Plasma-based ion implantation of oxygen in stainless steel: influence of ion energy and dose

doi:10.1016/S0257-8972(02)00093-2

Surface and Coatings Technology

Volume 156, Issues 1–3, 1 July 2002, Pages 225-228

https://doi.org/10.1016/S0257-8972(02)00093-2 Get rights and content

Abstract

Implantation of oxygen in stainless steel (15% Cr) via plasma-based ion implantation in a distributed ECR plasma reactor has been studied as functions of ion energy and dose. Due to the formation at the surface of dielectric films with optical index and thickness depending on the implantation time and pulse voltage (up to 44 kV), various colorations can be obtained. The experimental results demonstrate the feasibility of uniform processing, the possibility of reaching perfect control of the coloring through the dose and energy of implanted ions, and that the resulting coloration varies monotonically when increasing the dose and penetration depth of implanted oxygen. Characterization of the films by scanning electron microscopy and X-ray microanalysis shows that oxygen implantation results in strong surface oxidation, but without any significant degradation of the surface aspect. The thickness and composition profiles of the oxide layers determined using X-ray photoelectron spectroscopy combined with ellipsometry indicate that the thickness, of course, increases with ion energy and dose, but that the composition of the oxide layer resulting from the implantation process is not uniform. A uniform iron oxide layer (with a stoichiometry closed to Fe₂O₃), free from chromium, is formed in the near-surface region, while chromium segregates at the interface between the oxide layer and the bulk stainless steel to form chromium oxide Cr₂O₃.

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 kT_e=1.6 eV; plasma potential V_p=+10 V; floating potential V_f=+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 R_p+ΔR_p=71 nm calculated (using the trim program [7]) for 45-keV O⁺ ions in stainless steel, i.e. R_p=44 nm and ΔR_p=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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