Plasma-based ion implantation of oxygen in stainless steel: influence of ion energy and dose
Introduction
Implantation of oxygen in stainless steel is a possible method for the elaboration of decorative thin films or the formation of passivation oxide layers that prevent further surface corrosion [1], [2]. The desired coloration can be obtained either by implantation of elements able to form surface films exhibiting well-defined intrinsic colors, or by the formation of dielectric films of given optical index and thickness. In the first case, the coloration is independent of the thickness of the surface layer. In contrast, coloration strongly varies with the index and thickness of the dielectric film formed on the surface of a metallic substrate, so that the color uniformity strongly depends on the uniformity of the dielectric film. Generally, implantation of oxygen in metals produces surface oxide compounds with well-defined dielectric permittivity or optical index, and the resulting coloration, of course, depends on ion dose and energy implantation. These oxide films can also act as a diffusion barrier in order to protect the bulk material against corrosion [3].
The aim of this article is to present the experimental results concerning the implantation of oxygen in stainless steel using plasma-based ion implantation in a distributed ECR (DECR) plasma reactor. Therefore, after a short presentation of the characteristics of the plasma-based ion implantation (PBII) system used, the oxide layers achieved under different implantation operating conditions are characterized. The results are then analyzed and discussed in terms of ion implantation energy and dose.
Section snippets
Set-up and experimental procedure
The experiments were performed in the DECR plasma reactor previously described [4]. The oxygen plasma parameters obtained at a pressure of 0.8 mtorr (1 torr=133 Pa) and 1.5 kW of microwave input power are as follows: electron temperature kTe=1.6 eV; plasma potential Vp=+10 V; floating potential Vf=+5 V; and plasma density n=1.4×1010 cm−3. The ion distribution in the oxygen plasma, as measured by mass spectrometry [5], is 78% for O+ and 22% for O2+ ions. This plasma parameter is quite important,
Experimental results
In this experimental study, five samples were implanted with ion energy between 22 and 44 keV. With the exception of sample 2, the implantation time, i.e. the dose of oxygen implanted, was correlatively increased to the penetration depth in order to maintain a stoichiometry of the same order of magnitude in the surface layer. The parameters of the PBII process, i.e. the pulse voltage and the effective implantation time T, are summarized in Table 1.
The first spectacular feature observed is that
Discussion
Considering the above results, three important points are worthy of discussion. The first one is the thickness of the oxide layer achieved via oxygen implantation. At V0=−44 kV for the pulse voltage, the thickness of the oxide layer is 67 nm. Such a value is in good agreement with the maximum penetration depth Rp+ΔRp=71 nm calculated (using the trim program [7]) for 45-keV O+ ions in stainless steel, i.e. Rp=44 nm and ΔRp=27 nm. In fact, in the oxide layer, the penetration depth of oxygen ions
Conclusion
Implantation of oxygen in stainless steel via plasma-based ion implantation in a distributed ECR plasma reactor has been studied as functions of ion energy and dose. A uniform Fe2O3 layer, free from chromium, is formed in the near-surface region, while chromium diffuses towards the interface between the oxide layer and the bulk stainless steel to form the chromium oxide Cr2O3. The thickness of the oxide layer increases with increasing implantation time and pulse voltage. In this way, perfect
Acknowledgements
This work was supported by a grant from the Région Rhône-Alpes, France.
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