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Nitrogen migration during wear in an implanted steel
Scripta METALLURGICA
NITROGEN
Vol. 17, pp. 459-462, 1983 P r i n t e d in the U.S.A.
MIGRATION
DURING
WEAR
P e r g a m o n Press Ltd. All rights r e s e r v e d
IN A N I M P L A N T E D
STEEL
S. F A Y E U L L E , D. T R E H E U X , P. G U I R A L D E N Q D ~ p a r t e m e n t Mdtallurgie Physique-Mat~riaux, E. R . A . N o 7 3 2 du C N R S Ecole Centrale de Lyon, B P I 63, 69131 E C U L L Y C e d e x France T. B A R N A V O N , J. T O U S S E T Institut de Physique Nucldaire (et IN2P3),Universit~ CI. B e r n a r d Lyon-I 4 3 , B d du 11 N o v e m b r e 1918, 69622 V I L L E U R B A N N E C e d e x France M. ROBELET Centre de R e c h e r c h e d'Unieux (Creusot-Loire) 42701 F I R M I N Y France
( R e c e i v e d N o v e m b e r 8, 1982) ( R e v i s e d J a n u a r y 27, 1983) Introduction T h e effectiveness of nitrogen implantation in improving the tribological properties of steels under s o m e conditions has been well established (l, 2), but m e c h a n i s m s for this i m p r o v e m e n t are not well known. Nitrogen migration during wear, though a s s u m e d for several years (3,4), is still not well understood and few studies have been able to d e a r l y s h o w it. Indeed two m a j o r difficulties appear in its study : classical friction experiments like pin and disk test cause important modification of the surface states of the antagonists. Particularly, scratches appear very quickly, altering the m e a s u r e m e n t of w o r n depth and the analyses of implanted layers w h o s e depth (100 n m ! is lower than the depth of the scratches; methods for the determination of nitrogen profiles require an ionic erosion technique (Auger, E S C A . . . ). In addition to the perturbations that it can introduce in the distribution and the nature of t h e implanted nitrogen, this destructive erosion imposes the use of different samples for each test and so, does not permit the exact determination of a nitrogen profile during a tribological test. This paper gives results about the study of this migration in a Z 200 C 13 steel (13 ~o Cr, 2 ~ C) depending on the nitrogen fluence. The two problems mentioned previously are resolved by the use of a non destructive nuclear m e t h o d for analysis and of a purely erosion test that does not modify the surface state for wear. Experimental procedure Cubic sam p l e s ( 1 2 x 1 2 x I m m 3) of Z 2 0 0 C 13 steel (austenitisedat 1 2 7 0 K , quenched in oil and t e m p e r e d at 470 K for 2 hours, hardness 60 H R C ) w e r e implanted on one side after polishing in 1/4 ~ m diamond, with 40 k e V 15 N ions by uslng the isotope separator of the Institut de Physique Nucl~aire de Lyon. The ions fluences w e r e 10 17 , 2 x 10 17 15 N + ions c m -2 and 4 x 1017 " 5 N + ions c m -2. T h e samples w e r e implanted at r o o m temperature (~- 25°C) maintained by a water circulating device. The profiles of penetration w e r e obtained without erosion with the l ~ ( p c ~ ~)12C reaction (5) that presents an isolated resonance at 429 keV. The width of this peak, 0.9 keV, a11ows a depth resolution of 5 n m . The sample was b o m b a r d e d with protons of energy E. A s long as E is lower than 429 k e V (ER) the re-~
,
459 0036-9748/83/040459-04503.00/0 C o p y r i g h t (c) 1983 P e r g a m o n Press
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a c t i o n d o e s n o t h a p p e n . W h e n E i s e q u a l to E R, t h e r e s o n a n c e o c c u r s , a n d n i t r o g e n is d e t e c t e d on the s u r f a c e . W h e n E b e c o m e s h i g h e r t h a n E R, p r o t o n s l o s e a p a r t o f t h e i r e n e r g y to r e a c h E R at the depth x , at which n i t r o g e n is a n a l y s e d . T h u s , the r e a c t i o n is p r o g r e s s i v e l y "pushed" into the sample and, for each energy of t h e i n c i d e n t p r o t o n s , t h e ~( e m i s s i o n i s r e c o r d e d . T h e p r o t o n s w e r e a c c e l e r a t e d w i t h a 2. 5 MV V a n de G r a a f f g e n e r a t o r ; t h e ~( r a y s w e r e d e t e c t e d w i t h a 10 x 7 . 5 c m N a I ( T I ) c r y s t a l A f t e r i m p l a n t a t i o n , t h e s a m p l e s w e r e w o r n a c c o r d i n g to a t e c h n i q u e s i m i l a r to t h a t u s e d b y B o l s t e r e t a l . (6). The s a m p l e s w e r e p o l i s h e d f o r a g i v e n p e r i o d on a f a b r i c a t e d n o n - w o v e n s y n t h e t i c t e x tile c h a r g e d with 0 . 2 5 ~ m d i a m o n d p o w d e r . T h e y w e r e t h e n c l e a n e d u l t r a s o n i c a l l y and in t r i c h l o r o e t h y l e n e , a n d r i n s e d . T h e w e i g h t l o s s e s w e r e m e a s u r e d w i t h 10 -6 g p r e c i s i o n a n d w e r e a c t u a l l y p r o p o r t i o n a l to w o r n d e p t h b e c a u s e o f t h e u n i f o r m r e m o v a l o f t h e s u r f a c e , a s s h o w n o n the next figure :
I ~m
I
2b0-~m Fig.
1
: Typical profilometer
trace of surface after wear
As the a n a l y s i s is non d e s t r u c t i v e , one can follow the e v o l u t i o n of the n i t r o g e n p r o f i l e s a f t e r d i f f e r e n t s t a g e s o f w e a r on t h e s a m e s a m p l e . Results E a c h s a m p l e w a s a n a l y s e d a f t e r i m p l a n t a t i o n ( c u r v e n ° I), a f t e r a f i r s t s m a l l w e a r ( l o w e r t h a n 25 n m ) ( c u r v e n ° 2) a n d a f t e r a m o r e e x t e n s i v e w e a r ( m o r e t h a n 60 n m ) ( c u r v e n ° 3). R e s u l t s a r e p l o t t e d v e r s u s t h e f l u e n c e o n f i g u r e s Z, 3, 4. T h e i n f l u e n c e o f t h e n i t r o g e n d o s e i s n o t v i s i b l e i n t h e c a s e o f s m a l l w e a r ( c u r v e n ° 2) : f o r t h e t h r e e c a s e s , t h e p r o f i l e s a r e not v e r y m u c h m o d i f i e d . The m a x i m a a r e a little m o r e p r o n o u n c e d and the r e s i d u a l n i t r o g e n a m o u n t i s a l w a y s m o r e t h a n 95 ~0 o f t h e i n i t i a l a m o u n t . W h e n t h e w e a r i n c r e a s e s , n o t i c e a b l e d i f f e r e n c e s a p p e a r ( c u r v e n ° 3~ a n d t h e n i t r o g e n t r a n s p o r t in t h e b u l k i s t h e m o r e i m p o r t a n t f o r t h e 2 x 1017 15N+ i o n s c m - - f l u e n c e : t h e p r o f i l e d e p l a c e m e n t i s a b o u t 4 0 n m a n d t h e n i t r o g e n q u a n t i t y i s s t i l l a b o u t 55 ~o o f t h e i n i t i a l quan%ity. O n t h e c o n t r a r y , f o r b i g g e r f l u e n c e s | t h e p e n e t r a t i o n i s n e a r l y n u l l a n d l e s s t h a n 30 of the n i t r o g e n is s t i l l p r e s e n t . T h e 1017 i o n s c m -2 f l u e n c e g i v e s i n t e r m e d i a t e r e s u l t s ( d i s p l a c e m e n t o f a b o u t 25 n m , 45 ~o o f r e s i d u a l n i t r o g e n ) . Dis cuss ion T h e previous results s h o w that the nitrogen penetration passes through a m a x i m u m as a f u n c t i o n o f t h e f l u e n c e . We h a v e f o u n d t h a t t h e m a x i m u m o c ~ I r ~ n e a r Z x I 0 17 1 5 N + c m - 2 f o r Z 200 C 13 s t e e l . F o r e q u i v a l e n t w e a r , t h e r e s i d u a l a m o u n t i s b i g g e r a n d t h e p e n e t r a t i o n more important. T h i s f l u e n c e a g r e e s n e a r l y w i t h t h e o p t i m u m i n f r i c t i o n p r o p e r t i e s f o r s e v e r a l s t e e l s (2) a n d i s t h e s a t u r a t i o n d o s e f o r a 40 k e V i m p l a n t a t i o n a s s h o w n o n f i g u r e n ° 5, w h i c h g i v e s t h e . e v o l u t i o n o f t h e i m p l a n t e d n i t r o g e n a m o u n t v e r s u s t h e f l u e n c e . B e y o n d Z x 1 017 l 5 N ~ : i o n s c r n -Z, a s a t u r a t i o n d u r i n g i m p l a n t a t i o n i s o b t a i n e d s o t h a t t h e r e i s t h e s a m e a m o u n t o f n i t r o g e n
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introduced in the s a m p l e as is r e m o v e d (7). Previous studies (8, 9) have s h o w n that the nitro gen implantation in steel leads to the formation of ~' F e 4 N and e F e 2 + x N nitrides. At 2 x 1017 1 5 N + ions c m -2 the nitrogen is mainly c o m p o u n d e d as e F e 2 + x N while there would be m o r e and m o r e unbounded nitrogen w h e n the fluence increases. That point explains w h y at 4.1 017 1 5 N + ions c m -2 nitrogen is quickly eliminated, the unbound nitrogen counterbalancing the beneficial effect c~ the nitrides (pa rticula rly if it is p r e s e n t as clusters in the material (8) ~. With the other fluences and especially at 2 x 10 I T w h e n the nitride am o u n t is the m o r e important, the ¢ F e 2 + x N, a very interesting c o m p o u n d for friction (I0), causes the onset of w e a r to be delayed. But this wear, by a t h e r m o m e c h a n i c a l effect, probably causes nitride destabilisation and its decomposition to unbound nitrogen (with perhaps intermediate stages of F e 3 N , Fe4N). This nitrogen can then diffuse in the material. This phen o m e n o n , on a very small scale, explains the little m a x i m a in the case of small wear. O n a m o r e important scale, w e find the later evolution of the curves : there is competition between the bulk diffusion of nitrogen and the progression of the w e a r surface, less and less delayed by the nitride w h o s e a m o u n t is m o r e and m o r e decreasing.
Acknowledgements We w i s h to e x p r e s s o u r t h a n k s to A. P l a n t i e r This w o r k was funded by D . G . R . S . T .
(IPN) f o r c a r r y i n g out the i m p l a n t a t i o n .
References I.
2. 3.
H. H e r m a n , N u c l . I n s t r . Meth. 182/183, 887, (1981). M . C . Loys, Th~se L y o n (1 982). J.K, Hirvonen, Treatise on Material Science and Technology, Vol. 18, Acad. Press,
N.Y. 4.
5. 6. 7. 8. 9. I0.
(1 980).
G. Dearneley,N.E.W. H a r t l e y , Thin S o l i d F i l m s 54, 215-232, (1978). F. Schultz, K. Wittmaack, Rad. Elf., 23, 31, (1976). R. Bolster, I.L. Singer, A . S . L . E . , Trans. Vol. 24.4, 526-532, (1981). T. B a r n a v o n et al. To be published. G. Longworth, N.E.W.Hartley, Thin Solid Films, 48, (1978). G. M a r e s t et al., Nucl. Instr. Meth., I . B . M . M . Grenoble (1982). J.P. Peyre and C. Tournier, CetimInformation, 74, 50, (1982).
462
WEAR IN IMPLANTED STEEL
Vol.
17,
No.
40.
20, x'~ 4
I
/
3x~*
10~ q
tJ
o(J
o ~J
o.j
O '0
Z
Z
Z
IX)
r
o
Z
o Depth (nm)
D e p t h (nm)
Fig. 2 : Nitrogen profile evolution during w e a r a f t e r 10 ~ 7 i o n s c m -2 i m p l a n t a t i o n .
)
Fig. 3 : N i t r o g e n profile evolution during w e a r after Z x 1 0 17 ions c m -2 implantation.
40.
i
30. ~10"
v
~0.
.o
2x10'
2x10"
fd
0
~1
10.
10" f4
o
~
U
Z
0
/ I0~
'
Z
50
N i t r o g e n fluence
(cm -z)
Depth (nm) Fig. 4 : Nitrogen profile evolution during w e a r a f t e r 4 x 1 01 7 i o n s c m -2 i m p l a n t a t i o n ,
Fig. 5 fluence.
: Nitrogen
dose retained versus
4
NITROGEN
Vol. 17, pp. 459-462, 1983 P r i n t e d in the U.S.A.
MIGRATION
DURING
WEAR
P e r g a m o n Press Ltd. All rights r e s e r v e d
IN A N I M P L A N T E D
STEEL
S. F A Y E U L L E , D. T R E H E U X , P. G U I R A L D E N Q D ~ p a r t e m e n t Mdtallurgie Physique-Mat~riaux, E. R . A . N o 7 3 2 du C N R S Ecole Centrale de Lyon, B P I 63, 69131 E C U L L Y C e d e x France T. B A R N A V O N , J. T O U S S E T Institut de Physique Nucldaire (et IN2P3),Universit~ CI. B e r n a r d Lyon-I 4 3 , B d du 11 N o v e m b r e 1918, 69622 V I L L E U R B A N N E C e d e x France M. ROBELET Centre de R e c h e r c h e d'Unieux (Creusot-Loire) 42701 F I R M I N Y France
( R e c e i v e d N o v e m b e r 8, 1982) ( R e v i s e d J a n u a r y 27, 1983) Introduction T h e effectiveness of nitrogen implantation in improving the tribological properties of steels under s o m e conditions has been well established (l, 2), but m e c h a n i s m s for this i m p r o v e m e n t are not well known. Nitrogen migration during wear, though a s s u m e d for several years (3,4), is still not well understood and few studies have been able to d e a r l y s h o w it. Indeed two m a j o r difficulties appear in its study : classical friction experiments like pin and disk test cause important modification of the surface states of the antagonists. Particularly, scratches appear very quickly, altering the m e a s u r e m e n t of w o r n depth and the analyses of implanted layers w h o s e depth (100 n m ! is lower than the depth of the scratches; methods for the determination of nitrogen profiles require an ionic erosion technique (Auger, E S C A . . . ). In addition to the perturbations that it can introduce in the distribution and the nature of t h e implanted nitrogen, this destructive erosion imposes the use of different samples for each test and so, does not permit the exact determination of a nitrogen profile during a tribological test. This paper gives results about the study of this migration in a Z 200 C 13 steel (13 ~o Cr, 2 ~ C) depending on the nitrogen fluence. The two problems mentioned previously are resolved by the use of a non destructive nuclear m e t h o d for analysis and of a purely erosion test that does not modify the surface state for wear. Experimental procedure Cubic sam p l e s ( 1 2 x 1 2 x I m m 3) of Z 2 0 0 C 13 steel (austenitisedat 1 2 7 0 K , quenched in oil and t e m p e r e d at 470 K for 2 hours, hardness 60 H R C ) w e r e implanted on one side after polishing in 1/4 ~ m diamond, with 40 k e V 15 N ions by uslng the isotope separator of the Institut de Physique Nucl~aire de Lyon. The ions fluences w e r e 10 17 , 2 x 10 17 15 N + ions c m -2 and 4 x 1017 " 5 N + ions c m -2. T h e samples w e r e implanted at r o o m temperature (~- 25°C) maintained by a water circulating device. The profiles of penetration w e r e obtained without erosion with the l ~ ( p c ~ ~)12C reaction (5) that presents an isolated resonance at 429 keV. The width of this peak, 0.9 keV, a11ows a depth resolution of 5 n m . The sample was b o m b a r d e d with protons of energy E. A s long as E is lower than 429 k e V (ER) the re-~
,
459 0036-9748/83/040459-04503.00/0 C o p y r i g h t (c) 1983 P e r g a m o n Press
.
Ltd.
°
460
WEAR
IN I M P L A N T E D
STEEL
Vol.
17,
No.
4
a c t i o n d o e s n o t h a p p e n . W h e n E i s e q u a l to E R, t h e r e s o n a n c e o c c u r s , a n d n i t r o g e n is d e t e c t e d on the s u r f a c e . W h e n E b e c o m e s h i g h e r t h a n E R, p r o t o n s l o s e a p a r t o f t h e i r e n e r g y to r e a c h E R at the depth x , at which n i t r o g e n is a n a l y s e d . T h u s , the r e a c t i o n is p r o g r e s s i v e l y "pushed" into the sample and, for each energy of t h e i n c i d e n t p r o t o n s , t h e ~( e m i s s i o n i s r e c o r d e d . T h e p r o t o n s w e r e a c c e l e r a t e d w i t h a 2. 5 MV V a n de G r a a f f g e n e r a t o r ; t h e ~( r a y s w e r e d e t e c t e d w i t h a 10 x 7 . 5 c m N a I ( T I ) c r y s t a l A f t e r i m p l a n t a t i o n , t h e s a m p l e s w e r e w o r n a c c o r d i n g to a t e c h n i q u e s i m i l a r to t h a t u s e d b y B o l s t e r e t a l . (6). The s a m p l e s w e r e p o l i s h e d f o r a g i v e n p e r i o d on a f a b r i c a t e d n o n - w o v e n s y n t h e t i c t e x tile c h a r g e d with 0 . 2 5 ~ m d i a m o n d p o w d e r . T h e y w e r e t h e n c l e a n e d u l t r a s o n i c a l l y and in t r i c h l o r o e t h y l e n e , a n d r i n s e d . T h e w e i g h t l o s s e s w e r e m e a s u r e d w i t h 10 -6 g p r e c i s i o n a n d w e r e a c t u a l l y p r o p o r t i o n a l to w o r n d e p t h b e c a u s e o f t h e u n i f o r m r e m o v a l o f t h e s u r f a c e , a s s h o w n o n the next figure :
I ~m
I
2b0-~m Fig.
1
: Typical profilometer
trace of surface after wear
As the a n a l y s i s is non d e s t r u c t i v e , one can follow the e v o l u t i o n of the n i t r o g e n p r o f i l e s a f t e r d i f f e r e n t s t a g e s o f w e a r on t h e s a m e s a m p l e . Results E a c h s a m p l e w a s a n a l y s e d a f t e r i m p l a n t a t i o n ( c u r v e n ° I), a f t e r a f i r s t s m a l l w e a r ( l o w e r t h a n 25 n m ) ( c u r v e n ° 2) a n d a f t e r a m o r e e x t e n s i v e w e a r ( m o r e t h a n 60 n m ) ( c u r v e n ° 3). R e s u l t s a r e p l o t t e d v e r s u s t h e f l u e n c e o n f i g u r e s Z, 3, 4. T h e i n f l u e n c e o f t h e n i t r o g e n d o s e i s n o t v i s i b l e i n t h e c a s e o f s m a l l w e a r ( c u r v e n ° 2) : f o r t h e t h r e e c a s e s , t h e p r o f i l e s a r e not v e r y m u c h m o d i f i e d . The m a x i m a a r e a little m o r e p r o n o u n c e d and the r e s i d u a l n i t r o g e n a m o u n t i s a l w a y s m o r e t h a n 95 ~0 o f t h e i n i t i a l a m o u n t . W h e n t h e w e a r i n c r e a s e s , n o t i c e a b l e d i f f e r e n c e s a p p e a r ( c u r v e n ° 3~ a n d t h e n i t r o g e n t r a n s p o r t in t h e b u l k i s t h e m o r e i m p o r t a n t f o r t h e 2 x 1017 15N+ i o n s c m - - f l u e n c e : t h e p r o f i l e d e p l a c e m e n t i s a b o u t 4 0 n m a n d t h e n i t r o g e n q u a n t i t y i s s t i l l a b o u t 55 ~o o f t h e i n i t i a l quan%ity. O n t h e c o n t r a r y , f o r b i g g e r f l u e n c e s | t h e p e n e t r a t i o n i s n e a r l y n u l l a n d l e s s t h a n 30 of the n i t r o g e n is s t i l l p r e s e n t . T h e 1017 i o n s c m -2 f l u e n c e g i v e s i n t e r m e d i a t e r e s u l t s ( d i s p l a c e m e n t o f a b o u t 25 n m , 45 ~o o f r e s i d u a l n i t r o g e n ) . Dis cuss ion T h e previous results s h o w that the nitrogen penetration passes through a m a x i m u m as a f u n c t i o n o f t h e f l u e n c e . We h a v e f o u n d t h a t t h e m a x i m u m o c ~ I r ~ n e a r Z x I 0 17 1 5 N + c m - 2 f o r Z 200 C 13 s t e e l . F o r e q u i v a l e n t w e a r , t h e r e s i d u a l a m o u n t i s b i g g e r a n d t h e p e n e t r a t i o n more important. T h i s f l u e n c e a g r e e s n e a r l y w i t h t h e o p t i m u m i n f r i c t i o n p r o p e r t i e s f o r s e v e r a l s t e e l s (2) a n d i s t h e s a t u r a t i o n d o s e f o r a 40 k e V i m p l a n t a t i o n a s s h o w n o n f i g u r e n ° 5, w h i c h g i v e s t h e . e v o l u t i o n o f t h e i m p l a n t e d n i t r o g e n a m o u n t v e r s u s t h e f l u e n c e . B e y o n d Z x 1 017 l 5 N ~ : i o n s c r n -Z, a s a t u r a t i o n d u r i n g i m p l a n t a t i o n i s o b t a i n e d s o t h a t t h e r e i s t h e s a m e a m o u n t o f n i t r o g e n
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introduced in the s a m p l e as is r e m o v e d (7). Previous studies (8, 9) have s h o w n that the nitro gen implantation in steel leads to the formation of ~' F e 4 N and e F e 2 + x N nitrides. At 2 x 1017 1 5 N + ions c m -2 the nitrogen is mainly c o m p o u n d e d as e F e 2 + x N while there would be m o r e and m o r e unbounded nitrogen w h e n the fluence increases. That point explains w h y at 4.1 017 1 5 N + ions c m -2 nitrogen is quickly eliminated, the unbound nitrogen counterbalancing the beneficial effect c~ the nitrides (pa rticula rly if it is p r e s e n t as clusters in the material (8) ~. With the other fluences and especially at 2 x 10 I T w h e n the nitride am o u n t is the m o r e important, the ¢ F e 2 + x N, a very interesting c o m p o u n d for friction (I0), causes the onset of w e a r to be delayed. But this wear, by a t h e r m o m e c h a n i c a l effect, probably causes nitride destabilisation and its decomposition to unbound nitrogen (with perhaps intermediate stages of F e 3 N , Fe4N). This nitrogen can then diffuse in the material. This phen o m e n o n , on a very small scale, explains the little m a x i m a in the case of small wear. O n a m o r e important scale, w e find the later evolution of the curves : there is competition between the bulk diffusion of nitrogen and the progression of the w e a r surface, less and less delayed by the nitride w h o s e a m o u n t is m o r e and m o r e decreasing.
Acknowledgements We w i s h to e x p r e s s o u r t h a n k s to A. P l a n t i e r This w o r k was funded by D . G . R . S . T .
(IPN) f o r c a r r y i n g out the i m p l a n t a t i o n .
References I.
2. 3.
H. H e r m a n , N u c l . I n s t r . Meth. 182/183, 887, (1981). M . C . Loys, Th~se L y o n (1 982). J.K, Hirvonen, Treatise on Material Science and Technology, Vol. 18, Acad. Press,
N.Y. 4.
5. 6. 7. 8. 9. I0.
(1 980).
G. Dearneley,N.E.W. H a r t l e y , Thin S o l i d F i l m s 54, 215-232, (1978). F. Schultz, K. Wittmaack, Rad. Elf., 23, 31, (1976). R. Bolster, I.L. Singer, A . S . L . E . , Trans. Vol. 24.4, 526-532, (1981). T. B a r n a v o n et al. To be published. G. Longworth, N.E.W.Hartley, Thin Solid Films, 48, (1978). G. M a r e s t et al., Nucl. Instr. Meth., I . B . M . M . Grenoble (1982). J.P. Peyre and C. Tournier, CetimInformation, 74, 50, (1982).
462
WEAR IN IMPLANTED STEEL
Vol.
17,
No.
40.
20, x'~ 4
I
/
3x~*
10~ q
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o ~J
o.j
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Z
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o Depth (nm)
D e p t h (nm)
Fig. 2 : Nitrogen profile evolution during w e a r a f t e r 10 ~ 7 i o n s c m -2 i m p l a n t a t i o n .
)
Fig. 3 : N i t r o g e n profile evolution during w e a r after Z x 1 0 17 ions c m -2 implantation.
40.
i
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v
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N i t r o g e n fluence
(cm -z)
Depth (nm) Fig. 4 : Nitrogen profile evolution during w e a r a f t e r 4 x 1 01 7 i o n s c m -2 i m p l a n t a t i o n ,
Fig. 5 fluence.
: Nitrogen
dose retained versus
4