PHOTOTHERMAL DEGRADATION OF BLACK CHROME - A POSSIBLE CONTRIBUTING FACTOR

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PHOTOTHERMAL DEGRADATION OF BLACK

CHROME - A POSSIBLE CONTRIBUTING FACTOR

P. Driver, P. Mccormick

To cite this version:

.CoZbque suppZ6ment au nO1, Tome 42, janvier 1981

PHOTOTHERMAL D E G R A D A T I O N OF B L A C K CHROME

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A P O S S I B L E C O N T R I B U T I N G FACTOR P.M. D r i v e r and P.G. McCormick

Department of Mechanical E n g i n e e r i n g , T h e U n i v e r s i t y of W e s t e r n A u s t r a l i a , N e d l a n d s , W e s t e r n A u s t r a l i a 6009

~6sumS.- Une revue du processus p a r moyen duquel s ' a c c o m p l i t l e dSp6t du chrome n o i r a permis l ' i d e n t i f i c a t i o n d ' u n f a c t e u r , au- paravant inconnu, q u i c o n t r i b u e S l a d s g r a d a t i o n photothermique du chrome n o i r . Des msthodes s o n t proposses pour r s d u i r e ou 6 l i - miner l e s e f f e t s des 6lSments s e n s i b l e s

a

l a s t i m u l a t i o n photother- mique

.

A b s t r a c t . - By a review o f t h e e l e c t r o c h e n i s t r y of t h e e l e c t r o d e - p o s i t i o n of black chrome, it has been p o s s i b l e t o i d e n t i f y a pre- v i o u s l y unrecognised f a c t o r which i s almost c e r t a i n t o c o n t r i b u t e t o t h e photothermal degradation of black chrome. Methods havebeen proposed f o r reducing o r e l i m i n a t i n g t h e e f f e c t s o f t h e s e photo- thermally s e n s i t i v e c o n s t i t u e n t s .

1. I n t r o d u c t i o n . - For t h e l a s t 5 y e a r s black chrome s e l e c t i v e s u r f a c e s have h e l d t h e promise of being t h e most s u i t a b l e c o a t i n g s f o r a wide range of low t o medium temperature a p p l i c a t i o n s due t o t h e i r e x c e l l e n t o p t i c a l p r o p e r t i e s and a p p a r e n t high degree of s t a b i l i t y under o p e r a t i n g c o n d i t i o n s . I n p a r t i c u l a r , McDmald / l - 4 / r e p o r t s t h a t black chrome " i s n o t s u s c e p t i b l e t o d e g r a d a t i o n i n humid atmospheres" and p o s s e s s e s excel-

l e n t s e l e c t i v i t y . Mattox /S/ r e p o r t e d e x c e l l e n t s e l e c t i v i t y and thermal s t a b i l i t y i n a i r and vacuum up t o 350°c, and o t h e r a u t h o r s have r e p o r t e d s i m i l a r c h a r a c t e r i s t i c s f o r black chrome /6-20/.

However, r e c e n t s t u d i e s of b l a c k chrome i n d i c a t e t h a t t h e long term photothermal s t a b i l i t y of t h e s e d e p o s i t s i s i n doubt, even f o r ope- r a t i n g temperatures a s low a s 200°C /21-24/.

S i g n i f i c a n t l y , most r e p o r t s on black chrome d e a l only with d e p o s i t s from t h e Harshaw chromonyxR b a t h /12, 24-27/, although t h e a u t h o r has i- d e n t i f i e d i n e x c e s s of 7 0 d i f f e r e n t black chrome p a t e n t s and " r e c i p e s " . Can it be expected t h a t d e p o s i t s from a l l of t h e s e b a t h s w i l l have iden- t i c a l c h a r a c t e r i s t i c s of s e l e c t i v i t y and d u r a b i l i t y ? Probably not.

I n a d d i t i o n , q u a n t i t a t i v e s u r f a c e a n a l y s i s o f black chrome depo-

s i t s

i s extremely d i f f i c u l t a s h a s been shown by t h e range of c o n f l i c t i n g concepts p r e s e n t e d i n r e c e n t papers /8, 1 1 - 1 7 , 1 9 , 22, 28-37/. Unfortuna- t e l y a v i t a l f a c t o r has o f t e n been overlooked o r underestimated

-

t h a t

i s t h e degree of s t a b i l i t y of black chrome under t h e c o n d i t i o n s prevalent

JOURNAL DE PHYSIQUE

d u r i n g a s u r f a c e a n a l y s i s o p e r a t i o n - I f it c o u l d b e assumed t h a t b l a c k chrome c o n s i s t e d e s s e n t i a l l y o f C r m e t a l and Cr203, t h e n r e a s o n a b l e s t a - b i l i t y d u r i n g a n a l y s i s c o u l d b e e x p e c t e d . However, i f hydrous o x i d e s , h y d r o x i d e s o r more complex chromium compounds a r e p r e s e n t , such s t a b i - l i t y c o u l d n o t be a s s u r e d .

F o r t h i s r e a s o n , a comprehensive r e v i e w o f b l a c k chrome e l e c t r o - d e p o s i t i o n mechanisms h a s been c a r r i e d o u t i n o r d e r t o o r e d i c t t h e l i b e - l y d e p o s i t c h a r a c t e r i s t i c s and d e g r e e of s t a b i l i t y o f b l a c k chrome. How- e v e r , i t soon became a p p a r e n t t h a t such mechanisms had y e t t o be q u a n t i - t a t i v e l y d e t e r m i n e d even f o r b r i g h t chromium d e p o s i t i o n - a p r o c e s s which h a s b e e n commercial f o r o v e r 55 y e a r s - l e t a l o n e f o r t h e r e c e n t l y deve- l o p e d b l a c k chrome d e p o s i t i o n p r o c e s s .

T h e r e f o r e , i n o r d e r t o make p r e d i c t i o n s a b o u t t h e l i k e l y c h a r a c - t e r i s t i c s o f a g i v e n b l a c k chrome d e p o s i t , one needs t o f i r s t r e v i e w t h e e m p i r i c a l l y d e t e r m i n e d p a r a m e t e r s known t o f a v o u r b l a c k chrome d e p o s i - t i o n , and from t h e s e t o d e t e r m i n e t h e s o l u t i o n c h e m i s t r y o f t h e p l a t i n g b a t h , and t h e n t o a t t e m p t t o i d e n t i f y t h e e l e c t r o c h e m i c a l mechanisms l e a d i n g t o s u c c e s s f u l d e p o s i t i o n . The f i n a l s t e p i s t o t i e t o g e t h e r t h e s e t h r e e a r e a s of u n d e r s t a n d i n g - e m p i r i c a l , c h e m i c a l , and e l e c t r o - c h e m i c a l - i n o r d e r t o make o r e d i c t i o n s a b o u t t h e d e n o s i t c h a r a c t e r i s t i c s and t o i d e n t i f y ~ o s s i b l e p h o t o t h e r m a l l y s e n s i t i v e c o n s t i t u e n t s .

Such a s u r v e y h a s r e c e n t l y been c a r r i e d o u t /38/ and r e a c h e d a number o f c o n c l u s i o n s r e g a r d i n g t h e p r o b a b i l i t y t h a t p h o t o t h e r m a l l y un- s t a b l e compounds w i l l be i n c o r p o r a t e d i n t o b l a c k chrome d e p o s i t s . The main f a c t o r s a r i s i n g from t h i s s t u d y a r e d i s c u s s e d below, w i t h an i n d i - c a t i o n o f t h e i r p o s s i b l e e f f e c t s on t h e c o m p o s i t i o n o f b l a c k chrome se- l e c t i v e s u r f a c e s .

2. D e p o s i t i o n and d e p o s i t c h a r a c t e r i s t i c s . - There a r e many f a c t o r s w h i c h i n f l u e n c e t h e d e p o s i t i o n mechanisms of b l a c k chrome p l a t i n g b u t o f par- t i c u l a r i n t e r e s t w i t h r e g a r d t o s t a b i l i t y s t u d i e s o f b l a c k chrome i s

t h e v i t a l r o l e o f C r I I I i o n s and complexes i n t h e p l a t i n g s o l u t i o n , i n t h e c a t h o d e f i l m , and i n t h e b l a c k chrome d e p o s i t /8, 1 6 , 1 9 , 22, 30,

3 7 , 39-45/. I n p a r t i c u l a r , i n c r e a s i n g c o n c e n t r a t i o n s o f C r I I I i n t h e p l a t i n g s o l u t i o n have been shown t o l e a d t o d e c r e a s i n g p h o t o t h e r m a l s t a b i l i t y o f t h e d e p o s i t s /22/.

R e a c t i o n s o f C r I I I i o n s a r e e x t r e m e l y slow i n aaueous s o l u t i o n s due t o t h e i r h i g h c o - o r d i n a t i n g bendency and low l i g a n d s u b s t i t u t i o n l i a b i l i t y /30, 39, 4 4 , 46-50/. However, d e s p i t e t h e i r s l m n e s s , such r e a c t i o n s c a n c o n t i n u e f o r many h o u r s , even months, l e a d i n g f o r example, t o slow and o f t e n i r r e v e r s i b l e a g i n g o f chromium p l a t i n g s o l u t i o n s /51/.

r e g i o n and p a r t i c u l a r l y i n t h e cathode f i l m ) a r e u n l i k e l y t o be f u r t h e r reduced by t h e e l e c t r o d e p o s k t i o n p r o c e s s , b u t r a t h e r t o be i n c o r p o r a t e d i n t o t h e t o p o g r a p h i c a l l y rough black chrome d e p o s i t by mechanical i n - c l u s i o n o r by complex-deposit b r i d g i n g by l a r g e l i g a n d s such a s a c e t a t e o r s i l i c o f l u o r i d e i o n s u t i l i s e d i n t h e c a t a l y s i s of b l a c k chrome p l a t i n g s o l u t i o n s .

2.2.- The o p e r a t i n g environment of a black chrome s e l e c t i v e s u r f a c e with i t s i n c o r p o r a t e d low r e a c t i v i t y

er111

c o m ~ l e x e s

w i l l be very d i f f e r e n t

from t h a t i n t h e p l a t i n g s o l u t i o n , and w i l l t e n d t o p u t " p r e s s u r e " on t h e s e s p e c i e s t o r e a c t t o form o t h e r s p e c i e s (such a s Cr203) which a r e msre s t a b l e i n t h e o p e r a t i n g environment. I n p q r k i c u l a r , ~ o l y m e r i c C r I I I

complexes such a s

A c r / A \ c r d

/ where Ai = OH-

,

0 2 -

,

o r c a t a l y s t

/

\ ~ 2 /

\

a n i s n s

could be expected t o slowly decay t o Cr203 i n t h e long term, and more q u i c k l y when exposed t o e n e r g e t i c r a d i a t i o n o r h e a t .

Although t h e e x i s t e n c e of such polymers i n black chrome d e p o s i t s h a s . y e t t o be proved, t h e r e i s a c o n s i d e r a b l e body of evidence which would s u p p o r t such a p o s s i b i l i t y / 3 8 / .

The behaviour o f t h e l a r g e C r I I I complex s g e c i e s i n s o l i d black chrome d e p o s i t s i s a l s o undefined a t t h i s s t a g e , a l t h o u g h i t i s h i g h l y l i k e l y t h a t slow i r r e v e r s i b l e changes w i l l occur i n t h e long term. Since t h e complexes a r e now bound i n t h e d e p o s i t , t h e y w i l l i n v a r i a b l y be d i s - t o r t e d from t h e i r normal o c t a h e d r a l c o n f i g u r a t i o n , s o overcoming t o some e x t e n t t h e r e a c t i o n r a t e l i m i t a t i o n s impssed on them i n s o l u t i o n by t h e Frank Condon p r i n c i p l e . E l e c t r o n i c rearrangement w i l l now be favoured, and on exposure t o vacuum, high temperatures, o r e n e r g e t i c r a d i a t i o n

(photons, x-rays, e l e c t r o n s o r i o n s ) chemical decomposition

w i l l

be pro- moted, a l b e i t slowly i n some c a s e s , l e a d i n g e o a Subsequent g r ~ d u a l c h a n - ge i n t h e d e p o s i t s o p t i c a l c h a r a c t e r i s k i c s .

3. P o s s i b l e s o l u t i o n s t o t h e photothermal i n s t a b i l i t v of black chrome d e p o s i t s . - There a r e two p o s s i b l e s o l u t i o n s t o t h e problem of b l a c k chrome photothermal i n s t a b i l i t y :

JOURNAL DE PHYSIQUE

c a l d i p t o e i t h e r decompose t h e u n s t a b l e c o n s t i t u e - n t s i n such a way a s t o produce an optimised s t a b l e s u r f a c e , o r t o s t a b i l i z e t h e u n s t a b l e c o n s t i t u e n t s . For t h i s t o be achieved, d e g r a d a t i o n r a t e s would have t o be aetermined f o r t h e given t y p e of b l a c k chrome under t h e s g e c i f i c o p e t r a t i n g ( i n c l u d i n g no l o a d ) c o n d i t i o n s t o which it was l i k e l y t o be ex- posed. E x t r a p o l a t i o n from s h o r t term a c c e l e r a t e d t e s t s may n o t necessa- r i l y be v a l i d i n t h i s c a s e due t o t h e slow, b u t non-zero, r e a c t i o n r a t e s of t h e decomposing s p e c i e s .

3.2.- I t i s probable t h a t t h e photothermal s t a b i l i t y o f d i f f e r e n t C r I I I

complexes with black chrome p l a t i n g s o l u t i o n i o n s w i l l vary c o n s i d e r a b l y . Also, f o r example, t h e t y p e s of polymers and bonds formed i n complexes with a c e t a t e i o n s may vary c o n s i d e r a b l y from t h o s e formed with s i l i c o - f l u o r i d e i o n s , s o t h a t t h e r o l e of " c a t a l y s t " anions may be very s i g n i - f i c a n t . (Note t h a t t h e term " c a t a l y s t " i s s t r d c t l p a misnomer, s i n c e t h e s e i o n s a r e slowly consumed d u r i n g t h e d e p o s i t i o n p r o c e s s ) . I n p a r t i c u l a r , with c a t a l y s t c o n c e n t r a t i o n s ranging from 400 m l / l of a c e t i c a c i d /52/

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down t o 0.45g/l of S I F 6 /53/, i t i s probable t h a t t h e number of c a t a l y s t anions i n c o r p o r a t e d i n t o black chrome d e p o s i t s w i l l vary f r o m b a t h t o bath. Thus i n c r e a s i n g l y s t a b l e black chrome d e p o s i t s c a n a l m o s t c e r t a i n l y be o b t a i n e d by t h e a p p r o p r i a t e choice of d e p o s i t i o n c o n d i t i o n s and c a t a - l y s t s , i n such

a

way a s t o reduce t h e l i k e l i h o o d o f l a r g e q u a n t i t i e s of photothermally u n s t a b l e c o n s t i t u e n t s being i n c o r p o r a t e d i n t o t h e depo- s i t s .

The range o f p o s s i b i l i t i e s i s enormous, b u t t h e p r o s p e c t s a r e en- couraging.

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