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Improving the mechanical properties of steels using low energy, high temperature nitrogen ion implantation
Surface and Coatings Technology 83 (1996) 257-262
Improving the mechanical properties of steelsusing low energy, high temperature nitrogen ion implantation S.J. Bull’, A.M. Jones, A.R. McCabe AEA Techology,
Harwell, Oxfordshire, OX1 1 ORA, UK
Abstract The use of nitrogen ion implantation to increase the surface hardness of structural steels is well documented. Traditionally this involves the use of high energy nitrogen ion beams (approximately 100 keV), with a relatively low beam current density because high energy beams are necessary to produce the required penetration into the material to achieve a significant depth of hardened material. Hardening needs to occur in a region whose size is comparable with the scale of the deformation associated with the tribocontact. 100 keV nitrogen ions typically penetrate into steels only about 0.1 urn and the range of possible tribological applications is thus restricted by this shallow treatment depth. In plasma nitriding processes the nitrogen ions approach the substrate with much lower energies but the ion currents are sufficiently high to cause considerable substrate heating. In this study an ion beam process has been used which more closely approximates plasma nitriding conditions. This involves the use of comparatively low energy nitrogen ion beams (1-2 IceV), with higher beam current densities and, where necessary, additional heating to increase the sample temperature to approximately 400 “C at the surface. Secondary ion mass spectrometry has been used to assessthe extent of nitrogen diffusion into a range of steels. The nitrogen penetration depth (and profile shape) depends both on the ion beam parameters and the structure and composition of the steel. Additional experiments conducted with a supplementary supply of nitrogen gas in the form of a vacuum chamber backfill demonstrate that increasing the nitrogen supply can increase nitrogen take-up for some materials. For a 30 min implantation at 263 uA cm-’ the penetration depth was a substantial fraction of a micrometre and is greater than the profile produced by high energy implantation in some cases.The hardness of the implanted steels can be correlated with the extent of nitrogen take-up, the amount of diffusion during implantation and softening induced by annealing of the steel substrates. Considerable hardening is achieved for some steels (e.g. M2 high speed steel) whereas for other steels the improvements are less obvious (e.g. 304 stainless). These observations are discussed in the light of the nitridability of the steels. Ke~~or& Hardness; Ion implantation; Depth profiling; Nitriding; Steels
1. Introduction The use of ion implantation
to improve the mechanical
and tribological properties of metals is well documented: see for example Refs. [l-3]. Typically high energy (greater than 50 keV) ion beams are used in conventional ion implantation in order to achieve a sufficient thickness of treatment to have a measurable effect on the hardness or wear performance of the implanted surface. For instance nitrogen implantation in steels can lead to considerable increases in wear resistance and hardness [4-61. This is generally related to the formation of nitride precipitates [7-101 which depends on the precise composition of the steel; iron nitrides can form but 1 Present address: School of Materials, The University, Newcastle upon Tyne NE1 7RU, UK.
alloying elements which have hard, stable nitrides (e.g. titanium, chromium, aluminium, molybdenum and tungsten) can have a more significant effect. As the energy of ion implantation is reduced, the thickness of the implanted layer and hence the magnitude of any improvement in tribological properties produced is also reduced. Furthermore, it becomes increasingly difficult to measure the changes to surface properties which have occurred and low load microhardness testers or nanoindenters are necessaryto monitor the effects of the treatment [ 11-131. However, low energy ion sources are available which can deliver very high beam currents compared with conventional implantation sourceswhich could result in a greatly improved process efficiency if sufficient tribological benefits can be generated. Thus, in order to be effective, low energy, high current ion implantation needs to work in a different manner to
258
S.J. Bull et al./Swface and Coatings Technology 83 (1996) 257-262
ature rise of about 180“C at the sample. Additional radiant heating was supplied to raise the temperatureof the samplesto approximately 220 “C prior to implantation which rose to around 400 “C within 1 min of exposureto the ion beam and remained at that temperature for the duration of the implantation. A further set of materials was treated in the same way but with a backfill of nitrogen gas in the vacuum chamber, to an additional 2 x 10s4Torr, to assessthe additional effects of nitrogen supply from the environment (LEB). To establish the influence of the vacuum heat treatment alone, a duplicate set of steelswas treated for the same length of time, at the sametemperaturein a vacuum of approximately 1 x lo-” Torr. Comparison sampleswere produced by conventional ion implantation (CII) using 76 keV nitrogen (Nzi) from a plasma bucket ion source in the Harwell Blue Tank facility at a current density of about 2.5 1tAcm-‘. A dose of 5 x 1017nitrogen ions cni-2 was implanted over 6.7 h. The rise in sample temperature caused by implantation under these conditions was measured to be less than 180“C in similar runs and was less than 200 “C (i.e. below the minimum temperature limit of a control pyrometer) in this experiment.
conventional ion implantation. One such way in which this can be achievedis to allow the temperature of the component to rise so that the implantation takes place at about 300-500 “C rather than at less than 100“C which is normal for conventional ion implantation. This results in a processwhich is intermediate between ion implantation and plasma nitriding. Previous work has shown that considerablediffusion of implanted nitrogen into steel occurs for 2 kV treatment [ 141 at elevated temperature. In this study we have reduced the implantation energy to 1 kV and ~ investigated the effectsof nitrogen implantation into a range of steelsat 400 “C as a function of implantation environment. The effectsof the treatmentson near surfacestructure and hardnesshave been determinedand comparedwith those achievablefrom conventional ion implantation. 2. Experimental details 2.1. Sampleproduction
Metals were machined from standard bar stock and polished down to a 3 pm finish using diamond paste. The steels examined were divided into three groups: (1) tool steels(M2 and M42 in the unhardened state and Ol(NSOH) in the hardened and tempered condition); (2) engineering steels(En40B, 708 407 and 817 407); (3) stainlesssteels(304 BA, 316 2B and 430 BA). The nominal compositionsof thesematerialsare given in Table 1. All steelswere used in the as-receivedcondition and were given no heat treatments to changetheir original hardness. Low energy(LE) nitrogen ion implantation treatments were undertaken at normal incidence using a Kaufman type ion source running at 1 keV. The samples were implanted at a current density of approximately 265 PA cm-’ for 30 min to give a total dose of approximately 1.5 x 10” nitrogen ions cmW2 at a residual chamber pressure of approximately 2 x 10e4Torr. Implantation under theseconditions leads to a temper-
2.2. Depthp~ojiling Depth profiles were produced by secondaryion mass spectrometry(SIMS) using a caesiumprimary beam.To assessthe nitrogen profile the mass147CsN” molecular ion was used which was found to be not significantly influenced by interference from molecular ions from substrateelements.The depth scalewas establishedfrom the depth of the pit left after sputter profiling and assumes that a constant etch rate was maintained throughout the experiment.The retaineddosesestimated for the conventional ion implanted samplesfrom the SIMS traces were up to a factor of four less than the nominal implanted dose, whereas for low energy implanted samplesthey were about an order of magnitude lower. More precisemeasurementsof retained dose were not possiblewith the method used in this study.
Table 1 Nominal compositions of steels investigated (wt.%) Steel M2 -Ii%42 CKNS0H En40B BS 708 407 BS 817 407 304 BA 316 2B 430 BA
Fe Balance -Balance Balance Balance Balance Balance Balance Balance Balance
C 1.28 -1:05 0.95 0.25 0.38 0.36 0.045 0.05 0.1
Cr
MO
W
V
Co
Ni
P
s
Si
Mn
Mg
4.2 - 3.9 0.5 3.2 1.05 1.2 18.25 17.5 17.0
5.0 9.5 0.55 0.2 0.3 2.6 -
6.4 1.5 0.5 -
3.1 1.15 0.2 -
8.0 -
-
0.05 0.035 0.035 -
0.05 0.4 0.4 -
0.4 0.25 025 0.25 0.6 0.6 -
1.2 0.55 0.46 1.25 1.25 -
0.85 -
0.4 1.5 10.0 12.0 0.5
-
S.J. Bull et al./Surface and Coatings Technology83 (1996) 257-262
2.3. Hardness measurements
Knoop hardness measurementswere made on all treated surfacesusing a Matsuzawa DMH-2 instrument. Measurementsweremade at 25 g load and eachhardness value was an average from five separate indents. A constant dwell time of 15 s was used to minimize the effectsof creep during indentation.
r-C
I-----
N
3. Results 3.1. Composition
There is a considerabledifferencein the depth profiles for low energy, high temperature and high energy, low temperature implantation as might be expected(Fig. 1). For the low temperature,high energy implantation case (CII) the peak of nitrogen concentration occurs about 70 nm below the surface and there is an oxygen-rich layer nearer to the surface.This is probably due to a combination of recoil implantation of surfacecontamination during implantation and enhancedoxidation of the active surfaceduring implantation or after removal from the vacuum system.The vacuum quality was relatively poor in the machine used for this treatment compared with that in the low energy implantation machine. The thickness of this oxide layer is about 100nm for all steelsinvestigated(seeTable 2). This is comparable with previous observations,e.g. [ 111. For the high temperature, low energy implants the peak in the nitrogen distribution is close to the surfaceand the oxide layer is relatively thin. This is consistentwith nitrogen diffusion into the material from the surfaceand the formation of a thin native oxide after removal from the implanter.
2w
0
m
4co
Depth (nm)
(4
-C
Austenitic stainless (304)
Treatment Oxide thickness bJ4
CII LE LEB CII Ferritic stainless (430) LE LEB CII Tool Steel LE 042) LEB Protie code CII prediction [ 151 LE/LEB
102 14 14 111 25 12 94 <1
Nitrogen peak depth b)
Total implanted layer thickness b-4
79 7 I 111 20 10 12
613 175 130 463 300 500 195 100 300 160 10
CII conventional ion implantation, LE low energy ion implantation, LEB low energy ion implantation with a nitrogen backm.
----
N
.-.....-.
0
-'-'-'-'-
Cr
-----
Fe
r-l
Table 2 Measured SIMS profile parameters for selected nitrogen implanted steels Material
100:
800
0
(b)
103
2Jx
xi0
Depth
403
SW
600
700
(nm)
Fig. 1. Secondary ion mass spectrometry depth profiles for 304 stainless steel, (a) conventional high energy ion implantation and (b) low energy, high temperature ion implantation.
Surface sputtering removes rather than implants any surfaceadsorbates. Detailed nitrogen depth profiles have been obtained
S.J. Bull et al.JSurfaceand Coatings Technology83 (1996) 257-262
260
for three materials (Fig. 2): (a) austenitic stainlesssteel (304), (b) ferritic stainless steel (430); (c) high speed steel (M2). In nearly all casesconventionalion implantation leads to the thickest treated layer but a considerabletreatment depth is achievedfor the low energy implants. In the case of austenitic stainlesssteel (Fig. 2(a)) the
_““”
304 I
“I.
0
103
200
300
400
SW
m
loo
Depth (nm) ICKQ
430 I
0
100
2m
303
400
SW
603
Death bn)
use of the nitrogen backfill reduces the amount of nitrogen which can diffuseinto the bulk. This is because the presenceof the backfill generatesan oxide layer on the surfacewhich providesa barrier to nitrogen diffusion. This oxide layer is sputtered off continually during implantation but forms at sufficient rates when the backfill is present that this is counteracted. Since the backfill gas was not completely purified prior to use,it is probably the small amount of oxygen and water vapour impurities within it that are responsiblefor this behaviour. The total thickness of the conventionally implanted layer (definedas when the profile falls to 10% of its maximumvalue)is much greaterthan that expected from the profile code [15] though the peak occurs at about the predicted depth; it is thus likely that some diffusion has occurred in this case.This is not unlikely since no attempt was made to control the sample temperaturesduring the conventional ion implantation. However, the temperaturerise was still lessthan 200 “C and the amount of diffusion is expectedto be considerably lessthan in the low energy implantation cases.No evidencefor chromium nitride formation was found for any of the treatmentsusing X-ray diffraction. For the ferritic stainless steel (430) the depth of nitrogen penetration increaseswith the use of a nitrogen backfill (Fig. 2(b)). There is a crossoverof the nitrogen profiles with the no backfill casehaving a higher surface concentration than when a backfill is used. For these materials the surface oxide peak is less well defined so the layer does not offer the same sort of barrier to nitrogen as in the austenitic case.The total implanted layer thicknessesin the low energy treatmentsare much greater than for the other steels,Again there is evidence that some diffusion has occurred during conventional implantation. For the high speed steel (M2), diffusion was much lessapparentin conventionalion implantation (Fig. 2(c)) and the low energy ion treatments produce nitrogen profiles with depths comparable with those of the high energy implant. The use of a nitrogen backfill leads to an increasein the depth of nitrogen penetration. 3.2. Hardness
0
100
200
300
4w
DeDth (nm)
Fig. 2. Secondary ion mass spectometry nitrogen depth profiles as a function of implantation conditions, (a) 304 austenitic stainless steel, (b) 430 ferritic stainless steel and (c) M2 high speed steel (CII conventional high energy ion implantation, LE low energy ion implantation, LEB low energy ion implantation with a nitrogen backfill).
Knoop hardness measurements of the tool steel samples are shown in Fig. 3. For M2 and M42, the vacuum anneal does not affect hardness,while the low energy ion implantation treatment only gives a small increasein hardness.A significant improvementis seen -for both materials when the nitrogen gas backfill is introduced in addition to the ion implantation. The 01 tool steelshows a minor, but not significant,increasein hardnessin the two low energynitrogen ion implantation treatments(with and without a nitrogen gas backfill). Nitrogen ion implantation of the En403 steelshowed no benefits over the untreated case;however, both the
S.J. Bull et al.lSusface and Coatings Technology 83 (1996) 257-262
261
implant-no backfill instance which was greater than the increase observed for conventional implantation.
4. Discussion
Fig. 3. Knoop hardness of M2, M42 and 01 tool steels at 25 g load as a function of treatment.
implant with gas backfill and vacuum anneal treatments resulted in a significant reduction in the observed hardness. Both of the bright steel materials assessed(types 708 and 817) showed no significant changes in hardness from any of the surface modification processes used in this work, when compared with the untreated materials. The stainless steels examined showed differing effects from the various modifying conditions, as shown in Fig. 4. The 304 BA type showed practically no change in hardness for the low energy ion implantation conditions but a slight increase in hardness was measured after conventional ion implantation. 316 2B stainless steel showed a marginal increase in hardness for the low energy implant-no backfill and conventional implantation cases over all other cases. The 430 BA steel displayed some improvement for the implant-backfill specimen and a more significant rise in hardness for the
I . f
l
5
Fig. 4. Knoop hardnesses of 304 BA, 316 2B and 430 BA stainless steels at 25 g load as a function of treatment.
From the results presented here it can be seen that increases in near-surface hardness are possible in some steels after low energy nitrogen ion implantation. For 1 keV nitrogen ion implantation at normal incidence in steels, the nitrogen peak is at the surface, with approximately 150 nm surface material lost by sputtering. Without taking temperature into consideration, most of the implanted surface region is sputtered away, leaving a layer of about 10 nm thickness (Table 2). The elevated temperature anneal (400 “C in a vacuum) does not itself improve the hardness, nor soften appreciably most of the steels. This conclusion is as expected from the literature [ 16-191. Generally, there are a number of conditions required for successful nitriding. The presence of (hard) nitride formers is required as alloy additives, including aluminium, titanium, chromium, molybdenum, tungsten and vanadium. The first two are especially desirable as they easily form the nitride phase and can improve the hardness of steels when present in very low amounts (around lo/). The presence of the latter three elements in such small concentrations is not generally so effective. For chromium, the beneficial effect of nitriding on hardness increases with the amount present. A typical composition for a nitriding steel is as follows [19]: Fe balance; C 0.2%-0.3%; Mn 0.4%-0.6%; Al 0.9%-1.4%; Cr 0.9%-1.4%; MO 0.15%-0.25%. Typically nitriding takes place at temperatures greater than 500 “C which is higher than the temperature used in the present study. Long nitriding times are often used so that the thickness of the nitrided layer can be a substantial fraction of a millimetre. With reference to Table 1, it can be seen that none of the steels used in this study contain any aluminium or titanium, even in small quantities. A gas backfill appears to benefit steels M2 and M42, and these both contain the alloying elements chromium, molybdenum, vanadium and tungsten which are good nitride formers. 01 tool steel has low amounts of these desirable elements present, and this is mirrored in the results where only a small hardness improvement is observed. En40B steel shows a distinct drop in hardness when vacuum annealed or treated with a nitrogen gas backfill. This is probably due to grain growth and implies that the material is not suitable for this high temperature treatment. The two bright engineering steels (types 708 and 817) have low quantities of most additives present, including approximately 1% Cr and 0.25% MO; together with the low temperature and time of treatment, this explains why little hardness change is observed.
262
S.J. Bull et al./Susface and Coatings Technology83 (1996) 257-262
The three stainless steels all have high chromium concentrations-several times greater than most commonly nitrided steels.Certainly, the nitrogen gasbackfill provides no benefit to the hardness level but does influence the nitrogen depth profile below the surface. In the case of both the 430 BA and 316 2B steels,the low energy and high energy nitrogen implants do give some improvement in hardness, whereas no improvement at all was observedfor the 304 BA material, for any treatment but high energy nitrogen implantation. The differences in the composition between these stainlesssteelsis not sufficientto explain theseobserved effects.Typically, the austenitic stainlesssteels(e.g.304, 316) are more difficult to nitride than the ferritic types (e.g.430). The increaseddepth of nitrogen penetration in the 430 BA steel compared with 304 treated under the sameconditions is consistentwith this observation. The depth of the nitrogen profile below the surfacefor the austenitic stainlesssteelsseemsto be controlled by the rate at which a protective chromium oxide forms on their surfacewhich provides a barrier to the passageof nitrogen. In this casehigher energy implantation treatments which can deposit nitrogen beneath the oxide are preferred. The limited temperature and time of treatment used in this study has resulted in thin treated layers.A much greater process temperature (greater than or equal to 500 “C) and/or a longer exposure time (tens of hours) or higher nitrogen arrival rate (beam current density) is probably required to generatesignificant nitriding and more significant hardnesschanges.
5. Conclusions
The following conclusions may be drawn from the work describedhere. (i) Low energy,high temperaturenitrogen ion implantation can lead to considerableincreasesin hardness. This is becausenitrogen penetration occurs deeper into the materials than would be expectedfrom the low energy of the implants owing to diffusion at elevatedtemperature. (ii) The most significant hardness increasesoccur in steels with alloying elements that can form hard nitrides (e.g.chromium, aluminium, titanium).
(iii) Further work is neededto determine the optimum treatment conditions sincemore significantchanges will be produced by longer treatment times and higher temperaturesand nitrogen arrival rates. Acknowledgements
This work is part of the long term Corporate Research Programme of AEA Technology. The authors thank Mrs J. Cullen for preparing the samples and Mr G. Angood for the polishing treatmentsand Dr H.E. Bishop for the SIMS analysis. References [l]
G.D. Dearnaley, in CM. Preece and J.K. Hirvonen (eds.), Ion Implplantation Metalhrg~~ Proc. SJW~. hrm. Meet. MRS, Cambridge, fifA, 30 Norember 1979, Metallurgical Society of AIME, Warrendale, PA, 1980,p. 1. [2] Chr.A. Straede, N~rcl.Instrum. Methods B, 68 (1992) 380. [3] W.L. Lin, X.J. Ding, J.M. Sang, J. Xu and X.M. Yuan, J. ,%ter. Eng. Pef$, 3(5) (1994) 587. [4] T.M. Wang, B.Q. Li and J. Shi., Sn$ Coat Tecllriol., 50 (1991) 63. [S] J. Dryzek, J. Wiezorek, S. Wollschlligcr, J. Lekki, A. Gottdang and B. Cleff, hfoter. Lett., 12 (1991) 16. [6] R. Hutchings, Mater. Sci. Eng., AI84 (1994) 87. [7] S. Aggarwal, A.K. Gael, R.K. Mohindra, P.K. Ghosh and A. Chand, T/tin Solid Films, 233 (1993) 72. [S] M. Gan, W. Pu, L. Shen, P. Li, M, Gen, Y. Jang and F. Wang, Surf, Coat. Technol., 66 (1994) 288. [9] 0. Nishimura, K. Yabe, K. Saito, T. Yamashina and M. Iwaki, Surf. Coat. Technol., 66 (1994) 403. [lo] C. Cordier-Robert, L, Bourdeau, T. Magnin and J. Fact, Mater. Lett., 20 (1994) 113. [ll] S. Shrivastava, R.D. Tarey, MC. Bhatnagar, A. Jain and K.L. Chopra, Sw$ Coat. Technol., 50 (1991) 41. [12] M. Samandi, B.A. Shedden, D-1. Smith, G,A. Collins, R. Hutchings and J. Tendys, SUI$ Coot. Tecl~nol.,59 (1993) 261. [13] F. Alonso, A. Arizaya, A. Garcia and J.1. Onate, Surf. Coot TechnoI., 66 (1994) 291. [ 141 H.-J. Fusser and H. Oechsner,Surf. Coat. Technol., 48 (1991) 97. [ 151 PROFILE Code, Implant Sciences,Danvers, MA, 1991. [16] D.K. Bullens, Steel and its Heat Treatment, Vol. I, Prir~ciples, Processes,Control, Wiley, London, 4th edn, 1944,p. 350. [ 171 L. Aitchison and W.I. Pumphrey, Engineering Steels,MacDonald & Evans, London, 1st edn, 1953,p, 479. [18] J. Woolman and R.A. Mottram, Tile ,%fecechariical and Physical Properties of‘ the BtWh Stanrhd En Stds (BS 970-1955), Vol. 3, En40 to En363, Pergamon, Oxford, 1st edn, 1969,p. 3. Cl91 Metals Handbook, Vol. 4, Wear Treatment, ASM, Metals Park, OH, 1981,p, 191.
Improving the mechanical properties of steelsusing low energy, high temperature nitrogen ion implantation S.J. Bull’, A.M. Jones, A.R. McCabe AEA Techology,
Harwell, Oxfordshire, OX1 1 ORA, UK
Abstract The use of nitrogen ion implantation to increase the surface hardness of structural steels is well documented. Traditionally this involves the use of high energy nitrogen ion beams (approximately 100 keV), with a relatively low beam current density because high energy beams are necessary to produce the required penetration into the material to achieve a significant depth of hardened material. Hardening needs to occur in a region whose size is comparable with the scale of the deformation associated with the tribocontact. 100 keV nitrogen ions typically penetrate into steels only about 0.1 urn and the range of possible tribological applications is thus restricted by this shallow treatment depth. In plasma nitriding processes the nitrogen ions approach the substrate with much lower energies but the ion currents are sufficiently high to cause considerable substrate heating. In this study an ion beam process has been used which more closely approximates plasma nitriding conditions. This involves the use of comparatively low energy nitrogen ion beams (1-2 IceV), with higher beam current densities and, where necessary, additional heating to increase the sample temperature to approximately 400 “C at the surface. Secondary ion mass spectrometry has been used to assessthe extent of nitrogen diffusion into a range of steels. The nitrogen penetration depth (and profile shape) depends both on the ion beam parameters and the structure and composition of the steel. Additional experiments conducted with a supplementary supply of nitrogen gas in the form of a vacuum chamber backfill demonstrate that increasing the nitrogen supply can increase nitrogen take-up for some materials. For a 30 min implantation at 263 uA cm-’ the penetration depth was a substantial fraction of a micrometre and is greater than the profile produced by high energy implantation in some cases.The hardness of the implanted steels can be correlated with the extent of nitrogen take-up, the amount of diffusion during implantation and softening induced by annealing of the steel substrates. Considerable hardening is achieved for some steels (e.g. M2 high speed steel) whereas for other steels the improvements are less obvious (e.g. 304 stainless). These observations are discussed in the light of the nitridability of the steels. Ke~~or& Hardness; Ion implantation; Depth profiling; Nitriding; Steels
1. Introduction The use of ion implantation
to improve the mechanical
and tribological properties of metals is well documented: see for example Refs. [l-3]. Typically high energy (greater than 50 keV) ion beams are used in conventional ion implantation in order to achieve a sufficient thickness of treatment to have a measurable effect on the hardness or wear performance of the implanted surface. For instance nitrogen implantation in steels can lead to considerable increases in wear resistance and hardness [4-61. This is generally related to the formation of nitride precipitates [7-101 which depends on the precise composition of the steel; iron nitrides can form but 1 Present address: School of Materials, The University, Newcastle upon Tyne NE1 7RU, UK.
alloying elements which have hard, stable nitrides (e.g. titanium, chromium, aluminium, molybdenum and tungsten) can have a more significant effect. As the energy of ion implantation is reduced, the thickness of the implanted layer and hence the magnitude of any improvement in tribological properties produced is also reduced. Furthermore, it becomes increasingly difficult to measure the changes to surface properties which have occurred and low load microhardness testers or nanoindenters are necessaryto monitor the effects of the treatment [ 11-131. However, low energy ion sources are available which can deliver very high beam currents compared with conventional implantation sourceswhich could result in a greatly improved process efficiency if sufficient tribological benefits can be generated. Thus, in order to be effective, low energy, high current ion implantation needs to work in a different manner to
258
S.J. Bull et al./Swface and Coatings Technology 83 (1996) 257-262
ature rise of about 180“C at the sample. Additional radiant heating was supplied to raise the temperatureof the samplesto approximately 220 “C prior to implantation which rose to around 400 “C within 1 min of exposureto the ion beam and remained at that temperature for the duration of the implantation. A further set of materials was treated in the same way but with a backfill of nitrogen gas in the vacuum chamber, to an additional 2 x 10s4Torr, to assessthe additional effects of nitrogen supply from the environment (LEB). To establish the influence of the vacuum heat treatment alone, a duplicate set of steelswas treated for the same length of time, at the sametemperaturein a vacuum of approximately 1 x lo-” Torr. Comparison sampleswere produced by conventional ion implantation (CII) using 76 keV nitrogen (Nzi) from a plasma bucket ion source in the Harwell Blue Tank facility at a current density of about 2.5 1tAcm-‘. A dose of 5 x 1017nitrogen ions cni-2 was implanted over 6.7 h. The rise in sample temperature caused by implantation under these conditions was measured to be less than 180“C in similar runs and was less than 200 “C (i.e. below the minimum temperature limit of a control pyrometer) in this experiment.
conventional ion implantation. One such way in which this can be achievedis to allow the temperature of the component to rise so that the implantation takes place at about 300-500 “C rather than at less than 100“C which is normal for conventional ion implantation. This results in a processwhich is intermediate between ion implantation and plasma nitriding. Previous work has shown that considerablediffusion of implanted nitrogen into steel occurs for 2 kV treatment [ 141 at elevated temperature. In this study we have reduced the implantation energy to 1 kV and ~ investigated the effectsof nitrogen implantation into a range of steelsat 400 “C as a function of implantation environment. The effectsof the treatmentson near surfacestructure and hardnesshave been determinedand comparedwith those achievablefrom conventional ion implantation. 2. Experimental details 2.1. Sampleproduction
Metals were machined from standard bar stock and polished down to a 3 pm finish using diamond paste. The steels examined were divided into three groups: (1) tool steels(M2 and M42 in the unhardened state and Ol(NSOH) in the hardened and tempered condition); (2) engineering steels(En40B, 708 407 and 817 407); (3) stainlesssteels(304 BA, 316 2B and 430 BA). The nominal compositionsof thesematerialsare given in Table 1. All steelswere used in the as-receivedcondition and were given no heat treatments to changetheir original hardness. Low energy(LE) nitrogen ion implantation treatments were undertaken at normal incidence using a Kaufman type ion source running at 1 keV. The samples were implanted at a current density of approximately 265 PA cm-’ for 30 min to give a total dose of approximately 1.5 x 10” nitrogen ions cmW2 at a residual chamber pressure of approximately 2 x 10e4Torr. Implantation under theseconditions leads to a temper-
2.2. Depthp~ojiling Depth profiles were produced by secondaryion mass spectrometry(SIMS) using a caesiumprimary beam.To assessthe nitrogen profile the mass147CsN” molecular ion was used which was found to be not significantly influenced by interference from molecular ions from substrateelements.The depth scalewas establishedfrom the depth of the pit left after sputter profiling and assumes that a constant etch rate was maintained throughout the experiment.The retaineddosesestimated for the conventional ion implanted samplesfrom the SIMS traces were up to a factor of four less than the nominal implanted dose, whereas for low energy implanted samplesthey were about an order of magnitude lower. More precisemeasurementsof retained dose were not possiblewith the method used in this study.
Table 1 Nominal compositions of steels investigated (wt.%) Steel M2 -Ii%42 CKNS0H En40B BS 708 407 BS 817 407 304 BA 316 2B 430 BA
Fe Balance -Balance Balance Balance Balance Balance Balance Balance Balance
C 1.28 -1:05 0.95 0.25 0.38 0.36 0.045 0.05 0.1
Cr
MO
W
V
Co
Ni
P
s
Si
Mn
Mg
4.2 - 3.9 0.5 3.2 1.05 1.2 18.25 17.5 17.0
5.0 9.5 0.55 0.2 0.3 2.6 -
6.4 1.5 0.5 -
3.1 1.15 0.2 -
8.0 -
-
0.05 0.035 0.035 -
0.05 0.4 0.4 -
0.4 0.25 025 0.25 0.6 0.6 -
1.2 0.55 0.46 1.25 1.25 -
0.85 -
0.4 1.5 10.0 12.0 0.5
-
S.J. Bull et al./Surface and Coatings Technology83 (1996) 257-262
2.3. Hardness measurements
Knoop hardness measurementswere made on all treated surfacesusing a Matsuzawa DMH-2 instrument. Measurementsweremade at 25 g load and eachhardness value was an average from five separate indents. A constant dwell time of 15 s was used to minimize the effectsof creep during indentation.
r-C
I-----
N
3. Results 3.1. Composition
There is a considerabledifferencein the depth profiles for low energy, high temperature and high energy, low temperature implantation as might be expected(Fig. 1). For the low temperature,high energy implantation case (CII) the peak of nitrogen concentration occurs about 70 nm below the surface and there is an oxygen-rich layer nearer to the surface.This is probably due to a combination of recoil implantation of surfacecontamination during implantation and enhancedoxidation of the active surfaceduring implantation or after removal from the vacuum system.The vacuum quality was relatively poor in the machine used for this treatment compared with that in the low energy implantation machine. The thickness of this oxide layer is about 100nm for all steelsinvestigated(seeTable 2). This is comparable with previous observations,e.g. [ 111. For the high temperature, low energy implants the peak in the nitrogen distribution is close to the surfaceand the oxide layer is relatively thin. This is consistentwith nitrogen diffusion into the material from the surfaceand the formation of a thin native oxide after removal from the implanter.
2w
0
m
4co
Depth (nm)
(4
-C
Austenitic stainless (304)
Treatment Oxide thickness bJ4
CII LE LEB CII Ferritic stainless (430) LE LEB CII Tool Steel LE 042) LEB Protie code CII prediction [ 151 LE/LEB
102 14 14 111 25 12 94 <1
Nitrogen peak depth b)
Total implanted layer thickness b-4
79 7 I 111 20 10 12
613 175 130 463 300 500 195 100 300 160 10
CII conventional ion implantation, LE low energy ion implantation, LEB low energy ion implantation with a nitrogen backm.
----
N
.-.....-.
0
-'-'-'-'-
Cr
-----
Fe
r-l
Table 2 Measured SIMS profile parameters for selected nitrogen implanted steels Material
100:
800
0
(b)
103
2Jx
xi0
Depth
403
SW
600
700
(nm)
Fig. 1. Secondary ion mass spectrometry depth profiles for 304 stainless steel, (a) conventional high energy ion implantation and (b) low energy, high temperature ion implantation.
Surface sputtering removes rather than implants any surfaceadsorbates. Detailed nitrogen depth profiles have been obtained
S.J. Bull et al.JSurfaceand Coatings Technology83 (1996) 257-262
260
for three materials (Fig. 2): (a) austenitic stainlesssteel (304), (b) ferritic stainless steel (430); (c) high speed steel (M2). In nearly all casesconventionalion implantation leads to the thickest treated layer but a considerabletreatment depth is achievedfor the low energy implants. In the case of austenitic stainlesssteel (Fig. 2(a)) the
_““”
304 I
“I.
0
103
200
300
400
SW
m
loo
Depth (nm) ICKQ
430 I
0
100
2m
303
400
SW
603
Death bn)
use of the nitrogen backfill reduces the amount of nitrogen which can diffuseinto the bulk. This is because the presenceof the backfill generatesan oxide layer on the surfacewhich providesa barrier to nitrogen diffusion. This oxide layer is sputtered off continually during implantation but forms at sufficient rates when the backfill is present that this is counteracted. Since the backfill gas was not completely purified prior to use,it is probably the small amount of oxygen and water vapour impurities within it that are responsiblefor this behaviour. The total thickness of the conventionally implanted layer (definedas when the profile falls to 10% of its maximumvalue)is much greaterthan that expected from the profile code [15] though the peak occurs at about the predicted depth; it is thus likely that some diffusion has occurred in this case.This is not unlikely since no attempt was made to control the sample temperaturesduring the conventional ion implantation. However, the temperaturerise was still lessthan 200 “C and the amount of diffusion is expectedto be considerably lessthan in the low energy implantation cases.No evidencefor chromium nitride formation was found for any of the treatmentsusing X-ray diffraction. For the ferritic stainless steel (430) the depth of nitrogen penetration increaseswith the use of a nitrogen backfill (Fig. 2(b)). There is a crossoverof the nitrogen profiles with the no backfill casehaving a higher surface concentration than when a backfill is used. For these materials the surface oxide peak is less well defined so the layer does not offer the same sort of barrier to nitrogen as in the austenitic case.The total implanted layer thicknessesin the low energy treatmentsare much greater than for the other steels,Again there is evidence that some diffusion has occurred during conventional implantation. For the high speed steel (M2), diffusion was much lessapparentin conventionalion implantation (Fig. 2(c)) and the low energy ion treatments produce nitrogen profiles with depths comparable with those of the high energy implant. The use of a nitrogen backfill leads to an increasein the depth of nitrogen penetration. 3.2. Hardness
0
100
200
300
4w
DeDth (nm)
Fig. 2. Secondary ion mass spectometry nitrogen depth profiles as a function of implantation conditions, (a) 304 austenitic stainless steel, (b) 430 ferritic stainless steel and (c) M2 high speed steel (CII conventional high energy ion implantation, LE low energy ion implantation, LEB low energy ion implantation with a nitrogen backfill).
Knoop hardness measurements of the tool steel samples are shown in Fig. 3. For M2 and M42, the vacuum anneal does not affect hardness,while the low energy ion implantation treatment only gives a small increasein hardness.A significant improvementis seen -for both materials when the nitrogen gas backfill is introduced in addition to the ion implantation. The 01 tool steelshows a minor, but not significant,increasein hardnessin the two low energynitrogen ion implantation treatments(with and without a nitrogen gas backfill). Nitrogen ion implantation of the En403 steelshowed no benefits over the untreated case;however, both the
S.J. Bull et al.lSusface and Coatings Technology 83 (1996) 257-262
261
implant-no backfill instance which was greater than the increase observed for conventional implantation.
4. Discussion
Fig. 3. Knoop hardness of M2, M42 and 01 tool steels at 25 g load as a function of treatment.
implant with gas backfill and vacuum anneal treatments resulted in a significant reduction in the observed hardness. Both of the bright steel materials assessed(types 708 and 817) showed no significant changes in hardness from any of the surface modification processes used in this work, when compared with the untreated materials. The stainless steels examined showed differing effects from the various modifying conditions, as shown in Fig. 4. The 304 BA type showed practically no change in hardness for the low energy ion implantation conditions but a slight increase in hardness was measured after conventional ion implantation. 316 2B stainless steel showed a marginal increase in hardness for the low energy implant-no backfill and conventional implantation cases over all other cases. The 430 BA steel displayed some improvement for the implant-backfill specimen and a more significant rise in hardness for the
I . f
l
5
Fig. 4. Knoop hardnesses of 304 BA, 316 2B and 430 BA stainless steels at 25 g load as a function of treatment.
From the results presented here it can be seen that increases in near-surface hardness are possible in some steels after low energy nitrogen ion implantation. For 1 keV nitrogen ion implantation at normal incidence in steels, the nitrogen peak is at the surface, with approximately 150 nm surface material lost by sputtering. Without taking temperature into consideration, most of the implanted surface region is sputtered away, leaving a layer of about 10 nm thickness (Table 2). The elevated temperature anneal (400 “C in a vacuum) does not itself improve the hardness, nor soften appreciably most of the steels. This conclusion is as expected from the literature [ 16-191. Generally, there are a number of conditions required for successful nitriding. The presence of (hard) nitride formers is required as alloy additives, including aluminium, titanium, chromium, molybdenum, tungsten and vanadium. The first two are especially desirable as they easily form the nitride phase and can improve the hardness of steels when present in very low amounts (around lo/). The presence of the latter three elements in such small concentrations is not generally so effective. For chromium, the beneficial effect of nitriding on hardness increases with the amount present. A typical composition for a nitriding steel is as follows [19]: Fe balance; C 0.2%-0.3%; Mn 0.4%-0.6%; Al 0.9%-1.4%; Cr 0.9%-1.4%; MO 0.15%-0.25%. Typically nitriding takes place at temperatures greater than 500 “C which is higher than the temperature used in the present study. Long nitriding times are often used so that the thickness of the nitrided layer can be a substantial fraction of a millimetre. With reference to Table 1, it can be seen that none of the steels used in this study contain any aluminium or titanium, even in small quantities. A gas backfill appears to benefit steels M2 and M42, and these both contain the alloying elements chromium, molybdenum, vanadium and tungsten which are good nitride formers. 01 tool steel has low amounts of these desirable elements present, and this is mirrored in the results where only a small hardness improvement is observed. En40B steel shows a distinct drop in hardness when vacuum annealed or treated with a nitrogen gas backfill. This is probably due to grain growth and implies that the material is not suitable for this high temperature treatment. The two bright engineering steels (types 708 and 817) have low quantities of most additives present, including approximately 1% Cr and 0.25% MO; together with the low temperature and time of treatment, this explains why little hardness change is observed.
262
S.J. Bull et al./Susface and Coatings Technology83 (1996) 257-262
The three stainless steels all have high chromium concentrations-several times greater than most commonly nitrided steels.Certainly, the nitrogen gasbackfill provides no benefit to the hardness level but does influence the nitrogen depth profile below the surface. In the case of both the 430 BA and 316 2B steels,the low energy and high energy nitrogen implants do give some improvement in hardness, whereas no improvement at all was observedfor the 304 BA material, for any treatment but high energy nitrogen implantation. The differences in the composition between these stainlesssteelsis not sufficientto explain theseobserved effects.Typically, the austenitic stainlesssteels(e.g.304, 316) are more difficult to nitride than the ferritic types (e.g.430). The increaseddepth of nitrogen penetration in the 430 BA steel compared with 304 treated under the sameconditions is consistentwith this observation. The depth of the nitrogen profile below the surfacefor the austenitic stainlesssteelsseemsto be controlled by the rate at which a protective chromium oxide forms on their surfacewhich provides a barrier to the passageof nitrogen. In this casehigher energy implantation treatments which can deposit nitrogen beneath the oxide are preferred. The limited temperature and time of treatment used in this study has resulted in thin treated layers.A much greater process temperature (greater than or equal to 500 “C) and/or a longer exposure time (tens of hours) or higher nitrogen arrival rate (beam current density) is probably required to generatesignificant nitriding and more significant hardnesschanges.
5. Conclusions
The following conclusions may be drawn from the work describedhere. (i) Low energy,high temperaturenitrogen ion implantation can lead to considerableincreasesin hardness. This is becausenitrogen penetration occurs deeper into the materials than would be expectedfrom the low energy of the implants owing to diffusion at elevatedtemperature. (ii) The most significant hardness increasesoccur in steels with alloying elements that can form hard nitrides (e.g.chromium, aluminium, titanium).
(iii) Further work is neededto determine the optimum treatment conditions sincemore significantchanges will be produced by longer treatment times and higher temperaturesand nitrogen arrival rates. Acknowledgements
This work is part of the long term Corporate Research Programme of AEA Technology. The authors thank Mrs J. Cullen for preparing the samples and Mr G. Angood for the polishing treatmentsand Dr H.E. Bishop for the SIMS analysis. References [l]
G.D. Dearnaley, in CM. Preece and J.K. Hirvonen (eds.), Ion Implplantation Metalhrg~~ Proc. SJW~. hrm. Meet. MRS, Cambridge, fifA, 30 Norember 1979, Metallurgical Society of AIME, Warrendale, PA, 1980,p. 1. [2] Chr.A. Straede, N~rcl.Instrum. Methods B, 68 (1992) 380. [3] W.L. Lin, X.J. Ding, J.M. Sang, J. Xu and X.M. Yuan, J. ,%ter. Eng. Pef$, 3(5) (1994) 587. [4] T.M. Wang, B.Q. Li and J. Shi., Sn$ Coat Tecllriol., 50 (1991) 63. [S] J. Dryzek, J. Wiezorek, S. Wollschlligcr, J. Lekki, A. Gottdang and B. Cleff, hfoter. Lett., 12 (1991) 16. [6] R. Hutchings, Mater. Sci. Eng., AI84 (1994) 87. [7] S. Aggarwal, A.K. Gael, R.K. Mohindra, P.K. Ghosh and A. Chand, T/tin Solid Films, 233 (1993) 72. [S] M. Gan, W. Pu, L. Shen, P. Li, M, Gen, Y. Jang and F. Wang, Surf, Coat. Technol., 66 (1994) 288. [9] 0. Nishimura, K. Yabe, K. Saito, T. Yamashina and M. Iwaki, Surf. Coat. Technol., 66 (1994) 403. [lo] C. Cordier-Robert, L, Bourdeau, T. Magnin and J. Fact, Mater. Lett., 20 (1994) 113. [ll] S. Shrivastava, R.D. Tarey, MC. Bhatnagar, A. Jain and K.L. Chopra, Sw$ Coat. Technol., 50 (1991) 41. [12] M. Samandi, B.A. Shedden, D-1. Smith, G,A. Collins, R. Hutchings and J. Tendys, SUI$ Coot. Tecl~nol.,59 (1993) 261. [13] F. Alonso, A. Arizaya, A. Garcia and J.1. Onate, Surf. Coot TechnoI., 66 (1994) 291. [ 141 H.-J. Fusser and H. Oechsner,Surf. Coat. Technol., 48 (1991) 97. [ 151 PROFILE Code, Implant Sciences,Danvers, MA, 1991. [16] D.K. Bullens, Steel and its Heat Treatment, Vol. I, Prir~ciples, Processes,Control, Wiley, London, 4th edn, 1944,p. 350. [ 171 L. Aitchison and W.I. Pumphrey, Engineering Steels,MacDonald & Evans, London, 1st edn, 1953,p, 479. [18] J. Woolman and R.A. Mottram, Tile ,%fecechariical and Physical Properties of‘ the BtWh Stanrhd En Stds (BS 970-1955), Vol. 3, En40 to En363, Pergamon, Oxford, 1st edn, 1969,p. 3. Cl91 Metals Handbook, Vol. 4, Wear Treatment, ASM, Metals Park, OH, 1981,p, 191.