corrosion resistant coatings al2o3 produced by metal organic chemical vapor deposition using...

8/10/2019 Corrosion Resistant Coatings Al2O3 Produced by Metal Organic Chemical Vapor Deposition Using Aluminum-tri-sec-

1/7

138

Thin Solid Films, 230

(1993) 138-144

Corrosion resistan t coatings A 120 3) produced by m etal organic

chemical vapour deposit ion using a l um i n i um t r i sec b u t ox i d e

V . A . C . H a a n a p p e l , H . D . v a n C o r b a c h , T . F r a n s e n , a n d P . J . G e l l i n g s

University of Twente, Department of Chemical Technology, P.O. Box 217, 7500 AE Enschede Netherlands)

(Received February 9, 1993; accepted Marc h 25, 1993)

A b s t r a c t

The m eta l organic chemical vapo ur depos i t ion (MO CV D) of am orphous a lumina f ilms on s tee l was pe r formed in

ni t rogen a t a tmospher i c pres sure. Thi s M OCV D process i s based on the the rmal decompos i t ion o f a luminium -

tri-sec-butoxide (ATSB) . The e ffec t of the depos i t ion t em pera ture (wi th in the range 290 -420 C) , the precursor

vap our pressure (5.33 x 10 - 3_2.67 x 10 - 2 kPa), and the gas f low (6.5-1 2.5 1 min - t ) of the M OC VD process have

been s tudied in relat ion to corros io n propert ies a t high temp eratures . The corros io n experiments were perfor med at

450 C in a gas a tmosphere containing 1% H2S, 1% H20, 19% H2, and balanced Ar.

I t was found tha t the amou nt o f cor ros ion produc t s on an a lumina f ilm (0 .20__. 0 .05 mg cm -2 ) -A IS I 304

combinat ion decreased with increas ing deposi t ion temperature of the coat ing. This was more pronounced for the

products formed through the coat ing owing to a certa in poros i ty. The crack densi ty, where products were also

formed, was almost unaffected.

1 I n t r o d u c t i o n

F o r t h e d e p o s i t i o n o f a l u m i n a s e v e r al r e a c ta n t s h a v e

b e e n i n v e s ti g a t e d [ 1 - 1 1 ] , f o r e x a m p l e A 1 C 1 3 w h i c h

c a n r e a c t w i t h c a r b o n d i o x i d e a n d h y d r o g e n u n d e r

t h e f o r m a t i o n o f a l u m i n i u m o x i d e . T h e s e la y e r s a r e

m o s t l y u s e d i n t h e e l e c t r o n i c i n d u s t r y f o r t h e f a b r i c a -

t i o n o f m e t a l o x i d e s e m i c o n d u c t o r - f i e l d - e f f e c t tr a n s is -

t o r s [ 1 2 - 1 4 ] . T h e p r o c e s s h a s t o b e c a r r i e d o u t a t

h i g h t e m p e r a t u r e s , n o r m a l l y b e t w e e n 8 5 0 a n d 1 6 00 C ,

w h i c h d e m a n d s a h i g h en e r g y c o n s u m p t i o n . F u r t h e r -

m o r e , t h e e x p e r i m e n t a l p r o c e d u r e n e e d s s e v e r a l g a s

s t r ea m s , l e a d i n g to a n i n c re a s e d a m o u n t o f p a r a m e t e r s

i n f lu e n c i n g th e p r o p e r t i e s o f t h e a l u m i n a f il m s . F o r

e x a m p l e , o w i n g t o d i f f e r e n t d e p o s i t i o n t e m p e r a t u r e s ,

s e v e r a l w e l l - d e f i n e d o x i d e f i lm s c a n b e o b t a i n e d w i t h a

d e s ir e d s t ru c t u r e . A t t e m p e r a t u r e s o f a b o u t 8 5 0 C a n

a m o r p h o u s l a y e r [ 1 2 ] c a n b e o b t a i n e d , a s c a n a f i n e

g r a i n e d p o l y c r y s t a l l i n e s t r u c t u r e [ 1 3 ] a t t e m p e r a t u r e s

u p t o 1 0 0 0 C , a n a l p h a p h a s e a t t e m p e r a t u r e s u p t o

1 4 0 0 C , a n d a t h i g h e r t e m p e r a t u r e s a s i n g l e c r y s t a l

s t r u c t u r e [ 1 4 ] c a n b e o b t a i n e d .

T h e d e p o s i t io n b y m e a n s o f c o m p o u n d s s u c h a s

A IC I 3, is c a l l e d c h e m i c a l v a p o u r d e p o s i t i o n ( C V D ) . I t is

a l s o k n o w n t h a t m e t a l a l k o x i d e s a r e s o m e t i m e s e x c e l -

l e n t p r e c u r s o r s f o r t h e d e p o s i t i o n o f m e t a l o x i d e s s u ch

a s S i O 2 , T i O 2 , C r 2 0 3 a n d A 1 2 0 3 [ 1 5 - 1 8 ] . T h e u s e o f

* Autho r to who m all correspond ence should be addressed.

t h e s e p r e c u r s o r s o f f e r s s e v e r a l a t t r a c t i o n s : t h e m e t a l

a l k o x i d e s c a n b e p u r i fi e d t o a h i g h d e g r e e , t h e y c o n t a i n

m o r e t h a n e n o u g h o x y g e n f o r th e f o r m a t i o n o f t h e de s ir e d

o x i d e , a n d , l a s t b u t n o t l e a s t, t h e m u c h l o w e r d e p o s i t i o n

t e m p e r a t u r e c o m p a r e d w i th n o r m a l C V D c a n a ls o b e v e r y

a t t r a c t i v e i f t h e s u b s t r a t e d o e s n o t s u f f e r u n d e r h i g h

t e m p e r a t u r e s . T h e d e p o s i t i o n o f t h i n o x i d e f il m s b y

t h e r m a l d e c o m p o s i ti o n o f o r g a n o m e t a l li c c o m p o u n d s

i s n o r m a l l y c a l l e d m e t a l o r g a n i c c h e m i c a l v a p o u r d e p o -

s i t i o n ( M O C V D ) . T h e t r i v a l e n t m e t a l a l k o x i d e s w i t h

n o a l k y l g r o u p s a r e f r e q u e n t l y c o n s i d e r e d t o b e r e l a -

t i v e ly st a b l e a t e l e v a te d t e m p e r a t u r e s . T h e y c a n b e h e a t e d

w i t h o u t d e c o m p o s i t i o n . T h u s f o r o u r e x p e r i m e n t s o n l y

m e t a l ( a l u m i n i u m ) a l k o x i d e s a r e r e l e v a n t s u c h a s A 1 -

tri- tert-butoxide ( A T T B ) , Al- tr i - sec-butoxide ( A T S B ) ,

A l - t r i -n - b u t o x i d e ( A T N B ) , A l - t r i - is o p r o p o x i d e ( A T I ) ,

a n d A l - a c e t y l - a c e t o n a t e ( A A A ) .

T h e u s e o f A T S B w a s r e p o r t e d o n l y b y O k u y a m a et

al.

[ 1 9 ] a n d K o d a s

et al.

[ 2 0 ] f o r t h e p r o d u c t i o n o f

u l t r a f i n e a l u m i n i u m o x i d e a e r o s o l p a r t i c l e s b y t h e r m a l

d e c o m p o s i t io n o f A T S B . P h y s ic a l p ro p e r t ie s o f A T S B

a n d s o m e o t h e r a l u m i n i u m a l k o x id e s w e re r e p o r t e d b y

S l a d e k [ 2 ] a n d W i l h o i t [ 2 1 ] r e g a r d i n g t h e e q u i l i b r i u m

p a r t i a l p r e s s u r e .

F r o m t h e l i t e ra t u r e i t is c le a r t h a t u n t i l t o d a y n o t h i n g

h a s b e e n r e p o r te d a b o u t M O C V D u s in g A T S B a s th e

m e t a l a l k o x id e f o r t h e f o r m a t i o n o f a n a d h e r e n t a n d

d e n s e a l u m i n a f i l m .

T h e a i m o f o u r s t u d y w as t h e d e v e l o p m e n t a n d

t e s ti n g o f d e n s e a n d a d h e r e n t t h i n a l u m i n a f il m s , a p -

0040-6090/93/ 6.00 1993 - - Elsevier Seq uoia . All rights reserved


2/7

11 . A. C. H aanappel et al . / MO CV D production of A1203 coatings using A TS B 139

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

s h o u l d p r o t e c t t h e u n d e r l y i n g s u b s t r a t e a g a i n s t t h e

c o r r o s i v e e n v i r o n m e n t s a t h i g h t e m p e r a t u r e s s u c h a s

c o a l g a s i f ic a t i o n a n d g a s t u r b i n e a t m o s p h e r e s . I n t h i s

p a p e r t h e r e s u l t s a r e p r e s e n t e d r e g a r d i n g t h e d e p o s i t i o n

o f a l u m i n a b y M O C V D f r o m A T S B . T h e k i n e ti c a s -

p e c t s in r e l a t i o n t o t h e p r o t e c t i v e n e s s o f t h e c o a t i n g i n

s i m u l a t e d c o a l g a s i f i c a t i o n a t m o s p h e r e s a r e d i s c u s s e d .

2 Expe rimen tal details

D e p o s i t i o n s w e r e p e r f o r m e d o n A I S I 3 04 ( 1 8 C r ,

8 N i , 0 . 0 8 C , 1 S i, 2 M n , 7 1 F e ) . T h e A I S I 3 0 4

s a m p l e s w e r e c u t f r o m a n e l e c t r o p o l i s h e d s h e e t a n d

s u b s e q u e n t l y u l t r a s o n i c a l l y c l e a n e d f o r 3 0 m i n i n R B S

s o a p , h e x a n e a n d e t h a n o l . F i n a l l y , t h e s a m p l e s w e r e

i m m e r s e d in S t ru e r ' s e t c h i n g f l u id ( 5 s o l u t i o n o f 3 M

n i t r i c a c i d i n e t h a n o l ) f o r 1 5 m i n , w a s h e d w i t h p u r e

e t h a n o l a n d d r i e d i n a i r a t r o o m t e m p e r a t u r e . F o r t h e

c o r r o s i o n e x p e r i m e n t s , t h e s a m p l e s w e r e c o a t e d w i t h

0 . 2 0 _ 0 . 05 m g c m - 2 a l um i n a .

A l u m i n a f il m s w e r e p r o d u c e d b y d e c o m p o s i n g A 1 -

t r i sec butoxide ( A T S B , J a n s s e n C h i m i c a ) u n d e r a t m o -

s p h e r i c c ir c u m s t a n c e s . A s c h e m a t i c v i e w o f t h e

e x p e r i m e n t a l a p p a r a t u s i s g i v e n i n F i g . 1 . A s t r e a m o f

p u r e n i t r o g e n g a s w a s p a s s e d t h r o u g h a c o i l i n a s i l i c o n

o i l b a t h a t 1 38 C w h e r e t h e n i t r o g e n g a s w a s s a t u r a t e d

w i t h t h e A T S B . T h e r e s u l t i n g g a s w a s m i x e d w i t h

a n o t h e r n i t r o g e n g a s s t r e a m t o a c h i e v e t h e r e q u i r e d

c o n c e n t r a t i o n o f t h e p r e c u r s o r b e f o r e e n t e r i n g th e r e a c -

t i o n t u b e . T h e s a m p l e w a s a t t a c h e d t o a c e r a m i c t u b e

w i t h a t h e r m o c o u p l e o n t h e i n s i d e f o r a c c u r a t e m e a -

s u r e m e n t o f t h e s a m p l e t e m p e r a tu r e s . T h e s t a n d a r d

c o n d i t i o n s f o r t h e M O C V D p r o c e ss w e r e a l so u s e d f o r

F i g s. 5 - 1 2 , a n d e x c e p t i o n s ar e m e n t i o n e d i n t h e f i g u re

c a p t i o n s : s i l i c o n o i l b a t h w i t h A T S B , 1 3 8 C ; c a r r i e r g a s

f l o w (N 2 ) , 1 3 00 m l m i n - 1 ( S T P ) ; d i l u e n t g a s f l o w ( N 2 ) ,

5 2 0 0 m l m i n - ~ ( S T P ) ; s u b s t r a t e te m p e r a t u r e , 3 3 0 C .

T h e c o r r o s i o n e x p e r i m e n t s w e r e c a r r i e d o u t f o r 2 4 h

i n a c l o s e d s y s t e m . B e f o r e s t a r t i n g t h e e x p e r i m e n t , t h e

s y s t e m w a s f l u s h e d w i t h a r g o n ( s a t u r a t e d w i t h w a t e r a t

1 5 C ) f o r 2 0 h w i t h a f l o w r a t e o f 7 1 h - ~ ( S T P ) . T h e

m i x t u r e o f g a s e s ( 1 H 2 S , 1 H 2 0 , 1 9 H 2 , b a l . A r . )

w a s i n t r o d u c e d a f t e r w a r d s , w i t h a f l o w r a t e o f 1 6 1 h - 2.

A f t e r 2 h t h e f u r n a c e w a s h e a t e d t o t h e t e m p e r a t u r e a t

w h i c h t h e e x p e r i m e n t s w e r e p e r f o r m e d . T h e w h o l e s y s -

t e m w a s f l u s h e d f o r a n o t h e r 2 h a n d t h e n c l o s e d . I t w a s

p r e v i o u s l y f o u n d t h a t n o r e l e v a n t d i f f e re n c e i n c o r r o -

s io n r a t e a n d c o r r o s i o n p r o d u c t s w a s o b t a i n e d b e t w e e n

a c l o se d s y s t e m a n d a s y s t e m w i t h a c o n t i n u o u s f l o w ,

i f t h e r e a c t iv e g a s c o n s u m p t i o n d i d n o t e x c e e d m o r e

tha n 18 .

T h e m o r p h o l o g y a n d t h e c o m p o s i t i o n o f th e l a y er s

w e r e i n v e st ig a t ed b y m e a n s o f o p t ic a l m i c r o s c o p y a n d

At mo s p h e r i c MOCVD

__

1: Thermocouplewith

specimen

2: Furnace

3: Nitrogen5.0 carder

gas

4: Container with ATSB

at

constant tempera

ture

: N i troge n di lut ion gas

= == 3

4

Fig. 1. Schematic view of the MO CV D apparatus: 1, thermocouple

with specimen; 2, furn ace; 3, nitrogen 5.0 carrier gas; 4, con tainer with

ATSB at constant tem perature; 5, nitrogen dilution gas.

s c a n n in g e l ec t r o n m i c r o s c o p y ( S E M ) ( J E O L M 3 5 C F )

e q u i p p e d w i t h a n E D X a n a ly s is s y s t em ( K e v e x D e l t a ,

c l a ss I I I ) . T h e d e p o s i t i o n r a t e o f th e f il m s o n t h e m e t a l l i c

s u b s t r a t e w a s d e t e r m i n e d b y w e i g h i n g t h e s a m p l e s b e -

f o r e a n d a f t e r t h e d e p o s i t i o n .

3 Results and discussion

3.1. Deposition experiments

T h e e ff e ct o f t h e d e p o s i ti o n t e m p e r a t u r e ( 2 9 0 - 4 2 0 C )

o n t h e r a t e ( m g c m - 2 h - 1 ) i s s h o w n i n F ig . 2 . T h e o v e r a l l

a c t i v a t i o n e n e r g y f o r t h e h e t e r o g e n e o u s r e a c t i o n i s

8 3 _+ 5 k J m o l - t ( c a l c u l a t e d f o r t h e r e a c t i o n r a t e l i m i t e d

f i l m g r o w t h ) . A t h i g h e r t e m p e r a t u r e s t h e d e p o s i t i o n r a t e

i n c r e a s e s m o r e s l o w l y t h a n a t l o w e r t e m p e r a t u r e s . A

f u r t h e r i n c r e a s e i n t e m p e r a t u r e w i l l r e s u l t in a d e c r e a s e d

d e p o s i t i o n r a t e a n d b e c o m e s i r r e p r o d u c i b l e w i t h in c r e a s -

i n g t e m p e r a t u r e . T h i s m e a n s t h a t t h r e e r e g i o n s c a n b e

d i s t i n g u i s h e d : r e g i o n 1 , r e p r e s e n t i n g t h e d e p o s i t i o n b e -

h a v i o u r a t l o w t e m p e r a t u r e s , s h o w s a d e p o s i t i o n r a t e

c o n t r o l l e d b y r e a c t i o n s b e t w e e n t h e s u b s t r a t e a n d t h e

p r e c u r s o r ( r e a c t i o n r a t e l i m i t e d f i l m g r o w t h ) ; r e g i o n

2 , a t h i g h e r t e m p e r a t u r e s , r e p r e s e n t s t h e d e p o s i t i o n

r a t e c o n t r o l l e d b y m a s s t r a n s p o r t o f t h e r e a c t a n t s i n t h e

g a s p h a s e ( d i f f u s i o n r a t e l i m i t e d f i l m g r o w t h ) ; a n d

r e g i o n 3 , a t th e h i g h e s t t e m p e r a t u r e s , r e p r e s e n t s m a s s


3/7


4/7

V A C Haanappel et al / M OC VD production of AI203 coatings using ATS B

141

d e p o s i t i o n r a t e s b e t w e e n 3 7 0 a n d 4 0 0 C i s mu c h h i g h e r

t h a n b e t w e e n 4 0 0 a n d 4 2 0 C . T h i s c a n b e e x p l a in e d b y :

( I ) a c h a n g e o f th e d e p o s i t i o n r e g i me f r o m r e a c t i o n

l imi ta t ion to d i f fus ion l imi ta t ion ; and (2 ) a homo-

g e n e o u s r e a c t i o n ( a l u mi n a p o w d e r f o r ma t i o n i n t h e g a s

p h a s e ) w h i c h w i l l b e c o me mo r e p r o n o u n c e d a t h i g h e r

t e mp e r a t u r e s .

3.2. orrosion experiments

In F igs . 5 -7 the re l a t ive weigh t ga in i s shown,

def ined as 100 x ( the ra t io be tween the weigh t ga in o f

a c o a t e d a n d a n u n c o a t e d s a mp l e ) . F r o m F i g . 5 i t i s

c l e a r t h a t t h e a mo u n t o f c o r r o s i o n p r o d u c t s ( r e l a t iv e

weigh t ga in ) t ends to decrease wi th increas ing depos i -

t i o n t e mp e r a t u r e . A t d e p o s i t i o n t e mp e r a t u r e s a b o v e 3 7 0

C the re l a t ive weigh t ga in sca t t e rs somewhat .

In F igs . 6 and 7 the re l a t ive weigh t ga in i s shown as

a f u n c t i o n o f t h e i ni ti a l p r e c u r s o r c o n c e n t r a t i o n a n d t h e

gas f low. The depos i t ion ra t e i s d i rec t ly re l a t ed to the

par t i a l p ressu re o f the p recurso r , a s seen in F ig . 3.

From these f igures i t i s c l ear tha t the re l a t ive weigh t

1

8O

(.0 6O

i o

20

290 ] 310 r 330 ' 350 ~ 370 390

blank 300 320 340 360 380

Deposition temperature ( C)

42

4

Fig . 5 . Bar d iag ram o f the re la tive we igh t ga in ( ) o f the spec imens

(a f te r 24 h su lph ida t ion a t 450 C) as a func t ion o f the depos i t ion

tempera tu re ( C) .

100

blank

Dep. Temp. (*C)

W ~

to

[] 4ao

0.2 0.4 0.6 0,8 1

Vepour Pressure ATS B ram Hg)

Fig . 6. Bar d iag ram of the re la t ive we igh t ga in ( ) o f the spec imens

(a f te r 24 h o f su lph ida t ion a t 450 C) as a func t ion o f the vapour

pressu re o f ATSB (kPa) and the depos i t ion temp era tu re ( C) .

100 ~ ~ ' i

blank

Dep.Temp.

( ' C )

370

I I ~ o

E ~o

6 .5

8 10 12

Gas

Flow (Lmin-1)

Fig . 7 . Bar d iag ram of the re la t ive we igh t ga in ( ) o f the spec imens

(a f te r 24 h o f su lph ida t ion a t 450 C) as a fun c t ion o f the gas f low (1

m in- t ) and the depos i t ion tempe ra tu re ( C) .

gain i s a lmos t unaf fec ted by chang ing the par t i a l p res -

s u r e s o f t h e p r e c u r s o r ( c o n c e n t r a t i o n ) b e t w e e n 5 . 3 3 x

10 -3 a nd 2 .67 10 -2 kP a and a gas f low be tw een 6 .5

a n d 1 2 .5 1 mi n - ~ .

I n F ig s . 8 ( a ) - 8 ( d ) SE M p i c t u re s o f t h e s u lp h i d i ze d

s a mp l e s a r e s h o w n . T h e s e f i g u r e s r e p r e s e n t t h e s u r f a c e

o f a s a m p le c o a t e d u n d e r s t a n d a r d c o n d i t i o n s a n d a t

d e p o s i t i o n t e mp e r a t u r e s r a n g in g f r o m 3 3 0 C t o 4 2 0 C .

A t h i g h e r d e p o s i t i o n t e mp e r a t u r e s t h e c o r r o s i o n a t t a c k

decreases , espec ia l ly by a reduced sca le poros i ty . Espe-

c i a l l y , t h e Fe S f o r ma t i o n b e t w e e n t h e c h a i n - l i k e p r o d -

uc t s i s reduced . Cracks in the a lumina f i lms a re

r e s p o n s i b l e f o r t h e c h a i n - l i k e c o r r o s i o n p r o d u c t s , a n d

a r e mo r e p r o n o u n c e d n e a r t h e e d g e s .

H i g h e r ma g n i f i c a t i o n s ( F i g . 9 ) s h o w t h a t Fe s u l p h i d e

i s a l so fo rme d th roug h the a lumin a f i lm, ind ica t ive o f a

p o r o u s s t r u c t u r e o f th e f il m ( d e p o s i t i o n u n d e r s t a n d a r d

c o n d i t i o n s a n d a f t e r 2 4 h s u l p h i d a t i o n ) . T h i s i s mo r e

p r o n o u n c e d a t l o w e r d e p o s i t i o n t e mp e r a t u r e s .

E x p e r i me n t s w e r e a l s o : p e r f o r me d b y c h a n g i n g t h e

A T SB c o n c e n t r a t i o n a n d g a s f l o w . F r o m t h e w e i g h t

ga in bar d iag rams i t i s c l ear tha t the re l a t ive weigh t

g a i n w a s n o t s t r o n g l y a f f e c t e d b y t h o s e v a r i a t i o n s . O n

t h e c o n t r a r y , SE M o b s e r v a t i o n s s h o w t h a t a t l o w e r

d e p o s i t i o n te mp e r a t u r e s t h e s u r f a c e mo r p h o l o g y o f t h e

c o r r o d e d s p e c i me n s h a s c h a n g e d s o me w h a t w i t h r e s p e c t

t o d i f f e r e n t A T SB c o n c e n t r a t i o n s . D e p o s i t i o n s p e r -

f o r me d a t l o w d e p o s i t i o n r a t e s r e s u l t in a s u r f a c e w h e r e

c o r r o s i o n p r o d u c t s a r e ma i n l y c o n c e n t r a t e d n e a r t h e

e d g e s . A t h i g h e r d e p o s i t i o n r a t e s c o r r o s i o n p r o d u c t s

w e r e f o u n d a l l o v e r t h e s p e c i me n s , a n d t h i s w a s mo r e

p r o n o u n c e d a t l o w e r d e p o s i t i o n t e mp e r a t u r e s t h a n

a t h igher ones . See Figs . 10 and 8 (c) showing the

s u r f a c e mo r p h o l o g y a f t e r s u l p h i d a t i o n ( d e p o s i t i o n a t

400 C and a par t i a l p ressu re o f AT SB of 5 .33 x 10 -3

and 2 .67 x 10 -2 kPa , respec t ive ly ) .

T h e b e s t p r o t e c t i o n o f t h e a l u mi n a f il ms a g a i n st h i g h

t e mp e r a t u r e c o r r o s i o n w a s o b t a i n e d a t h i g h d e p o s i t i o n


5/7

142

V A C Haanappel et al / M OC VD production of Al203 coatings using AT SB

(a) (b)

c) d)

F ig . 8 . SEM image o f the su r face mo rpho lo gy o f coa ted samples a f te r 24 h o f su lph ida t ion a t 450 C : a ) depos i t ion tempe ra tu re , 330 C ; b )

370 C; c) 400 C; and d) 420 C.

F i g . 9. S E M i m a g e o f t h e s u r f ac e m o r p h o l o g y o f a c o a t e d s a m p l e

a f te r 24 h o f su lph ida t ion a t 450 C showing Fe- r ich su lph ides

produc ts wh ich were fo rmed th roughou t the a lumina sca le .

t empera tu res bo th cons ider ing the re l a t ive weigh t ga in o f

t h e s p e c i me n s a n d t h e s u r f a c e mo r p h o l o g y a f t e r t h e

c o r r o s i o n e x p e r ime n t s . T w o t y p e s o f c o r r o s i o n p r o d u c t s

cou ld s t il l be fou nd on the su r face o f the su lph id ized

spec imens : ( l ) cha in - l ike co r ros ion p roduct s (F ig . 8 (d ) ) ;

and (2 ) s ing le c rys ta l s , a ll cons i s t ing o f Fe- r i ch su lph ides

(Fig. 11).

Cracks in the a lumina f i lms a re respons ib le fo r the

cha in - l ike co r ros ion p roduct s . Owing to these c racks in

t h e a l u mi n a c o a t i n g t h e a g g r e s s i v e g a s c o mp o n e n t s c a n

reach the under ly ing imetal li c su r face . The ex i s t ence o f

the c racks i s genera l ly re l a t ed to the in te rna l s t res s in

the thin oxide f i lms [24]. Oxide f i lms appl ied by

M O C V D s u ff e r i n g en e r a l f r o m t h e r ma l a n d i n t ri n si c

s t res ses , accord ing to :

O i n t e r n a l = O i n t r in s i c - A i - O t h e r m a I

These in te rna l s t res ses may be genera ted by d i f fe ren t

s o u rc e s : d u r i n g f il m f o r ma t i o n a n d / o r o w i n g t o t h e r ma l

mism atch . The therm al ly induced par t o f the st res s in

a lumina on s t ee l can be ca lcu la ted f rom the d i f fe rence

betwee n the therma l expan s ion coef f i c ien t s o f the f ilm

and the meta l , t he d i f fe rence be tween the depos i t ion

a n d t h e a c t u a l t e mp e r a t u r e , a n d t h e Y o u n g s mo d u l u s

of the fi lm. Th e in t r ins i ca lly in duced par t i s caused

b y th e g r o w t h m e c h a n i sm o f t he a m o r p h o u s a l u m i na

film.


6/7

~ N . - . ~ ) . - : ~ ~ . . ., , . ~ > 7 ) ;

J t

: ' < ~ 7 - ~ - X ~

V . A . C . H a a n a p p e l e t a l. [ M O C V D p r o d u c t i o n o r A l 2 0 3 c o a t in g s u si n g A T S B

143

a )

' :' ,, - .- ~ ,~ .- , ' < . - - z,~ ,'1 ,~

..-E~.? ~-.,' \ %:~7~ -~- ,. , (~ Lk--


7/7

144

V. A . C . Haanappe l e t a l . / MO CV D product ion o f A IzO 3 coa tings us ing A TS B

4 Conclusions

T h i n f il ms o f a l u mi n i u m o x i d e c a n b e o b t a i n e d b y

p y r o l y t i c d e c o mp o s i t i o n o f

Al- t r i - s ec -bu tox ide

T h e

ac t iva t ion energy fo r the he te rogeneous reac t ion i s 83 +

5 k J m o l - 1 . T h e s u r f a c e mo r p h o l o g y o f th e t h i n f il ms,

d e p o s i t e d o n me t a l l i c s u b s t r a t e s , s h o w e d c r a c k s a n d a

p o r o u s s t r u c t u r e , w h i ch w e r e mo r e p r o n o u n c e d a t l o w

d e p o s i t i o n t e mp e r a t u r e s . E v e n t h o u g h t h e c o a t e d s p e c i -

me n s a r e mu c h l e s s a t t a c k e d t h a n u n c o a t e d o n e s ,

t h e a p p l i c a t i o n o f t h e s e l a y e r s f o r c o r r o s i o n p r o t e c t i o n

of the under ly ing meta l l i c mater i a l s i s as ye t su f fe r ing

f rom fas t d i f fus ion pa ths such as c racks and pores . The

a g g r es s iv e g a s c o m p o n e n t s c a n c o n s e q u e n t l y e a s i ly r e a c h

the under ly ing subs t ra t e , fo rming , fo r example , su lph ide

p r o d u c t s . H i g h e r d e p o s i t i o n t e mp e r a t u r e s , c o r r e s p o n d -

ing to the reg ion o f d i ffus ion- l imi ted f i lm g rowth , l eads

t o a l es s u n i f o r m b u t a m o r e p r o t e c t i v e b e h a v i o u r o f t h e

a lumina f i lm owing to a denser s t ruc tu re .

Mo r e r e s e a r c h i s n e c e s s a r y f o r t h e p r e p a r a t i o n o f

un i fo rm th in a lumina f i lms wi th wel l -def ined chemica l

a n d me c h a n i c a l p r o p e r t i e s b y s e v e r a l me t h o d s s u c h a s

t h e a d d i t i o n o f f o r e i g n e le me n t s , t h e rma l a n n e a l i n g a n d

t h e a d d i ti o n o f o t h e r c o m p o u n d s t o t h e g a s s t re a m.

Acknowledgments

T h i s r e s e a r c h w a s s u p p o r t e d b y t h e I n n o v a t i v e Re -

s e a r c h P r o g r a m o n T e c h n i c a l Ce r a mi c s I O P - T K ) w i t h

t h e fi n a nc i al a i d o f t h e D u t c h Mi n i s t r y o f E c o n o mi c

Affa i r s . The au thors thank J . Wassens , A . Th ie l , and J .

B. Rem fo r the t echn ica l as s i s t ance .

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