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A comparison of plasma immersion ion implantation with conventional ion implantation
Nuclear Instruments and Methods in Physics Research B80/81 (1993) 262-266 North-Holland
HB
Beam Intsraotions with Materials &Atoms
A comparison of plasma imalersion ion implantation with conventional ion implantation M.J. Kenny and L .S . Wielunski
CSIRO Division of Applied Physics, Linditeld, NSW, Austraha
J . Tendys and G .A. Coliins
Adi-ced Materials, ANSTO, Menai, NSW, Austmha
Plasma immersion ion implantation has a potential advantage over conventional line-of-sight ion implantation in being able t" provide a uniform d se over a nonplanar surface. Operating regimes are different for each process leading to different surface modification characteristics . Comparisons have been made of the two techniques for mild steel, tungsten carbide and graphitic carbons . For nitrogen implanted mild steel, the PI' process gives a nitrogen depth distribution at least an order of magnitude greater than in CI`. In graphitic carbons, C1 2 is more effective in producing radia,..ion damage and improving resistance to wear.
1 . Introduction
2 . Mild steel
Plasuna immersion ion implantation (PI') involves the application of a pulsed higo voltage bias to components located its an rf plasma [1] . The resultant high energy ion bombardment modifies the surfaces of the components and can lead to significant increases in wear resistance and surface hardness. Although similar to plasma source ion 6,rplantation (PSII) [2], PI' differs in the method of plasma generation and th_ behaviour of the cathodic sheath which forms around the components. In PSII the high voltage pulse length is restricted to prevent the sheath contacting the s.,caths of other target objects or the chamber ;walls . In PI' the sheath comes to a steady state position quickly, placing no restriction on the pulse length. PI' was conceived as a non-line-of-sight alternative to zonvc .,tional ion implantation (CV) . Hower er the uperali~~g regimes for these two implantation processes are quite different and can result in differences in the characteristics of the modified surfaces . This paper examines the effects of CI' and PI' on tht . tribowgical characteristics of mild steel, tungsten carbide an .: vitreous carbon . Table 1 summarises the essen .ial differences in implantation conditions between tiie two processes. In addit,cn to a direct comparisoi ; of the two processes, the temperature and amvient pressure in the conventional implanter were :1 to snatch those of Pi -2' in order to bett .,r under:vas st-tn 1 the mechanisms by which PI ; modifi , surt c.s .
Cold drawn, polished (1 win finish), medium carbon (0.?%) mild steel discs were prepared and nitrogen implanted by the Pl' process _ rploying a bias voltage of 25 kV [31. Molecular nitrogen ions (N ;) are the predominant ionised species in the plasma . On impact with the surface they dissociate into two N+ ions each of energy 12 .5 keV. Implantation temperatures were between 200 and 340°C for nominal doses in the range 1-7 X 10 1 ' atoms cm -= . Imp!anted nitrogen concentration/depth profiles were measured by nuclear reaction analysis (NRA) employing the reaction t 4 N(d, 0) t `C anu a deuteron energy of 1 .2 MeV . Fig . 1 shows the concentration/ depth profile for s,ample tcmncraturcs of 200 and 340°C and nominal dose 7 x !art' atoms cm -Z . At 200°C the nitrogen is confined to a 100 nm surface layer . This is in contrast with a calculated range of 25 nrn for 12 .5 keV nitrogen ions. At 340°C the peak has significantly broadened and a substantial tail extending beyond 1000 nm is observed . The median depth of nitrogen ions is now 300 nm which is more than an order of magnituac greater than the calculated range . The tail is indicative of a diffusion enhanced process. Ball-on-disc wear measurements were made using a tungsten carbide ball (10 mm diameter 50 g load), 0 .8 m s - i contact velocity and paraffin oil lubrication. All samples showed increased resistance to wear (f?g . 2b) . The high dose, high temperature plasma modified sam-
0168-583X/93/$06 .00 (0 1993 -
Elsevier Science Pubi,shers B .V . All rights reserved
MJ. Kenny et al. / Comparison of P1 3 and CJz
20
263
(a) 200oC l.
is
High dose/high T
y 0
I
C
.Ç 7
i 0.0
0
0.2
0.4
Depth (gm)
0.6
0.8
20
C v Ô
Û xU
')cptll (Pill) Fig. 1 . Nitrogen depth profiles from NRA i4 N(d, a) i `C for treatment of mild steel at 200 and 340°C . ple had an order of magnitude enhanced wear resistance 131 over unimplanted samples . In support of the wear data, ultramicrohardness measurements (fig. 2a) showed that the modified surface had a hardness more than twice that of the bulk material . Similar samples were conventionally implanted employing 50 keV N+ and similar nominal doses. Con, entration/depth profiles were consistent with both range energy calculations and the nominal implanted dose. Wear resistance measurements indicated an enhancement factor of 3 following implantation . In order to imitate same of the p1 3 implant conditions, the conventional implantation system was then modified to allow 50 keV nitrogen implantation at
F
Wear Time (Hours) Fig . 2. Surface hardness (a) and pin-on-disc wear (b) for mild steel . temperatures in the range 50 to 400°C and pressure in the range 10 -6 to 5 x 10 -4 mbar. The nitrogen concentration . .=is found to vary as a function of temperature and background gas pressure as shown in fig . 3 . With the sample temperature at 260°C and a background pressure 10 -1 mbar, no nitrogen was retained, but with the background pressure raised to 1-4 nitrogen was retained. However no substantial evidence for a diffusion controlled deep nitrogen profile
Table I Operating conditions of Cl 2 and Pl 3 Va :unm
Ion species
ci , < 10 -' mbar tingle-magnet seleci,d
Sample temperature
Independently variable ( -100 to 600°C)
Excited neutrals and electrons Implanted region Implant process
Minimal presence Line-of-sight Continual
Pl ,
t0 -j to i0 -4 mbar Various ionised atomic and molecular species Controlled by plasma and pulse repetition rate (100 to 400°C) Significant pres-ce Fntire cnrface Puls~d - short duty cycle Ila . ME:'AL MODIFICATION (a)
264
M.J. Kenny et al. / Comparison of Pli and CIz Depth Inm) 500
150 Impl.m,
0
Rc~iJu.~l
loo 0
V
Channel Number
Fig. 3. Depth profiles from 14N(d, a) 12 C NRA on mild steel t-eated t "y CI Z at different temperatures and background pressures .
was observed. It should be no, ed that these conventional implantations were carried out using a beam current density of about 25 WA cm -2 . These results contrast with the phenomenon of deep nitrogen implants at elevated temperatures in steel reported with CI Z by Wei et al . [4] . The common feature of deep implants produced by PI 3 and by Wei et al . is the use of high current densities and increased surface temperature ;. In the work of Wei et al., a current density of 0.3 to 0.5 mA cm -2 was used while in PI 3 pulse currents are of the order of 5 mA cm -2 . These high currents can create high nitrogen densities in the surface region of the sample. A possible explanation for the observed differences between CIZ and PI 3 is as follows . At elevated temperatures (> 300°C) nitrogen can diffuse in steel as interstitials and transfer fre-n solid solution to E-FC Z N I _x or y'-Fe 4N or vice versa [5] . During C1Z at elevated temperatures and high vacuum, the high mobility of solid solution nitrogen can lead to outdiffusion and nitrogen loss from the sample. In contrwd during PI 3 the high concentration of nitrogen ions and atoms in the near surface environment not only suppresses outdiffusion of nitrogen, but can be a source of atomic nitrogen for indiffusion . Thus a mechanism exists for deep indiffusion of nitrogen which can form a solid solution with iron and chemical compound ; at significant depths below the surface. It is not surprisi-ig, therefore, that the depth of nitrogen is determined by diffusion parameters (atomic nitrogen density, rime and temperature) rather than by simple range-energy calculations .
3. Tungsten carbide Po'.ished cemented tungsten carbide (6 wt.% Co) discs ware PI-1 treated at temperatures up to 400°C, bias voltages of 1, 10 and 40 kV and nominal nitrogen doses up tc ? x 10, 1' agues cru - `. Con-ccntration/ depth profiles were measured by NRA . For all the conditions chosen, the retained nitrogen concentration was well below the nominal dose and a maximum retained nitrogen concentration of 6 x 10 1" atoms CM-2 was obtained using 40 kV plasma voltage . Modification, as measured by imrlant nitrogen depth, was confined to the first 100 nm of the surface . Diamond abrasive wear testing (1 Wm diamond on a nap cloth) revealed no enhancement in wear resistance of the implanted sample . In contrast, conventional implantation at ambient temperature with 50 keV N + showed the' up to 1 x 10 17 ions cm -2 could be retained . Diamond abrasive wear testing revealed moderately enhanced wear resistance following, implantation resulting in a 0 .5 gm high step between implanted and unimplanted zones before breakup of the implanted zone began . SEM examination of the PI 3 samples showed that a preferential sputtering of the Co binder left the WC grains exposed, allowing them to be dislodged during the wear testing. The more severe sputtering during PI3 is the likely reason for the lower saturation nitrogen concentration, and explains the absence of enhanced wear resistance in the 1,13 samples.
4. Graphitic carbons Conventional implantation of carbon materials employing, for exampie 50 keV N+ and a nominal dose of 10 1e ions cm -- , is capable of creating a wear resistant surface layer [6] with hardness approaching that of diamond . In contrast to the modification mechanisms in metals where alloying is predominant, in carbon r!-mar Hence materials the effect is due to rsd, it occurs for many ion species and at much lover doses . It has been established for C1 2 that a radiation damaged layer at least 100 nm thick is required for enhanced wear resistance to diamond abrasion. Vitreous carbon samples were plasma implanted using helium or neoa as the ion species with respective energies of 25 and 42 keV . Energy and dose were chosen to ensure sufficient damage :n a layer thicker than 100 rim . For 25 keV helium the damaged layer begins 130 run below the surface and is 140 run thick and a dose of 5 x 10 'b ions CM - Z was used. Using a standard diamond polishing procedure the sample showed only a slight enhancement in wear resistance.
AL,i. Kenny et al. / Comparison of PI 3 and C1 2 For neon . the damaged layer extends from the surface to 1":5 nm and nominal doses of 1 X 10!6 and 2 X 10 !6 ions cm -2 were used. The step differences between tf:e implanted and unimplanted regions of the PI 3 samples were 1 .5 and 2.5 pnt, respectively . This step height is in contrast to the 5 to 10 pni which is normally observed following conventional implantation under conditions chosen to generate a 100 me damage zone [7] . These results .,`tow tl'Lit PI 3 requires a dose two to three times greater than CI' and results in a much smaller improvement in wear resistance. This suggests that radiation damage is partly annealed during urzplantation in PI 3. With CIZ enhanced wear resistance has been observed in samples implanted at temperatures up to 600°C. During PI 3 treatment the sample temperature was held below Z00°C to limit any effects of thermal annealing . However the plasma process is pulsed with typical pulse lengths of 50 p.s and repetition rate < 100 Hz. It is possible that a partial relaxation of radiation damage occurs either due to the intense nature of the pulse or the high activity associated with the plasma . Rutherford backscattering spectroscopy showed a retained PI 3 neon dose in each case which was comparable to the nominal implant dose. It is therefore unlikely that the Surface is being sputtered away during the PI 3 process . Another sample which was implanted initially with nitrogen by CIZ and ttlen with neon by PI 3 retained its
265
wear resistance, indicating that t,te plàsma process did not anneal the damage or remove the damaged surface layer which was already present. 5. Implantation of complex surfaces `or°_^!, :)val nnniantation is a line-of-sight process requiring sample or beam manipulation for complex shapes. Plasma implantation is potentially able to implant complex surfaces without manipulation. The effectiveaess of this capability was assessed by using PI 3 nn a 10 mm diameter carbon ball with 25 keV nitrogen ions (50 keV N 2 )at a nominal dose of 1 .5 x 10! 6 ions cm -2. NRA at several points on the surface showed a uniform dose within t 10%. This is consistent with observations by Chen et al . [S] when using PSIL E. Conclusions The differences in the effects of CI Z and p;3 on the materials studied an table 2. Clearly plasma implantation is superior to conventional implantation for the production of enhanced wear resistance in mild steel. The combination of temperature, pressure and energetic plasma lead to radiation erhanced diffusion and a modified layer an order of magnitude thicker than expected from ion range theory. In low allov tool steels treated by PI 3 at 500'C
Table 2 Summary of effects of CI Z and Pi a Parameter
CI Z
PI 3
Nitrogen retention
Agrccst%ith rangeenergy calculations No! at implant temperatures > 200°C
Hardness increase Improved wear resistance
50% 2-3
Order of maf,nitude greater than range-energy calculations Total nitrogen conct ntration exceeds nominal dote and is retaiacd at all temperature 100% 10
Mild steel lot, depth
Tungsten carbide Maximum nitrogen reténtion Improved wear resistance Sputtering Graphitic carbons Wear resistance Radiation damage Suitable species Dose required
10 17 atoms Small Negligible
CM-2
Improved by two orders of magnitude [6] Easily produced Most ions (H, He, N, O, Ne, Ar etc .) = 10 16 atoms cm -2
6 x 10 16 atoms cm - I None Substantial of Co bi ider Improved, but less effective than C1 2 Partial annex ling slows radiation damage Limited ions (Ne) > 3 x 10 16 atoms cm -2 Ila . METAL MODIFICATION (a)
266
M.J. Kem?yet al. / Cumpnrison o(Pl3 and Cl =
diffusion of nitrogen to depths of 150 p,m has been recently observed [9] . Variation of tem;eratare and pressure during CIZ did not reproduce the deep im. plant conditions observed with P: or reported by "some other workers in line-of-sight implantation . It is likely that the ion current density is the critical param-
eter for deep implants using CIZ. Compared with the CIZ results of this work and ref. u6] PI 3 of tungsten carbide and carbon materials is much iess effective . In the care trt tungsten carbide, this is likely to be the result of severe sputtering during plasma imp!antation. In the case of carbon materials
where wear resistance is'cnhanced by radiatioa damage, it is suggested that the encigetic species present in the plasma generate a dynamic annealing process which reduces the cumulative radiation damage responsible for radiation induced hardness .
Acknowledgements The authors acknowledge discussions with J .T .A . Pollock ant' considerable assistance from R.A. Clissold in carryipy out wear measurements and preparing samples .
Support for this work was provided under the Generic Technology component of the Australian Industry Research and Development Act 1986 .
Refert"nces [11 J. Tendys, I.J. Donnelly. M.J. Kenny and J.T.A . Pollock, Appl . Phys . Lett . 53 (1988) 2143 . [2] J.R. Conrad, J.L. Radtke, R.A. Dodd, F..i Worzata and N.C . Tan, J. Appl . Phys. 62 (1987) 4591 . [3] G.A. (ollias, R. Hutchings and J. Tendys, Mates . Sci . Eng. A139 (1991) 171. [41 R. Wei, P.L . Wilbur, W.S . Sampatl,, U.L. Williamson, Y. Qu and L. Wang, J. Triboiogy 112 (1990) 27. [5] D. Nicholls, in : Comprehensive Inorganic Chemistry ed. A.F. Trotman-Dickensca (Pergamon Press 1974` p. 101(1. [nj M.J . Kenny, J.T .A. Poilock and L.S . Wielunski, Nucl. Insir. and Meth. B39 (1989) 704. (7j J.L.A. Pollock, RA. Clissold and M. Farrelly, J. Mater . Sci. 6 (1987) 1023 . [8] A. Chen, J.T . Scheuer, C. Ritter, R.B. Alexander and J.R . Conrad, J. Appl. Phys. 70 (199i) 6757. [91 M. Sam3ndi, A. Pauza, G. Hatziandunion, It . Yasbandha, R. Hutchings, G.A. Collins and J. Tendys, Surf. Coat. Technol . 54/55 (1992) 447.
HB
Beam Intsraotions with Materials &Atoms
A comparison of plasma imalersion ion implantation with conventional ion implantation M.J. Kenny and L .S . Wielunski
CSIRO Division of Applied Physics, Linditeld, NSW, Austraha
J . Tendys and G .A. Coliins
Adi-ced Materials, ANSTO, Menai, NSW, Austmha
Plasma immersion ion implantation has a potential advantage over conventional line-of-sight ion implantation in being able t" provide a uniform d se over a nonplanar surface. Operating regimes are different for each process leading to different surface modification characteristics . Comparisons have been made of the two techniques for mild steel, tungsten carbide and graphitic carbons . For nitrogen implanted mild steel, the PI' process gives a nitrogen depth distribution at least an order of magnitude greater than in CI`. In graphitic carbons, C1 2 is more effective in producing radia,..ion damage and improving resistance to wear.
1 . Introduction
2 . Mild steel
Plasuna immersion ion implantation (PI') involves the application of a pulsed higo voltage bias to components located its an rf plasma [1] . The resultant high energy ion bombardment modifies the surfaces of the components and can lead to significant increases in wear resistance and surface hardness. Although similar to plasma source ion 6,rplantation (PSII) [2], PI' differs in the method of plasma generation and th_ behaviour of the cathodic sheath which forms around the components. In PSII the high voltage pulse length is restricted to prevent the sheath contacting the s.,caths of other target objects or the chamber ;walls . In PI' the sheath comes to a steady state position quickly, placing no restriction on the pulse length. PI' was conceived as a non-line-of-sight alternative to zonvc .,tional ion implantation (CV) . Hower er the uperali~~g regimes for these two implantation processes are quite different and can result in differences in the characteristics of the modified surfaces . This paper examines the effects of CI' and PI' on tht . tribowgical characteristics of mild steel, tungsten carbide an .: vitreous carbon . Table 1 summarises the essen .ial differences in implantation conditions between tiie two processes. In addit,cn to a direct comparisoi ; of the two processes, the temperature and amvient pressure in the conventional implanter were :1 to snatch those of Pi -2' in order to bett .,r under:vas st-tn 1 the mechanisms by which PI ; modifi , surt c.s .
Cold drawn, polished (1 win finish), medium carbon (0.?%) mild steel discs were prepared and nitrogen implanted by the Pl' process _ rploying a bias voltage of 25 kV [31. Molecular nitrogen ions (N ;) are the predominant ionised species in the plasma . On impact with the surface they dissociate into two N+ ions each of energy 12 .5 keV. Implantation temperatures were between 200 and 340°C for nominal doses in the range 1-7 X 10 1 ' atoms cm -= . Imp!anted nitrogen concentration/depth profiles were measured by nuclear reaction analysis (NRA) employing the reaction t 4 N(d, 0) t `C anu a deuteron energy of 1 .2 MeV . Fig . 1 shows the concentration/ depth profile for s,ample tcmncraturcs of 200 and 340°C and nominal dose 7 x !art' atoms cm -Z . At 200°C the nitrogen is confined to a 100 nm surface layer . This is in contrast with a calculated range of 25 nrn for 12 .5 keV nitrogen ions. At 340°C the peak has significantly broadened and a substantial tail extending beyond 1000 nm is observed . The median depth of nitrogen ions is now 300 nm which is more than an order of magnituac greater than the calculated range . The tail is indicative of a diffusion enhanced process. Ball-on-disc wear measurements were made using a tungsten carbide ball (10 mm diameter 50 g load), 0 .8 m s - i contact velocity and paraffin oil lubrication. All samples showed increased resistance to wear (f?g . 2b) . The high dose, high temperature plasma modified sam-
0168-583X/93/$06 .00 (0 1993 -
Elsevier Science Pubi,shers B .V . All rights reserved
MJ. Kenny et al. / Comparison of P1 3 and CJz
20
263
(a) 200oC l.
is
High dose/high T
y 0
I
C
.Ç 7
i 0.0
0
0.2
0.4
Depth (gm)
0.6
0.8
20
C v Ô
Û xU
')cptll (Pill) Fig. 1 . Nitrogen depth profiles from NRA i4 N(d, a) i `C for treatment of mild steel at 200 and 340°C . ple had an order of magnitude enhanced wear resistance 131 over unimplanted samples . In support of the wear data, ultramicrohardness measurements (fig. 2a) showed that the modified surface had a hardness more than twice that of the bulk material . Similar samples were conventionally implanted employing 50 keV N+ and similar nominal doses. Con, entration/depth profiles were consistent with both range energy calculations and the nominal implanted dose. Wear resistance measurements indicated an enhancement factor of 3 following implantation . In order to imitate same of the p1 3 implant conditions, the conventional implantation system was then modified to allow 50 keV nitrogen implantation at
F
Wear Time (Hours) Fig . 2. Surface hardness (a) and pin-on-disc wear (b) for mild steel . temperatures in the range 50 to 400°C and pressure in the range 10 -6 to 5 x 10 -4 mbar. The nitrogen concentration . .=is found to vary as a function of temperature and background gas pressure as shown in fig . 3 . With the sample temperature at 260°C and a background pressure 10 -1 mbar, no nitrogen was retained, but with the background pressure raised to 1-4 nitrogen was retained. However no substantial evidence for a diffusion controlled deep nitrogen profile
Table I Operating conditions of Cl 2 and Pl 3 Va :unm
Ion species
ci , < 10 -' mbar tingle-magnet seleci,d
Sample temperature
Independently variable ( -100 to 600°C)
Excited neutrals and electrons Implanted region Implant process
Minimal presence Line-of-sight Continual
Pl ,
t0 -j to i0 -4 mbar Various ionised atomic and molecular species Controlled by plasma and pulse repetition rate (100 to 400°C) Significant pres-ce Fntire cnrface Puls~d - short duty cycle Ila . ME:'AL MODIFICATION (a)
264
M.J. Kenny et al. / Comparison of Pli and CIz Depth Inm) 500
150 Impl.m,
0
Rc~iJu.~l
loo 0
V
Channel Number
Fig. 3. Depth profiles from 14N(d, a) 12 C NRA on mild steel t-eated t "y CI Z at different temperatures and background pressures .
was observed. It should be no, ed that these conventional implantations were carried out using a beam current density of about 25 WA cm -2 . These results contrast with the phenomenon of deep nitrogen implants at elevated temperatures in steel reported with CI Z by Wei et al . [4] . The common feature of deep implants produced by PI 3 and by Wei et al . is the use of high current densities and increased surface temperature ;. In the work of Wei et al., a current density of 0.3 to 0.5 mA cm -2 was used while in PI 3 pulse currents are of the order of 5 mA cm -2 . These high currents can create high nitrogen densities in the surface region of the sample. A possible explanation for the observed differences between CIZ and PI 3 is as follows . At elevated temperatures (> 300°C) nitrogen can diffuse in steel as interstitials and transfer fre-n solid solution to E-FC Z N I _x or y'-Fe 4N or vice versa [5] . During C1Z at elevated temperatures and high vacuum, the high mobility of solid solution nitrogen can lead to outdiffusion and nitrogen loss from the sample. In contrwd during PI 3 the high concentration of nitrogen ions and atoms in the near surface environment not only suppresses outdiffusion of nitrogen, but can be a source of atomic nitrogen for indiffusion . Thus a mechanism exists for deep indiffusion of nitrogen which can form a solid solution with iron and chemical compound ; at significant depths below the surface. It is not surprisi-ig, therefore, that the depth of nitrogen is determined by diffusion parameters (atomic nitrogen density, rime and temperature) rather than by simple range-energy calculations .
3. Tungsten carbide Po'.ished cemented tungsten carbide (6 wt.% Co) discs ware PI-1 treated at temperatures up to 400°C, bias voltages of 1, 10 and 40 kV and nominal nitrogen doses up tc ? x 10, 1' agues cru - `. Con-ccntration/ depth profiles were measured by NRA . For all the conditions chosen, the retained nitrogen concentration was well below the nominal dose and a maximum retained nitrogen concentration of 6 x 10 1" atoms CM-2 was obtained using 40 kV plasma voltage . Modification, as measured by imrlant nitrogen depth, was confined to the first 100 nm of the surface . Diamond abrasive wear testing (1 Wm diamond on a nap cloth) revealed no enhancement in wear resistance of the implanted sample . In contrast, conventional implantation at ambient temperature with 50 keV N + showed the' up to 1 x 10 17 ions cm -2 could be retained . Diamond abrasive wear testing revealed moderately enhanced wear resistance following, implantation resulting in a 0 .5 gm high step between implanted and unimplanted zones before breakup of the implanted zone began . SEM examination of the PI 3 samples showed that a preferential sputtering of the Co binder left the WC grains exposed, allowing them to be dislodged during the wear testing. The more severe sputtering during PI3 is the likely reason for the lower saturation nitrogen concentration, and explains the absence of enhanced wear resistance in the 1,13 samples.
4. Graphitic carbons Conventional implantation of carbon materials employing, for exampie 50 keV N+ and a nominal dose of 10 1e ions cm -- , is capable of creating a wear resistant surface layer [6] with hardness approaching that of diamond . In contrast to the modification mechanisms in metals where alloying is predominant, in carbon r!-mar Hence materials the effect is due to rsd, it occurs for many ion species and at much lover doses . It has been established for C1 2 that a radiation damaged layer at least 100 nm thick is required for enhanced wear resistance to diamond abrasion. Vitreous carbon samples were plasma implanted using helium or neoa as the ion species with respective energies of 25 and 42 keV . Energy and dose were chosen to ensure sufficient damage :n a layer thicker than 100 rim . For 25 keV helium the damaged layer begins 130 run below the surface and is 140 run thick and a dose of 5 x 10 'b ions CM - Z was used. Using a standard diamond polishing procedure the sample showed only a slight enhancement in wear resistance.
AL,i. Kenny et al. / Comparison of PI 3 and C1 2 For neon . the damaged layer extends from the surface to 1":5 nm and nominal doses of 1 X 10!6 and 2 X 10 !6 ions cm -2 were used. The step differences between tf:e implanted and unimplanted regions of the PI 3 samples were 1 .5 and 2.5 pnt, respectively . This step height is in contrast to the 5 to 10 pni which is normally observed following conventional implantation under conditions chosen to generate a 100 me damage zone [7] . These results .,`tow tl'Lit PI 3 requires a dose two to three times greater than CI' and results in a much smaller improvement in wear resistance. This suggests that radiation damage is partly annealed during urzplantation in PI 3. With CIZ enhanced wear resistance has been observed in samples implanted at temperatures up to 600°C. During PI 3 treatment the sample temperature was held below Z00°C to limit any effects of thermal annealing . However the plasma process is pulsed with typical pulse lengths of 50 p.s and repetition rate < 100 Hz. It is possible that a partial relaxation of radiation damage occurs either due to the intense nature of the pulse or the high activity associated with the plasma . Rutherford backscattering spectroscopy showed a retained PI 3 neon dose in each case which was comparable to the nominal implant dose. It is therefore unlikely that the Surface is being sputtered away during the PI 3 process . Another sample which was implanted initially with nitrogen by CIZ and ttlen with neon by PI 3 retained its
265
wear resistance, indicating that t,te plàsma process did not anneal the damage or remove the damaged surface layer which was already present. 5. Implantation of complex surfaces `or°_^!, :)val nnniantation is a line-of-sight process requiring sample or beam manipulation for complex shapes. Plasma implantation is potentially able to implant complex surfaces without manipulation. The effectiveaess of this capability was assessed by using PI 3 nn a 10 mm diameter carbon ball with 25 keV nitrogen ions (50 keV N 2 )at a nominal dose of 1 .5 x 10! 6 ions cm -2. NRA at several points on the surface showed a uniform dose within t 10%. This is consistent with observations by Chen et al . [S] when using PSIL E. Conclusions The differences in the effects of CI Z and p;3 on the materials studied an table 2. Clearly plasma implantation is superior to conventional implantation for the production of enhanced wear resistance in mild steel. The combination of temperature, pressure and energetic plasma lead to radiation erhanced diffusion and a modified layer an order of magnitude thicker than expected from ion range theory. In low allov tool steels treated by PI 3 at 500'C
Table 2 Summary of effects of CI Z and Pi a Parameter
CI Z
PI 3
Nitrogen retention
Agrccst%ith rangeenergy calculations No! at implant temperatures > 200°C
Hardness increase Improved wear resistance
50% 2-3
Order of maf,nitude greater than range-energy calculations Total nitrogen conct ntration exceeds nominal dote and is retaiacd at all temperature 100% 10
Mild steel lot, depth
Tungsten carbide Maximum nitrogen reténtion Improved wear resistance Sputtering Graphitic carbons Wear resistance Radiation damage Suitable species Dose required
10 17 atoms Small Negligible
CM-2
Improved by two orders of magnitude [6] Easily produced Most ions (H, He, N, O, Ne, Ar etc .) = 10 16 atoms cm -2
6 x 10 16 atoms cm - I None Substantial of Co bi ider Improved, but less effective than C1 2 Partial annex ling slows radiation damage Limited ions (Ne) > 3 x 10 16 atoms cm -2 Ila . METAL MODIFICATION (a)
266
M.J. Kem?yet al. / Cumpnrison o(Pl3 and Cl =
diffusion of nitrogen to depths of 150 p,m has been recently observed [9] . Variation of tem;eratare and pressure during CIZ did not reproduce the deep im. plant conditions observed with P: or reported by "some other workers in line-of-sight implantation . It is likely that the ion current density is the critical param-
eter for deep implants using CIZ. Compared with the CIZ results of this work and ref. u6] PI 3 of tungsten carbide and carbon materials is much iess effective . In the care trt tungsten carbide, this is likely to be the result of severe sputtering during plasma imp!antation. In the case of carbon materials
where wear resistance is'cnhanced by radiatioa damage, it is suggested that the encigetic species present in the plasma generate a dynamic annealing process which reduces the cumulative radiation damage responsible for radiation induced hardness .
Acknowledgements The authors acknowledge discussions with J .T .A . Pollock ant' considerable assistance from R.A. Clissold in carryipy out wear measurements and preparing samples .
Support for this work was provided under the Generic Technology component of the Australian Industry Research and Development Act 1986 .
Refert"nces [11 J. Tendys, I.J. Donnelly. M.J. Kenny and J.T.A . Pollock, Appl . Phys . Lett . 53 (1988) 2143 . [2] J.R. Conrad, J.L. Radtke, R.A. Dodd, F..i Worzata and N.C . Tan, J. Appl . Phys. 62 (1987) 4591 . [3] G.A. (ollias, R. Hutchings and J. Tendys, Mates . Sci . Eng. A139 (1991) 171. [41 R. Wei, P.L . Wilbur, W.S . Sampatl,, U.L. Williamson, Y. Qu and L. Wang, J. Triboiogy 112 (1990) 27. [5] D. Nicholls, in : Comprehensive Inorganic Chemistry ed. A.F. Trotman-Dickensca (Pergamon Press 1974` p. 101(1. [nj M.J . Kenny, J.T .A. Poilock and L.S . Wielunski, Nucl. Insir. and Meth. B39 (1989) 704. (7j J.L.A. Pollock, RA. Clissold and M. Farrelly, J. Mater . Sci. 6 (1987) 1023 . [8] A. Chen, J.T . Scheuer, C. Ritter, R.B. Alexander and J.R . Conrad, J. Appl. Phys. 70 (199i) 6757. [91 M. Sam3ndi, A. Pauza, G. Hatziandunion, It . Yasbandha, R. Hutchings, G.A. Collins and J. Tendys, Surf. Coat. Technol . 54/55 (1992) 447.