Tribological studies of ultrahigh dose nitrogen-implanted iron and stainless steel

Tribological studies of ultrahigh dose nitrogen-implanted iron and stainless steel

Nuclear Instruments and Methods in Physics Research B59/60 (1991) 731-736 North-Holland 731 Tribological studies of ultrahigh dose nitrogen-implante...

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Nuclear Instruments and Methods in Physics Research B59/60 (1991) 731-736 North-Holland

731

Tribological studies of ultrahigh dose nitrogen-implanted and stainless steel

iron

R. Wei a, P.J. Wilbur a, 0. Ozturk b and D.L. Williamson b a Colorado Stare University, Fort Collins, CO 80523, USA ’ Colorado School of Mines, Golden, CO 80401, USA

%e effects of nitrogen implantation to doses as high as 1 x 1019 Ions/cm* on the sliding wear resistance and nitrogen con~ntrat~on depth profiles are examined e~e~mentally. By maint~ning the proper implantation temperature, increases in dose induce the formation of thicker nitrogen-rich, wear-resistant layers. Several microns thick layers are demonstrated for both iron and stainless steel.

1. Introduction Nitrogen implantation has been shown to improve the tribological properties of iron and stainless steels [ 1,2] and a number of researchers have suggested implanted ion doses near 2 X lOI ions/cm2 produce the most wear-resistant surfaces [3-81. In contrast Dimigen et al. [9] suggest the optimum dose is 7 x 10” ions/cm* for stainless steel. Other researchers (Singer and Murday [lo] and Fujihana et al. [ll]) have implanted to but they did not higher doses (1 X 10 ” ions/cm2). report the associated tribological effects, Generally, however, it is argued that impl~tation to doses above 2 X lOI ions/cm2 does not produce deeper, more wear-resistant implanted layers because sputtering effects limit the nitrogen that can be retained at higher doses [12,13]. It is noted, however, that implantation processing is generally carried out under low dose rate, near-ambient temperature conditions. Previous work carried out by the authors under high implantation current density conditions, where surface temperatures rise substantially above ambient values, have shown that tribological performance can continue to improve as doses are increased above 2 x 10” ions/cm2 for both iron and stainless steels [14-161. In this article, experimental results confirming that observation are presented and the effects of iron and AISI 304 stainless steel (SS) implantation under conditions favorable for deep ovation and high retention of nitrogen at doses ranging to 1 x 1Or9 Nz/cm2 are examined.

2. Experimental apparatus aud procedures Pure iron (99.9%) was used in the study because it is a structurally simple constituent of steels (ferrite) that 0168-583X/91/$03.50

can be analyzed readily to obtain unambiguous data on the implanted ions and their effects on microstructure and tribological performance. AISI 304 stainless steel was used even though it is more complex because it represents the crystal structure of a steel constituent (austenite) and is a widely used engineering material. The tribological studies were conducted using an oscillating pin-on-disc wear tester shown to be well suited to the study of implanted surfaces [14-161. All of the 5 cm discs used were machined, polished mechanically to a mean roughness of 0.01 pm and cleaned before use. Iron pins, which were worn against the iron discs, and tungsten carbide pins, which were worn against the SS discs, were machined to a hemispherical tip with a radius of 3.2 mm and polished to the same finish. In order to reduce the implantation times required to deliver the high doses used in this study, a broad beam implantation system [17] operating at high current density (100 to 500 pA/cm’) was used. The implantation energy was fixed at 60 keV. The temperature history of each disc was controlled during implantation. because this history is known to influence the post-implantation chemical and microstructural states and wear characteristics of near-surface layers [14,15, 18,191. The nitrogen distribution near the surface was measured after implantation using Auger electron spectroscopy (AES) coupled with ion beam sputtering. Conversion electron Miissbauer spectroscopy (CEMS) and X-ray diffraction (XRD), which were used before and periodically during the wear tests to determine the nitride phases formed as consequences of implantation and wear-induced transformations, are discussed in a companion article [20]. Wear tests were conducted differently on the iron and SS discs. A 5 N normal load was applied between

0 1991 - Elsevier Science Publishers B.V. (North-Holland)

VII. METALS ,’ TRfBOLOGY

132

R. Wei et al. / Ultrahrgh dose Ni-implanted Fe and srainless steel

all iron pins and discs and the disc mass loss was measured after every few hours of wear testing using a microbalance accurate to within 10 pg to determine the mass removed over the active wear surface (1.7 cm i.d. to 4.5 cm o.d.). For the SS discs, on the other hand, a series of 1 h tests were run in which the normal load was increased incrementally for each test beginning at 0.1 N. Even at the lowest load, an unimplanted SS disc exhibited a substantial mass loss rate (mg/(N km)) and a rough surface suggestive of severe adhesive wear. In contrast, the mass loss rates from implanted discs were negligible and they remained relatively smooth until the applied normal load mcreased to the critical value where the implanted layer failed. The lubricant used in all tests was 10% oleic acid in kerosene. The average relative sliding speed between the pins and discs was 13 cm/s for all tests. More detailed information concerning the wear testing and implantation procedures is given elsewhere [14.15].

to about 38O“C. The figure also shows that this trend toward deeper penetration is facilitated by higher doses (to at least 1 X lOI9 N,/cm”) provided the temperature is maintained near 400 o C. The depth achieved at this high dose (1.7 pm) is an order of magnitude greater than the mean ballistic range of the ions. It is noted that temperatures substantially above 400 o C result in excessively rapid nitrogen diffusion and substantial nitrogen loss but slightly higher temperatures can be maintained provided the dose rate is increased [16]. Fig. 2 shows that AES concentration profiles measured in nitrogen-implanted SS exhibit essentially the same trends as those observed in iron. The following specific differences are, however, observed: (1) disc temperatures can be held higher during SS implantation without inducing excessive diffusion rates, (2) the nitrogen concentration at the ultrahigh dose is flat at about the value of the chromium concentration (18%). and (3) the thickness of the nitrogen-rich layer is greater for the ultrahigh dose SS. This thickness (> 11 km) is, in fact, two orders of magnitude greater than the ballistic range of the implanted nitrogen. Finally, it is noted that retained nitrogen doses determined from integration of profiles like those in figs. 1 and 2 and X-ray diffraction data are in good agreement with implanted doses for SS. For iron, retained nitrogen doses are generally less than corresponding implanted doses but they come into closer agreement as implantation temperatures approach 400 ’ C 1201.

3. Results and discussion 3. I. A ES [email protected] When iron is maintained at a low temperature while it is implanted with nitrogen, a conventional concentration profile [21] like the one corresponding to 1 x 1Ol7 N/cm2 and 90°C in fig. 1 is measured. If the dose is increased and the temperature is kept low (dashed line in fig. 1) the profile remains essentially unchanged. Fig. 1 shows, however, that increasing the dose beyond 1 X 10” N,/cm” induces deeper penetration of the nitrogen provided the implantation temperature is raised

3. -7. Wear testing

of irott

Typical wear test results obtained for iron implanted to 1 x 10” N,/cm’, which are shown in fig. 3, are =I-r

I

NITROGEN

IMPLANTED

N2/cmZ.

90

IRON

PURE

k10

‘C,

260

+I

380

+50

100

/.+rnZ

0 ‘C,

500

m/cm’

‘C.

500

~/‘crnz

430

L-50

Oc.

500

,uA/cm

0.2

,. 0.2

0.4

0.6

0.8 DEPTH

Fig. 1. Typical AES nitrogen

1 .o

1.2

1.4

profiles

measured

1.6

(pm)

concentration

in iron.

1.6

R. Wei er al. / Ultruhigh dose A’-rmplmred

NITROGEN

IMPLANTED

304

SS

340

*so

Oc,

500

,M/cm=

380

C5O

‘C,

100

/@.A/cm=

Nz/cm2,

380

k50

‘C.

100

,~+m*

N2/cm2,

510

?lO

‘C.

500

/la/cm2

N,/cm2,



0.4

0.2

2

I 4

I 6 DEPTH

Fig.

2. Typical

133

Fe md stainless steel

, 8

I 10

0.6

I 12

(pm)

AES nitrogen concentration profiles measured in AISI 304 stainless steel.

consistent with data obtained in previous tests [14.16]. They show that nitrogen implantation induces substantial improvements in the wear resistance of iron. Increasing the temperature at which the disc is maintained during implantation induces increased wear resistance up to a temperature of 380°C. When the implantation temperature is increased further (430 o C), the wear resistance degrades slightly. This suggests the most wearresistant surface develops on iron when it is held at a temperature near 380’ C during implantation to a high

dose (1 X 10” N,/cm’). Implantation at a low temperature (e.g. 260” C) to doses above - 1 x lOI N,/cmZ results in negligible nitrogen diffusion and Fe,N forms. When the temperature is increased to - 380 o C, sufficient nitrogen diffusion occurs so a thick layer of wearresistant nitride (Fe,N) forms 1201. Increasing the temperature beyond - 380 to 430°C appears to induce such rapid nitrogen diffusion that much less Fe,N is present [20] and the surface wear resistance decreases from its maximum value.

NITROGEN MEAN

IMPLANTED

SLIDING

SPEED

PURE

IRON

-

cm/s

13

LUERICATE~

-.I

NORMAL

LOAD

DOSE

1 X 10

-

-,85

N N&l2

4

ii 2 260

:: 2i

510

*G,

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/d/cm’ jLA/cm2 pA/cm2

430

+lO

*C.

500

380

+50

Oc,

100

TIME

(hr)

2

WEAR

Fig. 3. Effect of temperature during nitrogen implantation to a high dose on the wear behavior of iron. VII. METALS / TRIBOLOGY

R. Wei et al. / Ultmhigh dose Ni-implanted

134

Fe and stainless steel

NITROGEN MEAN

IMP~NTED

SLlDlNG

SPEED

PURE

IRON

-

cm/s

13

LUBRICATED UNIMPLANTED

20

during

~

-

1 X 10”

DOSE

RATE

nitrogen

implantation

5 N N&-nz2

500

/.&An/cm

100

80

(hr)

to an ultrahigh

dose on the wear behavior

NITROGEN

IMPLANTED

MAYAN SLIDING

SPEED

PURE

IRON

-

cm/s

13

LUERlCATED UNlMPL4NTET

NORMAL

LOAD

TEMPERATURE

-

5 N

-

430

+50

OC

z

r!

4

% s :: si

1 X 10”

N2/cm2,

100

/.&m2

1 X 10”

N2/cm2,

500

p/cm2

2

WEAR

Fig. 5. Effect of implanted

nitrogen

of iron.

43O’C after 100 h of wear testing is about 1 mg and that this corresponds to removal of a layer - 0.1 urn thick. Thus, the fig. 1 data suggest the 1.7 urn thick N-rich layer should wear for - 1700 h under the conditions of this accelerated wear test. The effect of dose on the wear resistance of iron discs implanted to doses ranging to an ultrahigh level at an implantation temperature of 430 o C is shown in fig. 5. It shows that the highest dose produces the most wear-resistant layer - CEMS and XRD again indicate it contains the nitride Fe,N [20]. Because fig. 1 shows that this dose also results in the thickest layer, the results of

I

6~

-

-

60 TIME

The beneficial effect of implantation temperature on the wear characteristics of iron discs implanted to an ultrahigh dose (1 X lOI Nz/cm2) is illustrated by the data of fig. 4. At this higher dose, however, increasing the implantation temperature from 380 to 430° C induces the formation of the more wear resistant surface. This result is consistent with previous results [14,16], the AES data of fig. 1 and CEMS/XRD data [20]. It is suggested that the higher temperature is beneficial in this case not only because of the higher dose, but also because the nitrogen was delivered at a higher dose rate. It is noted that the mass loss from the disc implanted at

0

LOAD

DOSE

40 WEAR

Fig. 4. Effect of temperature

NORMAL

TIME

(hr)

dose on the wear behavior

of iron.

R. Wei ei al. /

Ultrahigh dose Ni-implanted

the two figures indicate the lowest wear rate will endure for the longest time under the ultrahigh dose, high dose rate (500 uA/cm’), 430 o C implantation temperature conditions where a thick, wear-resistant N-containing layer is produced. Finally, it is noted from the data of figs. 3, 4 and 5 that the lowest initial ( < 20 h) wear rate is realized in the disc implanted to 1 X 10” N,/cm’ at 380” C. The small difference between this wear rate and that of the 1 x 1019 N,/cm2 at 43O’C disc is considered to be a consequence of the fact that more sputter-induced roughening occurred during implantation to the ultrahigh dose. It is also possible, however, that the higher initial wear rate on the ultrahigh dose disc was partially due to a lower near-surface concentration of nitrogen. 3.3.

Wear

testing

of

304 stainless steel

Nitrogen implantation has been shown to reduce the wear rate of SS dramatically by facilitating wear under mild adhesive conditions rather than the severe conditions that develop for unimpl~ted surfaces. This mild adhesive wear state persists until the critical load at which the implanted layer fails is exceeded ]1.5.16]. This study focused on the determination of this critical load and how it is affected by implantation parameters.

Fe und stcrinless steel

735

Typical results showing the effects of implanted nitrogen dose and the temperature at which the discs were maintained during implantation on the critical load are shown in fig. 6. In general they show that the critical load continues to increase with dose up to the maximum value investigated (1 X 1019 N,/cm*). It is particularly noteworthy that the greatest critical load for these nitrogen-implanted discs at > 120 N is three orders of magnitude greater than the critical load for unimplanted ones (< 0.1 N). The data also show that the critical load increases with implantation temperature up to the maximum value investigated (600 o C) at all but the highest dose level investigated where 500°C appears to be near optimal. These results are generally consistent with the AES data of fig. 2 which indicate that the higher dose and temperature levels result in the formation of thicker nitrided layers. It is noted that CEMS and XRD data [20] show that (1) c-phase nitrides tend to form in the SS under moderate dose (I 4 x IO” N,/cm2) low temperature conditions, (2) nitrogen is generally present in solid solution for surfaces implanted to high doses (4 x lOI N,/cm’ to 1 x 10’s N,/cm’ ) at temperatures 2 400 o C, and (3) v-CrN forms in SS discs implanted to 1 X 1019 N,/cm’ at high temperatures ( 2 500” C). 3.4. Surface sputtering and roughness induced ty ultrahigh dose implantation

TEST MACHINE LOAD LIMIT 1x ly

NITROGEN

IMF~NTED

MEAN SLIDING

304

SPEEO -

SS

13 cm/s

LUBRICATED

1 ocI-

BC I-

Ion implantation with 60 keV nitrogen ions to ultrahigh doses induces significant sputtering and an attendant increase in surface roughness (from 0.01 pm to - 0.1 pm for both materials). High-frequency acoustical noise. which was not apparent during wear testing of discs implanted to doses at or below 1 x 10” N,/cm’. was emitted from these discs during the early phase of their wear testing. This noise subsided after a few hours of wear testing.

9 2

s

4. Conclusions 6Cl-

2 g 0

4cl-

2( )-

s

I

I

I

18

10” DOSE (Ng’cm

210 )

lo’*

Fig. 6. Effect of implanted temperature and nitrogen dose on the critical load for AISI 304 stainless steel.

Increases in nitrogen dose implanted into iron continue to induce increased diffusion and thicker nitride layer formation to doses as high as 1 x lOI N,/cm’ provided the surface is maintained at an elevated temperature (near 400°C) during implantation. The layer formed is not only thick but it contains the nitride that is most resistant to sliding wear (Fe4N). Iron discs implanted under these conditions exhibit wear rates an order of magnitude below those for unimplanted discs. Increases in implanted nitrogen dose into SS continue to induce increased diffusion and thicker nitrogen-bearing layers to doses as high as 1 x 1019 Nz/cm’ provided the surface is maintained at an elevated temperature during impl~tation. Such layers increase the VII. METALS / TRIBOLOGY

736

R. Wei et al. / ~ifrahigh dose No-~~~lanfed Fe and sfainless steel

load associated with the transition between mild and severe sliding, adhesive wear by a factor greater than 1000. The implantation temperature that induces the maximum critical load in SS at this ultrahigh dose is near 500 o C. The ultrahigh doses that produced the most beneficial effects in this study are about two orders of magnitude greater than those identified as optimal by previous researchers who implanted at near-ambient temperatures.

Acknowledgements The financial support provided by the National Science Foundation (Grants MSS-87~8 and MSS8617811) and the Argonne National Laboratory (DOE Contract 0372401) and the technical support of Igor Ivanov (Charles Evans & Assoc.), who made the AES measurements are gratefully acknowledged.

References [I] S.R. Shepard and N.P. Suh, J. Lubr. Technol. 104 (1982) 29. [2] S. Fayeulie, in: Ion Implantation 1988, ed. F.H. WBhlbier (Tram Tech, 1988) p. 327. [3] S. Lo Russo, P. Mazzoldi. I. Scotoni, C. Tosello and S. Tosto, Appl. Phys. Lett. 34 (1979) 627. [4] R.N. Bolster and I.L. Singer. Appl. Phys. Lett. 36 (1980) 208. [5] G.K. Hubler and F.A. Smidt, Nucl. Instr. and Meth. B7/8 (1985) 151.

WI S. Fayeulfe, Wear 107 (1986) 61.

[71 S. Fayeulle and D. Treheux, Nucl. Instr. and Meth. B19/20 (1987) 216. 181S. Hogmark, H. Khosroupour and M. Braun, in: Ion Implantation and Plasma Assisted Processes, eds. B.F. Hochman, H. Solnick-Legg and K.O. Legg (ASM Intemational, 1988) p. 21. 191 H. Dimigen, K. Kobs, R. Leutenecker, H. Ryssel and P. Eichinger, Mater. Sci. Eng, 69 (1985) 181. DOI I.L. Singer and J.S. Murday, J. Vat. Sci. Technol. 17 (1980) 327. PII T. Fujihana, Y. Okabe and M. Iwaki, Mater. Sci. Eng. All5 (1989) 291. WI N.E.W. Hartley, Treatise on Mater. Sci. and Technol. 18 (1980) 321. 1131 R.B. Alexander, in: Ion Implantation and Piasma Assisted Processes, eds. B.F. Hochman, H. Solnick-Legg and K-0. Legs (ASM Intemation~, 1988) p. 17. D41 R. Wei, P.J. Wilbur, W.S. Sampath, D.L. Williamson, Y. Qu and L. Wang, J. Trib. 112 (1990) 27. 1151 R. Wei. P.J. Wilbur, W.S. Sampath, D.L. Williamson and L. Wang. Lubr. Eng. 47 (1991) 326. 1161 R. Wei, P.J. Wilbur, W.S. Sampath, D.L. Williamson and L. Wang, J. Trib. 113 (1991) 166. 1171 P.J. Wilbur and L-0. Daniels. Vacuum 36 (1986) 5. I181 D.L. Williamson, Y. Qu, R. Wei, W.S. Sampath, and P.J. Wilbur, Mater. Res. Sot. Symp. Proc. 128 (1989) 409. iI91 D.L. Williamson, L. Wang, R. Wei. and P.J. Wilbur, Mater. Lett. 9 (1990) 302. PO1 D.L. Williamson, 0. Ozturk, S. Glick, R. Wei and P.J. Wilbur, these Proceedings (7th Int. Conf. on Ion Beam Modification of Materials, Knoxville, TN, USA, 1990) Nucl. Instr. and Meth. B59/60 (1991) 737. [21] G. Terwage, M. Piette, F. Bodart and W. Mliller, Mater. Sci. Eng. All5 (1989) 25.