Controlling Nitrogen Dose Amount in Atmospheric-Pressure Plasma Jet Nitriding

Article Controlling Nitrogen Dose Amount in Atmospheric-Pressure Plasma Jet Nitriding

Ryuta Ichiki 1,*, Masayuki Kono 1, Yuka Kanbara 1, Takeru Okada 2 , Tatsuro Onomoto 3, Kosuke Tachibana 1, Takashi Furuki 1 and Seiji Kanazawa 1

1 Division of Electrical and Electronic Engineering, Oita University, Oita 870-1192, Japan; [email protected] (M.K.); [email protected] (Y.K.); [email protected] (K.T.); [email protected] (T.F.); [email protected] (S.K.) 2 Department of Electronic Engineering, Tohoku University, Sendai 980-8579, Japan; [email protected] 3 Material Technology Division, Fukuoka Industrial Technology Center, Kitakyushu 807-0831, Japan; onomoto@ﬁtc.pref.fukuoka.jp * Correspondence: [email protected]; Tel.: +81-97-554-7826

Received: 25 April 2019; Accepted: 24 June 2019; Published: 25 June 2019

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.

Keywords: plasma nitriding; atmospheric-pressure plasma; nitrogen dose amount; hydrogen fraction; void

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 [1–16]. 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 [4–6], 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 [17–22] 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

Metals 2019, 9, 714; doi:10.3390/met9060714 www.mdpi.com/journal/metals Metals 2019, 9, 714 2 of 11 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 2+ + + species such as N [2], N [9], and NHx [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 Figure1a. 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. Metals 2019, 9, 714 3 of 11 Metals 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 3 of 11

83 84 FigureFigure 1.1. Pulsed-arc plasmaplasma jet.jet. (a) Schematic ofof jetjet nozzle.nozzle. ( (b) Photograph of plasma jet.

85 For the spectroscopic experiment, the jet nozzle was ﬁttedfitted with a quartz pipe (30 mm in diameter 86 andand 500 mm in length) at the tip tip as as shown shown in in Figure Figure 11(b)b to to observe observe the the jet jet plume plume generating generating in in the the 33Π 33Σ- 87 operatingoperating gas.gas. The opticaloptical emissionemission ofof NHNH (A(A3ΠΠ–X–X3Σ-−) )of of 336 336 nm nm was was detected with aa spectrometerspectrometer 88 (Shamrock(Shamrock SR-500i,SR-500i, Andor, Belfast, UK). UK). We We collectedcollected the light emitted from the jet plumeplume atat thethe 89 distancedistance of 10 mm from the the nozzle nozzle tip. tip. 90 For the nitriding experiment, experiment, the the jet jet nozzle nozzle was was inse insertedrted into into a acylindrically cylindrically shaped shaped cover cover made made of 91 ofquartz quartz as asshown shown in inFigure Figure 2 to2 topurge purge residual residual ox oxygenygen from from the the treatment treatment atmosphere. atmosphere. The The height height and 92 anddiameter diameter of the of quartz the quartz cover cover was was85 and 85 and124 124mm, mm, respectively. respectively. The Thegas gaswas was exhausted exhausted through through the 93 the1-mm 1-mm gap gapbetween between the jet the nozzle jet nozzle and the and quartz the quartz cover. cover. The experimental The experimental system systemwas put was in a put simple in a 3 3 94 simplebooth (1.0 booth × (1.01.2 × 1.81.2 (height)1.8 (height) m3) surrounded m ) surrounded by a by vinyl a vinyl curtain curtain to tolead lead the the exhaust exhaust gas gas to to thethe × × 95 gas-treatmentgas-treatment equipment. Prior Prior to to generating the the plasma jet, residual oxygen inside the covercover waswas 96 gas-purgedgas-purged by the operating gas gas introduced introduced through through the the nozzle. nozzle.

97 Figure 2. Experimental Setup with the quartz cover to purge residual oxygen. (a) Schematic. 98 Figure 2. Experimental Setup with the quartz cover to purge residual oxygen. (a) Schematic. (b) (b) Photograph of plasma-jet nitriding. 99 Photograph of plasma-jet nitriding. The steel to be treated was cold roll steel JIS SPCC. The composition was as follows: 0.02% C, 100 The steel to be treated was cold roll steel JIS SPCC. The composition was as follows: 0.02% C,3 0.09% Mn, 0.017% P, 0.004% S, and the balance was Fe. The sample dimension was 25 25 1.2 mm3 . 101 0.09% Mn, 0.017% P, 0.004% S, and the balance was Fe. The sample dimension was 25× × 25 ×× 1.2 mm3. 101 The0.09% hardness Mn, 0.017% of base P, material0.004% S, was and ca. the 150 balance HV. The was surface Fe. The was sample mirror dimension ﬁnished withwas 25 alumina × 25 × 1.2 powder mm . 102 The hardness of base material was ca. 150 HV. The surface was mirror finished with alumina powder 102 (1Theµm) hardness and degreased of base material in an ultrasonic was ca. 150 acetone HV. bath.The surface was mirror finished with alumina powder 103 (1 µm) and degreased in an ultrasonic acetone bath. 103 (1 µm)To and make degreased the eﬀects in ofan nitrogen ultrasonic dose acetone amount bath. as conspicuous as possible, the surface temperature 104 To make the effects of nitrogen dose amount as conspicuous as possible, the surface temperature 104 was setTo intomake the the range effects of 1000 of nitrogen to 1100 Kdose during amount nitrogen as conspicuous doping and as the possible, doped sample the surface was immediatelytemperature 105 was set into the range of 1000 to 1100 K during nitrogen doping and the doped sample was 105 quenchedwas set into to invokethe range iron-nitrogen of 1000 to martensite 1100 K during transformation. nitrogen Suchdoping nitro-quenching and the doped treatment sample waswas 106 immediately quenched to invoke iron-nitrogen martensite transformation. Such nitro-quenching 107 treatment was known to form voids, which can be readily observed with a microscope, in the surface 108 when excess nitrogen was doped [26]. In addition, the formation of iron-nitrogen martensite indicated

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109 knownto us the to answer form voids, as to whether which can or benot readily a non-trivial observed amount with of a nitrogen microscope, had inbeen the doped surface even when when excess the 110 nitrogennitrogen wasdose doped amount [26 ].was In addition,intently thereduced formation to suppress of iron-nitrogen the formation martensite of voids. indicated The to surface us the 111 answertemperature as to whetherof ca. 1000–1100 or not a non-trivialK was maintained amount by of nitrogenthe plasma-jet had been spraying doped itself, even where when thethe nitrogendistance 112 dosebetween amount the nozzle was intently tip to the reduced surface to was suppress set to the7 mm. formation The treatment of voids. temperature The surface was temperature measured by of 113 ca.spraying 1000–1100 the jet K plume was maintained to a dummy by sample the plasma-jet with a thermocouple spraying itself, on wherethe surface. the distance The doping between duration the 114 nozzlewas 900 tip to to1800 the s. surface The doped was setsteel to was 7 mm. quenched The treatment by water temperature cooling, where was tap measured water was by sprayingpoured onto the 115 jetthe plume opposite to asurface dummy through sample the with hole a bore thermocoupled in the center on theof the surface. sample The stage. doping duration was 900 to 116 1800 s.The The doped doped nitrogen steel was concentration quenched by in water the steel cooling, surface where was tap detected water was by pouredan electron onto theprobe opposite micro 117 surfaceanalyzer through (EPMA, the JXA-8200SP, hole bored inJEOL, the center Tokyo, of theJapa samplen). The stage. void formation and the metallographic 118 structureThe dopedwere observed nitrogen concentrationwith an optical in themicroscope steel surface (VHX-5000, was detected KEYENCE, by an electron Osaka, probeJapan) micro to a 119 analyzercross-section (EPMA, of doped JXA-8200SP, steel surface. JEOL, Tokyo,The sample Japan). surface The voidwas formationetched in andnital the solution metallographic (3%) for 120 structureobserving werethe metallographic observed with anstruct opticalure. The microscope formation (VHX-5000, of iron ni KEYENCE,trides was Osaka,detected Japan) by X-ray to a 121 cross-sectiondiffraction (XRD, of doped SmartLab, steel surface. Rigaku, The Tokyo, sample Japan) surface using was etched Co K inα nitalradiation solution (λ (3%)= 0.179 for observingnm). The 122 thehardness metallographic profile of structure.the cross-sect Theion formation was measured of iron nitrides with a wasVickers detected microhardness by X-ray di ﬀtesterraction (FM-300, (XRD, 123 SmartLab,FUTURE-TECH, Rigaku, Kawasaki, Tokyo, Japan) Japan), using where Co the Kα indenterradiation load (λ =was0.179 0.098 nm). N and The the hardness loading proﬁle time was of the 10 124 cross-sections. 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. 125 3. Results and Discussions 3. Results and Discussions 126 3.1. NH Emission Intensity 3.1. NH Emission Intensity 127 Figure 3 shows the fH2 dependence of the emission intensity of NH radicals from the jet plume. Figure3 shows the f dependence of the emission intensity of NH radicals from the jet plume. 128 The NH emission intensityH2 increased with increasing fH2 up to 0.25%. On the contrary, the NH The NH emission intensity increased with increasing f up to 0.25%. On the contrary, the NH emission 129 emission intensity turned to decrease with increasingH2 fH2 over 0.25% up to 5%. The decreasing 130 intensitytendency turnedof the toemission decrease intensity with increasing suggestedf H2 theover likely 0.25% possibility up to 5%. that The the decreasing density of tendencyNH radical of the emission intensity suggested the likely possibility that the density of NH radical existing in the 131 existing in the jet plume decreased with increasing fH2 in the range more than 0.25%. Following this jet plume decreased with increasing f in the range more than 0.25%. Following this suggestion, 132 suggestion, we increased fH2 for the purposeH2 of decreasing the nitrogen dose amount. Incidentally, we increased f for the purpose of decreasing the nitrogen dose amount. Incidentally, the minimum 133 the minimum H2fH2 was set to 1% in this study because by our previous work, fH2 was less than 1% 134 fprovedH2 was to set result to 1% in in oxidization this study because of the steel by our surfac previouse due work, to thef H2lackwas of lessredu thanction 1% performance proved to result against in 135 oxidizationresidual oxygen of the [18]. steel surface due to the lack of reduction performance against residual oxygen [18].

136

137 FigureFigure 3.3. Emission intensity ofof NHNH radicalsradicals (336(336 nm)nm) inin jetjet plumeplume vsvs HH22 fraction inin thethe operatingoperating gas.gas.

138 3.2.3.2. Nitrogen Density of Treated SteelSteel SurfaceSurface 139 FigureFigure4 4a,ba,b showsshows thethe cross-sectionalcross-sectional mappingmapping andand depthdepth proﬁleprofile ofof nitrogennitrogen concentration,concentration, 140 respectively,respectively, wherewhere the observation point point was was at at the the center center of of plasma-jet plasma-jet spraying. spraying. For For fH2 fofH2 1%,of 141 1%,nitrogen nitrogen was wasconsiderably considerably condensed condensed in the in thesurface. surface. In the In vicinity the vicinity of th ofe theoutermost outermost surface, surface, the 142 thenitrogen nitrogen concentration concentration reached reached ca. 12 ca. at% 12 at%and andmono monotonicallytonically decreased decreased in the in depth the depth direction. direction. The 143 Theconcentration concentration gradient gradient was wasa typical a typical characteristic characteristic in such in sucha diffusion a diﬀusion treatment. treatment. On the On other the hand, other 144 hand,the nitrogen the nitrogen concentration concentration in the invicinity the vicinity of the of surface the surface became became obviously obviously less for less fH2 for off H22.5%of 2.5%even 145 though the gradient tendency was analogous. The maximum concentration was merely 4 at% in this 146 case. Moreover, further decreases in nitrogen concentration was seen for fH2 of 5%, where the

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even though the gradient tendency was analogous. The maximum concentration was merely 4 at% in Metals 2019, 9, x FOR PEER REVIEW 5 of 11 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 147 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 148 steel surface monotonically decreased with increasing fH2, while it maintained the gradient tendency in the depth direction. This result indicates that the nitrogen dose amount was successfully controlled 149 in the depth direction. This result indicates that the nitrogen dose amount was successfully by changing f H2. 150 controlled by changing fH2.

151

152 Figure 4.4. Distribution of nitrogennitrogen concentrationconcentration forfor severalseveral ffH2H2.. The The doping doping duration duration was was 1800 s. ( a) 153 Two-dimensional mapping mapping of sampleof sample cross-section cross-section in the vicinity in the of treatedvicinity surface. of treated (b) One-dimensional surface. (b) 154 depthOne-dimensional proﬁle. depth profile.

155 3.3. Formation of Voids 156 Figure5 5shows shows cross-sectional cross-sectional micrographs micrographs of dopedof doped steels steels observed observed in the in vicinity the vicinity of the centerof the 157 ofcenter plasma-jet of plasma-jet spraying. spraying. For f H2 Forof 1%,fH2 of a number1%, a number of black of dotsblack were dotsseen, werewhich seen, which corresponded corresponded to the 158 voidsto the due voids to excess due to nitrogen excess doping.nitrogen The doping. existence The of existence such voids of would such havevoids made would the materialhave made surface the 159 extremelymaterial surface brittle. extremely On the other brittle. hand, On the the number other ha ofnd, voids the tended number to of decrease voids tended with increasing to decreasef H2 withand 160 asincreasing a consequence, fH2 and as they a consequence, become invisible they inbecome the optical invisible microscopic in the optical scale microscopic for f H2 of 5%. scale This for resultfH2 of 161 indicates5%. This thatresult increasing indicatesf H2 thatcan increasing suppress thefH2 voidcan formationsuppress the in the void steel formation surface. Fromin the the steel tendency surface. of 162 nitrogenFrom the concentrationtendency of nitrogen described concentration in Section 3.2 describe, it followsd in thatSection decreasing 3.2, it follows the nitrogen that decreasing dose amount the 163 wasnitrogen the cause dose ofamount the suppression was the cause of void of the formation. suppression of void formation.

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164164 Figure 5. Micrographs of sample cross-section. The dots appearing in the vicinity of surface correspond 165165 FigureFigure 5.5. MicrographsMicrographs ofof samplesample cross-section.cross-section. TheThe dodotsts appearingappearing inin thethe vicinityvicinity ofof surfacesurface to voids. The doping duration was 900 s. 166166 correspondcorrespond to to voids. voids. The The do dopingping duration duration was was 900 900 s. s. 3.4. Formation of Compound Layer 167167 3.4.3.4. FormationFormation ofof CompoundCompound LayerLayer Figure6 shows XRD spectra of the treated steel surface, where the observation point was the 168 Figure 6 shows XRD spectra of the treated steel surface, where the observation point was the 168 sampleFigure surface 6 shows in the XRD vicinity spectra of the of center the treated of plasma-jet steel surface, spraying. where For thef H2 observationof 1%, the treated point was surface the 169 sample surface in the vicinity of the center of plasma-jet spraying. For fH2 of 1%, the treated surface 169 obviouslysample surface contained in the iron vicinity nitrides, of namelythe center Fe4 ofN( plasma-jetγ’ phase) and spraying. Fe2–3N( Forε phase). fH2 of 1%, On the othertreated hand, surface the 170 obviously contained iron nitrides, namely Fe4N (γ’ phase) and Fe2–3N (ε phase). On the other hand, 170 spectralobviously intensities contained of iron the ironnitrides, nitrides namely suddenly Fe4N decreased(γ’ phase) withand increasingFe2–3N (ε phase).f H2 and On as the a consequence, other hand, 171 the spectral intensities of the iron nitrides suddenly decreased with increasing fH2 and as a 171 theythe spectral became lessintensities than the of detection the iron limit nitrides for f H2suddenlyover 2%. decreased This result with indicated increasing that increasingfH2 and asf H2 a 172 consequence, they became less than the detection limit for fH2 over 2%. This result indicated that 172 reducedconsequence, the formation they became of iron less nitrides than inthe the detection steel surface. limit Fromfor fH2 the over tendency 2%. This of nitrogen result indicated concentration that 173 increasing fH2 reduced the formation of iron nitrides in the steel surface. From the tendency of 173 describedincreasing infH2 Section reduced 3.2 the, it followsformation that of the iron formation nitrides ofin ironthe steel nitrides surface. was suppressedFrom the tendency owing to of a 174 nitrogen concentration described in Section 3.2, it follows that the formation of iron nitrides was 174 decreasingnitrogen concentration nitrogen dose described amount. in Section 3.2, it follows that the formation of iron nitrides was 175175 suppressedsuppressed owingowing toto aa decreasingdecreasing nitrogennitrogen dosedose amount.amount.

176176

177177 FigureFigure 6. 6. XRD XRD spectra spectra of of treated treated steel steel surface surface in in the the vici vicinityvicinitynity of of the the center centercenter of ofof plasma-jet plasma-jetplasma-jet spraying. spraying.spraying.

178178 InInIn addition, addition,addition, thethe XRDXRDXRD peaks peakspeaks of ofof the thethe retained retainedretained austenite austeniteaustenite ( γ ((γγphase) phase)phase) were werewere clearly clearlyclearly seen. seen.seen. The TheThe formation formationformation of 179 of the retained austenite was attributed to the austenitic transformation over the critical 179 theof retainedthe retained austenite austenite was attributed was attributed to the austenitic to the transformationaustenitic transformation over the critical over temperature the critical A1 180 temperature A1 and an excess solution of nitrogen. The formation of retained austenite can be 180 andtemperature an excess A solution1 and an of excess nitrogen. solution The formation of nitrogen. ofretained The formation austenite of can retained be regarded austenite asanother can be 181181 regardedregarded asas anotheranother negativenegative effecteffect ofof excessexcess nitrogennitrogen supplysupply asas wellwell asas thethe voidsvoids andand ironiron nitrides.nitrides. 182 However, Figure 6 exhibits that the peak intensity of γ tended to decrease with increasing fH2, 182 However, Figure 6 exhibits that the peak intensity of γ tended to decrease with increasing fH2,

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Metals 2019, 9, x FOR PEER REVIEW 7 of 11 negative eﬀect of excess nitrogen supply as well as the voids and iron nitrides. However, Figure6 183 exhibitsindicating that that the the peak amount intensity of ofretainedγ tended austenite to decrease was with reduced. increasing Moreover,f H2, indicating the γ peak that shifts the amounttoward 184 ofhigh retained theta austenitewith fH2, wasthatreduced. resulted Moreover,from decreasing the γ peak the shiftslattice toward constant high due theta to withdecreasingf H2, that dissolved resulted 185 fromnitrogen decreasing concentration. the lattice From constant the relationship due to decreasing between dissolved the austenitic nitrogen lattice concentration. constant a From and thethe 186 relationshipdissolved nitrogen between concentration the austenitic X latticeN in atomic constant percentagea and the dissolved(a/nm = 0.3564 nitrogen + concentration0.00077XN) [26],XN wein 187 atomicobtained percentage the dependence (a/nm = of0.3564 XN on +fH20.00077 as shownXN)[ in26 Figure], we obtained7. For fH2 the= 1%, dependence XN = 9.6 at%. of XIncreasingN on f H2 asfH2 188 shownmonotonically in Figure decreased7. For f H2 =XN1%, andXN for= 9.6 fH2 at%. = Increasing4%, XN wasf H2 reducedmonotonically downdecreased to 0.26 at%.XN and These for 189 fcharacteristicsH2 = 4%, XN was of retained reduced downaustenite to 0.26 are at%.additional These characteristicsevidence for the of retainedcontrollability austenite of arenitrogen additional dose 190 evidenceamount. for the controllability of nitrogen dose amount.

191

192 FigureFigure 7.7. Nitrogen concentrationconcentration inin retainedretained austeniteaustenite calculatedcalculated fromfrom thethe XRDXRD spectralspectral shift.shift. 3.5. Hardness Proﬁle 193 3.5. Hardness Profile Figure8 shows the two-dimensional hardness proﬁles of the cross-section of treated steels. 194 Figure 8 shows the two-dimensional hardness profiles of the cross-section of treated steels. The The horizontal axis is the surface position of sample, the origin of which corresponds to the center 195 horizontal axis is the surface position of sample, the origin of which corresponds to the center of of plasma-jet spraying. The vertical axis is the depth from surface. Here the micro-Vickers hardness 196 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. 197 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. 198 The hardness is displayed by gray scale.

199 200 Figure 8. Two-dimensional hardness profiles of sample cross-section. The micro-Vickers hardness is 201 displayed by gray scale. The doping duration was 900 s.

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183 indicating that the amount of retained austenite was reduced. Moreover, the γ peak shifts toward 184 high theta with fH2, that resulted from decreasing the lattice constant due to decreasing dissolved 185 nitrogen concentration. From the relationship between the austenitic lattice constant a and the 186 dissolved nitrogen concentration XN in atomic percentage (a/nm = 0.3564 + 0.00077XN) [26], we 187 obtained the dependence of XN on fH2 as shown in Figure 7. For fH2 = 1%, XN = 9.6 at%. Increasing fH2 188 monotonically decreased XN and for fH2 = 4%, XN was reduced down to 0.26 at%. These 189 characteristics of retained austenite are additional evidence for the controllability of nitrogen dose 190 amount.

191

192 Figure 7. Nitrogen concentration in retained austenite calculated from the XRD spectral shift.

193 3.5. Hardness Profile 194 Figure 8 shows the two-dimensional hardness profiles of the cross-section of treated steels. The 195 horizontal axis is the surface position of sample, the origin of which corresponds to the center of 196 plasma-jet spraying. The vertical axis is the depth from surface. Here the micro-Vickers hardness Metals 2019, 9, 714 8 of 11 197 was measured at intervals of 2 mm in the horizontal direction and 10 µm in the vertical direction. 198 The hardness is displayed by gray scale.

199 200 Figure 8. Two-dimensional hardness proﬁlesprofiles of sample cross-section. The micro-Vickers hardness is 201 displayed by gray scale. The doping duration was 900 s.

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. Figure9 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 Figure8.) 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 Figure6. 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. Metals 2019, 9, x FOR PEER REVIEW 8 of 11

202 We see that for every fH2, the hardness beneath the center of plasma-jet spraying was increased 203 significantly beyond the original hardness. Here the area of ca. 5 mm in diameter was locally 204 hardened. The local hardening was most likely due to the limited heating ability of the plasma jet 205 only to a narrow area, not due to local nitrogen supply because of the following fact. It has already 206 been proved that nitrogen can be supplied to the circular area as large as 20 mm in diameter by 207 identical plasma-jet spraying, where the steel sample was heated up to ca. 800 K with the assistance 208 of an external heater [18]. The highest hardness was 815, 755, 815, 822, and 606 HV for fH2 = 1%, 2%, 209 3%, 4%, and 5%, respectively. Such hardness cannot be obtained without nitrogen doping because 210 the original carbon content was too low in the sample to invoke iron-carbon martensite 211 transformation. Although the hardness was relatively low when fH2 = 5%, the drastic increase in 212 hardness proved that the nitrogen dose amount was still appropriate. 213 Figure 9 shows the depth profile of hardness averaged within the range of ±2.5 mm of the 214 surface position. The error bar corresponds to the standard deviation of each of the three data sets. 215 We can see the typical trend of hardness gradient for every fH2. Note that for fH2 = 1%, obvious 216 softening occurs in the outermost surface within the depth profile from 10 to 30 µm. (This is clearly 217 seen also in Figure 8.) This softening was possibly due to the considerable amount of retained 218 austenite. For fH2 = 2%, the softening effect became much weaker and for more fH2 it was not seen 219 any more. The dependence of the softening effect on fH2 is consistent with the peak intensity of γ 220 Metalsshown2019 in, 9Figure, 714 6. The outermost hardness of fH2 = 3% and 4% reached the largest value of ca.9 of800 11 221 HV. For fH2 = 5% it became lesser, likely due to a lower nitrogen dose amount.

222

223 Figure 9.9. DepthDepth profile proﬁle of hardness of samplesample cross-section.cross-section. The The error error bars correspond to the 224 standard deviation.deviation.

3.6. Metallographic Structure 225 3.6. Metallographic Structure 226 Figure 10 showsshows thethe metallographicmetallographic micrographs of samplesample cross-section in in the vicinityvicinity of thethe 227 surface. Here Here the the martensite martensite layer layer and and the the compound compound layer layer areare denoted denoted by m by and m andc, respectively. c, respectively. For For f H2 = 1%, the compound layer was clearly seen as a discontinuous thin layer. However, no 228 MetalsfH2 = 20191%,, 9 , thex FOR compound PEER REVIEW layer was clearly seen as a discontinuous thin layer. However,9 of no11 discontinuous layer appeared for more f H2, being consistent with the behavior of γ’ and ε peaks shown 229 discontinuous layer appeared for more fH2, being consistent with the behavior of γ’ and ε peaks 239230 indecreaseshown Figure in 6in Figure. nitrogen 6. dose amount. The change in the layer thickness can be seen also in Figures 8 and 240 9. 231 The thickness of the martensite layer depended on fH2. That is, the thickness increased with 232 increasing fH2 up to 2%, and turned to a decrease from 2% to 4%, and then kept constant for more fH2. 233 We consider that the thickness change was caused by a shift of the surface temperature. The 234 temperature of jet plume tended to increase with increasing fH2 owing to the high thermal 235 conductivity of hydrogen, transporting the thermal energy from the pulsed arc to the jet plume. The 236 sample temperature (measured with a dummy sample) was ca. 1000 K for fH2 = 1% but increased up to 237 ca. 1100 K for more fH2. From this fact, it follows that the increase in the thickness from fH2 = 1% to 2% 238 was caused by the enhanced thermal diffusion and the subsequent decrease was caused by the

241

242 Figure 10. Metallographic micrograph of sample cross-section.cross-section. The arrow pairs denoted by m and c 243 specify the vertical range ofof thethe martensitemartensite layerlayer andand thethe compoundcompound layer,layer, respectively.respectively.

244 4. ConclusionsThe thickness of the martensite layer depended on f H2. That is, the thickness increased with increasing f up to 2%, and turned to a decrease from 2% to 4%, and then kept constant for more f . 245 To overcomeH2 the problem of excess nitrogen supply in the PA plasma-jet nitriding, a controllingH2 We consider that the thickness change was caused by a shift of the surface temperature. The temperature 246 method of nitrogen dose amount was proposed on the basis of NH radical emission and was of jet plume tended to increase with increasing f owing to the high thermal conductivity of hydrogen, 247 addressed to experimentally performed. ConsequeH2 ntly, we have demonstrated that the nitrogen transporting the thermal energy from the pulsed arc to the jet plume. The sample temperature 248 dose amount to the steel surface can be controlled by changing the hydrogen fraction in the (measured with a dummy sample) was ca. 1000 K for f = 1% but increased up to ca. 1100 K for more 249 operating gas. As a result of nitrogen dose control, undesirableH2 formation of voids and iron nitrides f . From this fact, it follows that the increase in the thickness from f = 1% to 2% was caused by the 250 wereH2 successfully suppressed, while a nitrogen dose enough to invokeH2 martensite transformation enhanced thermal diﬀusion and the subsequent decrease was caused by the decrease in nitrogen dose 251 was simultaneously achieved. amount. The change in the layer thickness can be seen also in Figures8 and9. 252 This achievement means that we have first obtained the technique to control “the nitriding 253 potential” even in the new plasma-jet nitriding. We believe that the upgraded controllability 254 presented here was of great help for future practical applications of the plasma-jet nitriding, 255 especially for applications to high-mix low-volume production of mechanical and medical 256 fabrications. Note that although the treatable area in this study seems too small for practical use, we 257 can practically treat the circular area of at least 20 mm in diameter for the ordinary nitriding 258 temperature at ca. 800 K [18].

259 Author Contributions: Conceptualization, all; data curation, R.I., M.K. and Y.K., methodology, R.I., M.K. and 260 Y.K.; investigation, R.I., M.K., Y.K., T.O. (Takeru Okada), T.O. (Tatsuro Onomoto), K.T., T.F. and S.K.; 261 writing—original draft preparation, R.I.; writing—review and editing, K.T. and S.K.; visualization, M.K., Y.K., 262 T.O. (Takeru Okada), T.O. (Tatsuro Onomoto) and T.F.; supervision, R.I.; project administration, R.I.; funding 263 acquisition, R.I.

264 Funding: This work was supported by JSPS KAKENHI Grant Number 15K17482.

265 Acknowledgments: We wish to acknowledge valuable discussions with Masahiro Okumiya, Toyota 266 Technological Institute, and Nobuyuki Kanayama, Santier Giken Co., Ltd. We are grateful to Masaki Sonoda, 267 Oita Industrial Research Institute, for their technical assistance.

268 Conflicts of Interest: The authors declare no conflicts of interest.

269 References

270 1. Sun, Y.; Bell, T. Plasma surface engineering of low alloy steel. Mater. Sci. Eng. 1991, 140, 419–434. 271 2. Czerwiec, T.; Michel, H.; Bergmann, E. Low-pressure, high-density plasma nitriding: Mechanisms, 272 technology and results. Surf. Coat. Technol. 1998, 108–109, 182–190. 273 3. Rie, K.-T. Recent advances in plasma diffusion processes. Surf. Coat. Technol. 1999, 112, 56–62

Metals 2019, 9, 714 10 of 11

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].

Author Contributions: Conceptualization, all; data curation, R.I., M.K. and Y.K., methodology, R.I., M.K. and Y.K.; investigation, R.I., M.K., Y.K., T.O. (Takeru Okada), T.O. (Tatsuro Onomoto), K.T., T.F. and S.K.; writing—original draft preparation, R.I.; writing—review and editing, K.T. and S.K.; visualization, M.K., Y.K., T.O. (Takeru Okada), T.O. (Tatsuro Onomoto) and T.F.; supervision, R.I.; project administration, R.I.; funding acquisition, R.I. Funding: This work was supported by JSPS KAKENHI Grant Number 15K17482. Acknowledgments: We wish to acknowledge valuable discussions with Masahiro Okumiya, Toyota Technological Institute, and Nobuyuki Kanayama, Santier Giken Co., Ltd. We are grateful to Masaki Sonoda, Oita Industrial Research Institute, for their technical assistance. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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