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When performing nitriding of stainless steels, there is a decrease or even complete depletion of chromium in the nitrogen rich region just beneath the surface, inhibiting the formation of passivated Cr oxide layer, beneficial to withstand corrosive attacks and Cr/Fe nitrides, responsible for hardness enhancement. To overcome this problem, a hybrid technique was used, consisting of depositing a thin chromium film on steel surface and then bombarding it with nitrogen ions. By a complex recoil process, chromium atoms are implanted into the steel matrix. Plasma immersion ion implantation (PIII) was used in this bombardment. The new Cr-rich layer allows the formation of Cr oxides and nitrides. Treated surfaces were characterized by Auger electron spectroscopy (AES) and conversion electron Mossbauer spectroscopy (CEMS), showing formation of a new Cr-rich layer. (c) 2005 Elsevier B.V. All rights reserved.

Ion nitriding is an important surface modification technique used routinely in the advanced technology industries aiming at producing high added value components with increased hardness and wear resistance. To further improve the performance of critical-part components, new methods for surface strengthening are being developed with success, like plasma immersion ion implantation (PIII) and hybrid surface treatments combining PIII and ion nitriding. In particular, a combination of high-pressure (40 Pa), high-temperature (up to 450 degreesC) glow discharge nitriding with low-pressure (8 x 10(-2) Pa) ion nitriding was implemented. Analysis of the SS304 samples treated by this hybrid method was carried out using the technique of grazing incidence X-ray diffraction (GIXRD) and the results were supported by both normal XRD and AES (Auger electron spectroscopy) measurements. Depending on the particular conditions of the treatment and the depth probed, mixed phases Of gamma(N), and gamma were measured. The improvement in hardness of the obtained surfaces (up to a factor of 2.7) was also in accordance with the results of other analysis.

Thermal or radiation enhanced diffusion of nitrogen are extensively utilized for the surface hardening of metallic components. Plasma-immersion ion implantation (PIII) is a newly developed technology, which provides ion implantation at moderate energy (10–50 keV), and thereby allowing penetration depths deeper than the surface oxide barrier. The damage caused by ion implantation together with the surface sputtering may create favorable boundary conditions for an efficient subsequent diffusive treatment such as nitriding. Surface modification of aluminum alloy 5052, Ti6Al4V alloy and steels (AISI 304 and H13) by a combination of PIII and plasma nitriding (PN) has been investigated. Nitrogen ions were implanted into specimens at 15 kV and then ion nitrided at low pressure with bias of −800 V. Compared to the untreated samples the hardness of Ti6Al4V alloy and AISI 304 steel could be improved significantly. The hardness of H13 steel can be increased by 20% using a duplex process with 4-h nitriding time. X-ray diffraction (XRD) results have shown some structural modification of the metallic samples and formation of a double-layer structure in AISI 304, treated by PIII and PN. Nitrogen depth profile of the same stainless steel sample, obtained by Auger electron spectroscopy (AES), shows two rather well-defined nitrogen enriched regions with different N contents: high (25–30 at.%) in a surface layer and medium (∼10%) in a subsurface layer.

Hybrid surface modification involving plasma immersion ion implantation (PIII) and ion nitriding has been attempted. DC cathode nitrogen glow discharges with 600–1000 V, 50–300 mA were typically obtained using the sample support as the cathode and the vacuum chamber as the anode. In this mode, sample temperatures of 500–600 °C can be reached easily while PIII at 10 kV, 50 μs and 400 Hz alone can provide about 300 °C heating. Combining these two highly conformal treatments and achieving high temperature, high efficiency three dimensional ion implantation is our objective. X-ray diffractions (XRD and high resolution XRD in the glancing mode) showed effects of ion nitriding in the AISI 304 stainless steel samples surface which indicate very thick nitrided layer, especially when operating the glow discharge in the low pressure but with high temperature nitriding mode. Auger electron spectroscopy and microhardness measurements confirmed nitriding in the sample surface. The hybrid treatment (PIII/ion nitriding) is expected to improve further these surfaces.

A dc glow discharge source with controlled plasma potential was developed for application in plasma immersion ion implantation processing of materials surfaces. This type of ion implantation system allows cost effective surface modification of workpieces with complex shapes. The effects of the nitrogen plasma etching during the plasma immersion ion implantation process was studied using Si wafers as monitors, as we varied the externally controlled plasma potential between 0 and 350 V. When the plasma potential is controlled below 70 V, the ion implantation is dominant, otherwise the etching overtakes. The nitrogen implanted silicon wafers were analyzed by high resolution x-ray diffraction and Auger electron spectroscopy which revealed successful implantation of ions with accumulated nitrogen dose of 1.5×1017 cm−2, for the low potential case.

As an effort to improve the corrosion behavior of the mild steel under work conditions, we attempted to produce a high chromium content layer on its surface by applying the recoil implantation process. After polishing, SAE 1020 construction steel samples were covered with chromium and then bombarded with ions. As recoil bombarding atom, we used nitrogen, a light mass specimen. Recoil atoms were applied either by ion beam (IB) accelerator or by plasma immersion ion implantation (PIII) method. Samples treated by the PIII process showed better results, presenting a thicker layer of high Cr content, as measured by Auger electron spectroscopy (AES). The presence of nitrogen, from hitting process, caused little effect on the surface hardness, as SAE 1020 steel is not suitable for nitriding hardening. Corrosion tests of the PIII-treated samples showed significant enhancement in the steel behaviour under chloride medium attack.

Implantation of nitrogen ions into stainless steel in combination with oxidation often results in a decrease or even complete removal of the chromium in the nitrogen containing outermost surface layer. While iron nitrides can be formed easily by this method, due to the absence of chromium, the formation of chromium nitrides is impossible and the beneficial influence of chromium in the steel for corrosion resistance cannot be used. To overcome this problem we use the following hybrid technique. A thin chromium layer is deposited on steel and subsequently implanted with nitrogen ions. Chromium can be implanted by recoil into the steel surface and thus the formation of iron/chromium nitrides should be possible. Both beam line ion implantation and plasma immersion ion implantation are used. Due to the variation of the process parameters, different implantation profiles and different compounds are produced. The produced layers are characterized by Auger electron spectroscopy, conversion electron Mossbauer spectroscopy and X-ray diffraction. The obtained results show that due to the variation of the implantation parameters, the formation of iron/chromium nitrides can be achieved and that plasma immersion ion implantation is the most suitable technique for the enrichment of chromium in the outermost surface layer of the steel when compared to the beam line implantation.

High-resolution X-ray diffraction methods have been used to characterize nitrogen-doped silicon obtained by plasma immersion ion implantation (PIII). The Si wafers were implanted with the plasma potential controlled at 70 V, and a plasma density of 1.5×10 10 cm −3 . The high voltage pulser was operated with peak voltage of 10 kV, 6 μs pulse duration and repetition frequency of 20 Hz. Auger electron spectroscopy (AES) measurements were carried out revealing successful implantation of ions with accumulated nitrogen dose of 1.5×10 17 cm −2 . The (0 0 4) Si rocking curve (ω-scan) was measured in a high resolution X-ray diffractometer equipped with a Ge(2 2 0) four crystal monochromator before and after implantation. A small distortion of the Si(0 0 4)-rocking curve was clearly observed for the as-implanted sample. This rocking curve was simulated by dynamical theory of X-ray diffraction, assuming a Gaussian strain distribution through the implanted region and using the data from the nitrogen profile obtained from the Auger measurements. With these assumptions, a good agreement between the measured and simulated rocking curves was obtained.

In the present study we use x-ray diffraction methods to characterize the surface of Si wafers irradiated with nitrogen by Plasma Immersion Ion Implantation. This is a non-line-of-sight ion implantation method, which allows three-dimensional treatment of materials including semiconductors, metals and dielectrics. The atomic concentration profiles in the implanted Si wafers were measured by Auger electron spectroscopy. The (004) Si rocking curve ( omega-scan) was measured in a high resolution x-ray diffractometer equipped with a Ge (220) four-crystal monochromator before and after implantation. A distortion of the Si (004)-rocking curve was clearly observed for the as-implanted sample. This rocking curve was successfully simulated by dynamical theory of x-ray diffraction, assuming a Gaussian strain profile through the implanted region. The analysis made by x-ray difraction and Auger electron spectroscopy revealed successful implantation of ions with ac- cumulated nitrogen dose of 1.5 x 10(17) cm-3 .The Si wafers can be used as high sensitivity monitors in the Plasma Immersion Ion Implantation process, especially at the low dose range.

A combined surface modification treatment consisting of ion nitriding at high pressure and high temperature, followed by a cycle at low pressure, both cycles using a gas mixture of (N-2/H-2):(50/50) in pressure, was applied to stainless-steel AISI 304. In the first cycle, in a glow discharge at 4x10(-1) mbar and temperatures of 400- 450 degreesC, high-pressure nitriding was applied to the samples. In the second cycle, in a glow discharge at 8x10(-4) mbar, low-pressure nitriding was applied to the samples. Applying this sequential hybrid treatment alternately, good nitriding was obtained. X-ray diffraction (XRD) measurements showed the effects of this hybrid ion nitriding in the AISI 304 surface, indicating thick nitrided layers, confirmed by Auger electron spectroscopy (AES). Conversion electron Mossbauer spectroscopy, combined with AES and XRD, showed phases and compounds induced by such treatments. Vickers hardness measurements showed great enhancement in the surface hardness. Applying other combinations of gas mixtures and cycles, produced diverse results in the surface, like the induction of alpha- and epsilon-phases.