Fourier transform infrared reflection absorption spectroscopy and microspectroscopy, a tool to investigate thermally grown oxide scales

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 alpha(2)-TiAl (Ti75-Al25 at.%) and gamma-TiAl (Ti50-Al50 at.%) alloys, carried out at 650 degrees 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 Al2O3 is the higher longitudinal optical (LO) mode, the position of which is 920 cm(-1) for alpha-Al2O3, 930 cm(-1) for gamma-Al2O3 or a broad band at 980 cm(-1) for amorphous Al2O3. In the case of TiO2/alloy system, the LO mode moves from 830 cm(-1) for rutile TiO2 to 880 cm(-1) for anatase TiO2. 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 gown on alpha(2)-TiAl phase, consists of alpha-Al2O3 and rutile TiO2 (24 h to 500 h of oxidation treatment), gamma-Al2O3 and rutile TiO2 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 alpha-Al2O3 and locally with an Al2O3/TiO2 mixture. In the case of gamma-TiAl oxidation, the oxide film consists of alpha-Al2O3, gamma-Al2O3 and rutile TiO2 mixture. FTIR spectroscopy provides important information about amorphous, poor or well crystallized materials when X-ray diffraction is only convenient for crystallized phases. (C) 2007 Elsevier B.V. All rights reserved.