ATOM-PROBE RESULTS SUPPORT THE SKELETON MODEL FOR WC-Co

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ATOM-PROBE RESULTS SUPPORT THE SKELETON MODEL FOR WC-Co

A. Henjered, M. Hellsing, H. Andrén, H. Nordén

To cite this version:

A. Henjered, M. Hellsing, H. Andrén, H. Nordén. ATOM-PROBE RESULTS SUPPORT THE

SKELETON MODEL FOR WC-Co. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-349-C9-

353. �10.1051/jphyscol:1984958�. �jpa-00224444�

30URNAL DE PHYSIQUE

Colloque C9, supplément au n°12, Tome 45, décembre 1984 page C9-3W

ATOM-PROBE RESULTS SUPPORT THE SKELETON MODEL FOR WC"Co

A. H e n j e r e d , M. H e l l s i n g , H.O. Andren and H. Norden

Department of Physics, Chalmers University of Technology, S-412 96 Goteborg, Sweden

Résumé - Les joints WC/WC dans des carbures frittes du type WC-Co ont été analyses par une sonde atomique. Les joints contenaient environ la moitié d'une couche atomique du cobalt (ou du Co + Cr) et donc peuvent §tre décrits comme joints de grains avec une ségrégation de cobalt. Ce résultat

confirme le modèle "squelette continu" pour WC-Co.

Abstract - WC/WC boundaries in WC-Co type cemented carbides have been analysed with the atom-probe instrument. The boundaries contained about half a

monolayer of cobalt (or Co + Cr) and can therefore be described as grain boundaries with cobalt segregation. The results support the "continuous skeleton" model of WC-Co.

INTRODUCTION

In order to explain the high hardness and rupture strength and the low plasticity of WC-Co cemented carbides, it was suggested (already in 1940) that a continous carbide

"skeleton" forms during liquid phase sintering of WC-Co with 10% or less Co /1/. As an alternative to the skeleton idea the view was also put forward that all WC grains were surrounded by a layer of cobalt binder phase; this would explain that the maximum rupture strength was obtained at some intermediate cobalt content, and also that WC-Co exhibits some limited plasticity /2/.

During the last three decades, a large amount of experimental data from WC-Co has been reported. These data have often been intepreted as supporting either the skeleton or the cobalt layer theory. The experimental methods that have been used to detect possible thin cobalt layers between WC grains - chemical dissolution of binder phase, electron microscopy of surface replicas, electron microscopy of thin foils, and electron microprobe analysis - all failed to detect any cobalt between what appeared as WC grain boundaries. Recently, however, both Auger spectroscopy

131 and analytical electron microscopy /4/ have indicated the presence of cobalt in WC/WC boundaries, but there is a disagreement over the amount of cobalt present - estimates range from a 2 nm thick layer to less than 1 monolayer.

We have managed to prepare and analyse three field-ion specimens containing a WC/WC boundary at the tip. Atom-probe analysis showed that all three specimens contained cobalt, in the form of a submonolayer grain boundary segregation.

EXPERIMENTAL

Two commercial cemented carbide materials were studied: Material A, containing (wt.%) 89.5 WC - 10 Co - 0.5 Cr C , and Material B containing 92 WC - 6 Co -

2 (Ta,Nb,Ti)C. The materials hld

been prepared by the usual liquid phase sintering at about 1400°C, followed by slow cooling, and they were supplied in the form of 0.8 .mm diameter rods.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984958

JOURNAL DE PHYSIQUE

F i e l d - i o n m i c r o s c o p e s p e c i m e n s w e r e p r e p a r e d by e l e c t r o p o l i s h i n g i n s u l p h u r i c a c i d 1 5 1 . The s p e c i m e n s w e r e t h e n f u r t h e r p o l i s h e d i n s m a l l s t e p s u n t i l a WCIWC b o u n d a r y a p p e a r e d c l o s e t o t h e t i p a p e x . A s p e c i m e n t e m p e r a t u r e o f a b o u t 90 K was u s e d b o t h f o r f i e l d - i o n i m a g i n g and atom-probe a n a l y s i s . S i n c e t h e e v a p o r a t i o n f i e l d o f WC i s a l m o s t 60 Vlnm 1 6 1 , b o t h h e l i u m a n d n e o n g a s c o u l d be u s e d f o r i m a g i n g .

A n a l y s e s were c a r r i e d o u t a t a p r e s s u r e o f l e s s t h a n 1 0 - ~ ~ a , and t h e p r o j e c t e d d i a m e t e r o f t h e p r o b e h o l e v a r i e d b e t w e e n 1 . 5 a n d 4 . 4 nm. T h i s d i a m e t e r was computed u s i n g t h e m a g n i f i c a t i o n f o r m u l a M = R l b r , w h e r e R is t h e t i p - t o - s c r e e n d i s t a n c e , b t h e image c o m p r e s s i o n f a c t o r ( 1 . 6 0 i n t h i s c a s e ) , a n d r t h e t i p r a d i u s o f t h e s p e c i m e n . T i p r a d i i were d e t e r m i n e d by e l e c t r o n m i c r o s c o p y . I n o n e c a s e r c o u l d a l s o be d e t e r m i n e d by r i n g c o u n t i n g i n t h e f i e l d - i o n m i c r o g r a p h .

RESULTS

T h r e e s p e c i m e n s w e r e s t u d i e d w i t h t h e a t o m - p r o b e i n s t r u m e n t . The f i r s t two w e r e made f r o m M a t e r i a l A . and t h e t h i r d f r o m M a t e r i a l B.

Specimen 1 c o n t a i n e d a n a x i a l WCIWC b o u n d a r y a t t h e t i p ( F i g . 1 ) . The b o u n d a r y was v i s i b l e a s a d a r k e r band i n t h e f i e l d - i o n m i c r o g r a p h (Fig. 2 ) . The q u a l i t y o f t h e f i e l d - i o n image o f WC i s r a t h e r g o o d , a n d t h e low i n d e x p o l e s o f t h e s i m p l e h e x a g o n a l s t r u c t u r e a r e c l e a r l y s e e n .

U s i n g a p r o j e c t e d p r o b e h o l e d i a m e t e r o f 1.5 nm, f o u r a t o m - p r o b e a n a l y s e s were made i n t h e b o u n d a r y r e g i o n . A f t e r e a c h a n a l y s i s t h e s p e c i m e n was t i l t e d i n a d i r e c t i o n p e r p e n d i c u l a r t o t h e b o u n d a r y , s o t h a t t h e c e n t r e o f t h e a r e a o f a n a l y s i s moved 2 nm. A t t h e b o u n d a r y 5 . 8 a t % c o b a l t a n d 2 . 6 a t % chromium was r e c o r d e d . No c o b a l t o r chromium was r e c o r d e d f r o m e i t h e r s i d e o f t h e b o u n d a r y . F i n a l l y , a n a n a l y s i s o f t h e b o u n d a r y was made u s i n g a 2 . 2 nm a p e r t u r e . 4 . 2 % Co a n d 1.7 % Cr was r e c o r d e d . Specimen 2 c o n t a i n e d a t r a n s v e r s e WCiWC b o u n d a r y c l o s e t o i t s t i p ( F i g . 3 ) , t h e b o u n d a r y p l a n e making a 65' a n g l e t o t h e t i p a x i s . A c o n t i n o u s atom-probe a n a l y s i s was made t h r o u g h t h e b o u n d a r y w i t h a p r o j e c t e d p r o b e h o l e s i z e o f 2 . 6 nm. The m i d d l e p o r t i o n o f t h e r e s u l t i n g f i e l d e v a p o r a t i o n s e q u e n c e c o n t a i n e d 17 c o b a l t , 4 chromium, 130 c a r b o n , a n d 128 t u n g s t e n i o n s . No c o b a l t o r chromium i o n s w e r e r e c o r d e d o u t s i d e t h i s p a r t o f t h e s e q u e n c e .

Specimen 3 c o n t a i n e d a n a x i a l b o u n d a r y . An a n a l y s i s o f t h e b o u n d a r y u s i n g a 4.4 nm p r o j e c t e d - a p e r t u r e y i e l d e d 3.1 % c o b a l t and n o chromium ( t h e s p e c i m e n was made f r o m t h e chromium f r e e M a t e r i a l 2 ) .

CISCUSSION

C o b a l t was p r e s e n t i n t h e WCIWC b o u n d a r y r e g i o n i n a l l t h r e e s p e c i m e n s a n a l y s e d . I n a d d i t i o n , chromium w a s a l s o p r e s e n t i n t h e b o u n d a r y i f t h e s p e c i m e n had b e e n made f r o m t h e chromium c a r b i d e c o n t a i n i n g M a t e r i a l A . From t h e a n a l y s e s o f s p e c i m e n 1 i t i s c l e a r t h a t c o b a l t a n d chromium a r e l o c a l i z e d t o a zone n o t w i d e r t h a n 2 nm a t t h e b o u n d a r y .

An e v e n h i g h e r s p a t i a l r e s o l u t i o n c o u l d be o b t a i n e d u s i n g t h e d a t a f r o m s p e c i m e n

The d i s t a n c e b e t w e e n t h e t i p a p e x and t h e b o u n d a r y p l a n e , a s d e t e r m i n e d by e l e c t r o n m i c r o s c o p y , was u s e d t o c a l i b r a t e t h e e v a p o r a t i o n d e p t h s c a l e . A s a c h e c k , t h e t o t a l number o f i o n s r e c o r d e d was compared w i t h t h e number e x p e c t e d f r o m t h e d e t e c t i v i t y o f o u r i o n d e t e c t o r . The a g r e e m e n t was g o o d . From t h e d e p t h

c a l i b r a t i o n i t was f o u n d t h a t c o b a l t a n d chromium was r e c o r d e d o v e r a d i s t a n c e o f

1 . 0 0 . 3 nm. S i n c e t h e b o u n d a r y c u t t h e s p e c i m e n a x i s a t a n a n g l e o f 65O, t h i s

d i s t a n c e s h o u l d be compared t o t h e d i s t a n c e o v e r which t h e b o u n d a r y p l a n e w a s u n d e r

t h e p r o b e h o l e , 2 . 6 c o t 65' o r 1 . 2 nm. C o n s e q u e n t l y , t h e e v a p o r a t i o n s e q u e n c e

r e c o r d e d s u g g e s t s t h a t a l l c o b a l t and chromium was l o c a l i z e d t o t h e b o u n d a r y p l a n e .

F i g . 1 - T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h o f s p e c i m e n 1 . Two W C g r a i n s f o r m a n -

a x i a l b o u n d a r y a t t h e s p e c i m e n t i p . A t h i r d WC g r a i n l i e s f u r t h e r down t h e s h a n k ; b e t w e e n t h e t h r e e W C g r a i n s c o b a l t r i c h b i n d e r p h a s e (A) c a n be s e e n .

F i g . - Neon f i e l d - i o n m i c r o g r a p h o f t h e s p e c i m e n i n F i g u r e 1 . The p r o b e h o l e

- ( a s t e r i s k ) is p o s i t i o n e d o v e r t h e b o u n d a r y ( a r r o w s ) b e t w e e n t h e two a d j a c e n t WC

% r a i n s . The p'yramidal p l a n e s o f t y p e I ( 1 0 7 1 ) a n d t y p e I1 ( 1 1 ~ 1 ) g i v e t h e most

p r o m i n e n t p o l e s i n t h e i m a g e .

C9-352 JOURNAL DE PHYSIQUE

F i g . 3 - Dark f i e l d e l e c t r o n m i c r o g r a p h s o f t h e two WC g r a i n s f o r m i n g t h e WC/WC

- b o u n d a r y i n s p e c i m e n 2 .

one monolayer

(calculated)

C DIAMETER OF APERTURE, 3 4 nm 5

F i g . 4 - ( F u l l l i n e : ) E x p e c t e d c o n c e n t r a t i o n when a n a l y s i n g a n a x i a l b o u n d a r y c o n t a i n i n g o n e m o n o l a y e r o f c o b a l t , a s a f u n c t i o n o f a p e r t u r e ( p r o b e h o l e ) p r o j e c t e d s i z e . ( C r o s s e s : ) C o b a l t a n d chromium c o n c e n t r a t i o n s m e a s u r e d w i t h t h e atom-probe.

(The c o n c e n t r a t i o n s m e a s u r e d i n s p e c i m e n 2 h a v e b e e n r e d u c e d t o c o m p e n s a t e f o r t h e

e f f e c t o f a n o n - a x i a l b o u n d a r y . )

Knowlng now t h e l o c a l i z a t i o n o f c o b a l t and chromium, i t i s i n t e r e s t i n g t o e s t i m a t e t h e amount o f t h e s e e l e m e n t s i n t h e boundary i n t e r m s o f monolayers o f atoms (we d e f i n e one monolayer a s a 0.2 nm t h i c k l a y e r h a v i n g t h e same d e n s i t y a s h . c . p . c o b a l t ) . The c o b a l t c o n c e n t r a t i o n e x p e c t e d when a n a l y s i n g a n a x i a l boundary c o n t a i n i n g one monolayer o f c o b a l t i s shown a s a f u n c t i o n o f probe h o l e s i z e i n F i g u r e 4 . I n t h e same F i g u r e , t h e measured c o b a l t and chromium c o n c e n t r a t i o n s a r e a l s o p l o t t e d . I t i s e v i d e n t t h a t t h e sum o f t h e c o b a l t and chromium c o n c e n t r a t i o n s i n a l l s p e c i m e n s c o r r e s p o n d s t o a p p r o x i m a t e l y o n e h a l f o f a monolayer.

We have a l s o s t u d i e d f i e l d - i o n specimens c o n t a i n i n g WCIWC b o u n d a r i e s w i t h a n a l y t i c a l e l e c t r o n microscopy (STEMIEDS). S i n c e t h e l o c a l i z a t i o n o f c o b a l t now i s known, i t was p o s s i b l e t o q u a n t i f y t h e r e s u l t s from t h e X-ray a n a l y s e s by c o r r e c t i n g f o r t h e e l e c t r o n beam b r o a d e n i n g i n t h e specimen. The d e t a i l s o f t h i s s t u d y a r e g i v e n e l s e w h e r e 1 7 1 ; t h e r e s u l t was t h a t a l l b o u n d a r i e s s t u d i e d c o n t a i n e d a b o u t 0.5 monolayer o f c o b a l t ( o r c o b a l t + chromium).

C o n s e q u e n t l y , t h e WCIWC b o u n d a r i e s c a n be d e s c r i b e d a s g r a i n b o u n d a r i e s , t o which c o b a l t ( a n d , i f p r e s e n t i n t h e m a t e r i a l , chromium) h a s s e g r e g a t e d t o a b o u t h a l f a monolayer c o v e r a g e . T h i s r e s u l t i s a d i r e c t e v i d e n c e f o r t h e c o n t i n u o u s

c a r b i d e s k e l e t o n model o f WC-Co t y p e cemented c a r b i d e s .

CONCLUSIONS

1 . Thin c o b a l t l a y e r s do n o t e x i s t around a l l WC g r a i n s i n l i q u i d p h a s e s i n t e r e d WC-Co. I n s t e a d WC g r a i n b o u n d a r i e s form d u r i n g s i n t e r i n g .

2. A l l WC g r a i n b o u n d a r i e s s t u d i e d c o n t a i n e d a b o u t 0.5 monolayer o f s e g r e g a t e d atoms.

3. I n t h e chromium f r e e m a t e r i a l , t h e s e g r e g a t i n g s p e c i e s was c o b a l t . I n t h e m a t e r i a l c o n t a i n i n g chromium c a r b i d e C r 3 C 2 , some o f t h e c o b a l t was r e p l a c e d by chromium.

4 . No c o b a l t o r chromium was o b s e r v e d i n t h e WC g r a i n s s u r r o u n d i n g t h e b o u n d a r i e s . 5. The p r e s e n t r e s u l t s s u p p o r t t h e s k e l e t o n model o f WC-Co.

Acknowledgements - T h i s work was s u p p o r t e d by AB Sandvik Hard M a t e r i a l s , Stockholm, and by t h e N a t i o n a l Swedish Board f o r T e c h n i c a l Development (STU).

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