Fourier transform infrared reflection absorption spectroscopy and microspectroscopy, a tool to investigate thermally grown oxide scales B. Lefez a, ⁎ , J. Lopitaux a , B. Hannoyer a , M.-P. Bacos b , E. Beucher c a Institut des Matériaux de Rouen, LASTSM, Université de Rouen, UPRES-EA 1290, 76801 Saint Etienne du Rouvray Cedex, France b Office National d'Etudes et de Recherches Aerospatiales (ONERA), BP 72-29, Avenue de la Division Leclerc, 92322 Châtillon Cedex, France c CRITT Analyses et surface, Site Industriel des Jonquets-2 chemin du Roy, 27400, Louviers, France Available online 20 June 2007 Abstract The main goal of this work is to show the interesting contribution of reflectance infrared spectroscopy to determine the composition of oxide films on metal surfaces. Illustration is reported on oxidation of α 2 -TiAl (Ti75-Al25 at.%) and γ-TiAl (Ti50–Al50 at.%) alloys, carried out at 650 °C under laboratory atmosphere. Reflectance infrared spectra were recorded on an FTIR 710 Nicolet spectrophotometer, equipped with a Nicolet IR plan microscope. The knowledge of the infrared optical constants makes it possible to predict the infrared reflection spectra of different thin layers. As there is a lack of data in literature, theoretical calculations have been made in order to study experimental spectra. To illustrate these computations the first band observed for a thin film (200 nm) of Al 2 O 3 is the higher longitudinal optical (LO) mode, the position of which is 920 cm - 1 for α-Al 2 O 3 , 930 cm - 1 for γ-Al 2 O 3 or a broad band at 980 cm - 1 for amorphous Al 2 O 3 . In the case of TiO 2 /alloy system, the LO mode moves from 830 cm - 1 for rutile TiO 2 to 880 cm - 1 for anatase TiO 2 . A peak fit program was applied to the spectra in order to extract each oxide contribution. The position of the fitted single absorption bands makes it possible to identify the different oxides. The oxide grown on α 2 -TiAl phase, consists of α-Al 2 O 3 and rutile TiO 2 (24 h to 500 h of oxidation treatment), γ-Al 2 O 3 and rutile TiO 2 for 1000 h of oxidation treatment. Chemical maps reveal heterogeneous distribution of the oxides in the layer. A detailed surface characterization showed that the surface was principally covered with α-Al 2 O 3 and locally with an Al 2 O 3 /TiO 2 mixture. In the case of γ-TiAl oxidation, the oxide film consists of α-Al 2 O 3 , γ-Al 2 O 3 and rutile TiO 2 mixture. FTIR spectroscopy provides important information about amorphous, poor or well crystallized materials when X-ray diffraction is only convenient for crystallized phases. © 2007 Elsevier B.V. All rights reserved. Keywords: TiAl alloys; Oxidation; Alumina; Titanium oxide; FTIR 1. Introduction The deterioration of alloys which occurs in air at elevated temperature is often complex and it is important to characterize oxides, especially at the beginning of the oxidation. Several tools have been used to identify the different types of oxides in the thermally grown scales as Scanning Electron Microscopy (SEM), X-ray diffraction to identify crystallographic phases or Transmis- sion Electron Microscopy to observe alumina phase morphology. Other techniques like X-ray photoelectron spectroscopy, Raman spectroscopy or luminescent spectroscopy are available [1–6]. For a better understanding of the oxidation behavior of Ti–Al alloys, knowledge of the chemical and physical properties of the oxide layer compounds formed at low temperature is necessary. From Scanning Electron Microscopy analysis, it follows that the oxidation of TiAl alloys generally leads to a complex scale with a layered structure consisting of an outer layer of mainly TiO 2 while an inner layer was composed of a mixture of TiO 2 and α-Al 2 O 3 . The main goal of this work is to show the complementary contribution offered by reflectance infrared spectroscopy (IRRAS) to determine the composition of oxide films on metal surfaces. The knowledge of the infrared optical constants makes it possible to predict the infrared reflection spectra of different thin layers. 2. Experiments The α2-Ti 3 Al and γ-TiAl polycrystalline substrates were grown at the ONERA by arc fusion under argon atmosphere. Their bulk composition (at.%) was measured to be Ti 75 Al 25 and Available online at www.sciencedirect.com Thin Solid Films 516 (2007) 349 – 354 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +33 2 35146244; fax: +33 2 35146259. E-mail address: benoit.lefez@univ-rouen.fr (B. Lefez). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.126