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Nitrogen implanted steels-aspects of oxidation and diffusion during wear
Nuclear Instruments and Methods in Physics Research B19/20 (1987) 263-267 North-Holland, Amsterdam
263
NITROGEN IMPLANTED STEELS- ASPECTS OF OXIDATION AND DIFFUSION DURING WEAR J.T.A. P O L L O C K and R.A. C L I S S O L D CS[RO Division of Chemical Physics, Lucas Heights Research Laboratories, Sutherland, NSW 2232, Australia
P.J. P A T E R S O N and C.J. V E I T C H RMIT, Applied Physics, Melbourne, Victoria 3001, Australia
An assumption of many nitrogen implanted wear studies has been the diffusion of nitrogen during the wear process; this migration was proposed to explain the retention of wear resistance at depths beyond that of the implant zone. Recent ion implantation studies have suggested a favourable role for modified oxidation wear, some associated with nitrogen implants, others with the implantation of known oxide-forming elements. Using Auger electron spectroscopy concentration/depth profiling data, aspects of oxidation and possible diffusion in mild steel are examined. It is shown that extensive diffusion does not occur but there is a modification of oxidation-wear in the presence of significant nitrogen.
1. Introduction Ion implantation of nitrogen has enhanced the resistance of steels to mild wear [1]. Ion energies of 50-100 keV are generally employed. At these energies and in the absence of abnormal thermal diffusion effects, implanted nitrogen is restricted to a depth of about 400 nm from the immediate surface. Obviously, wear causes a loss of surface material and reduced wear rates would not be long maintained if dependent upon the improved characteristics of the shallow implanted layer. However, many laboratory results and field trials [2,3] have demonstrated that the enhancement of wear resistance is retained when many times the implanted depth has been worn away. Explanations of this retention are based on the mechanism proposed by Dearnaley and Hartley [4] who, assuming that wear resistance was primarily the result of dislocation blocking, proposed the migration of nitrogen during the wear process. This migration mechanism was supported by the experimental evidence of Lo Russo et al. [5] who reported 20% nitrogen in a wear track cut greater than 20 times the implant depth. Other studies [3,6-11], although interpreted as being supportive of the mechanism, do not provide evidence of such deep migration. This is especially true of lubricated wear. In particular, the effect of wear induced surface roughness on the data has not been considered, although it can offer an equally satisfactory explanation for some data [12]. In this paper, Auger electron spectroscopy (AES) determined concentration/depth profiles of wear tracks 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
are used to examine whether nitrogen migration occurs and if implantation modifies the oxidative-wear of mild steel.
2. Experimental Disc samples of bright mild steel were polished to 1 /~m diamond before implanting with unanalysed nitrogen at 50 keV to a nominal dose of 2.5 × 1017 ions cm -2 using a beam current of about 10 /~A cm -2. Details of the alcohol lubricated ball-on-disc system have been reported elsewhere [11]. Wear tracks were cut using a light applied load of 20 g, which is suitable for mild steel. RouEhness profiles within the tracks were determined by optical interferometry using both white and m o n o c h r o m a t i c light. Wear tracks were concentration/depth profiled for N, O, C and Fe by AES in combination with argon sputtering using conditions described elsewhere [11]. Several samples, implanted and unimplanted, were heat treated in flowing oxygen at 375-550 K and similarly profiled to obtain a measure of comparative oxidation rates. A number of tracks cut in both implanted and unimplanted samples were examined using SCanning electron microscopy (SEM).
3. Results Auger electron spectroscopy concentrations are expressed as oxygen/iron or nitrogen/iron ratios normalII, METALS
264
z o z ILl L.J Z 8
J.T.A. Pollock et a L / Nitrogen implanted steels during wear
-~,.,~ - , ~ • 20 _ "+ - .
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' 2h~)/~SOK
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~
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, "'~-~----{~-,- - 200
I 300
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. o . - Jn"
Fig. 1. Normalised oxygen and nitrogen concentration/depth profiles for unimplanted and implanted mild steel heat treated at 475 K for 2 h in flowing oxygen.
I
b.30m - -~-_o . . . . . . o----_______~__
-~ lO
DEPTH (nm)
I
~o _~ ~ _ ~ ,~
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I
0
,
As-lmplanted
"~" 5x1017N. cm_z~ i 100
i 300
2~
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DEPTH (nmJ
Fig. 3. Normalised oxygen and nitrogen concentration/depth profiles at the centre of wear tracks. Track lengths as indicated (R~ = average depth _+roughness in/~ m).
2h (-~ 550K - - o - Unimptonted Oxygen - - + - Implonfed Oxygen lanfed Nifrogen
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Fig. 2. As for fig. 1, 2 h at 550 K.
ised to the background value of these ratios. In the as-implanted condition, nitrogen peaks of 28 at.~; were measured about 200 nm from the surface, and accompanied by prominent tails extending a further 200 nm. These depths are based on a sputtering rate measured with an iron standard and may be too large if the oxygen/nitrogen concentrations cause significant deviation. Surface nitrogen concentrations were about 12 at.%. Near surface oxygen and carbon concentrations were about 50 at.%, the result of beam heating in a moderate vacuum, falling to background within 30 nm. Oxygen concentration/depth (subsequently called oxide thickness) data measured with samples oxidised at 373 K and showing a clear blocking action for implanted nitrogen have been reported previously [12]. An exponential growth in oxide thickness with time was measured for unimplanted steel and compared with a near constant thickness for the implanted steel. Data
measured at 475 K (fig. 1) and 550 K (fig. 2) show that~ as was expected, this blocking action decreases with increasing temperature, although some blocking role is still evident at 550 K. At 475 K, the blocking action is clearly related to the presence of nitrogen which, apart from the near surface, is relatively unchanged from the as-implanted profile. The decrease in oxygen concentration is sharp and coincides with an increase in nitrogen concentration. However, at 550 K, as well as a doubling of the oxygen layer thickness, significant changes in the nitrogen distribution are observed due both to surface loss and in-diffusion. The peak concentration is substantially altered and the blocking action for oxide growth diminished. In contrast to the as-implanted steel, nitrogen is not observed at the sample surface at either temperature, appearing first about 50 nm below the surface. Data measured for a series of wear tracks cut in a single implanted sample are shown in fig. 3. Average depths and roughness measured from interferograms are included; a 100 m track length is equivalent to 21 minutes' wear time. An oxide growth restricting role for nitrogen is especially clear with the 30 and 125 m tracks. The data measured with the 250 m track, showing > 90% loss of implanted nitrogen give less clear support of a continuation of this blocking action. The coincidental fall in oxygen with a rise in nitrogen was observed with all samples containing significant residual nitrogen. Nitrogen was not detected in tracks with an average depth > 500 rim. Only one track had a significant surface concentration of nitrogen; all others were nitrogen free over the first 50 nm or so.
J.T.A. Pollocket aL/ Nitrogenimplantedsteelsduringwear 2L,
I
I
~ q , ~
i
30 & 60m TRACKS. --o-- Unimptonted + 2.5x1017Ncm-2
Pox 16 B
5 + O.1
--n < IK t~
o 8
100
200 DEPTH(nm)
300
Fig. 4. Normalised oxygen concentration/depth profiles at the centre of wear tracks cut in unimplanted and implanted mild steel. Track lengths as indicated. R a in/~m.
I
I
125m
24.
I
TRACK
16 o
"\'\'\.i°~±
265
The general blocking action of nitrogen during the wear process may be more easily assessed by the data presented in figs. 4-6, where oxide thicknessess measured with unimplanted samples are compared with those measured with unimplanted samples worn under identical conditions. The oxide profiles with the unimplanted sample are almost unchanged with track length. In contrast, the data measured with the implanted steels show significant changes, the oxide thicknesses approaching the values obtained with the unimplanted data with increasing wear time. Scanning electron microscopy reveal a trend towards a common surface appearance with wear time. The appearance of the 30 and 60 m tracks cut in implanted samples suggested a smeared soft layer supported by the harder underlying layer produced by nitrogen implantation [13]. When combined with AES results this suggested that the smeared layer, previously noted as a 'burnishing' [11], is the oxide coating or is controlled by the lubricating effect of the coating. In contrast, the unimplanted samples showed a finely ploughed appearance irrespective of wear time over the range investigated. With wear time, the appearance of the surfaces became similar as the modifying effect of implanted nitrogen disappeared and the unimplanted appearance predominated.
0.05
.,..i <
,',"
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8
z
Ra 035+iO6" " o , I
I
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200 DEPTH (nm)
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300
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Discussion
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Fig. 6. As fig. 4.
I
300
The AES data presented in this and an earlier study [12] clearly indicate that significant nitrogen diffusion does not occur during the wear of mild steel. Any apparent movement is over depths similar to sample roughness (see fig. 3). This agrees with the limited or lack of diffusion reports on SAE 3135 steel [14] and TiA1V [15]• However, it is clear that the presence of nitrogen has a modifying role in the wear of mild steel through its influence on oxidation processes. It is appropriate to discuss the AES data in the context of nitrogen-modified oxidation-wear processes in steel. The static oxidation data describing the restrictive role of nitrogen in oxide film growth at temperatures where nitrogen diffusion should be slow is not unexpected since oxide growth in iron occurs by cationic movement [16]. Presumably, the formation of nitrides and the general blocking action of the implanted nitrogen retards this process (at 375 and 475 K, fig. 1) until the temperature is sufficiently high (550 K, fig. 2) for a diffusion-controlled modification of the as-implanted profile. Of course, these comments are only for two hours at moderate temperature and much higher temperatures may be needed for similar modification during a dynamic wear process. Nevertheless, the oxide growth blocking action of implanted nitrogen is clear II. METALS
266
J.T.A. Pollock et aL / Nitrogen implanted steels during wear
and will take place during mild wear albeit within a complex tribological system. The role of oxide films in reducing friction between contacting metallic surfaces has been known since the vacuum-based studies of Jacob in 1912 [17]. However, detailed experimental investigation of the role of oxide films in wear have not been carded out with the zeal devoted to severe wear, where the generation of large metallic particles implies extensive plastic deformation and allows oxidation processes to be ignored. The significant exception has been the work carried out by Quinn and his coworkers since 1962 (see e.g. ref. [18]). Their work was founded on the premise that in the mild wear regime, oxidation processes are involved which generate both adherent oxide layers and particles. Wear rate is determined by the delamination of oxide layers when a critical thickness is reached. It is important to note that ion implantation is generally successful in applications or tests in which mild wear is predominant. Recent ion implantation-wear studies have contained ~uggesrions of an important role for implantation-modified oxidation wear. Some work has indicated that reduced wear is only obtained when an ion implantation induced oxide is present, e.g. a nitrogen-implanted TiA1V alloy [15]. Doyle et al. [19] have indicated variations in oxide thickness at the surface of stainless steels. According to Quinn [18], both oxide composition and growth rate control the wear rate during mild oxidation controlled wear. No data are available on the oxide compositions formed during the wear of nitrogenimplanted steels. It is, however, possible to make preliminary comments based upon the relative concentrations of oxygen and nitrogen at the wearing surface and variation with depth and wear time. Relarive proportions of oxygen and nitrogen are available for mild steel (current work), and 304 and 15-5PH stainless steels [19]. Oxide film thickness and variation of oxygen concentration with depth reveal an interdependence with the nitrogen implant layer. Studies on the soft steels 304 [18] and mild (current work) show reduced oxygen thickness layers when wear takes place within the implanted zone, e.g. figs. 3b and c. This suggests that the formation of iron or other transition metal nitrides as a result of implantation reduces the availability of oxide-forming elements. However, this reduction is not below the level needed to form a lubricating layer. These steels exhibit little change in friction characteristics after nitrogen implantation, supporting the argument that the same or similar oxide lubricating films are present. It should also be noted that, in the present study, both static and dynamic results show an absence of nitrogen in the near (-- 50 nan) region implying that the oxide film is the primary contact layer. As noted earlier, this nitrogen depletion is not unexpected since iron oxidises by cationic move-
ment [16] and confirms a blocking role for this outward movement of iron atoms. However, it may be a combination of chemical inhibition and mechanical hardening that causes the formarion of thinner, more adherent, oxide layers and an improved ramning-in period during the early stages of wear. The observation of 'burnished' tracks with implanted samples has been noted by ourselves and other workers. The retention of wear resistance in some steels when this layer of modified oxide is removed needs intense experimental study. There is a paucity of topographical or SEM studies of worn surfaces which seek differences or similarities between implanted and unimplanted samples when extensive wear has taken place. The SEM observations reported here suggest that the appearance of the wear surfaces is identical when nitrogen is absent.
5. Conclusions
1. It is unlikely that nitrogen migration occurs during sliding wear of implanted mild steel. 2. Implanted nitrogen creates a thinner, probal~ly more adherent lubricating oxide film during wear. The explanation for any retention of enhanced wear resistance beyond the implanted layer must be linked to this improved ramning-in period. 3. The SEM appearance of worn unimplanted and worn implanted surfaces are different when nitrogen is present, but similar when it has been worn away. The authors are grateful to M.D. Scott and D. Stevenson of the CSIRO Division of Chemical Physics for ion implantation and K. Watson of the Australian Atomic Energy Commission for scanning electron microscopy.
References
[1] N.E.W. Hartley, in: Treatise on Materials Science and Technology Vol. 18 (Academic Press, New York, 1980) p. 321. [2] G. Dearnaley, in: Ion Implantation Metallury, eds., C.M. Preece and J.K. Hirvonen (/LIME, New York, 1980) p. 1. [3] H. Dimigen, K. Kobs, R. Leutenecker, H. Ryssel and P. Eichinger, Mater. Sci. Eng. 69 (1985) 181. [4] G. Dearnaley and N.E.W. Hartley, in: Proc. 4th Conf. Scientific and Industrial Applications of Small Accelerators (IEEE, New York, 1976) p. 20. [5] S. Lo Russo, P. Mazzoldi, I. Scotoni, C. Tosello, and S.1 Tosto, Appl. Phys. Lett. 34 (1979) 629. [6] S. Fayeuille, D. Trelieux, P. Guiracdenq, T. Barabon, J. Touseet and M. Robelet, Scripta Metall. 17 (1983) 459.
J.T.A. Pollock et al. / Nitrogen implanted steels during wear
[7] G. Marest and N. Moncoffee, Appl. Surf. Sci. 20 (1985) 205. [8] R.N. Bolster and I.L. Singer, Appl. Phys. Lett. 36 (1980) 208. [9] Cui Fu-Zhai, U. Heng-de and Zhang Xias-Zhone, Nucl. Instr. and Meth. 209/210 (1983) 831. [10] G. Deamaley, Radiat. Eft. 63 (1982) I. [11] J.T.A. Pollock, M.J. Kenny and P.J. Patterson in: Ion Implantation and Ion Beam Processing of Materials, eds., S.K. Hubler, C.R. Clayton and C.W. White (NorthHolland, Amsterdam, 1984) p. 691. [12] J.T.A. Pollock, M.D. Scott, M.J. Kenny, P.J.K. Patterson and C.J. Veitch, Appl. Surf. Sci. 22/23 (1985) 128.
267
[13] J.B. Pethica, R. Hutchings and W.C. Oliver, Nucl. Instr. and Meth. 209/210 (1983) 995. [14] T.J. Sommerer, E.B. Hale, K.W. Burris and R.A. Kohser, Mater. Sci. Eng. 69 (1985) 149. [15] R. Hutchings and W.C. Oliver, Wear 92 (1983) 143. [16] A. Galerie, M. Caillet and M. Poms, Mater. Sci. Eng. 69 (1985) 329. [17] C. Jacob, Ann. Phys. (Leipzig) 38 (1912) 126. [18] T.F.J. Quinn, in: Microscopic Aspects of Adhesion and Lubrication, ed., J.M. Georges (Elsevier, Amsterdam, 1982) p. 579. [19] R.L. Doyle, D.M. Follstaedt, S.T. Picraux, F.G..Yost, L.E. Pope and J.A. Knapp, Nucl. Instr. and Meth. B7/8 (1985) 166.
Note added in proof (erratum) Due to a calibration error all depths given on Figs. 1-6 as well as within the text but drawn from these figures are a factor of 2 too large. All depths are equally affected and as the study is of comparative nature, the conclusions drawn remain unaltered.
II. METALS
263
NITROGEN IMPLANTED STEELS- ASPECTS OF OXIDATION AND DIFFUSION DURING WEAR J.T.A. P O L L O C K and R.A. C L I S S O L D CS[RO Division of Chemical Physics, Lucas Heights Research Laboratories, Sutherland, NSW 2232, Australia
P.J. P A T E R S O N and C.J. V E I T C H RMIT, Applied Physics, Melbourne, Victoria 3001, Australia
An assumption of many nitrogen implanted wear studies has been the diffusion of nitrogen during the wear process; this migration was proposed to explain the retention of wear resistance at depths beyond that of the implant zone. Recent ion implantation studies have suggested a favourable role for modified oxidation wear, some associated with nitrogen implants, others with the implantation of known oxide-forming elements. Using Auger electron spectroscopy concentration/depth profiling data, aspects of oxidation and possible diffusion in mild steel are examined. It is shown that extensive diffusion does not occur but there is a modification of oxidation-wear in the presence of significant nitrogen.
1. Introduction Ion implantation of nitrogen has enhanced the resistance of steels to mild wear [1]. Ion energies of 50-100 keV are generally employed. At these energies and in the absence of abnormal thermal diffusion effects, implanted nitrogen is restricted to a depth of about 400 nm from the immediate surface. Obviously, wear causes a loss of surface material and reduced wear rates would not be long maintained if dependent upon the improved characteristics of the shallow implanted layer. However, many laboratory results and field trials [2,3] have demonstrated that the enhancement of wear resistance is retained when many times the implanted depth has been worn away. Explanations of this retention are based on the mechanism proposed by Dearnaley and Hartley [4] who, assuming that wear resistance was primarily the result of dislocation blocking, proposed the migration of nitrogen during the wear process. This migration mechanism was supported by the experimental evidence of Lo Russo et al. [5] who reported 20% nitrogen in a wear track cut greater than 20 times the implant depth. Other studies [3,6-11], although interpreted as being supportive of the mechanism, do not provide evidence of such deep migration. This is especially true of lubricated wear. In particular, the effect of wear induced surface roughness on the data has not been considered, although it can offer an equally satisfactory explanation for some data [12]. In this paper, Auger electron spectroscopy (AES) determined concentration/depth profiles of wear tracks 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
are used to examine whether nitrogen migration occurs and if implantation modifies the oxidative-wear of mild steel.
2. Experimental Disc samples of bright mild steel were polished to 1 /~m diamond before implanting with unanalysed nitrogen at 50 keV to a nominal dose of 2.5 × 1017 ions cm -2 using a beam current of about 10 /~A cm -2. Details of the alcohol lubricated ball-on-disc system have been reported elsewhere [11]. Wear tracks were cut using a light applied load of 20 g, which is suitable for mild steel. RouEhness profiles within the tracks were determined by optical interferometry using both white and m o n o c h r o m a t i c light. Wear tracks were concentration/depth profiled for N, O, C and Fe by AES in combination with argon sputtering using conditions described elsewhere [11]. Several samples, implanted and unimplanted, were heat treated in flowing oxygen at 375-550 K and similarly profiled to obtain a measure of comparative oxidation rates. A number of tracks cut in both implanted and unimplanted samples were examined using SCanning electron microscopy (SEM).
3. Results Auger electron spectroscopy concentrations are expressed as oxygen/iron or nitrogen/iron ratios normalII, METALS
264
z o z ILl L.J Z 8
J.T.A. Pollock et a L / Nitrogen implanted steels during wear
-~,.,~ - , ~ • 20 _ "+ - .
J
' 2h~)/~SOK
'
~o
'
R, = O.L~._*0 06 ~ o
~
--o-- Unimptanfed-Oxygen - - + - - Implonfed Oxygen - . a . - Implanfed Nifrogen
~,= t.~
° 0
_ _ _ o
. . . .
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.
.
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.
.
.
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+/,- \
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d. 250 m.
~
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, "'~-~----{~-,- - 200
I 300
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. o . - Jn"
Fig. 1. Normalised oxygen and nitrogen concentration/depth profiles for unimplanted and implanted mild steel heat treated at 475 K for 2 h in flowing oxygen.
I
b.30m - -~-_o . . . . . . o----_______~__
-~ lO
DEPTH (nm)
I
~o _~ ~ _ ~ ,~
I
I
0
,
As-lmplanted
"~" 5x1017N. cm_z~ i 100
i 300
2~
"9"
DEPTH (nmJ
Fig. 3. Normalised oxygen and nitrogen concentration/depth profiles at the centre of wear tracks. Track lengths as indicated (R~ = average depth _+roughness in/~ m).
2h (-~ 550K - - o - Unimptonted Oxygen - - + - Implonfed Oxygen lanfed Nifrogen
z 20 pz z
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+
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I
200
I
300
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400
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Fig. 2. As for fig. 1, 2 h at 550 K.
ised to the background value of these ratios. In the as-implanted condition, nitrogen peaks of 28 at.~; were measured about 200 nm from the surface, and accompanied by prominent tails extending a further 200 nm. These depths are based on a sputtering rate measured with an iron standard and may be too large if the oxygen/nitrogen concentrations cause significant deviation. Surface nitrogen concentrations were about 12 at.%. Near surface oxygen and carbon concentrations were about 50 at.%, the result of beam heating in a moderate vacuum, falling to background within 30 nm. Oxygen concentration/depth (subsequently called oxide thickness) data measured with samples oxidised at 373 K and showing a clear blocking action for implanted nitrogen have been reported previously [12]. An exponential growth in oxide thickness with time was measured for unimplanted steel and compared with a near constant thickness for the implanted steel. Data
measured at 475 K (fig. 1) and 550 K (fig. 2) show that~ as was expected, this blocking action decreases with increasing temperature, although some blocking role is still evident at 550 K. At 475 K, the blocking action is clearly related to the presence of nitrogen which, apart from the near surface, is relatively unchanged from the as-implanted profile. The decrease in oxygen concentration is sharp and coincides with an increase in nitrogen concentration. However, at 550 K, as well as a doubling of the oxygen layer thickness, significant changes in the nitrogen distribution are observed due both to surface loss and in-diffusion. The peak concentration is substantially altered and the blocking action for oxide growth diminished. In contrast to the as-implanted steel, nitrogen is not observed at the sample surface at either temperature, appearing first about 50 nm below the surface. Data measured for a series of wear tracks cut in a single implanted sample are shown in fig. 3. Average depths and roughness measured from interferograms are included; a 100 m track length is equivalent to 21 minutes' wear time. An oxide growth restricting role for nitrogen is especially clear with the 30 and 125 m tracks. The data measured with the 250 m track, showing > 90% loss of implanted nitrogen give less clear support of a continuation of this blocking action. The coincidental fall in oxygen with a rise in nitrogen was observed with all samples containing significant residual nitrogen. Nitrogen was not detected in tracks with an average depth > 500 rim. Only one track had a significant surface concentration of nitrogen; all others were nitrogen free over the first 50 nm or so.
J.T.A. Pollocket aL/ Nitrogenimplantedsteelsduringwear 2L,
I
I
~ q , ~
i
30 & 60m TRACKS. --o-- Unimptonted + 2.5x1017Ncm-2
Pox 16 B
5 + O.1
--n < IK t~
o 8
100
200 DEPTH(nm)
300
Fig. 4. Normalised oxygen concentration/depth profiles at the centre of wear tracks cut in unimplanted and implanted mild steel. Track lengths as indicated. R a in/~m.
I
I
125m
24.
I
TRACK
16 o
"\'\'\.i°~±
265
The general blocking action of nitrogen during the wear process may be more easily assessed by the data presented in figs. 4-6, where oxide thicknessess measured with unimplanted samples are compared with those measured with unimplanted samples worn under identical conditions. The oxide profiles with the unimplanted sample are almost unchanged with track length. In contrast, the data measured with the implanted steels show significant changes, the oxide thicknesses approaching the values obtained with the unimplanted data with increasing wear time. Scanning electron microscopy reveal a trend towards a common surface appearance with wear time. The appearance of the 30 and 60 m tracks cut in implanted samples suggested a smeared soft layer supported by the harder underlying layer produced by nitrogen implantation [13]. When combined with AES results this suggested that the smeared layer, previously noted as a 'burnishing' [11], is the oxide coating or is controlled by the lubricating effect of the coating. In contrast, the unimplanted samples showed a finely ploughed appearance irrespective of wear time over the range investigated. With wear time, the appearance of the surfaces became similar as the modifying effect of implanted nitrogen disappeared and the unimplanted appearance predominated.
0.05
.,..i <
,',"
4.
8
z
Ra 035+iO6" " o , I
I
100
200 DEPTH (nm)
I
300
Fig. 5. As fig. 4.
I
I
2/. z
250m TRACK ---o- Unimptonfed ---D- 2,Sx1017N cm-2
•
x
16 C,3 --J
'N, . ~ 0 . 6 + - . O . 0 4
r," 0 Z
8
Ra = 0.44_+0.06~'.~. "~-...~.
I
100
Discussion
"°~o..........._
O ~
I
200 DEPTH(nm)
Fig. 6. As fig. 4.
I
300
The AES data presented in this and an earlier study [12] clearly indicate that significant nitrogen diffusion does not occur during the wear of mild steel. Any apparent movement is over depths similar to sample roughness (see fig. 3). This agrees with the limited or lack of diffusion reports on SAE 3135 steel [14] and TiA1V [15]• However, it is clear that the presence of nitrogen has a modifying role in the wear of mild steel through its influence on oxidation processes. It is appropriate to discuss the AES data in the context of nitrogen-modified oxidation-wear processes in steel. The static oxidation data describing the restrictive role of nitrogen in oxide film growth at temperatures where nitrogen diffusion should be slow is not unexpected since oxide growth in iron occurs by cationic movement [16]. Presumably, the formation of nitrides and the general blocking action of the implanted nitrogen retards this process (at 375 and 475 K, fig. 1) until the temperature is sufficiently high (550 K, fig. 2) for a diffusion-controlled modification of the as-implanted profile. Of course, these comments are only for two hours at moderate temperature and much higher temperatures may be needed for similar modification during a dynamic wear process. Nevertheless, the oxide growth blocking action of implanted nitrogen is clear II. METALS
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J.T.A. Pollock et aL / Nitrogen implanted steels during wear
and will take place during mild wear albeit within a complex tribological system. The role of oxide films in reducing friction between contacting metallic surfaces has been known since the vacuum-based studies of Jacob in 1912 [17]. However, detailed experimental investigation of the role of oxide films in wear have not been carded out with the zeal devoted to severe wear, where the generation of large metallic particles implies extensive plastic deformation and allows oxidation processes to be ignored. The significant exception has been the work carried out by Quinn and his coworkers since 1962 (see e.g. ref. [18]). Their work was founded on the premise that in the mild wear regime, oxidation processes are involved which generate both adherent oxide layers and particles. Wear rate is determined by the delamination of oxide layers when a critical thickness is reached. It is important to note that ion implantation is generally successful in applications or tests in which mild wear is predominant. Recent ion implantation-wear studies have contained ~uggesrions of an important role for implantation-modified oxidation wear. Some work has indicated that reduced wear is only obtained when an ion implantation induced oxide is present, e.g. a nitrogen-implanted TiA1V alloy [15]. Doyle et al. [19] have indicated variations in oxide thickness at the surface of stainless steels. According to Quinn [18], both oxide composition and growth rate control the wear rate during mild oxidation controlled wear. No data are available on the oxide compositions formed during the wear of nitrogenimplanted steels. It is, however, possible to make preliminary comments based upon the relative concentrations of oxygen and nitrogen at the wearing surface and variation with depth and wear time. Relarive proportions of oxygen and nitrogen are available for mild steel (current work), and 304 and 15-5PH stainless steels [19]. Oxide film thickness and variation of oxygen concentration with depth reveal an interdependence with the nitrogen implant layer. Studies on the soft steels 304 [18] and mild (current work) show reduced oxygen thickness layers when wear takes place within the implanted zone, e.g. figs. 3b and c. This suggests that the formation of iron or other transition metal nitrides as a result of implantation reduces the availability of oxide-forming elements. However, this reduction is not below the level needed to form a lubricating layer. These steels exhibit little change in friction characteristics after nitrogen implantation, supporting the argument that the same or similar oxide lubricating films are present. It should also be noted that, in the present study, both static and dynamic results show an absence of nitrogen in the near (-- 50 nan) region implying that the oxide film is the primary contact layer. As noted earlier, this nitrogen depletion is not unexpected since iron oxidises by cationic move-
ment [16] and confirms a blocking role for this outward movement of iron atoms. However, it may be a combination of chemical inhibition and mechanical hardening that causes the formarion of thinner, more adherent, oxide layers and an improved ramning-in period during the early stages of wear. The observation of 'burnished' tracks with implanted samples has been noted by ourselves and other workers. The retention of wear resistance in some steels when this layer of modified oxide is removed needs intense experimental study. There is a paucity of topographical or SEM studies of worn surfaces which seek differences or similarities between implanted and unimplanted samples when extensive wear has taken place. The SEM observations reported here suggest that the appearance of the wear surfaces is identical when nitrogen is absent.
5. Conclusions
1. It is unlikely that nitrogen migration occurs during sliding wear of implanted mild steel. 2. Implanted nitrogen creates a thinner, probal~ly more adherent lubricating oxide film during wear. The explanation for any retention of enhanced wear resistance beyond the implanted layer must be linked to this improved ramning-in period. 3. The SEM appearance of worn unimplanted and worn implanted surfaces are different when nitrogen is present, but similar when it has been worn away. The authors are grateful to M.D. Scott and D. Stevenson of the CSIRO Division of Chemical Physics for ion implantation and K. Watson of the Australian Atomic Energy Commission for scanning electron microscopy.
References
[1] N.E.W. Hartley, in: Treatise on Materials Science and Technology Vol. 18 (Academic Press, New York, 1980) p. 321. [2] G. Dearnaley, in: Ion Implantation Metallury, eds., C.M. Preece and J.K. Hirvonen (/LIME, New York, 1980) p. 1. [3] H. Dimigen, K. Kobs, R. Leutenecker, H. Ryssel and P. Eichinger, Mater. Sci. Eng. 69 (1985) 181. [4] G. Dearnaley and N.E.W. Hartley, in: Proc. 4th Conf. Scientific and Industrial Applications of Small Accelerators (IEEE, New York, 1976) p. 20. [5] S. Lo Russo, P. Mazzoldi, I. Scotoni, C. Tosello, and S.1 Tosto, Appl. Phys. Lett. 34 (1979) 629. [6] S. Fayeuille, D. Trelieux, P. Guiracdenq, T. Barabon, J. Touseet and M. Robelet, Scripta Metall. 17 (1983) 459.
J.T.A. Pollock et al. / Nitrogen implanted steels during wear
[7] G. Marest and N. Moncoffee, Appl. Surf. Sci. 20 (1985) 205. [8] R.N. Bolster and I.L. Singer, Appl. Phys. Lett. 36 (1980) 208. [9] Cui Fu-Zhai, U. Heng-de and Zhang Xias-Zhone, Nucl. Instr. and Meth. 209/210 (1983) 831. [10] G. Deamaley, Radiat. Eft. 63 (1982) I. [11] J.T.A. Pollock, M.J. Kenny and P.J. Patterson in: Ion Implantation and Ion Beam Processing of Materials, eds., S.K. Hubler, C.R. Clayton and C.W. White (NorthHolland, Amsterdam, 1984) p. 691. [12] J.T.A. Pollock, M.D. Scott, M.J. Kenny, P.J.K. Patterson and C.J. Veitch, Appl. Surf. Sci. 22/23 (1985) 128.
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[13] J.B. Pethica, R. Hutchings and W.C. Oliver, Nucl. Instr. and Meth. 209/210 (1983) 995. [14] T.J. Sommerer, E.B. Hale, K.W. Burris and R.A. Kohser, Mater. Sci. Eng. 69 (1985) 149. [15] R. Hutchings and W.C. Oliver, Wear 92 (1983) 143. [16] A. Galerie, M. Caillet and M. Poms, Mater. Sci. Eng. 69 (1985) 329. [17] C. Jacob, Ann. Phys. (Leipzig) 38 (1912) 126. [18] T.F.J. Quinn, in: Microscopic Aspects of Adhesion and Lubrication, ed., J.M. Georges (Elsevier, Amsterdam, 1982) p. 579. [19] R.L. Doyle, D.M. Follstaedt, S.T. Picraux, F.G..Yost, L.E. Pope and J.A. Knapp, Nucl. Instr. and Meth. B7/8 (1985) 166.
Note added in proof (erratum) Due to a calibration error all depths given on Figs. 1-6 as well as within the text but drawn from these figures are a factor of 2 too large. All depths are equally affected and as the study is of comparative nature, the conclusions drawn remain unaltered.
II. METALS