Oxidation behaviour of a Mo(Si,Al) 2 based composite at 1500  C L. Ingemarsson a, * , K. Hellström a , L.G. Johansson a , J.E. Svensson a , M. Halvarsson b a Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden b Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden article info Article history: Received 23 September 2010 Received in revised form 1 May 2011 Accepted 2 May 2011 Available online 2 June 2011 Keywords: A. Molybdenum silicides A. Multiphase intermetallics B. Oxidation D. Microstructure F. Electron microscopy, scanning abstract The oxidation of a Mo(Si,Al) 2 composite is investigated at 1500  C in dry air using exposure times from 1 to 1000 h. Cross sections are examined with Scanning Electron Microscopy (SEM) and the phase composition is analyzed by X-ray diffraction (XRD). The material forms a continuous and protective alumina layer, the growth of the alumina layer following parabolic kinetics. Immediately below the scale Mo(Si,Al) 2 is replaced by a Mo 5 (Si,Al) 3 layer due to the flux of aluminum to the scale. The Al concen- tration in the Mo(Si,Al) 2 phase in the underlying substrate decreases from 27% before exposure to 16e17% after 1000 h. The continuous alumina layer becomes covered by a top layer consisting of alumina grains embedded in a viscous melt with approximate composition 7 Na 2 Oe15 Al 2 O 3 e78 SiO 2 . With time, sodium is volatilized from the melt and the top scale layer transforms to a mixture of alumina, mullite and silica melt. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Molybdenum disilicide (MoSi 2 ) based materials combine some of the attractive properties of ceramics and metals which is why they are considered promising materials for high temperature structural applications [1,2]. Due to a moderate density (6.24 g/ cm 3 ), high melting temperature (2030  C) and metallic-like thermal and electrical conductivities [3e5], they are widely used as furnace heating elements at temperatures up to 1800  C in air [6,7]. The excellent oxidation resistance at high temperatures is due to the formation of a protective SiO 2 scale. However, one limitation of MoSi 2 is the poor oxidation resistance at intermediate temper- atures, showing severe oxidation between 400 and 700  C [8e11]. Since oxidation resistance is a key issue when developing high temperature materials, much effort has been put into finding ways to overcome the problem of low temperature oxidation. Some methods that may improve the oxidation resistance of MoSi 2 are additions of alloying elements, which have previously been studied by others [12e17] and the present authors [18] [19]. The best improvement in oxidation resistance at intermediate temperatures was reported to be with the addition of Al to MoSi 2 [10,20e22]. This yields an alumina forming Mo(Si,Al) 2 material with improved oxidation resistance due to the formation of a protective Al 2 O 3 scale. The Mo(Si,Al) 2 materials form a stable and adherent alumina with a close match of thermal expansion coefficient between the bulk material and Al 2 O 3 ; it can be used in both oxidizing and reducing atmospheres up to almost 1600  C in air [7,23]. This is a major benefit compared to silica forming MoSi 2 materials, which exhibit evaporation in reducing environments and spalling of the oxide scale [7,13,24]. The oxidation resistance of Mo(Si,Al) 2 material at high temperatures has been investigated by others [7,12,17,20,22,25] and, recently, by the present authors [26,27]. In our earlier study, the oxidation behaviour of a Mo(Si,Al) 2 based composite (with an Al content of 30 at.%) was investigated at 1450  C and 1500  C. It was found that the composite forms a-Al 2 O 3 scales during the initial oxidation (<1 h), and that the Al supply to the growing alumina scale is provided mainly by the Mo(Si,Al) 2 phase. The formation of the alumina scale is accompanied by the diffusion of aluminium towards the scale and by silicon diffusing within the silicide. The results showed that a Mo 5 (Si,Al) 3 layer developed directly below the Al 2 O 3 oxide scale. Moreover, a Mo(Si 1.4 ,Al 0.6 ) layer formed between the Mo 5 (Si,Al) 3 layer and the Mo(Si,Al) 2 bulk material. The presence of a Mo 5 (Si,Al) 3 layer is also reported by others [7,17,20,22]. Maruyama et al. [17] investigated the oxidation of Mo(Si,Al) 2 at 1550  C for 20 h. They observed an Al- depleted MoSi 2 layer between the Mo 5 (Si,Al) 3 layer and the Mo(Si 1x ,Al x ) 2 substrate. Ramberg et al. [20] observed the complete disappearance of the Mo(Si 1x ,Al x ) 2 phase from the sample after exposure at 1500  C for 1000 h. We have previously made a detailed microstructural investiga- tion of the Mo(Si,Al) 2 based composite exposed at 1500  C for 5 min * Corresponding author. Tel.: þ46 31 772 2859; fax: þ46 31 772 2853. E-mail address: linda.ingemarsson@chalmers.se (L. Ingemarsson). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.002 Intermetallics 19 (2011) 1319e1329