Improvement in Nano-hardness and Corrosion
Resistance of of XC48 Samples by Plasma Immersion Ion Implantation
M. M. ALIM
a*, N. SAOULA
a, R. TADJINE
a, A. KEFFOUS
band M. KECHOUANE
ca Centre de Développement des Technologies Avancées (CDTA), Cité 20Août 1956 B.P.17 Baba Hassan, Alger, Algérie.
b Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE), 2 Bd Frantz Fanon B.P.399 Alger, Algérie.
c Université des Sciences et des Technologies Houari Boumediene, Laboratoire de Physique des Matériaux, B.P. 32, El Alia, Bab Ezzouar Alger, Algérie.
*Corresponding author: malim@cdta.dz Abstract— In this work, Nitrogen Plasma Immersion Ion
Implantation (PIII) treatment of (XC48) Carbon steel was performed in a vacuum chamber with an inductive RF source (l3.56 MHz) at low pressure in order to investigate the influence of the process conditions on the corrosion properties of this studied material. The X-rays diffraction analysis revealed the presence of several nitride phases (Fe3N and Fe4N) formed onto implanted samples. The mechanical characterization of the processed samples showed an increase of 450 % in the nano- hardness. A low corrosion rate is obtained denoting an improvement in the corrosion resistance behavior. These results are particularly interesting since they were obtained for relatively low bias voltages and processing time.
Keywords— Plasma nitriding, Low carbon steel, Nano- hardness, Corrosion resistance, Negative DC bias voltage.
I. INTRODUCTION
Nitriding is one of the most widely used surface treatment methods that allow an improvement in hardness, wear and corrosion resistance of metallic materials [1]. This nitriding can be produced by different processes such as gas nitriding, salt bath nitriding, plasma nitriding, and laser nitriding [1-5]. Due to the fact that these surface treatments (nitriding) are carried out at higher temperatures (~ 500°C) and for a long time of treatment (20-80 h) [1], the deterioration of the surface, precipitation of carbides and grain growth may be possible, which would affect the mechanical properties of the material. Therefore, an improved process condition to overcome these limitations is not only necessary but also obligatory. Plasma Immersion Ion Implantation (PIII) which was introduced by Conrad et al. in 1988 appears to be the most suitable nitriding method [6]. Nowadays, this technique is used for the treatment of a large variety of materials [6-12]. PIII, which involves both the implantation and diffusion of Nitrogen, seems to be effective in the temperature range of 250-500°C for Carbon steels [11, 13].
This process shows a high ability in improving some proprieties of these materials. In this technique, the sample is immersed in plasma and is biased negatively with a high
voltage [14-16] to accelerate positive ions towards this sample.
In this study, we are interested to highlight the effect of self- heating by the high negative DC voltage applied to the substrate. The DC voltage is known to be able to rapidly heat the samples, on the contrary to the pulsed polarization which necessitates more elevated bias voltages [16-20]. Thus, the DC polarization enables us to avoid an ulterior step of heat treatment which is generally used to diffuse the implanted ions, which means that this type of treatment requires no oven.
To avoid the breakdown and arcs problem caused by the DC bias voltage [21], we used a quartz substrate holder shielded with a braid. To generate a high-density plasma (i.e. around 1012cm-3), we used homemade inductive source plasma at 13.56 MHz [22- 24]. This work aimed at improving the hardness and corrosion resistance of XC48 Carbon steel.
II. EXPERIMENTAL DETAILS
The experimental set-up of the PIII system used for the surface treatment of the material is shown in Fig.1. It consists of a spherical chamber of 930 mm diameter which is connected to a pumping unit designed to evacuate air until a vacuum of 10-6 mbar. The pumping unit involves a primary pump which allows achieving a primary vacuum of about 10-3 mbar and a secondary oil diffusion pump in order to achieve a high vacuum of the order 10-6 mbar. To create the plasma, we used an inductive excitation system. The inductive excitation consists of a Pyrex tube surrounded by a Copper coil polarized by an RF generator (13.56 MHz). A glass substrate holder with a length of 1500 mm and 13 mm of a diameter has been used. In order to bias our samples, we used a high negative voltage power supply (HCN 2800-6500) to reach values of the order 400 mA/6.5kVolt. XC48 Carbon steel samples of disc form (dimensions: 25 mm Ø and 8 mm thick; composition (in wt. %) C: 0.38; Si: 0.025; Mn: 0.66; P: 0.02; S: 0.0.16; Cr:
0.025; Mo: 0.02; Ni: 0.02) were used as substrate materials.
The samples were mechanically polished to a surface finish using Silicon carbide (SiC) emery papers starting from 200 to 2400 grade, followed by an ultrasonic cleaning in acetone and
alcohol bath for 10 min prior to treatment by PIII. The system was firstly evacuated to a low pressure of 8x10-6 mbar, and the operating pressure, from 3x10-2 to 12x10-2 mbar, was achieved by introducing Nitrogen into the system. The measurements of current density have been made by a planar probe of 10 mm diameter connected by a choke inductance to minimize disturbance caused by the action of radio frequency [25]. The probe is powered by a variable voltage generator from 0 to - 150 Volts, and measurements are collected by Voltmeter.
Before the start of ion implantation, the oxide layer surface was removed by sputtering with Argon gas for 20 min with a power 200 Watt. The substrate was negatively biased to a voltage ranging from 2.5 kVolt to 4 kVolt and the current was roughly around 10 mA. The implantation parameters are summarized in Table 1. Substrate temperature was measured by means of a K-type thermocouple.
The X-rays diffraction patterns were recorded from polished surface using BRUKERS D8 ADVANCE X-ray diffractometer operating at 40 kVolt, 40 mA with Cu Kα radiation. All the diffraction patterns were obtained by varying 2θ from 25° to 88° with a scan step of 0.05. The time spent for collecting the data per step was 1s. The nano-hardness was evaluated using a nano-indenter (CSM) system with a Berkovich-diamond indenter and a load of 350 mN for 10 s.
The corrosion resistance of untreated and nitrided samples was evaluated in 3.5% NaCl solution at 25°C by potentiodynamic polarization conducted on the PARSTAT 4000 electrochemical working station. The potentiodynamic experiments were conducted through a three-electrode cell: A saturated calomel electrode SCE was used as a reference to measure the potential across the electrochemical interface and Carbon sheet was used as a counter electrode. The sweep rate setting was 1mV.s-1. Data were automatically collected and analyzed with VersaStudio software. By using the Tafel extrapolation method, the corrosion potential (Ecorr) and corrosion current density (Icorr) were derived from the polarization curves.
A. X-Rays Diffraction Analysis and Auger Electron Spectroscopy
The X-ray diffraction (XRD) patterns associated to the untreated XC48 steel and Nitrogen-implanted samples for experimental conditions are given in Fig.2. The XRD patterns of the untreated sample (Fig.2 (a)) shows only three peaks related to the crystallographic planes (110), (200) and (211) of α-Fe phase denoted as ferrite, located at 44.67°, 65.02° and 82.33°, respectively. The experimental conditions of the Nitrogen-implantation are as follows: 4kVolt, 240Watt, 8 10-2 mbar and 120 min. The XRD pattern of the treated sample (Fig.2 (b)) shows a diminution in the intensity of the peak associated to the ferrite phase. Simultaneously to this, new phases (Fe3N (i.e. ε-Fe2N1−x) and γ-Fe4N) occurred with a strong relative intensity. These results are in good agreement with the binary phase diagram of Fe-N proposed by Perrot [28] which gives the presence of the iron nitride Fe3N and Fe4N at the temperature of 400°C. These phases were also
observed in α-Fe PIII implanted at 5 to 20 kVolt with respect to low temperatures range of 366 to 591°C [26, 30].
Fig.1. Schematic description of the experimental setup.
III. RESULTS AND DISCUSSIONS
B. Nano-hardness
The mechanical properties such as nano-hardness of the treated XC48 were measured and the influence of the implantation conditions on the nano-hardness are presented in Fig.3, 4 and 5. We note that all treated samples exhibit an evident increase in the nano-hardness compared to the untreated samples (roughly around 150 HV). Fig. 3 gives the influence of the power on the nano-hardness for 4 kVolt bias voltage, 8x10-2 mbar pressure and 120 min of treatment time.
The nano-hardness increases from 500 HV (at 160 Watt) to 700 HV (at 400 Watt). This trend seems obvious because it is due to implantation-induced defects. Also, an increase in the power has a direct effect on the plasma characteristics. Indeed, the plasma density increases with the injected power into the plasma and this is due to the increase of the electron energy.
Elastically anisotropic phases (such as γ-Fe4N) revealed in the XRD diffractogram (in the orientations (111) and (020) as shown in the figure 4) appear as a consequence of the implantation of reactive species into the substrate. Gressman et al. [31] and Yan et al. [32] used ab initio calculations to confirm the elastic anisotropy of this nitride phase. The γ- Fe4N phase that forms is believed to be the major source of hardening as indicated in the literature [33].
Fig.2. XRD patterns of XC48 steel, (a) untreated sample, (b) Nitrogen-implanted sample for experimental conditions
4kVolt, 240Watt, 8 10-2 mbar and 120min.
Fig.4 shows the nano-hardness as a function of pressure for 4 kVolt bias voltage, 240 Watt and 120 min of treatment time.
We noted that the nano-hardness increases with pressure, and this trend is due to the increase of the ionic density induced by the rise of ionizing collisions between particles in the plasma achieved by the decrease of the free mean path. Those species will be accelerated and implanted into the substrate to form hard phases such as γ-Fe4N. Consequently, the surface of the nitrided sample becomes harder. Furthermore, the nano- hardness depends strongly on the applied negative bias voltage. Fig.5. shows this evolution of nano-hardness versus negative bias voltage, for an injected power of 240 Watt, a pressure of 8 10-2 mbar and a treatment time of 120 min. For lower applied polarization (<3.5kVolt), the nano-hardness linearly increases with bias voltage. Beyond 3.5 kVolt, the slope of this increase diminishes, and the nano-hardness seems to stabilize at a plateau with a value around 610 HV (at 4kVolt). This behavior can be explained by the acceleration of positive ions created in the plasma source with increasing bias voltage; consequently, these ions will be implanted with a
high kinetic energy. Indeed, the concentration of Nitrogen increases at the surface and in depth, this fact reinforces the probability of forming the γ-Fe4N hard phase. Also, the increase in the bias voltage results in increasing the sample temperature (see section 3.1), and this promotes the formation of the γ-Fe4N hard phase and diffusion of Nitrogen.
Fig.3. Nano-hardness of the implanted sample XC48 versus the injected power.
C. POTENTIODYNAMIC POLARIZATION CHARACTERIZATION Fig.6. shows potentiodynamic curves recorded after immersion in a saline solution of 3.5 % NaCl at room temperature (25C°), for untreated and Nitrogen-implanted XC48 surfaces treated with different negative bias voltage (3, 3.5, and 4 kVolt) under the following operating conditions:
240Watt, 8 10-2 mbar and 120 min. The corrosion potential (Ecorr) and the corrosion current (Icorr) are listed in table 2.
From the curves (Fig.6), it is clear that the displacement of treated samples curves to lower values of the corrosion current and higher values of polarization voltage is the main indicator of the enhancement in corrosion performance of the treated samples.
Fig.4. Nano-hardness of the implanted sample XC48 versus the pressure.
Fig.5. Nano-hardness of the implanted sample XC48 versus the negative bias voltage.
The corrosion current density (Icorr) was significantly decreased after implantation compared to that of the untreated sample, which indicates that the implanted case impeded the anodic dissolution resulted in a very low corrosion current.
The corrosion potential (Ecorr) was shifted to higher values, e.g. from -507mV for the untreated sample to around of - 90mV for the implanted samples at 4 kVolt. However, the current densities decreased from the value corresponding to the untreated sample to those of the implanted samples. The corrosion current densities (Icorr) of all implanted samples were decreased in the range between 132.161 and 10.104 mA.cm-2 respectively for 3 kVolt and 4 kVolt of negative bias voltage. All of these values are below the corresponding values of untreated sample (319.216 mA.cm-2) which characterizes a significant increase in corrosion resistance as a result of nitriding implantation treatment. In other hand, it’s well established that the production of modified layers with the best corrosion performance is related to the preferred formation of Fe3N crystalline phase preference of nitrided layers [34, 35] which was confirmed by the XRD analysis.
Table 1: Electrochemical parameters deduced from the Tafel method, for the XC48 samples.
Sample Ecorr
(mVolt)
Icorr (µA. cm-2) Untreated sample -507.533 319.216 Implanted, 3 kVolt -589.983 132.161 Implanted, 3.5 kVolt -197.639 66.459 Implanted, 4 kVolt -90.088 10.104
IV. CONCLUSION
In this study, PIII Nitriding of XC48 steel was investigated under different operating conditions (pressure, power and negative bias voltage). Different techniques of characterization were used: X-ray diffraction, nano-hardness and the potentiodynamic polarization tests. The hardness increased up to around 450% for the Nitrogen-implanted samples compared
to the untreated one (150HV). The highest surface hardness value (709 HV) was obtained when PIII Nitriding process was carried out at 400°C using at 4 kVolt, 400 Watt, 120 min and 8 10-2 mbar. The XRD analysis revealed the presence of nitride Fe3N and Fe4N phases which were formed into implanted samples using our inductive plasma reactor. Corrosion resistance of XC48 Carbon steel was improved by plasma immersion ion implantation of Nitrogen.
Fig.6. Potentiodynamic polarization curves of untreated and treated XC48 samples, with different negative bias voltage (3, 3.5, and 4 kVolt), in 3.5 wt.% NaCl at room temperature (25°C).
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