THE CVI-PROCESSING OF CERAMIC MATRIX COMPOSITES

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Submitted on 1 Jan 1989

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THE CVI-PROCESSING OF CERAMIC MATRIX COMPOSITES

R. Naslain, F. Langlais, R. Fedou

To cite this version:

R. Naslain, F. Langlais, R. Fedou. THE CVI-PROCESSING OF CERAMIC MATRIX COMPOS- ITES. Journal de Physique Colloques, 1989, 50 (C5), pp.C5-191-C5-207. �10.1051/jphyscol:1989526�.

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JOURNAL D E PHYSIQUE

C o l l o q u e C 5 , s u p p l 6 m e n t au n 0 5 , Tome 50, m a i 1 9 8 9

THE CVI-PROCESSING OF CERAMIC MATRIX COMPOSITES

R. NASLAIN, F . LANGLAIS and R. FEDOU

Laboratoire des Composites Thermostructuraux (UM 47-CNRS-SEP-UBI) Europarc, 3 , Avenue Leonard de Vinci, F-33600 Pessac, France

Resume

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I,es c e r a m i q u e s p e u v e n t , d a n s d e s c o n d i t i o n s p a r t i c u l i e r e s , e t r e dkpos6es b p a r t i r d e p r e c u r s e u r s gazeux a u s e i n d e s u b s t r a t s p o r e u x . Ce p r o c 6 d 6 , d e s i g n e p a r i n f i l t r a t i o n c h i m i q u e e n p h a s e v a p e u r (CVI) e s t p a r t i c u l i e r e m e n t i n d i q u e pour l ' e l a b o r a t i o n d e s m a t e r i a u x c o m p o s i t e s b m a t r i c e c e r a m i q u e (CMC). Le r e m p l i s s a g e d ' u n p o r e p a r CV1 r e s u l t e d e deux phenomenes : ( i ) une r e a c t i o n d e s u r f a c e e t ( i i ) un t r a n s f e r t d e masse d e s r e a c t i f s e t d e s p r o d u i t s d a n s l a p h a s e g a z e u s e . En CV1 i s o t h e r m e / i s o b a r e , l e s t r a n s f e r t s d e masse se f o n t uniquement p a r d i f f u s i o n . I1 e n r e s u l t e q u e l a CV1 d o i t &tre c o n d u i t e b a s s e t e m p e r a t u r e e t p r e s s i o n r e d u i t e p o u r d o n n e r un d6pBt homogene e n B p a i s s e u r l e l o n g d e s p o r e s . En CV1 f o r c 6 e , les t r a n s f e r t s d e masse se f o n t p a r c o n v e x i o n f o r c e e d u e a un g r a d i e n t d e p r e s s i o n . De p l u s un g r a d i e n t i n v e r s e d e t e m p e r a t u r e est a p p l i q u e . I1 e n r e s u l t e une v i t e s s e d e dBp6t beaucoup p l u s & l e v e e . La f a i s a b i l i t e du p r o c e d e CV1 e s t e t a b l i e pour d i v e r s e s m a t r i c e s i n c l u a n t l e c a r b o n e e t S i c .

A b s t r a c t

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Under s p e c i f i c c o n d i t i o n s , c e r a m i c s c a n b e d e p o s i t e d from g a s e o u s p r e c u r s o r s w i t h i n p o r o u s s u b s t r a t e s . T h i s t e c h n i q u e , r e f e r r e d t o a s c h e m i c a l v a p o r i n f i l t r a t i o n (CVI) is p a r t i c u l a r l y s u i t e d t o t h e p r e p a r a t i o n o f c e r a m i c m a t r i x c o m p o s i t e s (CMC). P o r e f i l l i n g by CV1 r e s u l t s from two s i m u l t a n e o u s phenomena : ( i ) a s u r f a c e r e a c t i o n and ( i i ) mass t r a n s f e r s o f t h e r e a c t a n t s and p r o d u c t s i n t h e g a s p h a s e . I n i s o t h e r m a l / i s o b a r i c CVI, mass t r a n s f e r s o c c u r o n l y by d i f f u s i o n . A s a r e s u l t . ICVI h a s t o b e p e r f o r m e d a t low t e m p e r a t u r e s and u n d e r r e d u c e d p r e s s u r e s i n o r d e r t o l e a d t o a d e p o s i t homogeneous i n t h i c k n e s s a l o n g t h e p o r e s . I n forced-CVI, mass t r a n s f e r s a r e by f o r c e d c o n v e c t i o n d u e t o a p r e s s u r e g r a d i e n t . Moreover, an i n v e r s e t h e r m a l g r a d i e n t i s a p p l i e d r e s u l t i n g b o t h i n a much h i g h e r d e p o s i t i o n r a t e . The f e a s i b i l i t y o f t h e CV1 p r o c e s s i s e s t a b l i s h e d f o r a number o f c e r a m i c m a t r i c e s i n c l u d i n g c a r b o n and S i c .

1

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INTRODUCTION

Ceramic m a t e r i a l s a r e known f o r t h e i r r e f r a c t o r y c h a r a c t e r , t h e i r m e c h a n i c a l p r o p e r t i e s ( s t i f f n e s s , s t r e n g t h , wear r e s i s t a n c e ) b o t h a t a m b i e n t and h i g h t e m p e r a t u r e s , t h e i r low d e n s i t y a n d , i n many c a s e s , t h e i r r e s i s t a n c e t o s e v e r e c h e m i c a l e n v i r o n m e n t s ( e . g . o x y d i z i n g a t m o s p h e r e s a t h i g h t e m p e r a t u r e s ) . They a r e a l r e a d y w i d e l y u s e d i n many f i e l d s . e . g . a s c o a t i n g s r e s i s t a n t t o wear o r / a n d o x y d a t i o n . On t h e o t h e r h a n d , t h e i r u s e a s p r i m a r y s t r u c t u r a l p a r L s , e . g . i n advanced r e c i p r o c a L i n g e n g i n e s o r g a s t u r b i n e s , h a s been l i m i t e d up t o now by t h e i r b r i t t l e c h a r a c t e r . I t h a s been e s t a b l i s h e d , r a t h e r r e c e n t l y t h a t t h e t o u g h n e s s and r e l i a b i l i t y o f s t r u c t u r a l c e r a m i c s ( e . g . S i C , Si3N4, Si02-based g l a s s - c e r a m i c s , o x i d e s ) c a n b e d r a m a t i c a l l y improved by a p p l y i n g t o c e r a m i c s t h e c o n c e p t o f f i b e r - r e i n f o r c e m e n t . A s a m a t t e r o f f a c t , c e r a m i c m a t r i x c o m p o s i t e s (CMC) may e x h i b i t t o u g h n e s s c o m p a r a b l e t o t h a t o f l i g h t a e r o n a u t i c a l a l l o y s ( i . e . K I ~ v a l u e s o f

t h e o r d e r o f 30-50 MPa m$) when t h e y a r e c o r r e c t l y p r o c e s s e d / l - 4 / .

The p r o c e s s i n g o f s t r u c t u r a l c e r a m i c s i s known t o b e a d i f f i c u l t s u b j e c t i n m a t e r i a l s e n g i n e e r i n g . On t h e o n e h a n d , c e r a n i c m a t e r i a l s a r e c h a r a c t e r i z e d by a m e c h a n i c a l S e h a v i o r which i s v e r y s e n s i t i v e t o d e f e c t s even o f v e r y s m a l l s i z e ( i . e . o f t h e o r d e r o f a few pm and e v e n l e s s ) and t h u s , s h o u l d b e p r o c e s s e d v e r y c a r e f u l l y . On t h e o t h e r hand, c e r a m i c s a r e v e r y r e f r a c t o r y m a t e r i a l s ( m e l t i n g p o i n t s o f t e n h i g h e r t h a n 2 5 0 o 0 c ) , a f e a t u r e which p r e c l u d e s t h e i r p r o c e s s i n g and f o r m i n g i n t h e m o l t e n s t a t e . F u r t h e r m o r e , t h e y u s u a l l y d o n o t e x h i b i t any p l a s t i c i t y a t low o r medium t e m p e r a t u r e s ( a l t h o u g h s u p e r p l a s t i c c e r a m i c s h a v e b e e n r e c e n t l y m e n t i o n e d ) . T h u s , many c e r a m i c s ( e . g . c o v a l e n t S i c , Si3N4. B4C) c a n b e s i n t e r e d o n l y a t h i g h t e m p e r a t u r e s o r / a n d w i t h s i n t e r i n g a i d s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989526

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The main i n t e r e s t o f t h e chemical v a p o r p r o c e s s i n g r o u t e s l i e s i n t h e f a c t t h a t they allow t h e d e p o s i t i o n o f ceramics a t medium and even low t e m p e r a t u r e s , depending on t h e n a t u r e o f t h e a c t i v a t i o n mechanisms ( t y p i c a l l y 1 0 0 0 ' ~ f o r t h e r m a l l y a c t i v a t e d , .CVD and 3 0 0 - 5 0 0 ~ ~ f o r plasma a s s i s t e d CVD), whatever t h e m e l t i n g p o i n t and thermal s t a b i l i t y of t h e m a t e r i a l s . T h e r e f o r e , t h e CVD o f ceramics can be performed on a v a r i e t y o f s u b s t r a t e s i n c l u d i n g t h o s e with a l i m i t e d thermal s t a b i l i t y ( e . g . e l e c t r o n i c components, t h e r m a l l y t r e a t e d s t e e l s and a l l o y s , g l a s s e s , e t c . . . ) . Moreover, s i n c e i n CVD t h e s t a r t i n g m a t e r i a l s a r e gaseous s p e c i e s , a n o t h e r advantage o f t h e chemical v a p o r p r o c e s s i n g r o u t e s l i e s i n t h e f a c t t h a t t h e y may r e s u l t i n v e r y p u r e ceramics ( g a s and l i q u i d s a r e e a s i l y o b t a i n e d w i t h a h i g h d e g r e e of p u r i t y ) . F i n a l l y , CVD-processing may l e a d t o s o l i d s w i t h f i n e g r a i n m i c r o s t r u c t u r e s and t h u s w i t h good mechanical p r o p e r t i e s .

The advantages which have been mentioned above e x p l a i n why chemical vapor d e p o s i t i o n has been among t h e f i r s t p r o c e s s i n g r o u t e s s e l e c t e d f o r t h e p r e p a r a t i o n of CMC / 5 , 6 / . A s a m a t t e r o f f a c t , ceramic f i b e r s , e . g . carbon o r Sic-based f i b e r s , a r e v e r y s t r o n g and s t i f f b u t : ( i ) t h e i r d i a m e t e r i s v e r y s m a l l ( i . e . of t h e o r d e r of 1 0 pm and o f t e n l e s s ) and ( i i ) t h e y a r e v e r y s e n s i t i v e t o environmental e f f e c t s ( e . g . t o s u r f a c e d e f e c t s r e s u l t i n g from h a n d l i n g , t o t e m p e r a t u r e and t o o x y d a t i o n ) . Obviously, t a k i n g i n t o account t h e n a t u r e of ceramic f i b e r s , CMC s h o u l d be p r e p a r e d a c c o r d i n g t o s o f t p r o c e s s i n g r o u t e s a requirement which p r e c l u d e s t h e u s e of high t e m p e r a t u r e s ( e . g . s i n t e r i n g ) and a b r a s i v e p r o c e s s e s ( e . g . p r e s s i n g ) .

One of t h e o b j e c t i v e s of t h e p r e s e n t c o n t r i b u t i o n is t o show t h a t t h e s o - c a l l e d chemical vapor i n f i l t r a t i o n (CVI) p r o c e s s , which i s d i r e c t l y d e r i v e d from CVD, i s w e l l s u i t e d t o t h e s p e c i f i c r e q u i r e m e n t s o f t h e e l a b o r a t i o n of CMC. Inasmuch a s t h e s t a r t i n g m a t e r i a l i s u s u a l l y h e r e a porous preform made o f woven f i b e r s , a n o t h e r o b j e c t i v e of t h e p r e s e n t c o n t r i b u t i o n i s t o show how t h e d e p o s i t i o n p a r a m e t e r s commonly used i n CVD have t o be modified t o f a v o r i n - d e p t h d e p o s i t i o n ( i . e . t h e d e p o s i t i o n i n t h e p o r e network of t h e s u b s t r n t e ) o v e r e x t e r n a l s u r f a c e c o a t i n g i n o r d e r t o l e a d t o a f u l l y d e n s i f i e d m a t e r i a l . F i n n l l y , some i n f o r m a t i o n on t h e p r a c t i c a l a s p e c t s of CVI, t h e main p r o p e r t i e s of C V I - processed CMC and examples o f a p p l i c a t i o n w i l l be g i v e n .

2

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BASIS O F THE CVI-PROCESS 2 . 1

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D e f i n i t i o n

C V 1 i s a p r o c e s s i n g technique according t o which a s o l i d ( e . g . a ceramic m a t e r i a l ) i s d e p o s i t e d , w i t h i n t h e p o r e network o f a heated s u b s t r a t e , from a chemical r e a c t i o n t a k i n g p l a c e between gaseous s p e c i e s which flow ( e i t h e r by d i f f u s i o n o r c o n v e c t i o n ) i n t h e p o r e s . C V 1 can be i n p r i n c i p l e a p p l i e d t o any g i v e n porous s u b s t r a t e a s l o n g a s : ( i ) t h e p o r e s a r e i n t e r c o n n e c t e d and l a r g e enough i n d i a m e t e r and ( i i ) t h e s u b s t r a t e i s s t a b l e t h e r m a l l y and c h e m i c a l l y under t h e C V 1 c o n d i t i o n s . The aim i n C V 1 i s u s u a l l y t o d e n s i f y a s completely a s p o s s i b l e , t h e s u b s t r a t e b u t t h e p r o c e s s can a l s o b e stopped a t any d e s i r e d s t a t e o f d e n s i f i c a t i o n /7/.

To understand t h e s p e c i f i c r e q u i r e m e n t s o f t h e CVI-processing of porous s u b s t r a t e s , i t may be u s e f u l f i r s t t o r e c a l l b r i e f l y some of t h e f e a t u r e s of CVD i t s e l f .

2 . 2

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B a s i s o f CVD

Tn CVD, o s o l i d ( e . g . a c c m m i c ) is formed a s t h e r e s u l t o f a chemical r e a c t i o n t a k i n g p l a c e between gaseous s o u r c e s p e c i e s ( t h e p r e c u r s o r ) , t h e o t h e r p r o d u c t s o f t h e r e a c t i o n b e i n g gaseous under t h e CVD c o n d i t i o n s . T a b l e I g i v e s a few examples o f o v e r a l l chemical r e a c t i o n s commonly used f o r t h e d e p o s i t i o n o f c e r a m i c s . A s a m a t t e r o f f a c t , a v a r i e t y of s o u r c e s p e c i e s a r e a v a i l a b l e f o r most c o v a l e n t and i o n o - c o v a l e n t c e r a m i c s . H a l i d e s a r e o f t e n s e l e c t e d , f o r economical c o n s i d e r a t i o n s , b u t o r g a n o m e t a l l i c s p e c i e s a r e used a s w e l l . On t h e c o n t r a r y , CVD i s n o t well s u i t e d t o t h e formation o f i o n i c o x i d e s ( e . g . CaO o r M@) f o r l a c k of s o u r c e s p e c i e s ( e . g . a l k a l i n e e a r t h c h l o r i d e s a r e gaseous o n l y a t high t e m p e r a t u r e s ) .

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T a b l e I : Examples of o v e r a l l chemical r e a c t i o n s commonly used f o r t h e f o r m a t i o n o f ceramics by CVD/CVI

BF3 ( o r BC13) (g) + NH3(g)

-.

^BNls) ⁺^3HF(or

The chemical r e a c t i o n which l e a d s t o t h e d e p o s i t i o n of t h e s o l i d i s u s u a l l y n o t a s simple as t h o s e given i n t a b l e I : ( i ) i n t e r m e d i a t e s and by-products a r e o f t e n formed and ( i i ) r e a c t i o n s a r e o f t e n l i m i t e d . T h e r e f o r e , t h e d e p o s i t i o n o f a g i v e n ceramic may be t h e r e s u l t o f a v e r y complex heterogeneous chemical r e a c t i o n . A computerized thermodynamic approach i s f r e q u e n t l y used t o d e r i v e t h e main f e a t u r e s of a given CVD system, i . e . t h e n a t u r e and r e l a t i v e amounts of t h e gaseous and condensed s p e c i e s p r e s e n t a t e q u i l i b r i u m and t h u s t h e t h e o r e t i c a l y i e l d s , a s a f u n c t i o n of t h e CVD-parameters ( t e m p e r a t u r e , p r e s s u r e and f e e d g a s c o m p o s i t i o n ) , assuming t h a t e q u i l i b r i u m i s reached i n t h e CVD f u r n a c e (which i s n o t n e c e s s a r i l y t h e c a s e ) . An example of such a t r e a t m e n t i s given i n f i g . 1 f o r t h e CVD o f S i c from CH3SiC13/H2 mixtures /5/ whereas many o t h e r s a r e a v a i l a b l e from l i t e r a t u r e /8-IQ/. It c l e a r l y a p p e a r s from f i g . 1 t h a t t h e d e p o s i t , f o r a given temperature and p r e s s u r e , i s e i t h e r a s i n g l e phase o r a mixture of two phases ( c o d e p o s i t s ) depending on t h e i n i t i a l composition ( a = [H2]/[CH3SiC13] r a t i o ) . Moreover, a number of gaseous by-products a r e formed (e.g. s i l i c o n s u b - c h l o r i d e s , hydrocarbons, s i l a n e s ) which lower t h e y i e l d i n s o l i d /15/.

A s shown i n f i g . 2 a t h e mechanism a c c o r d i n g t o which a s o l i d i s d e p o s i t e d on a s u b s t r a t e is complex and c o n s i s t s of a t l e a s t t h r e e s t e p s : ( i ) t h e s o u r c e s p e c i e s d i f f u s e through a boundary l a y e r s u r r o u n d i n g t h e s u b s t r a t e , ( 2 ) t h e s o u r c e s p e c i e s a f t e r a d s o r p t i o n on t h e subsLraLe r e a c t among Lhemsclvcs t o g i v e r i s c t o t h c s o l i d and t o ndsorbcd gascous r e a c t i o n p r o d u c t s and f i n a l l y ( 3 ) t h e l a t t e r a f t e r b e i n g desorbed from t h e s u b s t r a t e d i f f u s e s through t h e boundary l a y e r . T h e r e f o r e , t h e d e p o s i t i o n r a t e may be c o n t r o l l e d e i t h e r by mass t r a n s f e r phenomena ( s t e p s ( 1 ) and ( 3 ) ) o r by t h e k i n e t i c s o f s u r f a c e phenomena ( s t e p ( 2 ) ) . S i n c e t h e s e two d i f f e r e n t phenomena do n o t obey t h e same laws with r e s p e c t t o t e m p e r a t u r e , p r e s s u r e and g a s flow, a t r a n s i t i o n between a domain where d e p o s i t i o n i s r a t e - c o n t r o l l e d by s u r f a c e phenomenon k i n e t i c s t o a domain where i t i s r a t e - c o n t r o l l e d by mass t r a n s f e r i s o f t e n observed on t h e V = f ( X ) c u r v e s (where V i s t h e d e p o s i t i o n r a t e and X one of t h e CVD-parmeters): An example o f such a t r a n s i t i o n i s given i n f i g .

3

f o r t h e d e p o s i t i o n of QC f r o a BC13-CH4-Hz /16/.

The chemical r e a c t i o n g i v i n g r i s e t o t h e d e p o s i t i o n of' t h e s o l i d s l ~ o u l d be a c t i v a t e d . I n most c a s e s and p a r t i c u l a r l y when t h e s u b s t r a t e i s s t a b l e enough, t h e a c t i v a t i o n i s o b t a i n e d by h e a t i n g t h e s u b s t r a t e t o a high enough t e m p e r a t u r e , i . e . 8 0 0 - 1 2 0 0 ' ~ f o r most i n o r g a n i c s o u r c e s p e c i e s and even much lower t e m p e r a t u r e s ( e . g . 4 0 0 - 7 0 0 ~ ~ ) when

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JOURNAL DE PHYSIQUE

-1 0 1

2

3

4 5

6

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Log alp

h

a (H2/CH3SiC13

1

F i g . 1 : CH3SiC13/H2 CVD/CVI system. C a l c u l a t e d thermodynamic y i e l d s f o r t h e s o l i d p h a s e s , as a f u n c t i o n o f a = [H2]/[MTS] f o r v a r i o u s t e m p e r a t u r e s from /15/

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^{feed gas} ^{feed gas}

I

heated substrate ^I ( a 1

steps 1,3,4,5 :mass transfers

by

diffusion step 2 : chemical reaction

boundary layer surface deposit

F i g . 2 : The d i f f e r e n t s t e p s i n CVD ( a ) and C V 1 ( b )

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F i g . 3 : Thermal v a r i a t i o n s of t h e d e p o s i t i o n r a t e of B4C from BC13-CH4-Hz showing a t r a n s i t i o n from a mass t r a n s f e r r a t e c o n t r o l l e d regime t o a regime r a t e c o n t r o l l e d by s u r f a c e r e a c t i o n k i n e t i c s 1161

o r g a n o m e t a l l i c p r e c u r s o r s a r e used. For s p e c i f i c a p p l i c a t i o n s , o t h e r k i n d s of a c t i v a t i o n may be p r e f e r r e d ( a s i n plasma a s s i s t e d CVD) / 7 / .

The m i c r o s t r u c t u r e of t h e d e p o s i t depends mainly on t h e n a t u r e o f t h e s u b s t r a t e (which may c o n t r o l t h e i n i t i a l n u c l e a t i o n mechanism) and t h e CVD-conditions ( p a r t i c u l a r l y t h e s u p e r s a t u r a t i o n ) . F a c e t t e d d e p o s i t s , well-developed columnar d e p o s i t s , a s w e l l a s f i n e g r a i n m i c r o s t r u c t u r e s a r e common, t h e l a t t e r b e i n g t h e most i n t e r e s t i n g a s f a r a s mechanical p r o p e r t i e s a r e concerned / 7 / .

2 . 3

-

Fundamentals of t h e CVI-process 2.3.1- I s o t h e z m a l / i s o b a r i c CV1 (ICVI)

The d e p o s i t i o n o f a s o l i d on t h e w a l l o f a p o r e w i t h a view t o f i l l , a s completely a s p o s s i b l e , t h a t p o r e i s s t i l l more complex and appears t o be p o s s i b l e o n l y under s p e c i f i c d e p o s i t i o n c o n d i t i o n s . I n i s o t h e r m a l / i s o b a r i c C V 1 (no t e m p e r a t u r e / p r e s s u r e g r a d i e n t s a l o n g t h e p o r e ) , t h e gaseous r e a c t a n t s and p r o d u c t s a r e t r a n s p o r t e d a l o n g t h e pore o n l y by d i f f u s i o n due t o c o n c e n t r a t i o n g r a d i e n t s between t h e e n t r a n c e and t h e bottom of t h e pore. Thus, a s shown i n f i g . 2 b , two new s t e p s must be added t o t h e t h r e e s t e p s a l r e a d y mentioned f o r CVD : ( i ) a f t e r having d i f f u s e d through t h e e x t e r n a l boundary l a y e r ( s t e p 1) t h e r e a c t a n t s must d i f f u s e along t h e pore l e n g t h ( s t e p 4 ) i n o r d e r t o r e a c h any p o i n t of t h e i n n e r s u r f a c e of t h e p o r e where t h e chemical r e a c t i o n g i v i n g r i s e t o t h e s o l i d - d e p o s i t takes p l a c e ( s t e p 2 ) and ( i i ) t h e gaseous by-products r e s u l t i n g from t h e

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C5-196 JOURNAL

DE

PHYSIQUE

d e p o s i t i o n r e a c t i o n must d i f f u s e i n t h e o p p o s i t e d i r e c t i o n , f i r s t a l o n g t h e pore l e n g t h towards t h e p o r e e n t r a n c e ( s t e p 5 ) and f i n a l l y a c r o s s t h e e x t e r n a l boundary l a y e r ( s t e p

3)

/17/.

From t h e above d i s c u s s i o n o f t h e k i n e t i c s o f CVD i t seems q u i t e obvious t h a t C V 1 should be performed under c o n d i t i o n s where t h e d e p o s i t i o n p r o c e s s i s r a t e - l i m i t e d by s u r f a c e phenomenon k i n e t i c s ( s t e p 2 ) and n o t by mass t r a n s f e r o f t h e r e a c t a n t s / p r o d u c t s by d i f f u s i o n i n t h e vapor phase ( s t e p s 1 , 3 , 4 and 5 ) . I f t h i s c o n d i t i o n i s n o t f u l f i l l e d , t h e d e p o s i t i o n on t h e e x t e r n a l s u r f a c e of t h e s u b s t r a t e and n e a r t h e p o r e e n t r a n c e w i l l be favored w i t h r e s p e c t t o t h a t t a k i n g p l a c e on t h e p o r e i n n e r s u r f a c e f a r from t h e p o r e e n t r a n c e , r e s u l t i n g i n an e a r l y s e a l i n g of t h e p o r e (which c o u l d be no l o n g e r d e n s i f i e d ) . T h e r e f o r e , i t i s imperative t h a t I C V I be performed a t low t e m p e r a t u r e s and p r e s s u r e s . U n f o r t u n a t e l y , under such c o n d i t i o n s : ( i ) t h e thermodynamic y i e l d i n s o l i d i s o f t e n low f o r many common CVD systems and ( i i ) t h e d e p o s i t i o n r a t e i s slow / 8 , 1 7 / .

The f i l l i n g o f a p o r e by I C V I i n v o l v e s two competing phenomena : ( i ) t h e mass t r a n s f e r s of t h e gaseous s p e c i e s a l o n g t h e p o r e , governed by d i f f u s i o n , which f e e d t h e r e a c t i o n s i t e s of t h e p o r e w a l l w i t h r e a c t a n t s and c o n v e r s e l y e v a c u a t e t h e gaseous p r o d u c t s ( s t e p s 4 and

5)

and ( i i ) t h e s u r f a c e r e a c t i o n which a b s o r b s t h e former and r e l e a s e s t h e l a t t e r ( s t e p 2 ) . The r e s u l t o f t h e c o m p e t i t i o n c a n b e a s s e s s e d by c o n s i d e r i n g d i m e n s i o n l e s s numbers which i n v o l v e t h e k i n e t i c c o n s t a n t of t h e s u r f a c e r e a c t i o n ks, an e f f e c t i v e d i f f u s i o n c o e f f i c i e n t De and a parameter r e p r e s e n t a t i v e of t h e pore geometry ( e . g . i t s r a d i u s R o r t h e L ~ / R r a t i o where L i s t h e p o r e l e n g t h ) / 6 , 19-21/. Two approaches w i l l be d i s c u s s e d assuming : ( i ) a f i r s t o r d e r r e a c t i o n , e . g . t h a t of f o r m a t i o n of S i c from CH3SiC13 mixed w i t h hydrogen and, ( i i ) a pore o f c y l i n d r i c a l geometry.

I n t h e approach proposed by Van den Breckel e t a l . /18/ f o r t h e CVD o f c e r a m i c s w i t h i n c y l i n d r i c a l t u b e s o f s m a l l d i a m e t e r s ( i . e . 0 . 1 <d< 1 mm) and t h e n extended by J . Y . Rossignol e t a l . /8/ t o porous f i b e r preforms with p o r e s of much s m a l l e r d i a m e t e r s ( 1 < d <

500 pm), t h e d i m e n s i o n l e s s number which has been s e l e c t e d i s t h e Sherwood number Sh (which i s independent o f both t h e r e a c t i o n o r d e r and c o n c e n t r a t i o n s i n t h e gas phase, f o r a f i r s t o r d e r r e a c t i o n ) :

where ks and D, depend on t h e d e p o s i t i o n c o n d i t i o n s , a s f o l l o w s :

with ko : frequency f a c t o r , E : a c t i v a t i o n energy and R* : t h e p e r f e c t g a s c o n s t a n t . Generally s p e a k i n g , t h e e x p r e s s i o n which has t o b e used f o r De s h o u l d i n c l u d e both t h e Fick d i f f u s i o n c o e f f i c i e n t DF and t h e Knudsen d i f f u s i o n c o e f f i c i e n t DK, which can be combined a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n :

For p o r e s of r a t h e r l a r g e d i a m e t e r s , Knudsen d i f f u s i o n c a n be n e g l e c t e d with r e s p e c t t o Fick d i f f u s i o n and D,, which i s equal t o DF i n a f i r s t approximation, i s known t o depend on both T and P f o r a g i v e n gaseous s p e c i e s , a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n :

where Do i s a c o n s t a n t and 1.5 < m < 2. I n such a c a s e , by combining e q u a t i o n s ( 1 ) . ( 2 ) and ( 4 ) . t h e Sherwood number can be r e w r i t t e n , a s a f u n c t i o n o f P , T and R , a s follows :

Sh

- .

P

.

exp (-E/R*T)

-

^Do ^Tm

The a b s o l u t e v a l u e of Sh governs t h e morphology of t h e d e p o s i t i n t h e p o r e . Small v a l u e s of Sh, which correspond t o experiments performed a t low T and P f o r a p o r e of given

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r a d i u s R, y i e l d more uniform d e p o s i t s a l o n g t h e p o r e l e n g t h /18/.

On t h e o t h e r hand, f o r p o r e s of s m a l l d i a m e t e r s ( t y p i c a l l y , 2R

<

10pm) Knudsen d i f , f u s i o n can no l o n g e r b e n e g l e c t e d ( i t can even be t h e o n l y mass t r a n s f e r mechanism f o r pore of very s m a l l d i a m e t e r s ) . Under such c o n d i t i o n s , both DF and DK must be taken i n t o account and e q u a t i o n ( 3 ) must be used f o r t h e c a l c u l a t i o n of D,, DK b e i n g g i v e n a c c o r d i n g t o t h e k i n e t i c t h e o r y o f g a s e s by :

f o r a gaseous s p e c i e s of molar mass M i n a p o r e o f r a d i u s R . I t i s worthy of n o t e t h a t DK does n o t depend on t h e t o t a l p r e s s u r e P. A s a r e s u l t , t h e e x p r e s s i o n f o r Sh i s more complex t h a n ( 5 ) b u t , g e n e r a l l y s p e a k i n g , t h e c o n c l u s i o n s drawn above remain v a l i d ( a t l e a s t a t a high enough t o t a l p r e s s u r e ) .

I n o r d e r t o i l l u s t r a t e t h e e f f e c t of T, P and R on t h e d e p o s i t p r o f i l e i n a c y l i n d r i c a l p o r e , a c a l c u l a t i o n h a s been done, on t h e b a s i s o f t h e Van den B r e c k e l / R o s s i g n o l model / 8 , 18/ f o r a symmetrical s t r a i g h t p o r e open a t b o t h e n d s , o f l e n g t h L = 1 0 mm, a c t i n g a s s u b s t r a t e f o r t h e d e p o s i t i o n of S i c from CH3SiC13/H2 p r e c u r s o r / 2 2 / . The c a l c u l a t i o n was done a c c o r d i n g t o an i n c r e m e n t a l procedure t o t a k e i n t o account t h e f a c t t h a t t h e p o r e e n t r a n c e r a d i u s r e g u l a r l y d e c r e a s e s v s time i n a I C V I experiment f i n a l l y becoming n i l when t h e pore i s s e a l e d by t h e d e p o s i t . whereas i n t h e o r i g i n a l Van den Brekel model /18/

t h e d e p o s i t t h i c k n e s s i s assumed t o remain s m a l l w i t h r e s p e c t t o t h e p o r e r a d i u s . For t h e i t e r a t i o n s t e p i , t h e r a d i u s o f t h e p o r e a t a d e p t h z ( t h e o r i g i n b e i n g t h e pore e n t r a n c e ) , i . e . r ( i , z ) , i s obtained by s u b t r a c t i n g t h e t h i c k n e s s of s o l i d G ( i , z ) ) c a l c u l a t e d a c c o r d i n g t o t h e Van den B r e k e l ' s model from t h a t c a l c u l a t e d f o r t h e s t e p i - l , i . e . r ( i - l , z ) :

where from /18/ ^:

with G ( i , O ) a s t h e t h i c k n e s s of s o l i d d e p o s i t e d a t z = 0 f o r s t e p i i n a p o r e assumed, i n a f i r s t approximation, t o be c y l i n d r i c a l and of r a d i u s R ( i ) . The i t e r a t i o n procedure i s stopped a t s t e p p , when t h e p o r e is s e a l e d , a t i t s e n t r a n c e s . by t h e d e p o s i t , i . e . when r ( p . 0 ) = 0 o r e ( p . 0 ) = R . The k i n e t i c d a t a f o r t h e CH3SiC1 /H2 system were taken from Schoch e t a l . 231. DK was c a l c u l a t e d a c c o r d i n g t o e q u a t i o n

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and found t o be e q u a l t o DK = 7.95 R T s m i - l f o r t h e CH3SiCl3 molecule whereas Dp was c a l c u l a t e d , f o r t h e CH3SiC13- H2 m i x t u r e s , a c c o r d i n g t o an e q u a t i o n o f type ( 4 )

,

DF = 5.59 1 0 - 5 ( ~ ) 3 / 2 ( ~ ) - 1 cm2. S-l / 2 4 / . The d e p o s i t p r o f i l e s a r e shown i n f i g . 4 and 5 f o r v a r i o u s v a l u e s of t h e pore d i a m e t e r (100 and l p m ) , t e m p e r a t u r e (800 ; 900 ; 1000 and 1 1 0 0 ' ~ ) and t o t a l p r e s s u r e ( 2 ; 20 and 100 k P a ) .

A s e x p e c t e d , t h e r e s u l t s o f t h e c a l c u l a t i o n show t h a t t h e t h i c k n e s s homogeneity of t h e d e p o s i t i s e x c e l l e n t when temperature and t o t a l p r e s s u r e a r e low enough ( e . g . T = 800 - 9 0 0 ' ~ and P = 2

-

20 kPa) a t l e a s t when t h e p o r e d i a m e t e r i s l a r g e (2R = 100 pm), i . e . f o r low Sherwood numbers ( e q u a t i o n ( 5 ) ) . On t h e c o n t r a r y , r a i s i n g b o t h T and P t e n d s t o f a v o r d e p o s i t i o n n e a r t h e p o r e e n t r a n c e . T h i s f e a t u r e is s t i l l more e v i d e n t f o r p o r e s of s m a l l d i a m e t e r s ( f i g . 4b and 5 b ) . A s a n example, almost no d e p o s i t i o n occurs i n a pore of 1 pm i n d i a m e t e r beyond L/10 from p o r e e n t r a n c e , f o r T = 1 0 0 0 ' ~ and P = 20 kPa ( f i g 4 b ) . Lowering t e m p e r a t u r e t o 8 0 0 ~ ~ o n l y s l i g h t l y improves t h e d e p o s i t p r o f i l e ( f i g . 4 b ) whereas l o w e r i n g t o t a l p r e s s u r e t o 2 kPa h a s no e f f e c t (inasmuch a s mass t r a n s f e r s a r e

thought t o b e a l r e a d y l i m i t e d by Knudsen d i f f u s i o n a t 20 kPa) ( f i g . 5 b ) .

I n a somewhat d i f f e r e n t approach based on t h e common f e a t u r e s t h a t e x i s t between I C V I and heterogeneous g a s c a t a l y s i s w i t h i n a porous c a t a l y s t . , F i t z e r and h i s coworkers have chosen t o u s e a n o t h e r d i m e n s i o n l e s s number, t h e second Damkohler number Da11 ( o r T h i e l e

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2 3

depth (mm)

2 3

depth (mm)

Fig. 4 : Computed in-pore deposit thickness profiles for various deposition temperatures and two pore diameters (i.e. 2R = 100 pm and 2R = lpm)

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depth (mm)

. .

Y

U) U)

Fig.

5

: Computed in-gore d e p o s i t thickness p r o f i l e s f o r various t o t a l p r e s s u r e s and two pore diameters ( i . e . 2R = 100 pm and 2R = 1 pm)

=

U

²⁰

5 .-

10 - ₁₀₀

CH3SiC13/ H2

- 2R= 100 pm T = 900" C

I I I I

0 1 2 3 4 5

depth (mm)

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JOURNAL

DE

PHYSIQUE

number G) d e f i n e d a s f o l l o w s f o r a f i r s t o r d e r r e a c t i o n / 6 , 19-21, 251 :

and which is, @S a m a t t e r o f f a c t , c l o s e l y r e l a t e d t o t h e Sherwood number a s emphasized by Fedou e t a l . /22/. F i t z e r e t a l . d e f i n e d an e f f e c t i v e n e s s f a c t o r r\ (which p l a y s a r o l e s i m i l a r t o t h e G ( i . z ) / G ( i . o ) r a t i o i n t h e p r e c e e d i n g model) a s t h e r a t i o between t h e r a t e of i n - p o r e d e p o s i t i o n and t h a t o f e x t e r n a l s u r f a c e d e p o s i t i o n . The rt f a c t o r i s r e l a t e d t o t h e d i m e n s i o n l e s s numbers by :

q : t a n h ~ a ; $ _{D ~ I I}

.

^{t a n h}_G^O

I

theoretical

-1

I

800 900 ("C) 1000 impregnation temperature

F i g . 6 : The model o f F i t z e r e t a l . : ( a ) v a r i a t i o n s of t h e e f f e c t i v e n e s s f a c t o r a s a f u n c t i o n o f t h e T h i e l e number ; ( b ) , V a r i a t i o n s o f t h e maximum d e p t h of impregnation a s a f u n c t i o n o f t h e impregnation t e m p e r a t u r e /26/

The v a r i a t i o n s o f q a s a f u n c t i o n o f O = ~ a a r e shown i n f i g . 6 a . I n o r d e r t o f a v o r ~ ~ f i n - p o r e d e p o s i t i o n . q s h o u l d be a s c l o s e a s p o s s i b l e t o u n i t y . The c u r v e shows t h a t t h i s c o n d i t i o n , e x p r e s s e d a s 0.95 <q

<

1, i s f u l f i l l e d when ~ a

<

~0.4. When combined with ~ 3 e q u a t i o n (g), t h i s c o n d i t i o n d e f i n e s a maximum d e p t h f o r impregnation. L, max :

L max

<

^0.4

^RD,

³

[

^2k.I

The a u t h o r s have c a l c u l a t e d L max f o r ( i ) model c y l i n d r i c a l p o r e s ( c l o s e d a t one end) of l a r g e d i a m e t e r s (0.4 < R < 1 m m ) and ( i i ) porous g r a p h i t e s w i t h mean p o r e d i a m e t e r s r a n g i n g from 1 t o 20 pm, f i l l e d by ICVI w i t h S i c d e p o s i t e d from CH3SiC13/H2 under c o n d i t i o n s c o r r e s p o n d i n g t o t h e regime r a t e - l i m i t e d by s u r f a c e r e a c t i o n . T h e r e s u l t s of t h e i r c a l c u l a t i o n s a s w e l l a s t h e i r e x p e r i m e n t a l d a t a a r e shown i n f i g . 6b f o r p o r e s o f s m a l l d i a m e t e r s / 2 6 / . L max a p p e a r s t o i n c r e a s e when t e m p e r a t u r e d e c r e a s e s and p o r e d i a m e t e r i n c r e a s e s , a f e a t u r e which is i n agreement w i t h t h e Van den B r e c k e l / R o s s i g n o l model, a s d i s c u s s e d above.

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2 . 3 . 2

-

Forced f l o w / t h e r m a l g r a d i e n t CV1 (FCVI)

I n CVI, t h e mass t r a n s f e r s o f r e a c t a n t s and p r o d u c t s a l o n g t h e p o r e a r e due only t o d i f f u s i o n w i t h t h e r e s u l t t h a t d e p o s i t i o n should be performed a t low t e m p e r a t u r e and p r e s s u r e i n o r d e r t o o b t a i n a d e p o s i t uniform i n t h i c k n e s s a l o n g t h e p o r e . Under such c o n d i t i o n s , t h e r a t e of i n f i l t r a t i o n i s n e c e s s a r i l y slow.

An a l t e r n a t i v e p r o c e s s , r e f e r r e d t o a s FCVI, h a s been worked o u t by Caputo e t a l . f o r t h e i n f i l t r a t i o n of S i c and Si3N4 m a t r i c e s i n d i f f e r e n t porous media, i n which t h e mass t r a n s f e r s a r e by f o r c e d c o n v e c t i o n r e s u l t i n g from a p r e s s u r e g r a d i e n t /27-29/. A s shown s c h e m a t i c a l l y i n f i g . 7 , t h e r e a c t a n t s a r e f o r c e d t o flow a l o n g t h e p o r e under a high p r e s s u r e (P1 = 100 t o 200 kPa) w h i l e t h e p r o d u c t s (and t h e u n r e a c t e d s p e c i e s ) a r e evacuated a t a lower p r e s s u r e P2. Moreover, s i n c e t h e g a s phase i s d e p l e t e d i n r e a c t a n t s a s i t flows i n t h e p o r e ( d u e t o t h e chemical r e a c t i o n t a k i n g p l a c e on t h e p o r e w a l l ) , an i n v e r s e thermal g r a d i e n t i s a p p l i e d along t h e pore. S i n c e a s d i s c u s s e d above, t h e s u r f a c e phenomena g i v i n g r i s e t o t h e d e p o s i t a r e t h e r m a l l y a c t i v a t e d ( e q u a t i o n ( 2 ) ) . t h e e f f e c t of t h e t e m p e r a t u r e i n c r e a s e may compensate, under optimized c o n d i t i o n s , t h a t of t h e g a s phase d e p l e t i o n i n r e a c t a n t s .

The competing e f f e c t s of t h e f o r c e d g a s flow and thermal g r a d i e n t on t h e d e p o s i t p r o f i l e have been j u s t i f i e d t h e o r e t i c a l l y , by S t a r r , f o r random s h o r t f i b e r preforms i n f i l t r a t e d with S i c d e p o s i t e d from CH3SiC13/H2 p r e c u r s o r , assuming a f i r s t o r d e r r e a c t i o n / 3 0 / . When T1 = T2 = 1 2 0 0 ' ~ (no thermal g r a d i e n t ) , d e p o s i t i o n i s l i m i t e d t o .the v i c i n i t y of t h e preform s u r f a c e through which t h e r e a c t a n t s a r e i n j e c t e d , due t o a v e r y r a p i d d e p l e t i o n of t h e g a s p h a s e o i n r e a c t a n t s . Furthermore, t h e d e p o s i t profi1.e a s shown i n f i g . 8, i s s i m i l a r t o t h a t c a l c u l a t e d f o r T = 1 1 0 0 ~ ~ a c c o r d i n g t o t h e Van den Breckel/Hossignol model ( f i g . 4.

5 ) .

On t h e c o n t r a r y , when T1 i s lowered t o ~ O O ' C , t h e d e p o s i t t a k e s p l a c e n e a r t h e o p p o s i t e s u r f a c e of t h e preform (maintained a t 1 2 0 0 ~ ~ ) s i n c e t h e g a s phase d e p l e t i o n i s now very l i m i t e d . F i n a l l y . a d e p o s i t o f almost uniform t h i c k n e s s i s o b t a i n e d when T1 i s a d j u s t e d t o about 1 0 0 0 ~ ~ .

One of t h e main advantage o f t h e FCVI p r o c e s s l i e s i n t h e f a c t t h a t t h e i n f i l t r a t i o n time n e c e s s a r y t o r e a c h a g i v e n s t a t e o f d e n s i f i c a t i o n f o r a g i v e n porous s u b s t r a t e i s reduced by one o r d e r o f magnitude w i t h r e s p e c t t o t h a t r e q u i r e d i n I C V I due t o ( i ) f a s t e r mass t r a n s f e r s ( f o r c e d c o n v e c t i o n ) and ( i i ) h i g h e r d e p o s i t i o n t e m p e r a t u r e s ( l i m i t e d only by t h e s t a b i l i t y o f t h e p r e f o r m s ) . On t h e o t h e r hand, t h e FCVI p r o c e s s h a s a l s o i m p o r t a n t drawbacks which w i l l b e d i s c u s s e d i n t h e n e x t s e c t i o n .

3

-

PRACTICAL ASPECTS OF THE CV1 PROCESS 3.1

-

Preforms

I n t h e C V 1 p r o c e s s i n g o f CMC, one of t h e i m p o r t a n t s t a r t i n g m a t e r i a l s i s t h e f i b r o u s preform ( t h e o t h e r b e i n g t h e gaseous p r e c u r s o r of t h e m a t r i x ) s i n c e i t s n a t u r e d i r e c t l y governs : ( i ) t h e volume f r a c t i o n s of f i b e r and m a t r i x i n t h e composite a s w e l l a s ( i i )

t h e f i b e r o r i e n t a t i o n and d e g r e e of a n i s o t r o p y . The f i b e r s a v a i l a b l e f o r t h e r e i n f o r c e m e n t o f ceramic m a t r i c e s a r e l i m i t e d t o carbon and S i c - o r Al2O3- based f i b e r s . From a mechanical and thermal s t a b i l i t y p o i n t o f view. t h e b e s t a r e t h e former b u t , u n f o r t u n a t e l y , t h e u s e o f carbon f i b e r s a t high Lemperaturcs i s l i m i tcd Lo aLmospheres which do n o t c o n t a i n oxygen u n l e s s a p r o t e c t i v e c o a t i n g , such a s S i c , has been d e p o s i t e d on t h e f i b e r s u r f a c e ( e . g . by CVD) / 3 1 / . I n t h e preform, t h e f i b e r s a r e e i t h e r s h o r t

(chopped f i b e r s o r w h i s k e r s ) o r continuous (woven o r non-woven).

A v e r y i m p o r t a n t parameter o f t h e preform i s i t s p o r o s i t y . On t h e b a s i s of t h e d i s c u s s i o n p r e s e n t e d i n s e c t i o n 2 , t h e p o r o s i t y of t h e preform s h o u l d o b v i o u s l y b e made of open i n t e r c o n n e c t e d p o r e s of l a r g e enough d i a m e t e r s ( i . e . r a n g i n g between a few pm and a few 100 pm). I n o r d e r t o allow an e a s y d i f f u s i o n / f l o w of t h e gaseous p r e c u r s o r , t h e pore spectrum of t h e preform must c o n t a i n a high enough p e r c e n t a g e o f p o r e s of l a r g e d i a m e t e r s .

S h o r t f i b e r preforms can b e made a c c o r d i n g t o t h e s l u r r y molding p r o c e s s /29/. Chopped f i b e r s ( o r w h i s k e r s ) a r e f i r s t suspended i n a l i q u i d c o n t a i n i n g a b i n d e r ( e . g . a p o l y c a r b o s i l a n e f o r a S i c m a t r i x ) . The s l u r r y i s then vacuum f i l t e r e d t o form a d i s k ( o r

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F i g .

temperature pressure

infiltrated cdmposite fibrous preform

~ n - p o r e mass transfers

coating

!l as pressure P,

7 : The CV1 p r o c e s s : ( a ) t h e t e m p e r a t u r e and p r e s s u r e g r a d i e n t s a l o n g t h e p o r e ; ( b ) t h e e x p e r i m e n t a l set u p ( s c h e m a t i c ) /27-29/

TI temperature ("C) T2=1200

I

T. L. S tarr, 1987

C

Q) d

C 0

position

F i g . 8 : The model o f S t a r r : d e p o s i t i o n p r o f i l e s f o r v a r i o u s v a l u e s o f T1 (T2 b e i n g e q u a l t o 1200'~) / 3 0 /

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any o t h e r g i v e n s h a p e ) which, a f t e r p r e s s i n g ( i n d i e s o r between p l a t e s ) and s i n t e r i n g , r e s u l t s i n a preform w i t h a f i b e r volume f r a c t i o n r a n g i n g between 15-25

5.

F e l t s a r e a l s o a v a i l a b l e on t h e market f o r mostcommon ceramic f i b e r s .

2D-preforms, made of a s t a c k o f f a b r i c s , a r e t h e most commonly used f i b r o u s preforms due t o : ( i ) t h e i r h i g h f i b e r volume f r a c t i o n s ( i . e . t y p i c a l l y 40-45 5 ) . ( i i ) t h e i r pore s p e c t r a v e r y w e l l s u i t e d t o C V 1 and ( i i i ) t h e i r e a s y p r e p a r a t i o n . I n t h e s o - c a l l e d dry preforms, t h e f a b r i c s a r e p r e s s e d t o g e t h e r w i t h a ceramic t o o l (which i s withdrawn a f t e r t h e f i r s t CV1 t r e a t m e n t ) . I n c o n s o l i d a t e d preforms, t h e f a b r i c s a r e bonded t o g e t h e r with an o r g a n i c o r o r g a n o m e t a l l i c b i n d e r ( e . g . a p o l y c a r b o s i l a n e ) t h a t a f t e r p y r o l y s i s w i l l y i e l d a s m a l l amount o f a ceramic m a t r i x , p r i o r t o t h e CVI-treatment. More complex nD preforms ( w i t h n

>

2 , n b e i n g t h e number of f i b e r o r i e n t a t i o n s ) can b e p r e p a r e d a c c o r d i n g t o a s i m i l a r procedure /3. 29, 32/.

F i n a l l y , ID-preforms a r e made from a l i g n e d f i b e r tows ( m a i n t a i n e d t o g e t h e r , a s s a i d above, e i t h e r w i t h a ceramic t o o l o r w i t h a b i n d e r ) . T h e i r main advantage l i e s i n t h e f a c t t h a t s t i l l h i g h e r f i b e r volume f r a c t i o n s can be achieved ( e . g . 50-60 X ) . On t h e o t h e r hand, i n s u c h p r e f o r m s , t h e p o r e s a r e e s s e n t i a l l y u n i d i r e c t i o n a l and d i f f i c u l t t o d e n s i f y . C r o s s p l y preforms a r e made a c c o r d i n g t o t h e same p r o c e s s i n g t e c h n i q u e .

3 . 2

-

D e n s i f i c a t i o n o f t h e preform by ICVI

I n ICVI, t h e preforms a r e s e t i n a h o t w a l l i s o t h e r m a l d e p o s i t i o n chamber f e d with a flow of t h e gaseous p r e c u r s o r ( s e e t a b l e I ) under a reduced p r e s s u r e whose v a l u e depends on t h e p o r e spectrum of t h e preform and n a t u r e o f t h e p r e c u r s o r / 3 2 / . A s d i s c u s s e d i n s e c t i o n 2 , t o o h i g h a t e m p e r a t u r e and a p r e s s u r e r a p i d l y r e s u l t i n an e a r l y p o r e s e a l i n g . T h e r e f o r e , t h e d e p o s i t i o n p a r a m e t e r s s h o u l d b e c o n t r o l l e d v e r y c a r e f u l l y d u r i n g t h e whole i n f i l t r a t i o n p r o c e s s . T a b l e I1 g i v e s t h e CVI-parameters f o r some ceramic m a t r i c e s f o r l a b - s c a l e a p p a r a t u s (most porous s u b s t r a t e s a r e 2D p r e f o r m s ) . The u n r e a c t e d s o u r c e s p e c i e s and t h e gaseous r e a c t i o n p r o d u c t s a r e pumped through t r a p s (most of t h e s e s p e c i e s being c o r r o s i v e when h a l i d e p r e c u r s o r s a r e u s e d ) .

T a b l e 11 : D e p o s i t i o n p a r a m e t e r s f o r t h e I C V I o r FCVI of ceramic m a t r i c e s i n porous s u b s t r a t e s .

Matrix P r e c u r s o r Temperature ( OC)

P r e s s u r e ( @ a ) 10

-

100

3 1

- 5

1 - 5

1 - 5 2 - 3 1 - 5

1

100

-

200

l

l l -

P r e c u r s o r Composition H2 : MTS = 5

-

^{1 0}

-

BC13 : H2 = 1 BC13 : CH4 = 4 H2 : CH4 = 1 0

H2 : C02 = 1 H2 : C02 = 1 H2 : MTS = l 0

C V I - type and r e f e r e n c e

I C V I /g/

I C V I 1331 I C V I /34/

FCVI /27, 281 FCVI /28/

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F i g . 9 : K i n e t i c s o f d e n s i f i c a t i o n o f a porous 2D-C-C preform by B4C shown on a semi- l o g a r i t h m i c s c a l e (Mo : maximum mass o f B4C c o r r e s p o n d i n g t o a t o t a l f i l l i n g of t h e p o r o s i t y ; MP : mass o f B4C d e p o s i t e d i n t h e p o r e s ; MS : mass o f B4C d e p o s i t e d on t h e e x t e r n a l s u r f a c e ) . I n s e r t : k i n e t i c s o f d e n s i f i c a t i o n shown on an a r i t h m e t i c s c a l e /16/

A s d i s c u s s e d i n s e c t i o n 2 and shown i n f i g . 9 ( i n s e r t ) , t h e r a t e of i n f i l t r a t i o n i n ICVI is slow /16, I T / . Furthermore, i t d e c r e a s e s r e g u l a r l y a s t h e i n f i l t r a t i o n proceeds ( t h e p o r e s becoming narrower, mass t r a n s f e r s by d i f f u s i o n , f o r g i v e n T, P, a r e more and more d i f f i c u l t ) . When t h e i n f i l t r a t i o n p a r a m e t e r s a r e p r o p e r l y o p t i m i z e d , t h e i n i t i a l open p o r o s i t y can b e a l m o s t t o t a l l y f i l l e d w i t h t h e ceramic d e p o s i t (down t o a r e s i d u a l p o r o s i t y o f 5-10 ,%) w i t h o u t s u r f a c e machining. However, i n o r d e r t o reduce t h e i n f i l t r a t i o n d u r a t i o n , i t may b e p r e f e r a b l e f o r economical c o n s i d e r a t i o n s t o re-open t h e p o r e s o r e n l a r g e t h e p o r e e n t r a n c e s by s u r f a c e machining o f t h e preforms. A s e a l i n g of t h e p o r e s is e a s i l y i d e n t i f i e d from a semi l o g - p l o t o f t h e v a r i a t i o n s o f t h e r e s i d u a l p o r o s i t y v s time ( d e v i a t i o n from a s t r a i g h t l i n e ) .

Even under o p t i m i z e d i n f i l t r a t i o n c o n d i t i o n s , t h e t h i c k n e s s o f t h e ceramic m a t r i x d e p o s i t e d i n t h e p o r e s i s u s u a l l y h i g h e r n e a r t h e e x t e r n a l s u r f a c e o f t h e preforms than i n t h e c o r e (due t o t h e d e p l e t i o n o f t h e g a s phase i n s o u r c e s p e c i e s , a s d i s c u s s e d i n s e c t i o n 2 ) . T h e r e f o r e , t h e r e is u s u a l l y ( i ) a d e n s i t y g r a d i e n t i n CMC o b t a i n e d by CV1

( t h e d e n s i t y b e i n g h i g h e r n e a r t h e e x t e r n a l s u r f a c e ) and ( i i ) some r e s i d u a l p o r o s i t y . The n a t u r e o f t h e m a t r i x o b t a i n e d by CV1 can b e e a s i l y modified by changing t h a t o f t h e p r e c u r s o r i n j e c t e d i n t h e i n f i l t r a t i o n chamber. A s an example, t h e i n t e r p h a s e m a t e r i a l , i . e . pyrocarbon o r hex-BN ( u s e d t o c o n t r o l t h e f i b e r - m a t r i x bonding and p r o t e c t t h e f i b e r s a g a i n s t t h e n o t c h e f f e c t a r i s i n g from t h e m i c r o c r a c k i n g o f t h e m a t r i x , when t h e composite i s loaded at" a high enough s t r e s s ) i s d e p o s i t e d f i r s t a s a t h i n l a y e r (from hydrocarbon o r BF3 ( o r BC13)/NH3 p r e c u r s o r s ) . Then t h e ceramic m a t r i x i t s e l f i s d e p o s i t e d by changing t h e p r e c u r s o r ( e . g . CH3SiC13/H2 f o r S i c ) . F i n a l l y a c o a t i n g ( d e p o s i t e d under CVD c o n d i t i o n s ) may b e a p p l i e d t o improve t h e r e s i s t a n c e o f t h e composite w i t h r e s p e c t t o

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t h e environmental e f f e c t s .

F i n a l l y , an i m p o r t a n t advantage o f t h e I C V I p r o c e s s i n g o f CMC l i e s i n t h e f a c t t h a t a l a r g e number o f preforms even o f complex s h a p e s can b e t r e a t e d s i m u l t a n e o u s l y i n t h e same i n f i l t r a t i o n chamber ( t h e number b e i n g l i m i t e d o n l y by s p a c e c o n s i d e r a t i o n s ) . This advantage compensates t h e low i n f i l t r a t i o n r a t e s due t o mass t r a n s f e r s by d i f f u s i o n . 3.3

-

D e n s i f i c a t i o n o f t h e preform by FCVI

A s shown s c h e m a t i c a l l y i n f i g . 7b, i n FCVI each preform has t o b e s e t i n a s p e c i f i c h o l d e r i n o r d e r t o g e n e r a t e t h e thermal g r a d i e n t and t o f o r c e t h e f e e d g a s t o flow i n t h e pore network under p r e s s u r e . A t t h e beginning o f a r u n , t h e r e a c t a n t s flow both a x i a l l y and r a d i a l l y i n t h e preform. However. t h e upper s u r f a c e o f t h e preform becomes r a p i d l y c o a t e d due t o t h e h i g h T2 t e m p e r a t u r e v a l u e ( t y p i c a l l y 1 1 0 0 - 1 2 0 0 ~ ~ f o r S i c d e p o s i t e d from CHjSiC13/H2). T h e r e f o r e , under s u c h c o n d i t i o n s t h e f e e d g a s must flow r a d i a l l y t o t h e void around t h e preform and e s c a p e through h o l e s i n t h e r e t a i n i n g r i n g . Moreover, s i n c e i n t h e p a r t i c u l a r c a s e o f S i c t h e d e p o s i t e d m a t r i x i s a good h e a t c o n d u c t o r , t h e h o t r e g i o n of t h e preform a t T2 moves from t h e t o p toward t h e bottom and c i r c u m f e r e n c e / 2 7 - 29/.

The p r e s s u r e which h a s t o b e a p p l i e d t o f o r c e t h e f e e d g a s t o flow a c r o s s t h e preform depends on t h e p o r e geometry and spectrum. P1 v a l u e s o f t h e o r d e r o f 100-200 kPa a r e r e p o r t e d f o r t h e i n f i l t r a t i o n of s a y 2D-Sic ( N i c a l o n ) preforms by S i c d e p o s i t e d from ClI3SiC13/H2 /27-29/.

The main advantage o f FCVI is t o s h o r t e n , by one o r d e r of magnitude, t h e d e n s i f i c a t i o n d u r a t i o n f o r a g i v e n preform w i t h r e s p e c t t o ICVI, due t o ( i ) h i g h e r d e p o s i t i o n t e m p e r a t u r e s and ( i i ) f a s t e r mass t r a n s f e r s of t h e r e a c t a n t s and p r o d u c t s by f o r c e d convection. However, t h e i n c r e a s e i n d e p o s i t i o n t e m p e r a t u r e may be l i m i t e d by t h e thermal s t a b i l i t y o f t h e f i b e r s . T h i s i s t y p i c a l l y t h e c a s e f o r t h e ex-polycarbosilane f i b e r s

(e.g. Nicalon f i b e r s ) whose m i c r o s t r u c t u r e begins t o c o a r s e n a t 1100'~ ( w i t h a lowering of t h e f a i l u r e s t r e n g t h ) . Under such c o n d i t i o n s , t h e o n l y advantage of t h e FCVI p r o c e s s is r e l a t e d t o f a s t e r mass t r a n s f e r s o f t h e gaseous s p e c i e s r e s u l t i n g from forced convection.

4 -

^{M A I N}PROPERTIES AND APPLICATIONS OF CVI-PROCESSED CMC

The main i n t e r e s t o f f i b e r r e i n f o r c e d ceramics l i e s i n t h e i r n o n - b r i t t l e mechanical behavior and improved r e l i a b i l i t y with r e s p e c t t o t h e i r u n r e i n f o r c e d c o u n t e r p a r t s , a s shown i n f i g . 1 0 /3. 4, 28,

35/.

However. t h i s n o n - b r i t t l e c h a r a c t e r i s observed o n l y f o r well-processed m a t e r i a l s , i.e. when : ( i ) t h e f i b e r s a r e n o t damaged d u r i n g t h e composite p r o c e s s i n g , ( i i ) t h e f i b e r s are o n l y weakly bonded t o t h e m a t r i x through a s o f t i n t e r p h a s e (e.g. a t h i n l a y e r o f pyrocarbon o r hex-BN) and ( i i i ) b o t h t h e f i b e r s and t h e i r i n t e r p h a s e s a r e p r o t e c t e d a g a i n s t environmental e f f e c t s e . g . by p r o t e c t i v e c o a t i n g s . Under s u c h c o n d i t i o n s . f i b r o u s ceramic m a t r i x composites : ( i ) obey a non l i n e a r s t r e s s - s t r a i n law ( f i g . 1 0 a ) and ( i i ) e x h i b i t b o t h a h i g h r e s i s t a n c e t o c r a c k p r o p a g a t i o n and a h i g h f a i l u r e energy ( f i g . l o b and 1 0 c ) due t o d i f f e r e n t damaging mechanisms ( e . g . m a t r i x m i c r o c r a c k i n g , f i b e r - m a t r i x debonding and f r i c t i o n , f i b e r p u l l o u t ) which a b s o r b energy. It i s worthy o f n o t e t h a t t h e p r o c e s s i n g r e q u i r e m e n t s mentioned above a r e p e r f e c t l y f u l f i l l e d by t h e C V 1 t e c h n i q u e .

Although t h e f e a s i b i l i t y o f t h e CVI-process h a s been e s t a b l i s h e d f o r d i f f e r e n t m a t r i c e s , t h e o n l y m a t e r i a l s which a r e produced on an i n d u s t r i a l b a s i s a r e carbon-carbon, on t h e one hand, and C-Sic o r S i c - S i c on t h e o t h e r hand. A s f a r a s we know. most of them a r e processed a c c o r d i n g t o t h e I C V I p r o c e s s . t h e low i n f i l t r a t i o n r a t e s b e i n g compensated by t h e f a c t t h a t l a r g e numbers o f p a r t s can b e s i m u l t a n e o u s l y t r e a t e d , a s d i s c u s s e d i n s e c t i o n 2 and

3 .

Carbon-carbon p a r t s a r e used i n r o c k e t e n g i n e s , h e a t s h i e l d s , b r a k e d i s k s and p r o s t h e t i c d e v i c e s . Sic-based composites a r e used i n g a s t u r b i n e s , r e u s a b l e thermal p r o t e c t i o n s and more g e n e r a l l y s p e a k i n g f o r s t r u c t u r a l p a r t s used a t high t e m p e r a t u r e s and under atmospheres c o n t a i n i n g oxygen.

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JOURNAL

DE

PHYSIQUE

strain

(%l

10000

7500

5000

2500

300 ¹

0 2 4 6

propagation

of

damage(mm1 Fig. 10 : The non-brittle behavior of Sic-Sic fibrous composites at room temperature : (a) stress-strain curve in tension / 3 / , (b) resistance to crack propagation / 4 /

strain (%l

Fig. 10 : (c) stress-strain curve in

3

point-bending /27, 28/

(18)

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