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Abstract. This paper reports the characterization of both barrier type and porous type anodic oxide films on aluminium by means of spectroscopic ellipsometry (SE). In order to show the capabilities of the technique for quantitative determination of the layer characteristics, results based on ellipsometric data are correlated with complementary information from the analytical techniques transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). It is concluded that ellipsometry yields an accurate characterization for the thicknesses and the interfacial properties of both the barrier layer and the porous layer. The porosity of the porous layer, determined with SE, is found to be in good agreement with the results obtained from TEM. 07.60Fs, 68.55Jk, 81.60Bn In the field of aluminium applications, a variety of surface treatments are used to provide the aluminium substrate with the required properties. Each of these treatments, roughening, polishing, anodizing etc., induces chemical or morphological changes at the surface, which are strongly correlated to the mechanical, chemical or thermal pretreatment of the substrate [1][2][3][4][5][6]. Therefore, the study of such treatments always requires a thorough characterization of the final, as well as the initial state of the surface in order to understand both the effect of the treatment itself and its correlation with the substrate history. PACS:To deal with this kind of investigations, a number of analytical techniques have been used successfully during the past years. Specifically in the field of the anodizing of aluminium, numerous studies were performed. This treatment consists in the controlled, electrochemical growth of an aluminium oxide film by the anodic polarization of the aluminium substrate in an electrolyte solution. By means of Auger electron spectroscopy (AES) [7], secondary ion mass spectroscopy (SIMS) [8], and mainly transmission electron microscopy (TEM) by Wood et al. [9], and Thompson etal. [8,[10][11][12], important aspects of the anodizing mechanism and of the morphological changes which take place at the surface were revealed. It was shown that depending upon the electrolyte type, different oxide layer structures were obtained [8]. In neutral solutions, in which the oxide is non soluble, a compact and uniform oxide layer, commonly called the barrier layer, is formed [8][9][10]. On the other hand, if the oxide is soluble in the electrolyte, like in acid solutions, a porous structure develops on top of the thin barrier layer [8,11,12]. This structure is characterized by a high density of linked hexagonal aluminium oxide cells with a cylindrical pore in their centre, perpendicular to the substrate surface [8,11,12].The purpose of this paper is to discuss the applicability of spectroscopic ellipsometry (SE) [13][14][15][16][17] for the characterization of barrier type and porous type anodic oxide films on aluminium. SE allows a fast and non-destructive surface analysis, which is a considerable advantage compared to TEM where a lo...

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Chromium phosphate conversion coatings on aluminum have been characterized with nondestructive optical techniques. Complementary vibrational spectroscopy techniques, i.e., Fourier transform infrared spectroscopy and confocal micro-Raman spectroscopy, prove the presence of chromium phosphate as principal component in the coating. Additionally, aluminum oxide and indications for the presence of chromium oxide and aluminum fluoride are found.Reflection/absorption infrared spectroscopy (RAIRS) allows analysis of coatings as thin as 40 nm, while confocal micro-Raman spectroscopy is limited to thicknesses above about 150 nm. Compared to RAIRS spectra, the interpretation of Raman spectra is easier due to the absence of peak shifts which occur in the RAIRS spectra. Spectroscopic ellipsometry allows determination of the morphological characteristics of the conversion coatings, e.g., the coating thickness, using a simulation and regression procedure based on a two-layer optical model. The optical constants of the upper layer, which in a first approximation can be attributed to the chromium phosphate part of the conversion coating, can explain the greenish appearance of the thickest conversion coatings. A linear relationship exists between the coating thickness and the conversion time. An analogous linear relation exists between the conversion time and the peak areas of most of the absorption peaks in the RAIRS spectra.