J. Vereecken

Effect of bath concentration and curing time on the structure of non-functional thin organosilane layers on aluminium

The corrosion resistance of aluminium alloys can be improved by different surface treatments such as painting. A pre-treatment based on chromate is the current method used to increase the corrosion resistance and the adhesion of the organic layer. Silane films seem to be an interesting alternative system to replace the toxic chromates. In this paper, the characterisation of bis-1,2-(triethoxysilyl)ethane (BTSE) thin layers has been evaluated by coupling optical techniques like spectroscopic ellipsometry (SE) and infra-red spectroscopic ellipsometry (IRSE) along with electrochemical methods (electrochemical impedance spectroscopy (EIS)). This approach has been chosen to have a better understanding of the protection provided by these organosilane thin films. It will be demonstrated that the BTSE bath concentration modifies the thickness of the layers and that the curing of this thin film can also improve the barrier properties by forming a denser layer.

Determination of the thickness of thin silane films on aluminium surfaces by means of spectroscopic ellipsometry

Abstract The thickness of thin films of non-functional silane bis-1,2-(triethoxysilyl)ethane (H 5 C 2 O) 3 Si-CH 2 CH 2 -Si(OC 2 H 5 ) 3 (BTSE) deposited on aluminium surfaces is investigated using spectroscopic ellipsometry (250–1700 nm). The data processing of the ellipsometry spectra is carried out by means of simulation and regression techniques. New advances in data processing, e.g. multiple sample analysis and determination of thickness non-uniformity, are applied to characterise these thin polymer films realistically. The influence of the concentration of the BTSE solution and the curing of the film is investigated. Optical thickness estimates are corroborated by independent auger electron spectroscopy and transmission electron microscopy analysis.

Composition and thickness of non-functional organosilane films coated on aluminium studied by means of infra-red spectroscopic ellipsometry

Surface treatments of metals, leading to the deposition of thin films, are generally performed in order to create or change certain surface properties such as corrosion resistance or adhesion. For the past few years, the use of organosilanes for aluminium surface treatments are more and more considered as they provide both corrosion protection and adhesion properties. It has been previously shown that the silane bath concentration strongly influences the thickness of the silane films deposited on aluminium substrates. A linear increase of the layer thickness with the silane bath concentration has been underlined. In this paper, the chemistry of the deposited silane films has been studied as a function of the silane bath concentration. For this, Infra red spectroscopic ellipsometry (IRSE) has been used. It is a method similar to reflection absorption infra red spectroscopy (RAIRS), which is a non-destructive infra red reflection technique, allowing chemical characterisation of thin films on metals. It will be shown that differences in the positions and strength of absorption bands are visible when the silane concentration of the solution is varied. Such changes can result in a difference in the film chemistry or in the film thickness. Since IRSE offers chemical information along with the thickness information, modifications occurring in an IRSE spectrum can be understood more easily. It will be demonstrated that the use of an optical model, describing the absorption bands and thickness of the films, permits the determination of the chemistry and the thickness of the silane films.

Chromate Conversion Coating on Aluminum Alloys I. Formation Mechanism

The formation of chromate conversion coatings (CCCs) on commercially pure Al (AA1050) during immersion in a chromic/ hydrofluoric acid solution has been investigated. A film formation mechanism is proposed based on the combination of open circuit potential measurements and surface analysis techniques, such as Auger electron spectroscopy and atomic force microscopy. It is suggested that the conversion of the Al surface takes place in three distinct stages: activation of the surface, initiation of the film formation, and growth of the conversion layer. In addition to the classical approach, a sol-gel model was considered for the initiation and growth of the film. The effect of the free fluoride and chromic acid concentration was studied using a thermodynamic model whereas the influence of the Al oxide film was investigated by forming an anodic barrier layer prior to the conversion process. It was shown that the rate-determining step in the CCC formation is the activation of the Al surface. Therefore, the morphology and structure of the conversion layer is determined not only by the bath composition but also by the thickness of the Al oxide film.

IRSE study on effect of thermal curing on the chemistry and thickness of organosilane films coated on aluminium

Abstract This paper focuses on the use of infra red spectroscopic ellipsometry (IRSE) to characterise thin organosilane films deposited on aluminium. IRSE combines in one measurement the possibilities of traditional FT-IR methods (molecular information) with those of visible spectroscopic ellipsometry (SE) (morphological—film thickness and optical—refractive index/extinction coefficient information). In this study, this optical method is used to get relevant informations concerning the influence of a thermal curing on various characteristics of the silane films. It will be shown that, by using an appropriated optical model for the interpretation of the data, IRSE permits to determine quantitatively the curing effect on the film thickness, non-uniformity and chemistry. It will be also demonstrated that the results obtained from the IRSE analysis correlate well those obtained from SE and electrochemical impedance spectroscopy (EIS).

Characterisation of conversion layers on aluminium by means of electrochemical impedance spectroscopy

Abstract This paper reports the characterisation of chemical conversion layers on aluminium by means of Electrochemical Impedance Spectroscopy. To fit the EIS spectra non-trivial dispersion phenomena, which are usually described by means of a constant phase element taking into account fractal dimensions or porosity effects, have to be considered. By investigating the influence of the nature and the conductivity of the EIS electrolyte on the impedance behaviour, the observed dispersion can be attributed to a ladder-network of electrolyte resistances and double layer capacitors in the pores. The proposed electric equivalent circuit allows to study the growth of the film as a function of the conversion parameters. In this paper the influence of the conversion time on the film thickness is worked out. In agreement with AES, SE and FTIRS the layer growth is in proportion to the conversion time.

Investigation of anodic aluminium oxide layers by electrochemical impedance spectroscopy

The effects of a.c.-electrochemical graining and anodizing of an aluminium substrate on the layer properties of both barrier and porous alumina layers are examined using electrochemical impedance spectroscopy (EIS). In order to show the capabilities of the technique for a quantitative determination, results based on impedance data are compared with complementary information from surface analytical techniques. Though the results for the determination of barrier layer thickness and dielectric constant look promising, calculations are troubled by non-trivial dispersion phenomena. This problem is treated using a fractal description of surface roughness of the substrate and of the layer thicknesses. Information on pore structure of porous oxide films could not be obtained from the approach considered in this study.

Development of an optical model for steady state porous anodic films on aluminium formed in phosphoric acid

Porous anodic oxide films on aluminium formed in phosphoric acid (PAA) have been characterized nondestructively by spectroscopic ellipsometry. Compared to previous studies on porous films formed in sulfuric acid, the optical behaviour of PAA films reveals new features which have been attributed to film-substrate interface roughness and optical anisotropy effects. On one hand relatively large interface roughness has been simulated by a graded index model. On the other hand, the implementation of uniaxial anisotropy in the optical model of the PAA film enables to interpret spectroscopic ellipsometry data acquired at multiple angles of incidence in terms of the morphology of the films. More specifically, accurate and physically realistic values are found for the porosity and porous film thickness. Although more difficult to interpret from the optical findings, the thickness of the barrier part of the porous film can also be estimated. The ellipsometry characterizations are confirmed by complementary TEM analysis of various films. Finally, the anisotropy exhibited by the PAA films is in line with recent theoretical predictions of the optical behaviour of arrays of parallel cylindrical capillaries in an isotropic medium proposed by other authors.

Synthesis and characterisation of chromium carbides

Abstract This paper presents the synthesis and the characterisation of various chromium carbide compounds. Thin Cr 23 C 6 films were deposited by reactive sputtering while Cr 7 C 3 films were formed by the carburisation of chromium films in a CH 4 /H 2 atmosphere. Cr x C y powders were synthesised from various precursors (Cr, CrN, Cr 2 O 3 ) by reaction with CH 4 /H 2 at high temperature. The samples were characterised by AES, XRD and electron diffraction. The effects of the experimental parameters (gas composition, temperature, reaction time) on the purity, the phase formed and the composition of the product of reaction are examined and discussed.

Auger Electron Spectroscopy Element Profiles and Interface with Substrates of Electroless Deposited Ternary Alloys

Electroless NiMeP alloys (Me = Cu, Sn, Sb) with high phosphorus content (∼11 weight percent) and a low weight percent of the third component (Me) have been plated in acidic baths onto aluminum, iron, and nickel. Scanning Auger electron spectroscopy is applied to study the element profiles and interface with the substrates. Generally the third component follows the profiles of Ni and P, which proves the alloy formation. A surface enrichment in the third element (Me) is observed in all cases. It is very strong when tin is the third component and very weak in the case of antimony, which is more or less uniformly distributed through the thickness. Decrease of surface concentration of phosphorus is noticed when NiSbP is plated. In Cu profiles three different zones exist : a surface enrichment zone, a groove (a zone of reduced concentration), and a plateau (a zone of almost constant concentration). There is no enrichment in the third element at the interface with the substrate for all three alloys and three substrates. At the interface with aluminum prepared for electroless plating with double zincate pretreatment complete dissolution of the zincate layer has been established.