CHEMICAL STATE ANALYSIS OF NITRIDED STEEL SURFACE BY MEANS OF CONVERSION ELECTRON MÖSSBAUER SPECTROMETRY

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CHEMICAL STATE ANALYSIS OF NITRIDED STEEL SURFACE BY MEANS OF CONVERSION

ELECTRON MÖSSBAUER SPECTROMETRY

Y. Ujihira, A. Handa

To cite this version:

Y. Ujihira, A. Handa. CHEMICAL STATE ANALYSIS OF NITRIDED STEEL SURFACE BY

MEANS OF CONVERSION ELECTRON MÖSSBAUER SPECTROMETRY. Journal de Physique

Colloques, 1979, 40 (C2), pp.C2-586-C2-588. �10.1051/jphyscol:19792204�. �jpa-00218583�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au n o 3, Tome 40, mars 1979, page

C2-586

CHEMICAL STATE ANALYSIS OF N I T R I D E D STEEL SURFACE BY MEANS OF CONVERSION ELECTRON MOSSBAUER SPECTROMETRY

Y. Ujihira and A. Handa

Department of Industria2 Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, 113 J a p m

REsum6.- La spectroscopie MEssbauer d'6lectrons de conversion a St6 employ6e pour caractgriser des surfaces d'acier nitrur6; lt6tat chimique des surfaces a 6t6 analys6 couche par couche. La surface d'acier nitrur6 par rdaction avec l'azote est composde de Fe2N-€, Fe3-xN(O<x<l) distordu, de la martensite Fe,N-Y et d'une couche de fer-a 16gSrement deform6 par l'azote introduit. La nitruration par attaque chimique conduit une surface de Fe3N-E, des couches intermediaires de Fe3N-€ et Fe,N-yt et du Fe-a distordu. La couche nitrur6e par 6lectrochimie a une Epaisseur de 2 5 3 microns de Fe,N-y:, Fe,N-y:et des couches de Fe-a distordu.

Abstract.- CEMS was used to characterize the nitrided steel surfaces and chemical states of surfaces estimated layer by layer. Gas nitrided steel surface is composed of €-Fe2N, distorted Fe,+xN ( o ~ x ~ I ) , y'-Fe,N martensite and a-Fe layer slightly disturbed by intruded nitrogen. Tuf ftrided steel surface consists of €-Fe3N, intermediate E-Fe3N and yl-Fe,N, y'-Fe,N and disturbed a-Fe layers. Ion nitrided steel surface has a 2-3 um thickness of y;-Fe,N (magnetically randomly oriented), y;-Fe,N and distorted a-Fe layers.

1. Introduction.- Though the potentiality of CEMS as an analytical tool for the characterization of oxidized iron surface or ion implanted semi-conduc- tor material has been widely demonstrated, little attention is orientated to the layer by layer analysis of the surface layers. Development of depth sen- sitive CEMS apparatus may meet the above request, there still need several years before depth sensi- tive spectrometer has come into practical use.

The present paper describes the results of chemical state analyses of nitrided steel surfaces achie,red by the combination of careful grinding of surface layer and the CEMS detecting 7.3 keV conversion electron and 5.6 keV Auger electron of iron-57.

2. Experimental.- The polished surface of NT70H (Fe:95.97at.%, C:0.56at.%, diameter:Scm, length:

0.5cm) was nitrided by the following three procedu- res.

(A) A sample was placed in the stream of ammonia gas and heated at 793 K for 30 hours. 30% of ammonia molecules were dissociated to feed nitrogen atoms (2NH3 + 2i+3~,) in the atmosphere

111.

(B) A sample was dipped into fused salts bath of potassium cyanide and potassium cyanate and nitrided for 7 hours 121.

(C) A sample was placed in the stream of nitrogen (0.35 Elmin.) and hydrogen (0.5 1 L/min.) mixture and ion nitrided for 4 hours at 793 K by the glow discharge of nitrogen 131.

The nitride surfaces were ground to the desi-

red depth utilizing the fine powder of Sic 500 and CEMS' of the exposed layers were measured by the backscatter electron detector (Austin Science Asso-

ciates BSD-2400). The ground thickness was monitored through the weighing of nitrided samples.

3. Results and discussion.-

3.1. Surface states of gas nitri:ded steel.- The CEMS of gas nitrided steel surface ( 3 0 ~ ) are shown in figure 1, which indicates the presence of 4 distinct layers on the surface.

The uppermost surface material is identified to paramagnetic €-Fe2N, the thickness of which is more than 10um.

The intensity ratio of the doublet peaks is 5:3. Although the conjecture that the efg of the layer could be oriented parallel to the surface is plausible, the idea that the doublet peaks are due to the superposition of several peaks of different iron species could not be ruled out.

CEMS of (b) reveals that the distorted Fe3+xN (O,<x,<l) layer exists between the c-Fe2N austenite layer and'y'-FesN martensite.

CEMS of (c) indicates that the crystal structu- re of y'-FesN is distorted considerably from that of pure y'-FesN /5/, / 6 / . The CEMS of the layer has the magnetically split six peaks with the intensity ratio 3:4:1:1:4:3, indicating the magnetic domain of the exposed y'-FebN layer to be oriented parallel to the surface.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792204

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(a) the uppermost surface

7.0

r 320 (c) ground 25 i r n

(d) ground 30 p m

F j g . I : CEMS' o f g a s n i t r i d e d s t e e l s u r f a c e ground w i t h S i c powder.

CEMS of ( d ) shows t h a t t h e c r y s t a l s t r u c t u r e o f a-Fe p h a s e c l o s e t o t h e y'-FesN i s d i s t o r t e d t o a c o n s i d e r a b l e e x t e n t by t h e i n t r u s i o n of n i t r o g e n atoms i n t o t h e l a t t i c e o f a-Fe. Yamakawa and F u j i t a r e p o r t e d t h a t 5-FezN was a l s o found b e s i d e s ~-Fe2-3N and yl-Fe4N i n t h e n i t r i d e d s t e e l o b t a i n e d by n i t r i - d i n g p u r e i r o n i n t h e ammonia g a s f o r 20 m i n u t e s 171 But we c o u l d n o t o b s e r v e t h e peaks o f 5-Fe2N o n t h e CEMS o f g a s n i t r i d e d s t e e l s u r f a c e . Our samples a r e produced by g a s n i t r i d i n g s t e e l s u r f a c e f o r 30 h r s , s o t h a t 5-Fe2N might have a l r e a d y changed i n t o E-Fez-3N o r y' -Fe4N.

3 . 2 . s u r f a c e s e a t e s o f t u f f p i d e d s t e e l . - The CEMS' o f t u f f t r i d e d s t e e l s u r f a c e (40pm) a r e shown i n F i g u r e 2 .

CEMS o f ( a ) s u g g e s t s t h e e x i s t e n c e o f two che- m i c a l l a y e r s a t t h e uppermost s u r f a c e . The peaks a r e a s s i g n e d t o t h e m i x t u r e of €-Fe3C and c-Fe3N, r e f e r - r i n g t o t h e i n f o r m a t i o n t h a t c a r b o n c o a g u l a t e s n e a r t h e s u r f a c e e-Fe2-3N p h a s e 181.

CEMS o f 20um-30pm l a y e r from t h e uppermost s u r - f a c e i n d i c a t e s t h e g r a d u a l d e c r e a s e o f n i t r o g e n

-8 -6 -4 -2 0 2 4 6 8 velocity /mms"

F j g . 2 : CEMS'of t u f f t r i d e d s t e e l s u r f a c e ground w i t h S i c powder.

atoms a s t h e d e p t h g o i n g on.

CEMS o f (d) i s a s s i g n e d a s t h e broadened peaks o f t h e d i s t o r t e d y'-FekN p h a s e .

M a g n e t i c a l l y s p l i t peaks o f a-Fe i s o b t a i n e d a t t h e 40pm d e p t h , t h e somewhat d i s t o r t e d p r o f i l e s r e - v e a l i n g t h e i n t r u s i o n o f n i t r o g e n atoms i n t h e a-Fe c r y s t a l l a t t i c e .

3.3. S u r f a c e s t a t e s of i o n i t r i d e d stget?.- The s u r f a - c e l a y e r c o n s i s t s o f 3 l a y e r s , y;-Fe,N, y:-Fe,N and a-Fe p h a s e ( F i g . 3)

.

The uppermost l a y e r is yi-Fe,N. R e l a t i v e i n t e n - s i t i e s of m a g n e t i c a l l y s p l i t peaks o f Yi-FekN sug- g e s t s t h a t t h e m a g n e t i c domain of t h e uppermost 10-100nm l a y e r i s o r i e n t e d a t random. The u n u s u a l e v i - d e n c e c a n b e e x p l a i n e d by t h e e v a p o r a t i o n o f s u r f a c e i r o n atoms by t h e impact o f n i t r o g e n i o n s , f o l l o w e d by t h e r e a c t i o n o f i r o n and n i t r o g e n i n t h e s p a c e and s p u t t e r i n g of produced i r o n n i t r i d e o n t o t h e s u r f a c e 191.

R e l a t i v e i n t e n s i t i e s o f CEMS o f t h e l a y e r 3pm below t h e s u r f a c e a r e 3:4:1:1:4:3, i n d i c a t i n g t h e m a g n e t i c domain of t h e exposed Y;-Fe,N l a y e r t o b e

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JOURNAL DE PHYSIQUE

orientated parallel to the surface. References

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(1977) 76.

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/5/ Foct, J., J. Physique Colloq.

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. .. '.

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8

45.0

: **

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(c) ground 10 Mm

Fig. 3 : CEMS' of ion nitrided steel surface ground with Sic powder.

Non-equivalent three sites of iron atoms in y'-FekN are clearly separated in CEMS (lattice site iron (FeI) and face-center sites (FeII-A and FeII-B).

Considerably discrepancy is observed in bhe quadru- pole splitting of FeII-A or FeII-B between y;-Fe,N phase and y;-Fe,N phase (Fig. 3 (a) and (b)).

a-Fe phase prevails in the 10um depth layer.