidates [4,5]. The melt- ing point of these materials exceeds 1750 °C and their densities are between 6.6 to 7.2 g cm -3 [6] versus 9–9.5 g cm -3 for nickel-based su- peralloys. These composites [7,8] consist of a ductile niobium solid solu- tion (Nb ss ) and of Nb 5 Si 3 silicides [9–11]. Unfortunately, niobium has substantial oxidation limitations in the monolithic form. For Nb-silicide composites, we showed [12] that the high temperature oxidation resistance depends both on the silicide ox- idation resistance and on its distribution [13]. The reaction of RMIC with oxygen led to the formation of unprotective oxide scales and to oxygen internal diffusion that is responsible of a significant embrittlement (pesting) of the subsurface [12,14]. Some improvements have been made by alloying, but their low oxidation resistance currently restraints their application. Therefore protective coatings are required to extend the lifetime of these alloys when exposed to air at high temperature. Refractory silicides have an attractive potential as protective coat- ings for niobium based material due to their compatibility with the Nb-silicide composites and to their high oxidation resistance [15,16]. However, the susceptibility of most refractory silicides (MoSi 2 , NbSi 2 , WSi 2 , etc. [17]) to catastrophic oxidation (pesting) at moderate temper- atures requires careful selection of the silicides to be deposited. Studies [18,19] demonstrate that the use of complex silicides can eliminate this type of oxidation behaviour. The slurry coatings R512E and R512A de- veloped by Priceman and Sama [20] and based on Fe–Cr–Si and Ti–Cr– Si systems, respectively, are not subject to pesting. Among the phases formed in these diffusion coatings, silicides with stoechiometries such as M 8 Si 7 ,M 7 Si 6 and M 11 Si 8 have been identified as having high oxida- tion resistance at temperature as high as 1300 °C [18,21] because of the formation of a protective silica scale [22,23]. Previous works [15,24–26] demonstrated that these ternary or qua- ternary silicides can be formed by codeposition of Ti, Fe, Cr and Si through the halide activated pack cementation (HAPC) technique. This gaseous process allows the manufacture of homogeneous coating on pieces with complex geometries. For this technique, the growth of the coating is governed by diffusion processes and the composition of the outer part of the coating is closely correlated to the composition of the master alloy employed. Thus, the manufacturing of this type of coat- ing is similar to a diffusion couple between the master alloy and the sample. For this reason, the knowledge of the thermodynamic data is necessary before envisioning the deposition of a specific phase. In the present study this phase corresponds to the Nb 4 Fe 4 Si 7 because of its Surface & Coatings Technology 235 (2013) 144–154 ⁎ Corresponding author. Tel.: +33 3 83 68 46 70. E-mail addresses: stephane.knittel@snecma.fr (S. Knittel), stephane.mathieu@univ-lorraine.fr (S. Mathieu), michel.vilasi@univ-lorraine.fr (M. Vilasi). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.07.027 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat