Materials Science and Engineering A 495 (2008) 259–264 Brazing of AlN to SiC by a Pr silicide: Physicochemical aspects A. Koltsov ∗ , F. Hodaj, N. Eustathopoulos SIMAP – UMR CNRS 5266, INP Grenoble-UJF, Domaine Universitaire, BP 75, 1130 rue de la Piscine, 38402 Saint Martin d’H` eres, Cedex, France Received 22 March 2007; received in revised form 19 November 2007; accepted 21 November 2007 Abstract In view of their very different thermomechanical properties, joining of metals to ceramics by brazing is usually performed by means of one or more interlayers. In a recent investigation AlN was chosen as interlayer material for brazing SiC to a superalloy. The aim of the present study is to determine an alloy with a high melting point (close to 1200 ◦ C) enabling brazing of AlN to SiC. Two types of experiments are performed with a Si-17 at.% Pr eutectic alloy (T m = 1212 ◦ C): sessile drop experiments to determine wetting and brazing of AlN and SiC plates to determine gap filling. Experiments are carried out in high vacuum to promote deoxidation. Interfacial reactivity, joint microstructure and type of failure occurring during cooling are examined by optical and scanning electron microscopy. © 2008 Elsevier B.V. All rights reserved. Keywords: Brazing; Ceramics; Deoxidation; Thermomechanical properties 1. Introduction This study is part of a project aimed at joining a Ni-based superalloy to SiC by brazing [1]. The assembly must subse- quently work under oxidizing conditions at high temperatures (up to 800 ◦ C). For the brazing alloy, such high temperatures imply a melting point close to 1200 ◦ C. Much higher tempera- tures are not acceptable as they can lead to irreversible changes of microstructure of the superalloy during the brazing process. Direct brazing of superalloys to SiC at 1200 ◦ C is very diffi- cult, if not impossible, due to two types of incompatibilities between these materials. The first is a chemical incompatibility whereby, during brazing, certain components of the superalloy (Ni, Co, etc.) can quickly reach the SiC, by diffusion through the molten braze, and react strongly with this ceramic forming brit- tle graphite precipitates weakening the joint [2–4]. The second is a mechanical incompatibility. Superalloys and SiC are high elastic modulus solids but their thermal expansion coefficients differ by a factor of 3–4 [5]. As a consequence, high thermal stress is generated during cooling leading invariably to failure. In order to overcome these difficulties, which are perfectly normal in metal-to-ceramic brazing, one or more interlayers are often used [6,7]. In the present project, an AlN interlayer was ∗ Corresponding author. Tel.: +33 387704895; fax: +33 387704713. E-mail address: alexey.koltsov@arcelor.com (A. Koltsov). used both as diffusion barrier and for dissipating thermomechan- ical stress by elastic deformation [8]. The purpose of the study is to determine a brazing alloy for the AlN/SiC couple satisfying the following criteria: melt- ing point close to 1200 ◦ C, high resistance to oxidation at high temperature, good wetting (contact angle θ ≪ 90 ◦ which is a nec- essary condition for obtaining complete filling of the gap) and mechanically strong interfaces. Although the study focuses on the physicochemical aspects of brazing, it will be seen that some qualitative information on the mechanical strength of interfaces can be obtained by observing samples of wetting and brazing experiments after cooling [9]. 2. Selection of a brazing alloy As a general rule, alloys based on high melting temperature metals such as Ni, Fe, Pt, etc. react strongly with silicon carbide to form large graphite precipitates [2–4]. In order to overcome this problem, a new family of brazing alloys with high melting point has been developed based on silicides of transition metals such as Co, Pd, or Pt, [10,11] because the addition of Si above a certain content to these metals, eliminates their reactivity with SiC and leads to good wetting (contact angle θ ≪ 90 ◦ ) and strong mechanical interfaces [4,10,11]. Moreover, these silicides present a high resistance to oxidation at high temperatures. However, the high melting tem- 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.092