The role of Si incorporation on the anodic growth of barrier-type Al oxide
Graphical abstract
Introduction
Light alloys like the Al-Si series are known to have good physical and mechanical properties (heat treatable, high strength and good elongation) which are very interesting for many industrial products [1], [2]. The purest industrially relevant and used for Al layer grade is 99.5 at.%. Even for these pure Al materials, Si is present in solid solution and in intermetallics. Si and other elements like Mg, Cu and Fe are introduced as alloy elements to enhance the mechanical strength of the otherwise too soft pure Al [3]. The high tensile strength along with low density gives aluminium alloys then a high specific strength. The interest in investigating supersaturated Al-Si alloy is twofold:
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from a fundamental point of view, to understand the influence of “small” amount of Si on the properties of the grown Al surface oxide, starting at the nm-thick passive film level.
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from the application point of view, related to the industrial processes of rolling (heating, deforming) or casting, during which Mg and Si are known to diffuse to the surface.
During alloy processing, elements, like magnesium, silicon and iron, are influenced differently by heat [4]. Up to 10 at.% Si and Mg enrichment can indeed be found on the surface layer [5] but while Mg can be easily removed by chemical etching, Si remains on the surface. As a consequence, surface properties including oxidation and reactivity will be different in presence of Si. The binary Al-Si phase diagram [6] is relatively simple with very little solubility at room temperature for Si in Al and for Al in Si. Thus, the terminal solid solutions are nearly pure Al and Si under equilibrium conditions. In the absence of other elements, Si additions to Al produce elemental Si particles and an Al-rich solid-solution phase. The highest solubility of Si in Al occurs at the eutectic temperature of and is 1.65 wt.% while the eutectic composition is 12.6 wt.% Si [7].
Anodic oxidation [8] is one of the widely used surface treatments in the Al industry to protect or functionalize the material surface. It also finds application as a cost-effective and readily implemented process in areas such as, membrane technology and microelectronics for production of ordered nanoscale structures, for its excellent dielectric behaviour and high thermochemical stability, to improve corrosion resistance [9] of metal surfaces and as dielectric materials in the form of porous or barrier type oxides [10], [11], [12], [13]. Surface protection properties in aggressive environments are strongly desirable for materials used in transportation combined with the light weight to reduce energy consumption. In these large scale components fields, the research and development are predominantly focused on tailoring the process to a desired pore and surface morphology. In the case of aluminium alloys, however, the process still encounters problems when related to the growth of sufficiently thick or hard layers (i.e. hard anodizing process). In industrially pure aluminium anodizing, an uniform anodic oxide morphology is usually achieved for thin barrier or nanoporous layer. One of the challenges in understanding and controlling the anodic surface oxide properties is the presence of a second material (alloying element dependent phases) introducing additional effects on the anodizing process which are dependent on particle composition, size and volume fraction. For alloy anodizing, understanding the relative contributions of the solid solution matrix and the second-phase particles is crucial to gain insight into the occurring process and hence for its further optimization. The homogeneous formation of anodic oxides with controlled composition on Al alloys is possible depending on the alloying elements and composition [14]. The oxide film grows by cooperative transport of metal and oxygen ions under the electric field with the formation of an oxide material by both inwards and outwards migration of oxygen and metal ions respectively. Anodizing behavior of different Al alloys (Cu, Cr, Fe containing) has been investigated revealing the development of an enrichment in alloying element layer [15], [16], [17] of these elements that are not easily anodized. Microscopy studies on binary Al-Si alloys have been done for porous anodizing [18], [19] but investigations related to initial growth stages and barrier type oxides are missing.
In this study, the effect of Si incorporation into Al PVD layers used as model system is investigated before and after the anodic oxide growth. The important role of Si in surface processes has been discussed previously in terms of surface enrichment but Si is also interesting in the sense that it will oxidize. Three different Al alloys with Si concentrations of 4, 7 and 10 at.% are subjected to anodizing potentials of 50, 100 and 150 V, producing oxide layers with a thickness ranging from tens to hundreds of nm. These PVD layers,the anodized oxides morphology and their electronic properties are then investigated by cross sectional SEM, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy/Scanning Kelvin probe force microscopy (AFM/SKPFM), energy dispersive X-ray spectroscopy (EDS) and electrochemical impedance spectroscopy (EIS). The microstructure, morphology and atomic distribution is derived for different Si content and compared to anodic films grown on pure Al. This investigation is essential to further understanding of surface functionalizing of commercial materials, integrating Al dependant surface composition/chemistries in anodic oxidation mechanisms with the prospect of improving anodizing technology.
Section snippets
Sample preparation
2.5 m-thick Al:Si samples were deposited on epi-polished -Al2O3(0 0 0 1) single-crystalline wafers by DC magnetron sputtering in a high vacuum chamber (base pressure mbar) from two confocally arranged unbalanced magnetrons equipped with targets of pure Al (99.99%) and Si (99.999%). During the deposition, a RF BIAS of 65 V was applied between the targets and the substrate holder. The two targets were maintained in a close-field configuration in order to minimize the thermal load on the
RBS and XRD analysis on not anodized PVD samples
The composition stoichiometry of the samples was checked by RBS. The results are shown in Table 1. There is very good agreement between the nominal (determined by calibrated deposition rates for both Si and Al) and measured Si composition values for Si content higher than 4 at.%. However, almost negligible Si content was measured for samples with nominal 4 at.%Si concentration. This could be due to a non-uniform depth distribution of Si and in particular lower concentration of Si on the surface.
Discussion
The introduction of Si into the Al PVD layers was found to have a significant effect on Al layer structure and crystallinity. XRD analysis shows that the incorporation of Si prevents the plane (1 1 1) being the most energetically favorable plane for the metallic Al layers. In presence of Si, Al layers are indeed polycrystalline and are no longer oriented out-of-plane and in-plane. This indicates a more disordered structure with a higher amount of defects and dislocations in the samples.
After the
Conclusions
The effect of Si incorporation (up to 10 at.%) in Al PVD grown films and in the electrochemical anodic oxidation has been discussed based on experimental results obtained by combining different techniques. The presence of Si was found to have a significant influence on the electronic properties and on the structure and morphology of the anodic oxide as well as on the metallic Al PVD layer. An amount of Si as small as 4 at.% is enough to completely change the crystallographic orientation of the Al
Acknowledgments
The authors thank Erwin Hack for the ellipsometry measurements and Roland Hauert for the XPS support. This work has been performed within the frame of the National Center of Competence in Research (NCCR) “Materials’ Revolution: Computational Design and Discovery of Novel Materials (MARVEL)” of the Swiss National Science Foundation.
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