2020 Volume 61 Issue 6 Pages 1115-1121
Low carbon steel sample S15C was nitrided by active screen plasma nitriding (ASPN) and direct current plasma nitriding with screen (S-DCPN) using a chromium screen to form simultaneously chromium nitride coating/nitrogen-diffusion layer on the sample surface. The sample was placed on the sample stage in a floating potential and a cathodic potential. A chromium screen was mounted on the cathodic stage parallel to the top of the sample. Plasma nitriding treatments in a floating potential (ASPN) and a cathodic potential (S-DCPN) were performed in a nitrogen–hydrogen atmosphere with 75% N2–25% H2 for 120 min at 773 K and 873 K under 200 Pa. After nitriding, the nitrided microstructure was examined with a scanning electron microscope, glow discharge optical emission spectroscopy and X-ray diffraction studies. In addition, the hardness and corrosion properties were also measured. The nitrided layer formed by S-DCPN consisted of a Cr-concentrated layer (∼750 HV) followed by a nitrogen-diffusion layer whereas that formed by ASPN consisted of deposited layer of chromium nitrides and iron nitrides without nitrogen-diffusion layer. Corrosion resistance of the samples treated by S-DCPN was better than that of untreated sample and sample treated by ASPN.
This Paper was Originally Published in Japanese in J. Jpn. Soc. Heat Treatment 59 (2019) 344–349.
For duplex hardening processes for steel materials, gas nitriding or plasma nitriding is often used as a method of hardening beneath the hard coatings, because: (1) the difference in hardness between the hard nitride film and the underlying matrix can be reduced, and the adhesion strength and durability can be improved; (2) it is easy to keep the quenching hardness of the products; and (3) there is no need for re-quenching, and it is advantageous for suppressing the amount of strain generated by heating and cooling.1–3) In the plasma nitriding method, the processing sample is heated by the collision energy of ions. Therefore, there is no external heating device required, and since active nitrogen atoms and ions are used, energy consumption is low, and the processing time is short. It is a very economical process. Furthermore, since a mixed gas of nitrogen and hydrogen is used, there is also an effect of cleaning the sample surface with hydrogen ions, and there is an advantage that there is little environmental load because no harmful substances such as cyanide that is used in salt bath nitriding are used.4–10) As described above, the plasma nitriding process is gaining superiority in the present background, in which environmental harmony in all processing steps, high-quality processing, and low cost are strictly required. The plasma nitriding method, therefore, is widely used in the industrial field as an excellent surface modification method, and its application is also expanded wide-ranging. Since the passive film is removed by the collision of ions in the plasma in plasma nitriding to stainless steel, there is a great advantage in that the pretreatment for removing the passive film, which was conventionally necessary, is unnecessary.9–15)
For the conventional direct current plasma nitriding (DCPN) method, applying a voltage with the sample as the cathode, however, causes a glow discharge on the sample surface to form plasma. This results in edge effects, arcing, and hollow cathode discharge, leading to defects in the processing sample.14–17) One of the methods for solving these problems is an active screen plasma nitriding (ASPN) method. In this method, a processing sample is insulated, a metal screen is provided around the processing sample, and a voltage is applied thereto so that plasma is formed on the screen surface instead of the processing sample surface. In addition to nitrogen molecules, atoms, ions, and electrons, the plasma formed on the screen contains a constituent material of the screen and its nitride. The nitride reaches the sample surface by the gas flow in the furnace, and nitriding is performed by diffusing the nitrogen generated by the decomposition of the nitride into the sample. In this method, since the sample is insulated, defects due to the edge effect, arc discharge, and other defects are considered to be eliminated.14,15,17–22)
ASPN treatment should form a surface-modified layer with higher functionality and quality by the duplex hardening process combined with vapor deposition.23–26) This duplex hardening process, however, is, in general, a two-stage process, in which the formation of the nitrogen diffusion layer and the formation of the hard coating film are performed in different apparatuses. Because the problem of increased apparatus cost and heating and cooling, must be performed in two steps, there is a problem that the total processing time becomes long and the energy consumption increases. To solve these problems, the author’s group has been working on a duplex hardening process using TiN and a nitrogen diffusion layer using the ASPN process.24) When ASPN treatment is performed using a titanium screen, the titanium on the screen is released into the plasma in the furnace by sputtering, and it is possible to form TiN in the plasma and coat the sample surface with TiN.24,27–29)
This ASPN treatment, however, still has many problems for industrial use. The indirect nitrogen supply from the screen, low nitride layer formation rate, non-uniformity of the nitriding effect due to the distance from the screen, and the formation of deposited layers from screens have been cited as some of the problems.30–32) In recent years, the author’s group, therefore, investigated the effects of installing a metal screen as an auxiliary cathode during DCPN processing.33) Consequently, for the DCPN process, a metal screen (S-DCPN process) was installed as an auxiliary cathode, the formation of a nitrided layer was significantly increased compared to the conventional DCPN process and ASPN processes.33) The reason for this is that plasma is formed in the two areas of the screen and the sample surface, so the amount of nitride and NH radicals that form the nitrogen supply source is increased and plasma is formed on the sample surface. Then, there is the removal of the surface-deposited layer, which inhibits nitrogen diffusion through sputtering. The screen as an auxiliary cathode also played a role as a heater by radiant heat, improving of the edge effect.33) Furthermore, several plasma nitriding treatments using different material screens for various metal materials were performed,33–37) but there are no reports of research using Cr for the screen.
This study, therefore, investigated a method for simultaneously forming a CrN film and a nitrogen diffusion layer using a Cr screen. In this research, ASPN and S-DCPN processes were applied to low carbon steel S15C using pure Cr as the top plate of the screen material and the phase identification of the formed material on the surface and the film formation rate measurement were performed.
As an experimental sample, low carbon steel S15C was used. S15C was cut from a rod material to a shape of ϕ25 mm × 5 mm. The surface was polished with water-resistant abrasive paper, finished to a mirror surface by buffing it with alumina powder with a particle size of 1.0 µm, and ultrasonically cleaned with acetone. Further, to analyze the deposited layer during ASPN, silicon substrate (Si) was also subjected to treatment. The Si single crystal (crystal axis ⟨100⟩) of 0.5 mm a wafer thickness was cut into the shape of 20 mm × 20 mm.
2.2 Experimental methodFor nitriding treatment, plasma nitriding furnace (NDK Inc., model JIN-1S) was used. For the screen in the apparatus, a pure Cr disk of ϕ100 mm × 1 mm was used as a top plate, and a SUS316L steel bar was used as a base to support the top plate. The voltage applied to the sample was performed under two conditions. (1) ASPN process was performed under the condition that voltage was applied only to the Cr screen using alumina (Al2O3) ceramics as the base of the insulator under the sample. (2) S-DCPN process was performed under the condition for applying a voltage to both the Cr screen and the sample. The sample was set so that the distance between the screen top and the sample was 10 mm. Figure 1 shows a schematic diagram of ASPN and S-DCPN process. The reaction gas has a gas composition of 75% N2–25% H2, and the mixed gas was introduced into the apparatus and evacuated to 10 Pa or less. The gas pressure was adjusted to a constant 200 Pa. The nitriding conditions have a processing temperature of 773 K and 873 K and processing time of 120 min.
Schematic illustration of plasma nitriding setups: ASPN and S-DCPN.
To identify the surface nitrided layer and the deposited layer formed on the sample surface, X-ray diffraction (XRD) measurement was performed using XRD apparatus (RIGAKU, RINT-2200). Cu-Kα rays (wavelength λ = 0.15405 nm) were used as an X-ray source, and measurement was performed at a tube voltage of 40 kV and tube current of 300 mA. To investigate the cross-sectional microstructure as per the processing conditions, an optical microscope (Keyence, VHX-200) was used to observe the cross-sectional microstructure. Further, to examine morphology due to the process conditions, a field emission scanning electron microscope (FE-SEM) (JEOL, JSM-6330FII) was subjected to a surface morphology observation. To compare the cross-sectional hardness of the samples, a micro Vickers hardness tester (Matsuzawa, PMT-X7A) was used for cross-sectional hardness measurement. Furthermore, a glow discharge optical emission spectroscopy (GD-OES) analysis was performed to investigate the elemental distribution in the depth direction of N, Fe, Cr, and O elements on the surface. A Marcus-type radio frequency glow discharge optical emission surface analyzer (Horiba, GD-Profiler2) was used for the analysis. Furthermore, the corrosion potential and anodic polarization curve were measured to evaluate corrosion resistance. The sample was spot-welded with a lead wire made of SUS304 steel, and the surface of the sample was covered with Teflon tape with a hole of ϕ6 mm. The liquid contact area was limited, and the central part was wetted. For the aqueous solution, 3.5 mass% NaCl was used, Ag/AgCl for the reference electrode, and Pt for the counter electrode. Nitrogen gas was introduced for the measurement to remove the dissolved oxygen in the solution. After degassing for 30 min, the samples were immersed, measured by a potentiostat (Hokuto Denko, HA-501G) controlled voltage from −1.0 V to +1.5 V using a data logger (Graphtec, GL200A-UM801).
Figures 2 and 3 show the XRD results of the sample after plasma nitriding. XRD results of S15C for the ASPN process shown in Fig. 2 indicated that the diffraction intensity of chromium nitride and iron nitride together increased by increasing the processing temperature. For the S-DCPN process, the diffraction intensity of the base material (α-Fe) decreased with an increase in the processing temperature, and the diffraction intensity of iron nitride (γ′-Fe4N) increased. On the other hand, because of XRD on the ASPN-treated Si substrate shown in Fig. 3, only the diffraction lines of the substrate were detected at 773 K, but the diffraction lines of chromium nitride and iron nitride were detected at 873 K. This indicates that a deposited layer was formed by the deposition of nitrogen compounds and constituent elements of the screen and cathode stage during ASPN.33)
X-ray diffraction pattern of the S15C sample treated by ASPN and S-DCPN.
X-ray diffraction pattern of the Si wafer sample treated by ASPN.
From the results of the cross-sectional microstructure observation with an optical microscope shown in Fig. 4, no nitrogen diffusion layer was observed with the ASPN process. This is because chromium nitride which has a strong bond with nitrogen was deposited on the sample surface and was hard to decompose in the deposited layer, and nitrogen was difficult to diffuse into the sample core. For the S-DCPN process, when the processing temperature increased, the thickness of the white compound layer on the top surface and the nitrogen diffusion zone increased. Furthermore, the needle-like structure of the nitrogen diffusion layer is because of the formation of iron nitrides (γ′-Fe4N and α′′-Fe16N2).38)
Cross-sectional micrograph of the S15C sample treated by ASPN and S-DCPN.
From the results of the cross-sectional hardness shown in Fig. 5, as the processing temperature increased, the surface hardness improved with the ASPN process and S-DCPN process. The surface hardness of ASPN was 200 HV or more and 700 HV or more for S-DCPN. For the S-DCPN process, the hardened zone increased as the processing temperature increased. It was also confirmed that the surface hardness of the S-DCPN process was similar to the hardness value (700 to 800 HV)39) as when applying nitriding to low carbon steel.
Cross-sectional hardness of the S15C sample treated by ASPN and S-DCPN.
Figures 6 to 8 show the elemental concentration distribution in the cross-section of the plasma-nitrided sample by GD-OES. The distribution of the nitrogen diffusion zone of S15C for the S-DCPN shown in Fig. 6 increased as the processing temperature increased whereas there was a little nitrogen diffusion zone for the ASPN process. For the ASPN process, the formation of the nitrided layer is inhibited by the deposited layer formed on the top surface of the sample.33) The depth profile from the top surface within 1.0 µm shown in Fig. 7 shows that the thickness of deposited layer for ASPN was 0.4 µm or less, and 50 nm or less for S-DCPN process. Moreover, Cr concentration was observed on the top surface under any condition. As the processing temperature increased, the N element increased for the ASPN process, and an increase in the Cr concentration at the top surface was observed in the S-DCPN process. Meanwhile, to do the elemental analysis of the deposited layer, the nitrogen distribution in the Si substrate for ASPN shown in Fig. 8 showed the same tendency as the process for S15C treated by ASPN. This is because the processing voltage is increased by setting the processing temperature high. Table 1 shows measurement data such as the processing voltage and current at a holding time of approximately 20 min for each processing condition. From Table 1, it was observed that the processing voltage increased by about 170 V when the processing temperature was set at 100 K higher for the ASPN. The sputtering rate, therefore, increases at 873 K, and deposits made of an SUS316L base that supports the Cr top plate are deposited on the sample surface.
GD-OES analysis of the surface region of the S15C sample treated by ASPN and S-DCPN.
GD-OES analysis of the region of top surface within 1.0 µm of the S15C sample treated by ASPN and S-DCPN.
GD-OES analysis of the region of top surface within 1.0 µm of the Si wafer sample treated by ASPN.
Figure 9 shows the surface morphology observation by FE-SEM. Large diameter particles of surface deposits and polygonal shapes were observed for the ASPN. On the other hand, for the S-DCPN process, fine deposits increased as the processing temperature increased. For the S-DCPN process, plasma is also formed on the sample surface, so the deposits on the sample surface are sputtered, the deposit becomes finer, and an extremely thin film is formed.33) Further, for the S-DCPN process, the amount of the deposits nitrided at 873 K was more than that nitrided at 773 K because the amount of deposits was more than the amount to be sputtered at a higher nitriding temperature.
FE-SEM image of the surface morphology of the S15C sample treated by ASPN and S-DCPN.
From the corrosion test results shown in Fig. 10, it is considered that the corrosion resistance decreases for the ASPN process because the current rises more rapidly than in the untreated sample. There are two probable reasons for the decrease in corrosion resistance. First, as shown in the schematic diagram in Fig. 11, many polygonal deposits were deposited as the processing temperature increased, the deposited layer became thicker, and the solution was concentrated in the deposits gap.40) Second, as shown in Fig. 6, no nitrogen diffusion layer was formed by the ASPN process. As a result, for the ASPN process, while chromium nitrides containing iron nitride were formed on the top surface of the sample, chloride ions (Cl−) and protons (H+) were concentrated in the gap in the deposited layer. The progress of corrosion was then accelerated because of a decrease in pH. Consequently, the corrosion resistance of the sample treated by the ASPN was more deteriorated than that of the untreated sample. Meanwhile, for the S-DCPN process, the rise of current was more gradual than that of the untreated sample, and it was considered that the corrosion resistance was improved because a passive state was formed. For the S-DCPN process at 873 K, two passivation zones of −0.4 V∼0 V and 0 V∼0.7 V were confirmed. From the GD-OES results shown in Fig. 7, because nitrides containing Cr were formed on the top surface, current density of the passivation zone decreased compared to the process at 773 K.41)
Polarization curve in the 3.5 mass% NaCl solution of the S15C sample treated by ASPN and S-DCPN.
Mechanism of corrosion in the NaCl solution of the untreated and ASPN-treated steel.
From the above results, the S-DCPN process using a Cr top plate as a screen material demonstrated that functionality such as hardness and corrosion resistance improved by forming a nitride layer containing Cr on the surface. In the future, to form a film with a higher Cr concentration, it is necessary to consider using a Cr plate for the side of the screen to prevent Fe from depositing on the surface during the ASPN process.
For the first time, plasma nitriding using a Cr top screen was attempted, and ASPN process and S-DCPN process were applied to carbon steel S15C to simultaneously form a chromium nitride film and a nitrogen diffusion layer on the sample surface. The results were obtained as follows:
