Polymeric Materials for Corrosion Control - ACS Publications

26 Performance Aspects of Plasma-Deposited Films 1

2

2

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Downloaded by GEORGE MASON UNIV on December 24, 2014 | http://pubs.acs.org Publication Date: October 14, 1986 | doi: 10.1021/bk-1986-0322.ch026

H. P. Schreiber , J. E. Klemberg-Sapieha , E. Sacher , and M . R. Wertheimer 1

Chemical Engineering Department, Ecole Polytechnique of Montreal, Montreal, Quebec, H3C 3A7, Canada Engineering Physics Department, Ecole Polytechnique of Montreal, Montreal, Quebec, H3C 3A7, Canada

2

Large volume microwave plasma apparatus (LMP) has been used to produce thin, integral films on a variety of substrates. Through control over such variables as substrate temperature and plasma power density, films have been produced from organo-silicones and inor ganic (SiN) starting materials. Measurements of mois ture permeation through such films indicate that the plasma deposited films are highly resistant to water transport, making them particularly attractive in applications calling for corrosion resistance. To illustrate the effect, the moisture transport charac teristics of polyimide substrate with deposited films some 0.5 µm in thickness were measured and found to give up to two orders of magnitude reductions in per meation coefficients. Silicon nitride films were particularly effective in this regard. In another illustration plasma-deposited organo-silicone polymers strongly inhibited the corrosive attack of metal films immersed in aggressive media. Deposition kinetics in microwave plasmas are roughly an order of magnitude greater than in lower frequency (e.g. r . f . ) plasmas. This, combined with the feasible scale-up of microwave plasma apparatus holds out the promise of larger-scale applications for plasma produced films in the corrosion protection area. Glow d i s c h a r g e o r " c o l d " plasmas a r e g a i n i n g i n c r e a s e d c u r r e n c y f o r the d e p o s i t i o n o f n o v e l and p o t e n t i a l l y v a l u a b l e m a c r o m o l e c u l a r c o a t i n g s . The range o f p r o p e r t i e s a t t a i n a b l e by a plasma-polymer i s w i d e , and depends c r i t i c a l l y on such v a r i a b l e s o f the plasma d e p o s i t i o n p r o c e s s as c h o i c e o f monomer, s u b s t r a t e temperature (T ) , power d e n s i t y (ρ), t h e e x c i t a t i o n f r e q u e n c y ( ν ) , and o t h e r s i n d u c t i n g mono mer f l o w r a t e , r e a c t o r geometry, e t c . . . C o n t r o l o v e r these v a r i a b l e s can produce c r o s s l i n k e d , dense d e p o s i t s w h i c h adhere t e n a c i o u s l y t o 0097-6156/ 86/ 0322-0291 $06.00/ 0 © 1986 American Chemical Society

In Polymeric Materials for Corrosion Control; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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POLYMERIC MATERIALS FOR CORROSION CONTROL

s u b s t r a t e s i n c l u d i n g m e t a l s , polymers, c e r a m i c s , e t c . To the e x t e n t t h a t t h i n p l a s m a - d e p o s i t e d f i l m s f u n c t i o n as e f f e c t i v e b a r r i e r s t o the d i f f u s i o n o f w a t e r and o t h e r a g g r e s s i v e f l u i d s , plasma-polymers may s e r v e a v a l u a b l e r o l e as c o a t i n g s f o r the p r o t e c t i o n o f m e t a l s against corrosion. The p o t e n t i a l u s e f u l n e s s o f plasmas as r o u t e s t o c o r r o s i o n p r o t e c t i o n has been r e c o g n i s e d f o r some t i m e , the work o f W i l l i a m s and coworkers (1,2) b e i n g an e a r l y example o f u s i n g glow d i s c h a r g e s t o o v e r c o a t s t e e l . Yasuda and coworkers (3) have made e x t e n s i v e use o f p l a s m a - d e p o s i t e d CH. s t r u c t u r e s as " p r i m e r s " f o r the enhanced adhe s i o n of c o n v e n t i o n a l t o p c o a t s , thereby i n d i r e c t l y a l s o improving the c o r r o s i o n r e s i s t a n c e o f t h e composite c o a t i n g . E a r l i e r r e s e a r c h from our l a b o r a t o r y (4) gave e m p i r i c a l e v i d e n c e o f t h e c o r r o s i o n p r o t e c t i o n due t o p l a s m a - d e p o s i t e d o r g a n o - s i l i c o n e polymers, and o f the e x c e l l e n t h i g h temperature s t a b i l i t y o f the plasma c o a t i n g s . The p r e s e n t paper i s d i r e c t e d m a i n l y a t the q u e s t i o n o f w a t e r perme a b i l i t y through plasma f i l m s , the presence o f w a t e r a t a s u r f a c e b e i n g c o n s i d e r e d e s s e n t i a l f o r the i n i t i a t i o n and p r o p a g a t i o n o f corrosion processes. As i n e a r l i e r r e p o r t e d work ( 4 ) , the p r e s e n t s t u d y has used a large-volume microwave plasma (LMP) f a c i l i t y . The c h o i c e i s b a s e d on t h e f a v o r a b l e d e p o s i t i o n k i n e t i c s a t microwave f r e q u e n c i e s , and on the r e l a t i v e ease o f s c a l i n g e x p e r i m e n t a l LMP apparatus t o i n d u s t r i a l l y useful size. Experimental A l l plasma experiments were performed w i t h an LMP r e a c t o r w h i c h has been d e s c r i b e d elsewhere i n d e t a i l ( 5 ) . The r e a c t o r i s equipped w i t h a h e a t a b l e , r o t a t i n g p l a t e n , 15 cm. i n d i a m e t e r , t o w h i c h a r e f a s t e ned specimens f o r s u r f a c e m o d i f i c a t i o n . The r e a c t o r o p e r a t e s a t 2.45 GHz frequency and a t power i n the range 0.1 - 2.5 kW. P r i m a r y i n t e r e s t was i n the b a r r i e r p r o p e r t i e s o b t a i n e d from plasma o r g a n o - s i l i c o n e s and from i n o r g a n i c " S i N " c o a t i n g s . Spec t r a l grade HMDSO was used i n the former c a s e , w h i l e m i x t u r e s o f S i H ^ and NH~ were used t o produce the S i N s t r u c t u r e s . The s u b s t r a t e i n much o r the work was DuPont Kapton type H p o l y i m i d e f i l m , 51 ym t h i c k . S u b s t r a t e temperatures extended t o 450 C, as d e s c r i b e d e a r l i e r ( 6 ) . The t h i c k n e s s o f plasma-polymer d e p o s i t s was about 0.5 ym. M o i s t u r e permeation was e v a l u a t e d by t h e r o u t i n e o f ASTME-96-53 Τ (water v a p o r t r a n s m i s s i o n o f m a t e r i a l s i n s h e e t f o r m ) . Additional, more p r e c i s e d a t a , were o b t a i n e d w i t h b o t h a Dohrmann E n v i r o t e c h Polymer Permeation A n a l y s e r , m o d i f i e d as p r e v i o u s l y d e s c r i b e d ( 6 ) , and a Mocon "Permatran W" m o i s t u r e permeation a p p a r a t u s . To t e s t t h e c o r r o s i o n p r o t e c t i o n c o n f e r r e d by LMP - produced f i l m s , g l a s s microscope s l i d e s b e a r i n g 5000 A - t h i c k l a y e r s o f aluminium (by vacuum e v a p o r a t i o n ) were o v e r c o a t e d w i t h P-PHMDSO f i l m s . I n t h i s e x p e r i m e n t a l s e r i e s plasma d e p o s i t s were m a i n t a i n e d a t t h i c k n e s s e s near 1000 A , and were produced a t Τ r a n g i n g from 100 C t o about 300 C. Plasma-coated and c o n t r o l samples were p l a c e d i n a b a t h o f a l k a l i n e c l e a n i n g f l u i d (pH 8.5) and i n s p e c t e d p e r i o d i c a l l y f o r l o s s o f A l , as d e s c r i b e d i n an e a r l i e r p u b l i c a t i o n ( 5 ) . F i n a l l y , s c a n n i n g e l e c t r o n microscopy (SEM) was used t o s t u d y the morphology o f plasma d e p o s i t s o b t a i n e d from HMDSO.


26.

SCHREIBER ET AL.

Performance Aspects of Plasma-Deposited Films

293

R e s u l t s and D i s c u s s i o n


a)

Moisture b a r r i e r properties

As s t a t e d , t h e c a p a b i l i t y o f plasma d e p o s i t s t o reduce t h e access o f w a t e r t o c o r r o s i o n - s e n s i t i v e s u r f a c e s may be an i m p o r t a n t m o t i v a t i o n f o r t h e i r a p p l i c a t i o n i n c o r r o s i o n p r o t e c t i o n . I n order t o study t h i s p r o p e r t y , Kapton p o l y i m i d e f i l m was s e l e c t e d as t h e s u b s t r a t e because o f i t s h i g h i n h e r e n t p e r m e a b i l i t y t o w a t e r and i t s a b i l i t y t o r e s i s t e l e v a t e d t e m p e r a t u r e s . The response o f Kapton f i l m o v e r c o a t e d by PPHMDSO t o t h e p e r m e a t i o n o f w a t e r v a p o r i s shown i n F i g . 1. C l e a r l y , t h e p r e s e n c e o f t h e o r g a n o - s i l i c o n e plasma f i l m g r e a t l y reduces w a t e r p e r m e a t i o n . The magnitude o f t h e e f f e c t i s much enhanced when plasma polymers a r e produced a t h i g h Τ and p. I t was shown e a r l i e r (4) t h a t as Τ and ρ i n c r e a s e , t h e plasma po lymer becomes denser and i n c r e a s e s c o n s i d e r a b l y i n t h e p r o p o r t i o n o f i n o r g a n i c l i n k a g e s . By way o f i l l u s t r a t i o n , a t Τ o i n t h e 400 C range, t h e d e n s i t y o f P-PHMDSO i s about 1.8 gm.cm (4); these r i g i d , g l a s s y s t r u c t u r e s t h e r e f o r e s e r v e p a r t i c u l a r l y w e l l as m o i s t u r e b a r r i e r s and, by i m p l i c a t i o n , as c o r r o s i o n i n h i b i t i n g c o a t i n g s f o r me t a l s . The r e s u l t s i n F i g . 1 suggest t h a t a t low power, t h e permea t i o n changes a r e r e l a t e d p r i m a r i l y t o d i e l e c t r i c h e a t i n g o f t h e sub s t r a t e , w h i l e a t h i g h power t h e p e r m e a t i o n changes a r e p r i n c i p a l l y S

p(W) 0

400

800

1200

T (°C) $

F i g u r e 1. P e r m e a t i o n c o e f f i c i e n t o f PPHMDSO-Kapton, as f u n c t i o n of Τ ( # ) , power ( · ) and d i e l e c t r i c h e a t i n g e f f e c t s ( A ) (from ref. 6). S


294

P O L Y M E R I C M A T E R I A L S FOR CORROSION C O N T R O L

t o be a s s o c i a t e d w i t h the changed c h e m i s t r y o f plasma d e p o s i t s . The changes i n d e p o s i t c h e m i s t r y , i n t u r n , may be a s s o c i a t e d w i t h i n c r e a s e d monomer f r a g m e n t a t i o n under these o p e r a t i n g c o n d i t i o n s . The change i n w a t e r vapor f l u x , A F , due t o t h e presence o f 0.5 ym plasma f i l m s may be e x p r e s s e d by F


AF =

— F

(—2—

) χ

100

(1)

° -2 -1 Here F i s t h e water vapor f l u x (mg.m day ) through a c o n t r o l f i l m , w h i l e F i s t h e c o r r e s p o n d i n g f l u x through an o v e r c o a t e d f i l m . V a l u e s o f AF a r e c o n v e n i e n t f o r a comparison o f t h e m o i s t u r e b a r r i e r e f f i c a c y o f S i N and o r g a n o - s i l i c o n e plasma d e p o s i t s . P e r t i n e n t d a t a are g i v e n i n T a b l e I . I t i s e v i d e n t t h a t S i N d e p o s i t s a r e p a r t i c u l a r l y e f f e c t i v e moisture b a r r i e r s , being s u p e r i o r t o o r g a n o - s i l i cone f i l m s under any g i v e n s e t o f plasma v a r i a b l e s . A g a i n , Τ and ρ a r e shown t o be v i t a l t o t h e c o n t r o l o f b a r r i e r f i l m , p r o p e r t i e s . Thus, a t c o n s t a n t t o t a l power (e.g. 600 w a t t s ) s t r o n g e r r e d u c t i o n i n w a t e r t r a n s p o r t i s produced a t i n c r e a s i n g Τ , t h e e f f e c t b e i n g e q u a l l y i m p o r t a n t f o r HMDSO and S i N d e p o s i t s , as shown i n t h e t a b l e . The i m p o r t a n t e f f e c t o f ρ a t c o n s t a n t Τ i s a l s o documented i n T a b l e I ( f o r Τ =250°C). A t a l l LMP o p e r a t i n g c o n d i t i o n s used i n t h i s work, the s u p e r i o r performance o f S i N as compared w i t h HMDSO d e p o s i t s i s m a n i f e s t . Indeed, when S i N f i l m s a r e produced a t Τ > 250 C, and ρ i n t h e range o f 1.0 kW. t h e r e d u c t i o n o f m o i s t u r e p e r m e a b i l i t y con f e r r e d t o Kapton i n c r e a s e s g r e a t l y . An e s t i m a t e o f t h e p e r m e a t i o n c o e f f i c i e n t s o f t h e plasma d e p o s i t s themselves ( i . e . o f the P-PHMDSO and t h e S i N f i l m s ) f u r t h e r i l l u s t r a t e s t h e s u p e r i o r i t y o f t h e S i N c o a t i n g . The permeation c o e f f i c i e n t , π o f a f i l m i s g i v e n by S

π = AW I

t

T a b l e 1„

A

_1 CH 0

(2)

2

Comparison o f w a t e r f l u x changes, A F , due t o plasma d e p o s i t s o f HMDSO and S i N on Kapton.

I n a l l cases plasma f i l m s a r e 0.5 ym t h i c k

Plasma O p e r a t i o n Τ (°C) p(W)

100 200 200 200 200 250 250 250 250

600 200 600 800 1200 400 600 1000 1200

AF

(PPHMDSO)

11 10 18 38 64 23 37 65 78

(%) f o r Ρ(SiN)

40

-85

70

93 >99


26.


SCHREIBER E T A L .

295

where AW i s t h e w e i g h t i n c r e m e n t due t o w a t e r s o r p t i o n , t i s t h e c o n t a c t t i m e , I and A a r e the t h i c k n e s s and a r e a o f t h e f i l m and C Φ i s t h e vapor p r e s s u r e o f w a t e r under c o n d i t i o n s chosen f o r ana lysis . S i n c e AW/At i s e q u a l t o the f l u x parameter F, i n e q u a t i o n ( 1 ) , i t follows that F.l


F

1

(3 )

= TT.C ^ h

0

χ

111

(3")

The l i n e a r i t y between F and ill has been documented by e x p e r i m e n t s t o be r e p o r t e d e l s e w h e r e . I n t h e case o f a composite f i l m , c o n s i s t i n g o f m a t e r i a l s , 1 and 2, (e.g. Kapton and S i N , o r PPHMDSO), we can write

I —

= - i ΐ

π

+ — 2

π

(4)

π

The p e r m e a t i o n c o e f f i c i e n t o f Kapton under t e s t c o n d i t i o n s s i m i l a r t o those used h e r e has been r e p o r t e d as 4.47 χ 10 (g.cm/cm . s e c . cmHg),in c l o s e agreement w i t h o u r own measurements. Thus, s o l v i n g e q u a t i o n (4) f o r π^, we f i n d

ïï

SiN

"

2

δ X

·

1

0

"

U

^ VHMDSO

=

2

·

5

K

1 0

, 2 g · cm/ cm . s . cmHg

"

The p e r m e a t i o n c o e f f i c i e n t s o f t h e two t h i n f i l m m a t e r i a l s d i f f e r by an o r d e r o f magnitude. To t h e e x t e n t t h e n , t h a t w a t e r i n i t i a t e s and p r o p a g a t e s t h e c o r r o s i o n o f m e t a l s u b s t r a t e s , plasma f i l m s o f SiH^/NH^, produced under t h e s t a t e d c o n d i t i o n s seem t o be p a r t i c u l a r l y promising corrosion i n h i b i t o r s . W h i l e , i n t h e case o f PPHMDSO, t h e h i g h e r i n o r g a n i c c o n t e n t produced a t h i g h Τ and ρ enhances f i l m b a r r i e r p r o p e r t i e s , an e q u a l l y i m p o r t a n t f a c t o r may be the dependence o f plasma f i l m morphology on t h e s e d e p o s i t i o n v a r i a b l e s . SEM w e l l documents t h e s e e f f e c t s ( 5 ) , as i l l u s t r a t e d i n F i g u r e 2 ( a ) and ( b ) . A t low T , f o r example n e a r room temperature as i n F i g . 2 ( a ) , PPHMDSO f i l m s c o n s i s t o f agglome rates of spheroidal p a r t i c l e s with c l e a r l y distinguishable i n t e r p a r t i c l e b o u n d a r i e s . T h i s type o f plasma p r o d u c t morphology has been p r e v i o u s l y d i s c u s s e d i n t h e l i t e r a t u r e ( 8 , j 0 . A t h i g h T, such as t h e 400°C used t o produce t h e specimens shown i n F i g . 2 ( b ) , much smoother f i l m s a r e formed, t h e few r e m a i n i n g d i s c r e t e p a r t i c l e s now b e i n g imbedded i n a c o n t i n u o u s m a t r i x o f plasma-polymer. L o g i c a l l y enough, the l a t t e r f i l m s would be much more s u i t a b l e as vapor o r f l u i d b a r r i e r s than the d i s c o n t i n u o u s s t r u c t u r e s e x e m p l i f i e d i n F i g . 2 ( b ) . g

b)

Corrosion protection

Q u a l i t a t i v e e v a l u a t i o n o f t h e specimens immersed i n a l k a l i n e c l e a n i n g f l u i d has demonstrated (5) the c a p a b i l i t y o f PPHMDSO t o p c o a t s t o protect t h e A l f i l m s s u p p o r t e d on g l a s s s l i d e s . The i n t r u s i o n o f



296


F i g u r e 2. Scanning e l e c t r o n m i c r o g r a p h s o f PPHMDSO f i l m s a t s u b s t r a t e Τ = near ambient ( t o p ) , and a t Τ = 400 °C ( b o t t o m ) . Reproduced w i t h p e r m i s s i o n from r e f . 5. f l u i d t o t h e A l - g l a s s i n t e r f a c e i n u n c o a t e d , c o n t r o l samples was r a p i d , w i t h s i g n i f i c a n t l o s s o f A l e v i d e n t a f t e r about 15 min. o f im mersion. I n t h e s e samples t o t a l l o s s o f A l o c c u r e d w i t h i n t h e f i r s t 60-75 min. o f c o n t a c t . P-PHMDSO d e p o s i t e d a t Τ < 100°C was i n e f f e c t i v e i n t h i s t e s t , b u t a t Τ > 100 C i n c r e a s i n g degrees o f p r o t e c t i o n were g i v e n by t h e plasma films. I n samples c o a t e d a t Τ = 150 C the A l l a y e r s remained i n t a c t f o r about 4h. immersion, w i t S g r a d u a l d e t e r i o r a t i o n noted t h e r e a f t e r . Samples c o a t e d a t Τ > 250 C showed no s i g n o f m e t a l l o s s even a f t e r s e v e r a l days o f immersion. Eviden t l y , i n t h e s e s y s t e m s , a s h i f t from m a r g i n a l t o e x c e l l e n t corrosion p r o t e c t i o n t a k e s p l a c e i n t h e c r i t i c a l T range o f 150-250 C. We e



26.

SCHREIBER ETAL.


297

a t t r i b u t e t h i s i n p a r t t o t h e d e n s e r , more i n o r g a n i c s t r u c t u r e o f the plasma d e p o s i t and t o s t r o n g e r b o n d i n g a t t h e plasma polymer/ m e t a l i n t e r f a c e . The i n c r e a s e d c o n t i g u i t y o f plasma f i l m s , d i s c u s sed i n terms o f SEM r e s u l t s above, w o u l d , o f c o u r s e , a l s o c o n t r i b u t e g r e a t l y t o t h e improved performance. The i m p l i e d c a p a b i l i t y o f t h e s e plasma d e p o s i t s t o i n h i b i t c o r r o s i o n a t m e t a l s u r f a c e s may be o f p r a c t i c a l as w e l l as o f b a s i c i m p o r t a n c e . An i m p o r t a n t c o n s i d e r a t i o n i n t h i s r e s p e c t i s t h e r a p i d r a t e o f d e p o s i t i o n f o r such p r o t e c t i v e c o a t i n g s a t t a i n a b l e a t m i c r o wave f r e q u e n c i e s . S i n c e plasma technology i s s t i l l i n a p r o c e s s o f e v o l u t i o n , optimum d e p o s i t i o n k i n e t i c s cannot y e t be s t a t e d ; however, the marked e f f e c t o f e x c i t a t i o n f r e q u e n c y on t h e d e p o s i t i o n o f o r g a n o - s i l i c o n e s can be documented (10), as i n F i g . 3. H e r e , u s i n g t e r m i n o l o g y and c o m p a r a t i v e d a t a due t o Yasuda e t a l . (3), i t i s shown t h a t d e p o s i t i o n r a t e s i n microwave plasmas exceed those a t l o w e r (e.g. r a d i o ) f r e q u e n c i e s by about an o r d e r o f magnitude. Coupled w i t h t h e r e l a t i v e ease o f s c a l i n g microwave plasma r e a c t o r s t o a s i z e p e r t i n e n t t o i n d u s t r i a l c o a t i n g o p e r a t i o n s (11), t h e s e f e a t u r e s p l a c e microwave plasma p r o c e s s e s i n t o t h e domain o f i n d u s t r i a l relevance.

10°

10*

P/ FM ( J/kg ) F i g u r e 3. D e p o s i t i o n r a t e s (D) f o r o r g a n o - s i l i c o n e s , t o f l o w r a t e (F) and monomer m o l e c u l a r w e i g h t (Μ), as plasma power ( P ) . Shaded develope: 13.56 MH plasmas; a r e f o r 2.45 GH plasmas a t monomer p r e s s u r e 0.1 T o r r T o r r ( · ) and 0?4 T o r r ( · ) .

normalized function of data p o i n t s ( A ) , 0.2

Acknowledgment

P a r t s o f t h i s work were s u p p o r t e d by t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l , Canada, by t h e N a t i o n a l Research C o u n c i l , Canada and by t h e M i n i s t r y o f E d u c a t i o n , Québec.


298


Literature cited 1. 2. 3. 4.


5. 6. 7. 8. 9.

T.Williams and M.W. Hayes, Nature 209, 769 (1966) and 261, 614 (1967). T. Williams and W.H. Edwards, Trans. Inst. Met. Finish. 44, 119 (1966). Y. Matsuda and H. Yasuda, This Solid Films, 118, 211 (1984). H.P. Schreiber, M.R. Wertheimer and A.M. Wrobel, Thin Solid Films, 72, 487 (1980). M.R. Wertheimer, J . E . Klemberg-Sapieha and H.P. Schreiber, Thin Solid Films 115, 109 (1984). E. Sacher, J . E . Klemberg-Sapieha, H.P. Schreiber and M.R. Wertheimer, J . Appl. Polym. S c i . , Polym-Symp, 38, 163 (1984). E. Sacher and J.R. Susko, J . Appl. Polym. S c i . , 23, 2355 (1979). L . F . Thompson and G. Smolinsky, J . Appl. Polym. S c i . , 16, 1179 (1972). J . Grebowicz, T. Pakula, A.M. Wrobel and M. Kryszewski, Thin

10.

Solid Films, 65, 351 (1980). M.R. Wertheimer and M. Moisan, J. Vac. Sci. Technol. A3, 2643 (1985).

11.

J . Kieser and M. Neusch, Thin Solid Films, 118, 203 (1984).

RECEIVED January 27, 1986


Polymeric Materials for Corrosion Control - ACS Publications

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