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ROLE OF IRON IN THE AMORPHIZATION PROCESS IN FRICTION-INDUCED PHOSPHATE
GLASSES
M. Belin, J. Martin, J. Mansot
To cite this version:
M. Belin, J. Martin, J. Mansot. ROLE OF IRON IN THE AMORPHIZATION PROCESS IN
FRICTION-INDUCED PHOSPHATE GLASSES. Journal de Physique Colloques, 1987, 48 (C9),
pp.C9-1147-C9-1153. �10.1051/jphyscol:19879210�. �jpa-00227331�
ROLE OF IRON IN THE AMORPHIZATION PROCESS IN FRICTION-INDUCED PHOSPHATE GLASSES
M. BELIN, J.M. MARTIN and J.L. MANSOT
Ecole Centrale de Lyon, Laboratoire de rechnologie des Surfaces, CNRS UA-855, 36, Avenue Guy de Collongue, BP 163, F-69131 Ecully Cedex
,
FranceI
-
INTRODUCTIONIt h a s been demonstrated i n p r e v i o u s work t h a t under boundary l u b r i c a t i o n , wear of s l i d i n g s t e e l s u r f a c e s can be decreased by t h e a d d i t i o n o f Zinc Dialkyl Dithiophosphate (ZDDP) i n t h e l u b r i c a n t 111. I t has been a l s o shown t h a t t h e wear d e c r e a s e mechanism i s a s s o c i a t e d with t h e build-up of an amorphous i n t e r f a c i a l m a t e r i a l (anti-wear f i l m ) . This s u r f a c e f i l m c o n t a i n s elements coming from t h e ZDDP molecules (phosphorus, s u l f u r , z i n c and oxygen) and from t h e f r i c t i o n s u r f a c e s ( i r o n ) . An
understanding o f t h e s t r u c t u r i n g r o l e of i r o n i n t h e s e g l a s s e s is important i n o r d e r t o e x p l a i n :
- t h e r h e o l o g i c a l and d i f f u s i o n p r o p e r t i e s of i n t e r f a c e m a t e r i a l r e l a t e d t o anti-wear p r o c e s s e s ,
- t h e mechanisms by which i n i t i a l i r o n o x i d e s a r e amorphized i n t h e presence of t h e a d d i t i v e .
I n o r d e r t o determine t h e r o l e of i r o n i n t h e amorphization p r o c e s s (network former o r modifier c a t i o n s ) , t h r e e typeq o f a n a l y t i c a l d a t a a r e r e q u i r e d :
-
t h e o x i d a t i o n s t a t e o f i r o n atoms,-
t h e symmetry o f i r o n s i t e s ,-
t h e c o o r d i n a t i o n number of i r o n surrounded by oxygen atoms.The experimental method used i s X-ray a b s o r p t i o n s p e c t r o s c o p y , t h e s p e c t r a a r e recorded i n t h e t r a n s m i s s i o n mode on t h e D C I s t o r a g e r i n g i n ORSAY ( p o s i t r o n s , 1 , 8 5 GeV) using a double c r y s t a l monochromator ( S i 220).
C r y s t a l l i n e body-centered i r o n , s i d e r i t e , ZnFe204, NiFe(NiCr)04 a r e used a s s t a n d a r d compounds f o r i r o n , r e s p e c t i v e l y a s Fe 0
,
Fez+ i n o c t a h e d r a l s i t e s , Fe3+ i n o c t a h e d r a l s i t e s and Fe3+ i n t e t r a h e d r a l s i t e s .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879210
C9-1148 JOURNAL
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PHYSIQUEThe g l a s s sample i s obtained a s submicronic wear fragments from a f r i c t i o n experiment i n which two f e r r o u s m a t e r i a l s a r e rubbed a g a i n s t each o t h e r ( s t e e l and c a s t i r o n ) i n t h e presence of a p a r a f f i n i c l u b r i c a n t (n- dodecan) c o n t a i n i n g 1% by weight z i n c d i - i s o p r o p y l dithiophosphate
(R0)-PSS-Zn 121. The t r i b o l o g i c a l parameters a r e :
-
s l i d i n g speed : 10 cm/s, -
mean c o n t a c t p r e s s u r e : 0 . 1 GPa,-
temperature 80°C.The t e s t e d l u b r i c a n t containirig wear p a r t i c l e s is c o l l e c t e d a t t h e end of t h e t e s t . The p a r t i c l e s a r e s e p a r a t e d by c e n t r i f u g a t i o n , washed i n pure n-hexane and d r i e d i n a primary vacuum.
I1
-
SURVEY ON NEAR EDGE STRUCTURE OF F e K-EDGEThe recorded Fe-K near-edge s t r u c t u r e extends from t h e beginning of t h e edge (around 7115 eV) t o s e v e r a l t e n t h s o f e l e c t r o n - v o l t s above. This r e g i o n corresponds t o t h e t r a n s i t i o n of t h e Fe-1s e l e c t r o n s t o empty l e v e l s . The shape and p o s i t i o n of peaks have been r e l a t e d t o :
( i ) t h e o x i d a t i o n s t a t e of i r o n a s a chemical s h i f t 131, ( i i ) t h e metal oxygen covalency 141,
( i i i ) t h e l o c a l symmetry and s i t e d i s t o r s i o n 141.
T h e r e f o r e , comparison between s t a n d a r d s and unknown specimens can g i v e t h e s t r u c t u r a l and chemical information which i s needed h e r e .
I11
-
RESULTS3.1. Oxidation s t a t e o f i r o n
3.1.1. edge p o s i t i o n
The o x i d a t i o n s t a t e o f i r o n induces a chemical s h i f t i n t h e edge and pre-edge p o s i t i o n s . The edge p o s i t i o n is d e f i n e d h e r e a s t h e h a l f h e i g h t of t h e absorbance jump which can be measured with an accuracy of 0 , 3 eV. A s a l r e a d y n o t i c e d 14, 51 r e s u l t s on s t a n d a r d s show an i n c r e a s e i n t h e edge energy from 7115.1 eV (Fe a s & - i r o n ) t o s e v e r a l e l e c t r o n - v o l t s h i g h e r f o r 0 o x i d i z e d s p e c i e s . R e s u l t s on our specimens a r e summarized i n t a b l e 1.
: ( e ~ a t m a x i m u m ) : ( ~ 0 . 3 e V ) : ( e V ) :
.---.---:---:---:
: Fe 0 ( o - i r o n ) :
-
7115.1 : r e f . :.---.---.---:---:
: Fe2+ ( s i d e r i t e ) : 7107.3 7116.1 : 1.0 :
: Fe
3+
( t e t r a ) : 7109.1 7117.8 : 2.7 :: f r i c t i o n g l a s s : 7109.1 7120.5 : 5.4 :
Table 1 : Values of e n e r g i e s of t h e pre-edge peak, t h e Fe-K edge and i t s energy s h i f t dE, f o r d i f f e r e n t compounds.
When comparing t h e g l a s s sample r e s u l t s with a p p r o p r i a t e s t a n d a r d s , we n o t i c e an important chemical s h i f t of 5.4 eV i n d i c a t i n g a high o x i d a t i o n s t a t e (Fe3+ ) 151. Nevertheless, t h e e x a c t p o s i t i o n of t h e edge is
d i f f i c u l t t o measure because of p o s s i b l e s u p e r p o s i t i o n of edge f e a t u r e s .
3.1.2. Pre-edge chemical s h i f t
A chemical s h i f t of t h e 1s-3d pre-edge peak energy r e l a t e d t o t h e o x i d a t i o n s t a t e of i r o n has been observed i n t h e l i t e r a t u r e 161 171.
Compared t o t h e s t a n d a r d Fez+ ( a s s i d e r i t e ) , we n o t i c e a 1 . 8 eV s h i f t i n our sample, i n d i c a t i n g a h i g h e r o x i d a t i o n s t a t e . The f a c t t h a t t h e pre-edge p o s i t i o n s recorded both with t h e g l a s s sample and Fe203 a r e i d e n t i c a l
( f i g u r e 1 1 , confirms t h e ~ e o x i d a t i o n s t a t e of i r o n i n t h e amorphous ~ + m a t e r i a l .
C9-1150 JOURNAL
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PHYSIQUE3.2. Symmetry of i r o n s i t e s
I n oxide systems, t h e weak pre-edge f e a t u r e corresponds t o t h e 1s-3d enhanced t r a n s i t i o n induced by t h e h y b r i d i z a t i o n of i r o n 3d l e v e l with oxygen 2p o r b i t a l s 161. The r e l a t i v e amplitude of t h i s pre-peak h a s been found t o i n c r e a s e i n t h e following o r d e r : r e g u l a r o c t a h e d r a l - r e g u l a r t e t r a h e d r a l . Normalized pre-edge peaks a r e p r e s e n t e d i n f i g u r e 1. From comparison with s t a n d a r d s , we deduce an o c t a h e d r a l symmetry of i r o n s i t e s i n our g l a s s . The d i s t o r t i o n of t h e s i t e has n o t been analysed here b u t i s i n c u r r e n t l y under c o n s i d e r a t i o n with a p p r o p r i a t e s t a n d a r d s by examining t h e s p l i t t i n g of t h e main a b s o r p t i o n l i n e 141
.
3.3. EXAFS
Information on Fe-0 covalency can a l s o be e x t r a c t e d from EXAFS modulations extending over a range of s e v e r a l hundred eV above t h e edge.
S t a r t i n g from a g i v e n s t a n d a r d p r o d u c t , it i s p o s s i b l e t o measure t h e Fe-0 d i s t a n c e and t o g e t an i d e a of t h e s h o r t range o r d e r . By applying t h e s t a n d a r d c a l c u l a t i o n method t o EXAFS r e c o r d i n g s both of t h e specimens and s t a n d a r d (Fe3+ i n o c t a h e d r a l s i t e ) , it is p o s s i b l e t o deduce t h e
c o o r d i n a t i o n number. The comparison o f t h e d a t a i s shown i n f i g u r e 2b : t h e
0
main peak i n t h e g l a s s i s e a s i l y a t t r i b u t e d t o t h e Fe-0 d i s t a n c e a t 2.00 A
0
which is c l o s e t o t h e 1 , 9 6 A d i s t a n c e measured i n t h e c r y s t a l l i z e d Fe203.
Minor peaks appear a t l a r g e r d i s t a n c e s and have a l r e a d y been d e p i c t e d , i n o t h e r phosphate g l a s s e s (71. They have been a t t r i b u t e d e i t h e r t o Fe-P p a i r s o r Fe-Fe d i s t a n c e s . But i n our c a s e s , some s m a l l c r i s t a l l i t e s of & i r o n might e x i s t , embedded i n t h e amorphous matrix.
The EXAFS r e s u l t s i n d i c a t e an 8-fold c o o r d i n a t i o n which does n o t match t h e 6-fold c o o r d i n a t i o n of t h e o c t a h e d r a l s i t e . This might be due t o t h e Fe20s s t a n d a r d i n which two iron-oxygen d i s t a n c e s e x i s t (1.91 and
0
2.06 A). I n our t r e a t m e n t we used t h e mean value o f t h o s e d i s t a n c e s ( 6
0
Oxygens a t 1.96 A) and introduced consequently a l a r g e Debye Waller f a c t o r i n t h e s t a n d a r d back s c a t t e r i n g amplitude c a l c u l a t i o n s . Future
i n v e s t i g a t i o n s u s i n g a more a c c u r a t e s t a n d a r d a r e under c o n s i d e r a t i o n ,
I V
-
CONCLUSIONFrom t h e Fe-K edge a b s o r p t i o n s t u d y of a f r i c t i o n induced phosphate g l a s s , t h e main a n a l y t i c a l d a t a o b t a i n e d a r e a s f o l l o w s :
-
The e x i s t e n c e o f i r o n i n t h e ~ e ' + o x ~ d a t i o n s t a t e evidenced by t h e chemical s h i f t s both of t h e K-edge and t h e pre-edge peak.of the pre-edge 1s-3d transition.
According to recent developments in the structural knowledge of mineral compounds in geology, it can be stated that iron cations in the Fe*
oxydation state act as network modifiers, the network being itself constituted by phosphate chains (poly or pyro-phosphates).
The point is very important for the understanding of friction and wear processes. Previous works have already attempted to relate the structuring role of each ion specie to the shear properties of a glass (8, 91.
ACKNOWLEDGMENTS
The authors thank Mrs 3 . PETIAU, P. LAGARDE, H. DEXPERT and D. SAYERS for their helpful comments and providing some reference compounds.
ENERGY
(eV)
Figure 1 : Normalized pre-edge peak of Fe K-edge for different compounds :
-
crystalline bcc iron (FeO ) ,-
amorphous NiFe(NiCr)04 (Fe 3+ tetra),-
amorphous ZnFe04 (Fe 3+ octa),
-
wear particles obtained with ZDDP lubricant,-
siderite (Fez+ octa).
C9-1152 JOURNAL
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PHYSIQUEE N E R G I E < r V >
Figure 2 : EXAFS of tine glassy wear particles with ZDDP lubricant.
2a : experimental RDF of iron showing the Fe-0 distance.
2b : experimental EXAFS modulations (full line) and simulation (dotted lines) with 8.0 neighbours, at a distance of
Q I
2.00 A , DO\ =
-
0,02, the reference compound being here Fez03.
REFERENCES
11) J.M. MARTIN, M. BELIN, J.L. MANSOT, "Friction-induced amorphization with ZDDP. An EXAFS study", ASLE Transactions, Vol 49, , O 4, pp. 523- 531, (1986).
12
1
J .M. MARTIN, J. L. MANSOT, I. BERBEZIER, M. BELIN, l~Microstructural aspects of lubricated mild wear with zinc dithiophosphate", Wear, Vol 107, pp. 3555-366, (1985).131 L.A. GRUNES, "Study of the K-edges of 3d transition metals in pure and oxide form by X-ray absorption spectroscopy", Physical Review B, Vol 27, no 4, pp. 2111-2131, (1982).
(4
1
J. PETIAU, G. GALAS, "Local structures about some transition elements in oxide glasses using X-ray absorption spectroscopy", Journal de Physique, Tome 43, pp. C9-47-50, (1982).151 M.H. TUILIER, B. BUFFAT, "Influence of oxidation degree and symmetry of the site on K-absorption spectra of six coordinated iron and cobalt in oxidesH, Journal de Physique, Tome 47, pp. C8-771-775, (1986).
high-resolution K-edge spectra : information from the pre-edge", Solid State Communication, Vol 48, n o 7, pp. 625-629, (1983).
17) F. STUDER, A. LE BAIL, "EXAFS of mixed valence iron potassium phosphate glasses", Journal de Physique, Tome 47, C8-781-785.
181 N. BINSTED, G.N. GREAVES, C.M.B. HENDERSON, "Fe K-edge X-ray
absorption spectroscopy of silicate minerals and glasses", Journal de Physique, Tome 47, pp. C8-837-840, (1986).
191 B.O. MYSEN, D. VIRGO, I. KUSHIRO, I. Amer. Mineral., 66, P. 678, (1981).
