2022 Volume 63 Issue 6 Pages 939-947
Active screen plasma nitriding (ASPN) is a nitriding method that avoids the edge effects and arcing that occur during conventional direct current plasma nitriding (DCPN). Furthermore, applying voltage to a sample during ASPN allows the nitriding rate to be increased (S-DCPN). While steel is the predominant screen material, there are few reports of non-ferrous material screens. Therefore, in this study, we investigated the effect of a Ni screen on ASPN and S-DCPN. Low carbon steel S15C was treated by ASPN and S-DCPN using a Ni screen. A steel plate cold commercial (SPCC) screen was also prepared for comparison. Plasma nitriding was performed at 773 K for 240 min under an atmosphere of 75% N2 + 25% H2 at a gas pressure of 200 Pa. After the nitriding treatment, X-ray diffraction (XRD), glow discharge optical emission spectrometry (GD-OES), cross-sectional microstructure observation, surface microstructure observation, Vickers hardness test, and corrosion test were performed. As a result, when the Ni screen was used for S15C steel nitriding, more nitrogen atoms diffused into the sample than that when the SPCC screen was used; furthermore, nickel atoms diffused within the samples treated by both ASPN and S-DCPN using the Ni screen. ASPN-treated samples had less surface hardness and were higher corrosion resistance when prepared using the Ni screen than the SPCC screen. S-DCPN-treated samples had greater surface hardness and were less corrosion resistance when prepared using the Ni screen than the SPCC screen.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 430–438.
Fig. 7 GD-OES nitrogen profiles of S15C samples treated by (a) ASPN and (b) S-DCPN using Ni and SPCC screen.
In direct current plasma nitriding (DCPN), a conventional plasma nitriding method, an activated N species is generated when a cathodic potential is applied to a sample surface to form plasma. Because a high cathodic potential is applied, sample melting and non-uniform nitriding treatment of the sample occur due to abnormal discharge and the edge effect.1–3) One of the treatment methods used to negate these effects is active screen plasma nitriding (ASPN), also called cathodic cage plasma nitriding (CCPN).2–15) In ASPN, a sample is insulated and a cathodic potential is applied to the surrounding metal screen to form plasma on the screen surface. Consequently, sample melting and the edge effect do not occur in ASPN. In this treatment, metal nitrides are formed by the bonding of metal atoms derived from the screen by sputtering and activated N species in the plasma. These metal nitrides are deposited on the sample surface by gas flow in the furnace, and N atoms diffuse into the sample by decomposition. In ASPN, a metal nitride layer (deposits layer) formed by the deposition of metal nitrides on the sample surface functions as a barrier to N diffusion; this results in a lower rate of nitriding than that in DCPN.16) Recently, the author’s group reported a plasma nitriding treatment in which a voltage was applied to the sample and screen (S-DCPN).16) In S-DCPN, the screen heats the sample, which suppresses the edge effect that occurs in DCPN. Further, sputtering on the sample surface enables an increase in the nitriding rate by removal of the deposits layer.
In both ASPN and S-DCPN, the predominant screen material is steel. However, there are a few reports on the use of non-ferrous material screens. The author’s group reported plasma nitriding treatment using Cr and Ti screens, and found that in ASPN, chromium and titanium nitrides from the screens deposited on the sample surface were difficult to decompose. This was due to the high affinity between these metal atoms and N atoms, which led to the negligible diffusion of N into the sample. In addition, previous studies reported that the constituent atoms of the screen diffused into the sample in ASPN and S-DCPN.17–20) A Ni screen is another non-ferrous material screen, with which the diffusion of Ni atoms into the sample is expected to improve the corrosion resistance and solid-solution strengthening. The diffusion of N into the sample is also expected, since Ni nitrides from the Ni screen are possibly easily decomposed on the sample surface. Ni nitrides are unstable, unlike chromium nitrides and titanium nitrides. However, there are few reports about ASPN using the Ni screens. Kovács et al. used a Ni electroplating screen and found that a nitrided layer did not form, although a Ni deposits layer from the screen was formed on the sample surface.21,22) In contrast, according to a report by Morell-Pacheco et al., a nitrided layer was formed with a Ni deposits layer on a sample surface.23) As described above, conflicting results for the formation of a nitrided layer were obtained previously, and there are many unclear variables regarding the use of a Ni screen in ASPN. In addition, there have been no reports of S-DCPN using a Ni screen.
In this study, the author’s group investigated the effect of a Ni screen on the formation of a nitrided layer, the behavior of Ni atoms from the Ni screen on the surface of and inside a sample, and the effect of Ni atoms on the hardness and corrosion resistance of the samples. To achieve these aims, low-carbon steel S15C was treated by ASPN and S-DCPN using a Ni screen and a steel plate cold commercial (SPCC) screen as the steel material, and the results of each treatment were compared.
To investigate the effects of a Ni screen, low-carbon steel S15C, a steel with a low amount of alloying elements, was used. The chemical compositions are listed in Table 1. A sample was cut from the S15C rod material to discs of dimensions ϕ25 mm × 5 mm, following which the top surface was wet-ground from #220 to #2000, and finally buffed to a mirror surface using alumina powder with a grain size of 1 µm.
A DC plasma nitriding furnace (NDK Inc., model: JIN-1S) was used for plasma nitriding. The screens were ϕ100 mm in diameter and 56 mm in height, were made of a Ni mesh (ϕ0.15 mm wire, 50 mesh, and 49.7% open area ratio) and an expanded SPCC (LW = 4.0 mm, SW = 8.0 mm, T = 0.5 mm, W = 0.8 mm, 66.3% open area ratio). In the ASPN experiments, an Al2O3 crucible was placed under the sample to insulate it, and in S-DCPN, a S15C rod of the same height as the Al2O3 crucible was placed under the sample. Figure 1 shows a schematic of the setup of the sample and the screen for the ASPN and S-DCPN experiments. The distance between the top of the samples and the top lid of the screens was 20 mm for ASPN and S-DCPN. Plasma nitriding was performed at 773 K for 240 min under a reaction gas mixture of 75% N2 + 25% H2 at a gas pressure of 200 Pa after the furnace was evacuated to below 10 Pa.
Schematic illustration of plasma nitriding setups: ASPN and S-DCPN.
To characterize the plasma-nitrided sample surface, X-ray diffraction (XRD) measurements were performed using an X-ray diffractometer (RIGAKU, model: RINT-2200). Cu-Kα radiation (wavelength λ = 0.15405 nm) was used as the X-ray source, and XRD measurements were acquired at a tube voltage of 40 kV and tube current of 300 mA. For elemental analysis from the surface to the depth direction, glow discharge optical emission spectroscopy (GD-OES) analysis was performed using a Marcus-type radio frequency GD optical emission surface analyzer (Horiba, model: GD-Profiler2). The cross-sectional microstructure was observed by scanning electron microscopy (SEM; JEOL, model: JSM-6060LV). The surface microstructure was observed by field-emission scanning electron microscopy (FE-SEM) (JEOL, model: JSM-6330FII). The hardness of the samples was measured using a micro Vickers hardness tester (Matsuzawa, model: PMT-X7A). To determine the corrosion resistance to salt water, corrosion tests were performed using a potentiostat (Hokuto Denko, model: HA-501G) and a data logger (Graphtec, model: GL200A-UM801) under the conditions shown in Table 2.
The S15C samples treated with ASPN and S-DCPN using a Ni screen are abbreviated as ASPN (Ni) and S-DCPN (Ni), respectively, and an SPCC screen are abbreviated as ASPN (SPCC) and S-DCPN (SPCC), respectively.
Figure 2 shows the XRD results of γ′-Fe4N and ε-Fe2–3N obtained for all the nitrided samples. The diffractions of Ni were detected in ASPN (Ni), which were concluded to be due to Ni deposits from the Ni screen; this result was comparable to the report using a Ni electroplated screen by Kovács et al.21) Figure 3 shows the XRD patterns of the S-DCPN-treated samples, restricted to a diffraction angle range of 42°–50°. For S-DCPN (Ni), the diffractions of α-Fe, γ′-Fe4N, and ε-Fe2–3N were shifted to a greater angle compared to those of S-DCPN (SPCC). For ASPN (Ni), such a shift in diffraction toward a greater angle was not observed.
X-ray diffraction patterns of S15C samples treated by ASPN and S-DCPN using Ni and SPCC screen.
X-ray diffraction patterns of S15C samples treated by S-DCPN using Ni and SPCC screen.
The results of the GD-OES measurement, i.e., the Ni, N, and Fe elemental profiles of ASPN (Ni), ASPN (SPCC), S-DCPN (Ni), and S-DCPN (SPCC), are shown in Fig. 4(a), Fig. 4(b), Fig. 4(c), Fig. 4(d), Fig. 5(a), Fig. 5(b), Fig. 5(c), and Fig. 5(d), respectively ((b) and (d) are figures in which the horizontal axes in (a) and (c) are restricted from 0 µm to 2.0 µm, respectively). Figures with low concentration ranges (0–10 at%) on the vertical axes were used to confirm the diffusion depths of Ni in ASPN (Ni) and S-DCPN (Ni) (Fig. 6(a) and Fig. 6(b), respectively). For ASPN (Ni), a Ni-rich deposits layer with a thickness of approximately 0.6 µm was observed (Fig. 4(a) and Fig. 4(b)). The boundary between the deposits layer and a nitrided layer was determined by a change in the concentration gradient of the N profile.16) The concentration gradients of not only N, but also Ni and Fe changed at a depth of approximately 0.6 µm from the sample surface. The Ni concentration decreased from approximately 90 at% to 20 at% toward the boundary, while the Fe and N concentrations increased from 0 at% to approximately 60 at% and 20 at% toward the boundary, respectively. The diffusion of N into the sample was observed, which, together with the detection of Fe nitride diffractions (XRD results in Fig. 2), indicate the formation of a nitrided layer. These results confirm the formation of a nitrided layer in ASPN using the Ni screen, and are different from reports by Kovács et al., which could not confirm a nitrided layer in ASPN using a Ni-electroplated screen.21,22) Figure 4(a), Fig. 4(b), and Fig. 6(a) show the diffusion of Ni (0.6–2.2 µm) from the deposits layer into the sample. These results indicate that Ni diffused from the deposits layer to the interior of the sample, whereas Fe and N diffused from the nitrided layer to the deposits layer. For ASPN (SPCC) shown in Fig. 4(c) and Fig. 4(d), a deposits layer with a thickness of approximately 0.8 µm was observed.16) The Fe and N concentrations in the deposits layer were almost constant, indicating the formation of an Fe nitride deposits layer. The diffusion of N into the sample was also observed.
GD-OES profiles of S15C samples treated by ASPN using (a) and (b) Ni screen and (c) and (d) SPCC screen.
GD-OES profiles of S15C samples treated by S-DCPN using (a) and (b) Ni screen and (c) and (d) SPCC screen.
GD-OES nickel profiles of S15C samples treated by (a) ASPN (b) S-DCPN using Ni screen.
For S-DCPN (Ni) shown in Fig. 5(a) and Fig. 5(b), a deposits layer with a thickness of approximately 0.1 µm was observed,16) and the Ni concentration decreased from approximately 35 at% to 25 at% toward the boundary of the deposits layer/nitrided layer. In contrast, the Fe and N concentrations increased from approximately 45 at% to 50 at% and from approximately 10 at% to 25 at% toward the boundary, respectively. This thin deposits layer and high Fe concentration in the deposits layer, unlike those of ASPN (Ni) shown in Fig. 4(a) and Fig. 4(b), are attributed to the removal of the deposits layer due to sputtering on the sample surface and formation of Fe nitride deposits from the S15C sample surface. Figure 5(a), Fig. 5(b), and Fig. 6(b) show the diffusion of N and Ni (0.1–3.8 µm) into the sample. The amount and diffusion depth of both elements were larger than those of ASPN (Ni), as shown in Fig. 4(a), Fig. 4(b), and Fig. 6(a). For S-DCPN (SPCC) shown in Fig. 5(c) and Fig. 5(d), a deposits layer with a thickness of approximately 0.1 µm was observed,16) and the amount of N diffusion into the sample was greater than that of ASPN (SPCC), as shown in Fig. 4(c). These results were attributed to the removal of the deposits layer by sputtering on the sample surface for S-DCPN, which increased the nitriding rate.16)
Figure 7 compares the N diffusion and depth between the SPCC and Ni screens for ASPN (Fig. 7(a)) and S-DCPN (Fig. 7(b)), considering only the N profiles shown in Fig. 4 and Fig. 5. Figure 7(a) shows that the N concentration for ASPN (SPCC) is higher than that of ASPN (Ni) at the top surface of the sample (approximately 0.7 µm), but after a depth of approximately 2 µm from the sample surface, that of ASPN (Ni) is higher than that of ASPN (SPCC), and both N concentrations become almost equal at approximately 10 µm. For S-DCPN (Fig. 7(b)), the N concentration of S-DCPN (SPCC) is higher than that of S-DCPN (Ni) at the top surface of the sample; however, after a depth of approximately 1 µm from the sample surface, that of S-DCPN (Ni) is higher than that of S-DCPN (SPCC). Furthermore, higher values (S-DCPN (SPCC): approximately 2 at%, S-DCPN (Ni): approximately 6 at%) are observed at 10 µm. These results indicate that when a Ni screen is used, more N diffusion into the sample occurs than when the SPCC screen is used, in ASPN and S-DCPN. In addition, for S-DCPN, the N diffusion depth also increases with the use of the Ni screen, indicating that the contribution of S-DCPN to this effect is higher than that of ASPN.
GD-OES nitrogen profiles of S15C samples treated by (a) ASPN and (b) S-DCPN using Ni and SPCC screen.
The cross-sectional microstructures observed by SEM for ASPN (Ni) and S-DCPN (Ni) are shown in Fig. 8(a) and Fig. 8(b), respectively. In these figures, the symbols ☆, ○, △, and ◇ indicate the points of elemental analysis by SEM-EDX; the results of ASPN (Ni) and S-DCPN (Ni) are shown in Table 3 and Table 4, respectively. In Fig. 8(a) and Fig. 8(b), the compound layer formed by γ′-Fe4N and ε-Fe2–3N and the needle-like precipitates of γ′-Fe4N are observed. Because the compound layer is more corrosion-resistant than the S15C matrix, it was necessary to etch the samples until the S15C matrix was over-corroded (area indicated in black) in order to observe it clearly. As shown in Fig. 8(a) and Table 3, for ASPN (Ni), a compound layer with a thickness of 2.0–2.5 µm is formed; elemental analysis within the compound layer reveals the presence of 1.7 at%Ni at approximately 2.5 µm from the sample surface, confirming the presence of Ni in the compound layer. However, in the Ni profile of Fig. 6(a) (the GD-OES results), the Ni concentration is almost 0 at% at approximately 2.5 µm from the sample surface, which is not consistent with the above result. This result is attributed to the low sensitivity of SEM-EDX for detecting light elements. The N concentration detected by SEM-EDX is lower than the actual value (as can be seen from the comparison between N concentrations in Fig. 4(a), Fig. 4(b), and Fig. 7(a), which are the results of GD-OES and N concentrations at each measurement point for the elemental analysis in Table 3). Therefore, Ni concentration was detected higher value than the actual value. A deposits layer mostly composed of Ni with a thickness of approximately 0.8 µm is observed at the top surface of the sample, which is approximately consistent with a deposits layer of approximately 0.6 µm thickness obtained from GD-OES in Fig. 4(b). For S-DCPN (Ni) (Fig. 8(b)), two compound layers are observed in regions ➀ and ➁. The cracks on the sample surface suggest that compound layer ➀ was hard; therefore, hardness tests were performed on compound layers ➀ and ➁. The hardnesses of these layers were approximately 900 HV and 750 HV, respectively. Table 4 shows that the Ni concentration in compound layer ➀ was 20.3 at% at the crack generation zone and 1.5 at% at the center of the layer, while the Ni concentration in compound layer ➁ was 0.4 at%, confirming the presence of Ni in the compound layer. These Ni concentrations were also considered to be higher than the actual values, as described in the results for ASPN (Ni) shown in Table 3.
Cross-sectional microstructure of S15C samples treated by (a) ASPN (b) and S-DCPN (b) using Ni screen.
The surface microstructures obtained by FE-SEM of ASPN (Ni), ASPN (SPCC), S-DCPN (Ni), and S-DCPN (SPCC) are shown in Fig. 9(a), Fig. 9(b), Fig. 9(c), and Fig. 9(d), respectively. Figure 9 shows deposit particles on the surfaces of all the nitrided samples. For ASPN (SPCC) in Fig. 9(b), the deposit particles on the sample surface are polygonal in shape, whereas for ASPN (Ni) in Fig. 9(a), they are round and small.16,19,20) For S-DCPN in Fig. 9(c) and Fig. 9(d), round deposit particles are non-uniformly distributed regardless of the screen material. Sputtering on the sample surface for S-DCPN resulted in sputtering also occurred on deposit particles on the sample and formation these particles.16,19,20)
Surface microstructure of S15C samples treated by ASPN using (a) Ni screen and (b) SPCC screen and S-DCPN using (c) Ni screen and (d) SPCC screen.
The results of the hardness tests on the surface and cross-sections of the samples are shown in Fig. 10. The increase in hardness near the surface (0–5 µm) is due to the formation of compound layers, while that after 10 µm is due to precipitates and the N solid solution intercalating into the α-Fe lattice in the N diffusion layer. The surface hardness of ASPN (Ni) is lower than that of ASPN (SPCC), whereas that of S-DCPN (Ni) is higher than that of S-DCPN (SPCC). The hardnesses of ASPN (Ni), ASPN (SPCC), and S-DCPN (SPCC) are almost the same at a depth of 10 µm, while that of S-DCPN (Ni) is higher than that in the other three conditions up to a depth of 15 µm.
Cross-sectional hardness of S15C samples treated by ASPN and S-DCPN using Ni and SPCC screen.
Figure 11 shows the polarization curves obtained from polarization tests conducted to determine the corrosion resistance. Compared to the untreated sample, the current density of all the nitrided samples decreased, indicating that the corrosion resistance was improved in these samples. For ASPN (Ni), a region of low current density was observed in the potential range of −0.6 V to −0.2 V, indicating that the corrosion resistance of ASPN (Ni) was higher than that of ASPN (SPCC). For S-DCPN (Ni), the current density was constant in a potential range of −0.4 V to −0.1 V; while for S-DCPN (SPCC), such the potential range was 0.1 V to 0.6 V, which was wider than that of S-DCPN (Ni). Additionally, the current density of S-DCPN (SPCC) was also smaller than that of S-DCPN (Ni) in these ranges. Therefore, it can be concluded that the corrosion resistance of S-DCPN (Ni) is lower than that of S-DCPN (SPCC).
Polarization curves in the 3.5 mass% NaCl of S15C samples treated by ASPN and S-DCPN using Ni and SPCC screen.
For ASPN (Ni), from the results of GD-OES in Fig. 4(a) and Fig. 4(b), the cross-sectional microstructure observation in Fig. 8(a), and the elemental analysis in Table 3, a Ni-rich deposits layer is formed on the sample surface. Due to the influence of this deposits layer, the surface hardness of ASPN (Ni) is lower than that of ASPN (SPCC), as shown in Fig. 10. In addition, the deposits layer acts as a protective film against corrosion.22) As shown in the polarization curves in Fig. 11, the current density is lower in the potential range of −0.6 V to −0.2 V, and the corrosion resistance of ASPN (Ni) is higher than that of ASPN (SPCC). As shown in the GD-OES results in Fig. 4(c) and Fig. 4(d), a deposits layer of Fe nitrides with a thickness of approximately 0.8 µm is formed for ASPN (SPCC). However, in the polarization curves shown in Fig. 11, the trend of a low current density for ASPN (Ni) is not observed for ASPN (SPCC). There are two possible reasons for this finding. (1) In the deposits layer of Fe nitrides, α-Fe, γ′-Fe4N, and ε-Fe2–3N deposits particles are mixed,5) resulting in regions with different corrosion resistances, and a likelihood of corrosion to progress from a region with a lower corrosion resistance. (2) For ASPN, crevice corrosion, which occurs between the deposit particles and the sample surface, causes a decrease in the corrosion resistance.19,24) As shown in Fig. 9, the deposit particles of ASPN (SPCC) are larger than those of ASPN (Ni) and are polygonal in shape. Therefore, the decrease in the corrosion resistance due to crevice corrosion for ASPN (SPCC) is larger than that for ASPN (Ni). Thus, it can be concluded that the deposits layer of ASPN (SPCC) possibly did not act as a protective film against corrosion as much as the deposits layer of ASPN (Ni) did.
For S-DCPN (Ni), as shown in the GD-OES results in Fig. 6(a) and Fig. 6(b), the diffusion depth of Ni into the sample is larger than that of ASPN (Ni) (ASPN (Ni): 0.6–2.2 µm, S-DCPN (Ni): 0.1–3.8 µm). Further, as shown in Table 3 and Table 4, the Ni concentration in the compound layer is also high (ASPN (Ni): 1.7 at%, S-DCPN (Ni): 20.3 at%). These results are attributed to the higher Ni concentration on the sample surface than for ASPN (Ni) due to the introduction of lattice defects and removal of the deposits layer caused by sputtering on the sample surface during S-DCPN. In the XRD results (Fig. 2 and Fig. 3), the shifts in the diffractions of α-Fe, γ′-Fe4N, and ε-Fe2–3N to a greater angle for S-DCPN (Ni) may be due to a decrease in their lattice parameters by the substitution of Ni atoms (atomic radius r = 0.125 nm) diffusion into the sample with Fe atoms (r = 0.126 nm) in these crystal lattices.25–29) The same tendency was not observed for ASPN (Ni) because the diffusion depth of Ni and the Ni concentration in the compound layer of ASPN (Ni) were lower than those of S-DCPN (Ni), as described above. Figure 8(b) indicates a likelihood for the formation of compound layers with different hardness values, because compound layer ➀ contains more Ni than compound layer ➁. As shown in Table 4, the solid-solution strengthening due to the substitutional solid solution of Ni atoms occurs in compound layer ➁. As shown in the hardness test results in Fig. 10, the surface hardness of S-DCPN (Ni) is higher than that of S-DCPN (SPCC) because of the formation of a hard compound layer (➀). The compound layer of S-DCPN (Ni) likely had a lower corrosion resistance than that of S-DCPN (SPCC), because the N concentration in the sample surface (0–1.0 µm) of S-DCPN (Ni) was lower than that of S-DCPN (SPCC), as shown in Fig. 6(b),30) and in Fig. 8(b), a region with a thin compound layer is observed, which is likely the origin of the corrosion.
4.2 Nitrogen diffusion mechanism in ASPN using a Ni screenFor ASPN (Ni), as shown in Fig. 4(a) and Fig. 4(b), the Ni concentration is approximately 90 at% and the N concentration is 0 at% at the top surface of the deposits layer (0 µm), suggesting that the deposits from the Ni screen are Ni and not Ni nitrides. According to the “sputtering and deposition” model proposed by some researchers, nitriding proceeds by the decomposition of nitride particles deposited on the sample surface in ASPN.2,5,9,10) With Fe nitride particles, diffractions of γ′-Fe4N and ε-Fe2–3N were detected in the XRD analysis of the deposited particles after ASPN, confirming the formation of Fe nitrides.5) From these observations, nitriding during ASPN with the Ni screen likely does not proceed because there is possibly no N supply and no Ni nitride deposits are observed. However, as shown in Fig. 4(a) and Fig. 4(b), N has diffused into the sample, and contradictory results are obtained. These are explained in Fig. 12, which is a schematic of the N diffusion mechanism for ASPN using a Ni screen. Ni nitrides may have formed in the plasma around the Ni screen during ASPN; because Ni nitrides are very unstable, unlike Fe nitrides, they may have completely decomposed into Ni and N on the sample surface. Subsequently, Ni likely formed a deposits layer and N was released into a furnace or diffused into the sample, leading to the conclusion that a nitrided layer was formed, despite their presence not being confirmed by XRD and GD-OES (Fig. 2, Fig. 4(a), and Fig. 4(b)) after ASPN using the Ni screen. Regarding the formation of Ni nitrides during processing in this study, although there are no reports on plasma nitriding using a Ni screen, there are reports on the fabrication of thin films of Ni nitrides of Ni4N, Ni3N, and Ni2N using reactive sputtering.31–34) Therefore, in this study, it was highly probable that Ni nitrides formed by the combination of Ni atoms from the Ni screen through sputtering and activated N species in the plasma. In contrast to this study, in reports by Kovács et al.,21,22) the absence of a nitrided layer despite the formation of a Ni deposit layer may be due to the absence of Ni nitride deposits during processing.
Diffusion mechanism of nitrogen into the sample in ASPN using Ni screen.
As shown in the GD-OES results in Fig. 7(a) and Fig. 7(b), for both ASPN and S-DCPN, the amount of N diffusing into the sample was larger when using a Ni screen than when using an SPCC screen. The Ni nitrides from the Ni screen were more unstable and decomposed more easily than the Fe nitrides from the SPCC screen (section 4.2). Thus, more N could be supplied to the sample. In addition, the effect of the Ni screen on the amount of N diffusion was larger for S-DCPN than for ASPN. There are two possible reasons for this finding. (1) For ASPN, N diffusion into the sample surface is inhibited by the deposits layer that becomes thicker with the passage of treatment time, whereas for S-DCPN, because the deposits layer is removed by sputtering on the sample surface, the distance between the deposited Ni nitride particles and the sample surface becomes smaller than that in ASPN. Therefore, the effect of enhanced N diffusion for S-DCPN was superior to that for ASPN. (2) In S-DCPN, plasma is also generated on the sample surface, unlike in ASPN where the sample is insulated; hence, the Ni on the sample from the decomposing Ni nitrides may have been tapped out by sputtering and combined with N-active species to form Ni nitrides again. Therefore, it was considered that more Ni nitrides were deposited on the sample surface in S-DCPN (Ni) than in ASPN (Ni). Thus, the effect of using the Ni screen on the increase in N diffusion was superior for S-DCPN than for ASPN.
In this study, to investigate the effect of a Ni screen on the plasma nitriding of carbon steel, S15C was subjected to ASPN and S-DCPN using Ni and SPCC screens, and the following conclusions were obtained.
The results demonstrate that the use of a Ni screen for plasma nitriding increases the amount of N diffusion into a sample compared to the use of a conventional steel screen. Nevertheless, it is necessary to consider the effect of the formation of a Ni-rich deposits layer and the diffusion of Ni atoms into the sample.
