Controlling Nitrogen Dose Amount in Atmospheric-Pressure Plasma Jet Nitriding

Abstract

A unique nitriding technique with the use of an atmospheric-pressure pulsed-arc plasma jet has been developed to offer a non-vacuum, easy-to-operate process of nitrogen doping to metal surfaces. This technique, however, suffered from a problem of excess nitrogen supply due to the high pressure results in undesirable formation of voids and iron nitrides in the treated metal surface. To overcome this problem, we have first established a method to control the nitrogen dose amount supplied to the steel surface in the relevant nitriding technique. When the hydrogen fraction in the operating gas of nitrogen/hydrogen gas mixture increased from 1% up to 5%, the nitrogen density of the treated steel surface drastically decreased. As a result, the formation of voids were suppressed successfully. The controllability of the nitrogen dose amount is likely attributable to the density of NH radicals existing in the plume of the pulsed-arc plasma jet.

1. Research Background

Plasma nitriding is a surface technology to dope nitrogen atoms into metal surfaces via plasma chemical reactions to improve wear resistance, and fatigue strength, etc., of materials [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. Plasma nitriding is now one of the essential surface treatments used in industry, especially in the automobile industry and die/mold fabrication. Conventional plasma nitriding uses low-pressure DC (or pulsed DC) plasmas in the abnormal glow discharge mode, where the batch process with a large vacuum furnace meets the purpose of mass production. In addition, a number of low-pressure plasma modes have recently been applied to nitriding treatment, e.g., active screen plasmas [4,5,6], electron cyclotron resonance plasmas [2,7], and radio-frequency plasmas [8], etc.

As another technological seed, nitriding methods using atmospheric-pressure plasmas have been developed, where the disuse of vacuum equipment makes the process much quicker and easier-to-operate. Two types of atmospheric-pressure plasmas are utilized to nitriding, namely the pulsed-arc (PA) plasma jet [17,18,19,20,21,22] and the dielectric barrier discharge (DBD) [23,24]. The PA plasma-jet nitriding has proved to be available to die steel [17,18,22], austenitic stainless steel [20], and titanium alloy [19,21], where the jet plume is sprayed onto the sample surface to thermally diffuse nitrogen atoms into it. Note that the nitrogen/hydrogen gas mixture is used as the operating gas. The plasma-jet nitriding will offer a drastically economical method to us compared with conventional plasma nitriding, especially when high-mix low-volume production is targeted.

The plasma-jet nitriding, however, is a relatively new, still developing technology. Thus, its controllability and reliability has to be improved further for practical application. For example, we had no methods to control the nitrogen dose amount from the jet plume to the metal surface, while such a method has been completed for conventional nitriding in which the nitrogen dose is well-controlled by adjusting the nitriding potential [25]. Due to the lack of dose controllability, the plasma-jet nitriding suffers from a problem of excess nitrogen supply due to the high pressure results in undesirable formation of voids and iron nitrides (the compound layer) attributed to nitrogen gas precipitation in the treated metal surface.

In this paper, a newly developed method is detailed to control nitrogen dose amount in plasma-jet nitriding to overcome the problem of excess nitrogen supply. A brief introduction of the method is as follows. The operating gas to generate the plasma jet is a nitrogen/hydrogen gas mixture. The optical emission spectroscopy proved that NH radical emission is dominant from the jet plume. In general, the NH emission intensity tends to decrease with increasing hydrogen fraction in the operating gas, f_H2. If NH is the key radical for plasma-jet nitriding and if the decreasing tendency of NH emission with f_H2 indicates decreasing NH density in the jet plume, we could decrease the nitrogen dose amount to metal surface by increasing f_H2. Following this assumption, we addressed to control the nitrogen dose amount by changing f_H2 in this study.

Prior to explaining our research, let us summarize here key species in various plasma nitriding techniques. As for low-pressure plasma nitriding methods, a comprehensive and systematic understanding of key species is not present. For example, several papers suggest the importance of ion species such as N²⁺ [2], N⁺ [9], and NH_x⁺ [10]. On the other hand, Matsumoto et al. proposed that neutral species govern the rate-limiting step [11]. For the radical nitriding, one of the low-pressure plasma nitriding methods using NH₃ and H₂ gas, NH radicals are considered to play a key role [12,13]. Besides, some other papers mention the importance of NH radicals for plasma nitriding [8,14]. Moreover, in gas nitriding, NH₃ dissociates on an iron surface to NH₂, NH₂ dissociates to NH, and NH dissociates to N and H, in order, indicating that the presence of NH is essential for gas nitriding [25]. As described above, a number of studies regard NH radicals as species effective to nitriding. Thus, our expectation that NH is key for plasma-jet nitriding is not peculiar.

2. Experimental Procedure

Experiments were performed with the PA plasma jet system shown in Figure 1a. The jet nozzle was composed of coaxial cylindrical electrodes. The grounded external electrode measured 35 mm in inner diameter. The discharge gap was approximately 20 mm. The nitrogen/hydrogen gas mixture was introduced into the nozzle at the total flow rate of 20 slm, where the hydrogen fraction, and the ratio of hydrogen flow rate to total flow rate, was f_H2. The low-frequency voltage pulse (5 kV in height and 21 kHz in repetition) was applied to the inner electrode, resulting in the maximum of the discharge current of ca. 1.2 A. The afterglow of the generated PA plasma was spewed out from the orifice and was 4 mm in diameter, forming the jet plume containing NH radicals.

For the spectroscopic experiment, the jet nozzle was fitted with a quartz pipe (30 mm in diameter and 500 mm in length) at the tip as shown in Figure 1b to observe the jet plume generating in the operating gas. The optical emission of NH (A³Π–X³Σ⁻) of 336 nm was detected with a spectrometer (Shamrock SR-500i, Andor, Belfast, UK). We collected the light emitted from the jet plume at the distance of 10 mm from the nozzle tip.

For the nitriding experiment, the jet nozzle was inserted into a cylindrically shaped cover made of quartz as shown in Figure 2 to purge residual oxygen from the treatment atmosphere. The height and diameter of the quartz cover was 85 and 124 mm, respectively. The gas was exhausted through the 1-mm gap between the jet nozzle and the quartz cover. The experimental system was put in a simple booth (1.0 × 1.2 × 1.8 (height) m³) surrounded by a vinyl curtain to lead the exhaust gas to the gas-treatment equipment. Prior to generating the plasma jet, residual oxygen inside the cover was gas-purged by the operating gas introduced through the nozzle.

The steel to be treated was cold roll steel JIS SPCC. The composition was as follows: 0.02% C, 0.09% Mn, 0.017% P, 0.004% S, and the balance was Fe. The sample dimension was 25 × 25 × 1.2 mm³. The hardness of base material was ca. 150 HV. The surface was mirror finished with alumina powder (1 µm) and degreased in an ultrasonic acetone bath.

To make the effects of nitrogen dose amount as conspicuous as possible, the surface temperature was set into the range of 1000 to 1100 K during nitrogen doping and the doped sample was immediately quenched to invoke iron-nitrogen martensite transformation. Such nitro-quenching treatment was known to form voids, which can be readily observed with a microscope, in the surface when excess nitrogen was doped [26]. In addition, the formation of iron-nitrogen martensite indicated to us the answer as to whether or not a non-trivial amount of nitrogen had been doped even when the nitrogen dose amount was intently reduced to suppress the formation of voids. The surface temperature of ca. 1000–1100 K was maintained by the plasma-jet spraying itself, where the distance between the nozzle tip to the surface was set to 7 mm. The treatment temperature was measured by spraying the jet plume to a dummy sample with a thermocouple on the surface. The doping duration was 900 to 1800 s. The doped steel was quenched by water cooling, where tap water was poured onto the opposite surface through the hole bored in the center of the sample stage.

The doped nitrogen concentration in the steel surface was detected by an electron probe micro analyzer (EPMA, JXA-8200SP, JEOL, Tokyo, Japan). The void formation and the metallographic structure were observed with an optical microscope (VHX-5000, KEYENCE, Osaka, Japan) to a cross-section of doped steel surface. The sample surface was etched in nital solution (3%) for observing the metallographic structure. The formation of iron nitrides was detected by X-ray diffraction (XRD, SmartLab, Rigaku, Tokyo, Japan) using Co Kα radiation (λ = 0.179 nm). The hardness profile of the cross-section was measured with a Vickers microhardness tester (FM-300, FUTURE-TECH, Kawasaki, Japan), where the indenter load was 0.098 N and the loading time was 10 s.

3. Results and Discussions

3.1. NH Emission Intensity

Figure 3 shows the f_H2 dependence of the emission intensity of NH radicals from the jet plume. The NH emission intensity increased with increasing f_H2 up to 0.25%. On the contrary, the NH emission intensity turned to decrease with increasing f_H2 over 0.25% up to 5%. The decreasing tendency of the emission intensity suggested the likely possibility that the density of NH radical existing in the jet plume decreased with increasing f_H2 in the range more than 0.25%. Following this suggestion, we increased f_H2 for the purpose of decreasing the nitrogen dose amount. Incidentally, the minimum f_H2 was set to 1% in this study because by our previous work, f_H2 was less than 1% proved to result in oxidization of the steel surface due to the lack of reduction performance against residual oxygen [18].

3.2. Nitrogen Density of Treated Steel Surface

Figure 4a,b shows the cross-sectional mapping and depth profile of nitrogen concentration, respectively, where the observation point was at the center of plasma-jet spraying. For f_H2 of 1%, nitrogen was considerably condensed in the surface. In the vicinity of the outermost surface, the nitrogen concentration reached ca. 12 at% and monotonically decreased in the depth direction. The concentration gradient was a typical characteristic in such a diffusion treatment. On the other hand, the nitrogen concentration in the vicinity of the surface became obviously less for f_H2 of 2.5% even though the gradient tendency was analogous. The maximum concentration was merely 4 at% in this case. Moreover, further decreases in nitrogen concentration was seen for f_H2 of 5%, where the maximum value was reduced down to 2 at%. In summary, the nitrogen concentration in the doped steel surface monotonically decreased with increasing f_H2, while it maintained the gradient tendency in the depth direction. This result indicates that the nitrogen dose amount was successfully controlled by changing f_H2.

3.3. Formation of Voids

Figure 5 shows cross-sectional micrographs of doped steels observed in the vicinity of the center of plasma-jet spraying. For f_H2 of 1%, a number of black dots were seen, which corresponded to the voids due to excess nitrogen doping. The existence of such voids would have made the material surface extremely brittle. On the other hand, the number of voids tended to decrease with increasing f_H2 and as a consequence, they become invisible in the optical microscopic scale for f_H2 of 5%. This result indicates that increasing f_H2 can suppress the void formation in the steel surface. From the tendency of nitrogen concentration described in Section 3.2, it follows that decreasing the nitrogen dose amount was the cause of the suppression of void formation.

3.4. Formation of Compound Layer

Figure 6 shows XRD spectra of the treated steel surface, where the observation point was the sample surface in the vicinity of the center of plasma-jet spraying. For f_H2 of 1%, the treated surface obviously contained iron nitrides, namely Fe₄N (γ’ phase) and Fe_2–3N (ε phase). On the other hand, the spectral intensities of the iron nitrides suddenly decreased with increasing f_H2 and as a consequence, they became less than the detection limit for f_H2 over 2%. This result indicated that increasing f_H2 reduced the formation of iron nitrides in the steel surface. From the tendency of nitrogen concentration described in Section 3.2, it follows that the formation of iron nitrides was suppressed owing to a decreasing nitrogen dose amount.

In addition, the XRD peaks of the retained austenite (γ phase) were clearly seen. The formation of the retained austenite was attributed to the austenitic transformation over the critical temperature A₁ and an excess solution of nitrogen. The formation of retained austenite can be regarded as another negative effect of excess nitrogen supply as well as the voids and iron nitrides. However, Figure 6 exhibits that the peak intensity of γ tended to decrease with increasing f_H2, indicating that the amount of retained austenite was reduced. Moreover, the γ peak shifts toward high theta with f_H2, that resulted from decreasing the lattice constant due to decreasing dissolved nitrogen concentration. From the relationship between the austenitic lattice constant a and the dissolved nitrogen concentration X_N in atomic percentage (a/nm = 0.3564 + 0.00077X_N) [26], we obtained the dependence of X_N on f_H2 as shown in Figure 7. For f_H2 = 1%, X_N = 9.6 at%. Increasing f_H2 monotonically decreased X_N and for f_H2 = 4%, X_N was reduced down to 0.26 at%. These characteristics of retained austenite are additional evidence for the controllability of nitrogen dose amount.

3.5. Hardness Profile

Figure 8 shows the two-dimensional hardness profiles of the cross-section of treated steels. The horizontal axis is the surface position of sample, the origin of which corresponds to the center of plasma-jet spraying. The vertical axis is the depth from surface. Here the micro-Vickers hardness was measured at intervals of 2 mm in the horizontal direction and 10 μm in the vertical direction. The hardness is displayed by gray scale.

We see that for every f_H2, the hardness beneath the center of plasma-jet spraying was increased significantly beyond the original hardness. Here the area of ca. 5 mm in diameter was locally hardened. The local hardening was most likely due to the limited heating ability of the plasma jet only to a narrow area, not due to local nitrogen supply because of the following fact. It has already been proved that nitrogen can be supplied to the circular area as large as 20 mm in diameter by identical plasma-jet spraying, where the steel sample was heated up to ca. 800 K with the assistance of an external heater [18]. The highest hardness was 815, 755, 815, 822, and 606 HV for f_H2 = 1%, 2%, 3%, 4%, and 5%, respectively. Such hardness cannot be obtained without nitrogen doping because the original carbon content was too low in the sample to invoke iron-carbon martensite transformation. Although the hardness was relatively low when f_H2 = 5%, the drastic increase in hardness proved that the nitrogen dose amount was still appropriate.

Figure 9 shows the depth profile of hardness averaged within the range of ±2.5 mm of the surface position. The error bar corresponds to the standard deviation of each of the three data sets. We can see the typical trend of hardness gradient for every f_H2. Note that for f_H2 = 1%, obvious softening occurs in the outermost surface within the depth profile from 10 to 30 μm. (This is clearly seen also in Figure 8.) This softening was possibly due to the considerable amount of retained austenite. For f_H2 = 2%, the softening effect became much weaker and for more f_H2 it was not seen any more. The dependence of the softening effect on f_H2 is consistent with the peak intensity of γ shown in Figure 6. The outermost hardness of f_H2 = 3% and 4% reached the largest value of ca. 800 HV. For f_H2 = 5% it became lesser, likely due to a lower nitrogen dose amount.

3.6. Metallographic Structure

Figure 10 shows the metallographic micrographs of sample cross-section in the vicinity of the surface. Here the martensite layer and the compound layer are denoted by m and c, respectively. For f_H2 = 1%, the compound layer was clearly seen as a discontinuous thin layer. However, no discontinuous layer appeared for more f_H2, being consistent with the behavior of γ’ and ε peaks shown in Figure 6.

The thickness of the martensite layer depended on f_H2. That is, the thickness increased with increasing f_H2 up to 2%, and turned to a decrease from 2% to 4%, and then kept constant for more f_H2. We consider that the thickness change was caused by a shift of the surface temperature. The temperature of jet plume tended to increase with increasing f_H2 owing to the high thermal conductivity of hydrogen, transporting the thermal energy from the pulsed arc to the jet plume. The sample temperature (measured with a dummy sample) was ca. 1000 K for f_H2 = 1% but increased up to ca. 1100 K for more f_H2. From this fact, it follows that the increase in the thickness from f_H2 = 1% to 2% was caused by the enhanced thermal diffusion and the subsequent decrease was caused by the decrease in nitrogen dose amount. The change in the layer thickness can be seen also in Figure 8 and Figure 9.

4. Conclusions

To overcome the problem of excess nitrogen supply in the PA plasma-jet nitriding, a controlling method of nitrogen dose amount was proposed on the basis of NH radical emission and was addressed to experimentally performed. Consequently, we have demonstrated that the nitrogen dose amount to the steel surface can be controlled by changing the hydrogen fraction in the operating gas. As a result of nitrogen dose control, undesirable formation of voids and iron nitrides were successfully suppressed, while a nitrogen dose enough to invoke martensite transformation was simultaneously achieved.

This achievement means that we have first obtained the technique to control “the nitriding potential” even in the new plasma-jet nitriding. We believe that the upgraded controllability presented here was of great help for future practical applications of the plasma-jet nitriding, especially for applications to high-mix low-volume production of mechanical and medical fabrications. Note that although the treatable area in this study seems too small for practical use, we can practically treat the circular area of at least 20 mm in diameter for the ordinary nitriding temperature at ca. 800 K [18].