Plasma nitriding of stainless steels at low temperatures

Plasma nitriding of stainless steels at low temperatures

Surface and Coatings Technology 116–119 (1999) 205–211 www.elsevier.nl/locate/surfcoat Plasma nitriding of stainless steels at low temperatures B. La...

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Surface and Coatings Technology 116–119 (1999) 205–211 www.elsevier.nl/locate/surfcoat

Plasma nitriding of stainless steels at low temperatures B. Larisch *, U. Brusky, H.-J. Spies TU Bergakademie Freiberg, Institute of Material Engineering, Gustav-Zeuner-Str. 5, 09596 Freiberg, Germany

Abstract To avoid the drop in corrosion resistance of stainless steels in conventional nitriding (precipitation of CrN ), low-temperature techniques like ion implantation, plasma immersion ion implantation (PIII, PI 3) and low-temperature plasma nitriding were developed. In this investigation, four stainless-steel grades (ferritic: X6Cr17, austenitic–ferritic: X2CrNiMoN22.5.3, austenitic: X8CrNiTi18.10 and X5CrNi18.10) were plasma-nitrided between 250 and 500°C. Nitrogen-enriched layers with a high nitrogen content were produced, leading to a significant increase in surface hardness. X-ray diffraction indicated that CrN did not precipitate if treatment temperatures did not exceed 400°C. ‘Expanded austenite’ formed in the austenitic and duplex steels and e-nitride (Fe N ) in the ferritic steel. The optically visible structure of the nitrided cases is comparable with that of the PIII layers, with 2 1−x higher charging densities being possible in the plasma nitriding. Also, in comparison to conventional ion implantation, large charges and parts with complicated shapes can be treated. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Expanded austenite; Plasma nitriding; S-phase; Stainless steels

1. Introduction Stainless steels have an excellent corrosion resistance because of their native passive layer. However, their load-bearing capacity is not very high. To increase the surface hardness, conventional nitriding in gas and plasma [1–4] was initially carried out leading to a drop in corrosion resistance due to the formation of CrN precipitates at temperatures above 400°C [3,5]. Plasma nitriding was therefore carried out at lower temperatures [3]. However, very long nitriding times were necessary to obtain only relatively thin layers (AISI 316; 400°C/60 h: 20mm [3]). While activities in the field of plasma nitriding had abated, techniques like ion implantation [6 ] and plasma immersion ion implantation (PIII or PI 3) [7–9] have been used more in recent times, with the former technique being taken up again, now leading to more optimal processes. Though all three techniques are used in the case of austenitic steel nitriding, ferritic and duplex [10] stainless steels have received less attention until now. Implantation is restricted by its line-of-sight nature. In contrast, PIII and plasma nitriding allow a simulta* Corresponding author. Tel.: +49-3731393102; fax: +49-373393657. E-mail address: [email protected] (B. Larisch)

neous nitriding of all sides of the workpiece. In ion implantation and PIII, the temperature regime strongly depends on the plasma parameters (i.e. ion energy, pulse frequency, etc.). This effect is not as strong in plasma nitriding due to lower ion energies. Low-temperature plasma nitriding (LTPN; T< 500°C ) had been carried out using d.c., triode [23] and r.f. systems as well as d.c. triode magnetron reactive sputtering ( Table 1). LTPN can produce nitrogenenriched layers with structures comparable to those of PIII layers [6 ]. These layers have a high hardness and are not only wear-resistant but, above all, corrosion resistant. In austenitic stainless steels and duplex steels, a phase forms, called ‘expanded austenite’ [8] or ‘S-phase’ [7,12,13], whereby nitrogen remains in solid solution in the f.c.c. lattice [6 ] (T≤450°C ). The real nature of this metastable phase is a controversial theme and is still an unknown territory [6,13–19]. Since the investigations mentioned above were conducted with laboratory equipment, little can be said about the possibility of producing such layers in devices of industrial size. A French group [4,16,30], already working for some time now in this field, as well as Menthe/Rie [15] and Dearnley [13], presented some screening results concerning layers produced in larger furnaces. Furthermore, there is a wide range of investi-

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Table 1 Literature review of nitriding conditions for stainless steels Reference

PN

Steels

T (°C )

t (h)

P (Pa)

N :H (%) 2 2

[20] [21]

AISI 304:X5CrNi18.10 18-8

400, 550 400+550/450/ 550

4 4; 40+4/2/4

667 667

80:20 80:20

[6 ] [22] [24] [11] [2]

400 400, 500 400 400 450, 500, 550

10 min, 30 min 0.5; 1

1200 332–465

r.f. r.f. d.c.

X5CrNi18 10 X5CrNi18 10 AISI 316: X5CrNiMo17.12.2 AISI 316:X5CrNiMo17.12.2 X3CrNi18 10

15:85 25:75 100% N

3 1; 5; 12; 30

400–670

[3] [4]

d.c. d.c.

AISI 316: X5CrNiMo17.12.2 X5CrNi18 10

400 500, 550, 600, 650

60 A few (3–16)

400–600 >133

[13]

d.c.

AISI 316: X5CrNiMo17.12.2

450–950

[25]

d.c.

[26 ]

d.c. triode

[27]

Pulse d.c.

13Cr; 13Cr +2Ti/3V/3W/2Nb/ 3Al/3Zr or 3Si AISI17/4PH; AISI 316: X5CrNiMo17.12.2 AISI 304: X5CrNi1810

[15]

Pulse d.c.

[10]

Pulse d.c.

AISI 316: X5CrNiMo17.12.2 AISI 321: X6CrNiTi18.10 AISI 304L:X3CrNi18.10 X20Cr13 X4CrNiMoN27 5 2 Ni42 X2CrNiMoN22 5 3

d.c. sputtering

X2CrNiMo17 13 2 X1CrNiMoNb28 4 2 AISI 310: X12CrNi25 20

[16 ]

300–400

Electrical parameter

300; 330; 395 V 21;25, 43 A m−2 400 V 1.6–3.2 mA cm−2 2

25:75 70:30 25:75 NH , N 3 2 NH ; 5N 3 2 +95H 2

c. 50 V 400–450 V

700 V 1 W cm−2 600–800 V

500, 550, 600

1; 3; 6

800

50:50

20 kW ( Klo¨ckner plant) c. 300–500 V

420

30

1000

25:75

500 V

380, 460

20, 48

H,N 2 2

10 kW (thyristor-controlled )

400–600

5

400

80:20

410 V PD: 50–200 ms PR: 50–1000 ms

350

40

150

2N :1H 2 2

470–480 V industrial plant

400

20 0.4–4

Ar; N

d.c. triode magnetron sputtering

300

gated process parameters in d.c., triode, r.f. or pulse plasma nitriding (Table 1) that do not allow a comparison of these processes in general. Pulse plasma nitriding is a well-established process in industry. The austenitic, duplex and ferritic stainless steels were therefore treated in such an industrial nitriding equipment.

2. Experimental Ferritic X6Cr17, austenitic steel grades X8CrNiTi18 10 and X5CrNi18 10 and duplex austenitic–ferritic steel X2CrNiMoN22 5 3 samples were ground or polished using 3-mm diamond paste and then degreased with alcohol before being plasma-nitrided. A commercial plasma-nitriding apparatus (pulsed d.c.) was used, with an auxiliary heater being used for sample heating. In contrast to other investigations ( Table 1), argon was used in addition to nitrogen and hydrogen with relatively

2

long nitriding times. The nitriding experiments were conducted in the temperature range of 250–500°C for 8–60 h at 300 Pa and 400–450 V. The pulse duration/ pulse repetition ratio was 50 ms/100 ms. Gas mixtures of 40–80% N , 10–30% H and 10–30% Ar were used. 2 2 After treatment, the samples were sectioned, nickelplated, ground and polished for optical microscopy and etched in Beraha II solution. Glow discharge optical spectroscopy (GDOS ) was conducted. The phases near to the surface were identified using glancing-angle X-ray diffraction ( Y- diffractometer method; radiation CuKa), whereby different slits had to be used for technical reasons. Furthermore, XRD measurements at incident angles (a) of 2, 5 and 12° were carried out on selected samples. The surface roughness was measured with a laser optical system (auto focus principle). The microhardness was estimated by load/displacement measurements using a Vickers indenter and a maximum load of 1 N. Five indentations were made on each sample from which average hardness values were calculated.

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3. Results and discussion 3.1. Diffusion depths Plasma nitriding in the temperature range 250–500°C can produce nitrogen-enriched layers with high nitrogen contents. The maximum concentrations of N mostly ranged between c. 20 and 40 at.%. The layer thickness and nitrogen concentration mainly depend on the temperature, the nitriding time and the steel grade. The higher concentration values and larger diffusion depths correspond to higher temperatures (Fig. 1) and longer nitriding times. As is known, the nitrogen diffusion is faster in the ferrite ( X6Cr17) than in the austenite ( X5CrNi18 10). The duplex steel showed the lowest case depths ( Fig. 2). The reason for the relatively low case depths in Fig. 2 will be discussed in the next section. To investigate the effect of gas composition, the ratio of N :H :Ar (4:3:3; 6:3:1; 6:1:3; 8:1:1) was varied at a 2 2 constant total flux. In contrast to the literature [31], no clear or significant influence of gas composition was found from the nitrogen concentration profiles (Fig. 3,

Fig. 1. GDOS depth profiles of plasma nitrided X6Cr17.

207

Table 2). However, from the way things stand at the moment, it seems that a gas mixture of 80% N +10% 2 H +10% Ar leads to a weaker reproducibility. Often, 2 lower diffusion depths and nitrogen concentrations were found when compared to the samples treated with the other gas compositions (Fig. 3, Table 2). This effect seems to be caused by the low proportions of argon or hydrogen. In agreement with Wei [14], it is assumed that the ion energy must be sufficiently high to sputter-remove the native oxide layer. Furthermore, the dissociative adsorption reaction of nitrogen could be assisted by hydrogen (formation of N–H radicals, according to the models of Szasz [14] and of gas nitriding). It has been observed that the addition of argon leads to a higher current density (higher effective currents). This could result in a higher plasma density that might lead to a more efficient dissociation or activation of nitrogen molecules and/or an increase in sputtering effects. Therefore, gas mixtures with higher argon and/or hydrogen proportions are recommended. If the surface barriers are overcome by sputtering, then further layer growth is diffusion-controlled. Also, a higher voltage leading to stronger bombardment of the surfaces improves the results ( Fig. 3, Table 2). Further experiments are necessary to investigate these effects in more detail. It seems that, especially for longer nitriding times, the same ‘growth rates’ (W ) can be achieved as much by plasma nitriding as by PIII or ion implantation ( Table 3). This supports the conclusion that the process is diffusion-controlled, although a longer incubation time cannot be excluded in plasma nitriding when compared to ion implantation and PIII. However, there is a wide scattering band for each technique, depending on the process parameters. Normally, probably due to the stronger heating effect of the ions with higher energies, lower treatment times are chosen for ion implantation and plasma immersion ion implantation (mostly 120 min max.) compared to plasma nitriding. In plasma nitriding and PIII, the temperature can be regulated independently of the plasma parameters (pulse frequency, power, etc.) due to the lower ion energies and the possibility of cooling the workpieces by an auxiliary cooling unit in the furnace wall during nitriding. This enables the production of thicker layers. 3.2. Micro-structure

Fig. 2. GDOS depth profiles of the plasma-nitrided samples (350°C; 26 h).

The polished cross-sections were etched in Beraha II solution. Micrographs of the structures of the nitrided layers are presented in Fig. 4a–c. The nitrided case in the X5CrNi18 10 ( Fig. 4a) is relatively thin (2.5 mm) with a sharp boundary to the core material. Due to better nitrogen diffusion in the b.c.c. lattice, a thicker layer (14.6 mm) grows in the X6Cr17 (Fig. 4b). The inner part of the nitrided, white

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Fig. 3. Influence of the gas composition on the nitrided case depth and the nitrogen content in the layer ( X5CrNi18.10; 350°C; 16 h; 400 V ). Table 2 Influence of gas composition (and voltage) on ‘growth rate’ Nitriding conditions: 350°C, 16 h, 400 V

‘Growth rate’: W=d/앀ta (mm h−0.5)

N :H :Ar 2 2

X5CrNi18 10

X6Cr17

40:30:30 40:30:30 (450 V ) 60:10:30 60:30:10 80:10:10 (450 V )

1.30 2.20 1.18 1.31 0.50

4.12 >5.95b 3.85 3.30 3.05

is also a transition zone to the core material. The austenitic–ferritic steel (Fig. 4c) shows different layer thicknesses (average value 1.7 mm) in the two phases due to the different diffusion velocities, as mentioned above. In contrast to the pure ferritic steel, the boundary line in the duplex steel is relatively sharp. For higher nitriding temperatures, the nitrided layers have a darker appearance, indicating less corrosion resistance. The layer thickness (Fig. 4) corresponds well with the nitrogen diffusion depths ( Fig. 2) estimated by GDOS.

a d: case depth; t: time. b The layer was thicker than the depth measurable by GDOS.

3.3. Phase analysis by X-ray diffraction Table 3 Comparison of the ‘growth rate’of the layers obtained by plasma immersion ion implantation (PIII ), ion implantation (II ), plasma nitriding (PN ) and gas nitriding (GN ) at 400°C ( X5CrNi18 10; AISI304, AISI 304L) X5CrNi18 10

W=d/앀t (mm h−0.5)

Technique

400°C

Reference

PIII/25–45 keV I /0.7 keV–60 keV PN GN Puls — PN/300°C Puls — PN/350°C Puls — PN/400°C GN/400°C

1.24–2.56 1.24–4.42 0.46–2.50 0.39–1.01 0.36–0.39 0.50–0.67 1.36–3.20 0.60–0.95

[22] [14,22,28,29] [14,15,20,22] [22] IWT Freiberg This investigation 80N :10H :10Ar (%) 2 2 IWT Freiberg

layer is not as white as the outer region (7 mm), and needle-like structures can be seen, which are interpreted as deformation bands. This deformation seems to occur as a result of the high internal stresses in the case. There

Diffraction patterns of the austenitic steel X8CrNiTi18 10 are shown in Fig. 5. Up to temperatures of 400°C, a set of broad peaks are observable together with the substrate austenite peaks. With increasing temperature and time, the substrate austenite peaks disappear due to increasing layer thickness. The broad peaks have been associated with a metastable phase called ’’expanded austenite‘‘ or ‘S-phase’. The lines of the austenite are shifted towards smaller angles, and the peaks become broader, indicating an expansion of the lattice due to nitrogen in the interstitial solution. Analogous patterns were found on ion-implanted samples, as shown in Fig. 5. As observed for austenitic steels nitrided by ion implantation, nitrogen remains in solid solution in the fcc lattice if the temperature is less than 400°C [7]. At 450°C, depending on the nitriding time, CrN begins to precipitate, and c∞-Fe N forms simultaneously. 4 For duplex steel plasma-nitrided below 350°C, a transformation of ferrite into expanded austenite was

B. Larisch et al. / Surface and Coatings Technology 116–119 (1999) 205–211

209

Fig. 5. XRD patterns obtained from ion-nitrided and ion-implanted X8CrNiTi18 10.

Fig. 6. XRD patterns of the X2CrNiMoN22 5 3, nitrided: 300°C/60 h.

Fig. 4. Microstructures of the nitrided samples (350°C; 26 h).

observed (Fig. 6). This has been attributed here to the reduction of ferrite content in the XRD patterns, especially compared to the untreated steel. The ferritic steel X6Cr17 and the ferritic phase in the duplex steel X2CrNiMoN22.5.3 behave differently. This can be

Fig. 7. XRD patterns of the X6Cr17, nitrided: 350°C/26 h.

ascribed to the fact that the ferritic phase in the duplex steel contains c. 4 wt% nickel, assisting the transformation into austenite.

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Table 4 Results of the XRD-phase analysis after plasma nitriding at different temperatures and times Steel grade

300°C/60 h

350°C/26 h

350°C/40 h

450°C/26 h

500°C/20 h

X6Cr17 (ferritic)

Ferrite Fe N 2 1−x

Ferrite Fe N 2 1−x

Austenite exp. aust. Ferrite austenite exp. aust.

Austenite exp. aust. Ferrite austenite exp. aust.

Ferrite Fe N 2 1−x Fe N CrN 4 Exp. aust. Fe N CrN 4

Fe N Fe N CrN 2 1−x 4

X8CrNiTi18 10 (austenitic) X2CrNiMoN22 5 3 (ferritic–austenitic)

Ferrite Fe N 2 1−x Fe N ? CrN ? 4 Austenite exp. aust. Austenite exp. aust.

At 350°C, the ferrite peaks disappear due to the transformation of ferrite into expanded austenite. At higher nitriding temperatures, the austenite (substrate) transforms into ferrite and CrN. As a result CrN, ferrite and c∞-Fe N can be seen in the XRD patterns 4 (500°C/20 h). No substrate austenite peak is observed because the layer is relatively thick. The XRD patterns of the ferritic steel show peaks of ferrite and e-Fe N (i.e. Fe N ) after plasma nitriding 2 1−x 3 for 26 h at 350°C (Fig. 7). With longer treatment times (40 h), the amount of e-Fe N increases. The formation of CrN and 2 1−x c∞-Fe N cannot be excluded, although the XRD patterns 4 show no clear evidence of these phases. After treatment at higher temperatures (450°C, 26 h) e-Fe N , c∞2 1−x Fe N and CrN were detected by XRD. 4 The results of the X-ray measurements are summarized in Table 4. X-ray measurements (sin2 Y method ) in the expanded austenite [(311) interference] showed evidence of high internal stresses. In the X8CrNiTi18 10, maximum values were measured in samples nitrided within the temperature range of 300°C (−3,3 GPa) to 350°C (−3.6 GPa). 3.4. Surface roughness Surface roughness increases with temperature and time ( Table 5). Plasma nitriding at low temperatures (<400°C ) and shorter nitriding times leads to a relatively low increase in surface roughness. For the industrial application of these layers, a further surface finish seems unnecessary.

Fe N CrN 4 ( Exp. aust.) CrN ferrite Fe N 4

3.5. Micro-hardness measurements The micro-hardness values are influenced by the core material and the thickness and structure of the layer. Because of the thin, modified near-surface regions, it is impossible to measure the hardness of the layer alone. Due to the surface roughness, higher loads (>1000 mN ) must be used. The hardness values increase with temperature and nitriding time ( Table 6) due to an increasing layer thickness (d : at 1 wt% nitrogen; GDOS N measurements).

4. Summary and conclusions These screening experiments show that nitrogenenriched layers of sufficient thickness can be produced in stainless steels in an industrial plant at temperatures at and below 400°C. The micro-structure of these layers is identical to that produced by ion implantation. In the austenitic and duplex steels, expanded austenite was observed with high concentrations of nitrogen in the solid solution. The XRD patterns of the ferritic steel show peaks of ferrite and Fe N (i.e. Fe N ) after 2 1−x 3 plasma nitriding at temperatures below 400°C. The micro-hardness of the steels increased with temperature and time, i.e. the nitrided layer thickness. To ensure a high process certainty, minimum hydrogen and argon percentages seem to be necessary. The optimum nitriding parameters have not yet been determined. In screening tests, no significant decrease in the corrosion resistance has been observed for samples nitrided at and below 350°C. These results will be presented elsewhere. The ‘growth rates’ in plasma nitriding are comparable

Table 5 Roughness values R (mm) before and after plasma nitriding of the a steels

Table 6 Hardness values, HV, of plasma-nitrided X8CrNiTi18 10 samples

Nitriding T (°C )/t (h)

X6Cr17

X8CrNiTi18 10 (ground )

X5CrNi18 10

Nitriding: T (°C )/t (h)

Max. N (wt%)

d (mm) N

HV (N mm−2) F=1000 mN

– 300/60 350/26 350/40 500/20

0.012 0.18 0.23 0.40 0.64

0.17 0.23 0.21 0.20 0.87

0.02 0.06 0.11 0.16 0.61

– 300°C/60 h 350°C/40 h 400°C/26 h 450°C/26 h

– 7.9 7.4 8.9 12.5

– 2.0 6.4 10 32.6

2350 3305 4285 6399 8292

B. Larisch et al. / Surface and Coatings Technology 116–119 (1999) 205–211

to those of PIII or ion implantation, although longer incubation times cannot be excluded. In comparison to PIII [7,9] or ion implantation [14] in pulsed d.c. plasma nitriding, the temperature regulation is more independent of plasma parameters. Therfore, longer nitriding times are possible, and thicker layers can be produced. In comparison to the conventional ion implantation, large charges and parts with complicated shapes can be treated. Compared to PIII, higher charging densities are possible in plasma nitriding. Stainless steels can be treated in available commercial plasma-nitriding units, thus avoiding higher investments.

Acknowledgements This investigations were supported financially by the Sa¨chsisches Ministerium fu¨r Wissenschaft und Kunst (SMWK; 4-7531.50-03-FZR/603) Dr E. Richter and S. Parascandola (Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf ) put the ion-implanted sample (Fig. 5) at our disposal.

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