CVD AND PVD LOW-FRICTION / ANTI-WEAR COATINGS

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CVD AND PVD LOW-FRICTION / ANTI-WEAR COATINGS

H. Boving, H. Hintermann

To cite this version:

H. Boving, H. Hintermann. CVD AND PVD LOW-FRICTION / ANTI-WEAR COATINGS. Journal

de Physique Colloques, 1986, 47 (C1), pp.C1-111-C1-118. �10.1051/jphyscol:1986117�. �jpa-00225544�

J O U R N A L D E P H Y S I Q U E

Colloque C1, supplement au n02, Tome 47, fbvrier 1986 page c1-ill

CVD AND PVD LOW-FRICTION / ANTI-WEAR COATINGS

H. BOVING and H.E. H I N T E R M A N N

Centre Suisse d'Electronique et de Microtechnique S.A.,

Recherche et DBveloppement, A.-L. Breguet 2 , Case Postale 41, CH-2000 Neuchdtel, Switzerland

Resum6

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La dur6e de v i e de composants mecaniques, q u i s o n t en c o n t a c t g l i s s a n t , r o u l a n t ou v i b r a n t , e s t souvent determinee p a r l e t a u x d ' u s u r e . Le p l u s i m p o r t a n t remede c o n t r e 1 'u s u r e e s t l a l u b r i f i c a t i o n .

Dans des cas 00 une l u b r i f i c a t i o n c o n v e n t i o n n e l l e ne p e u t 6 t r e u t i l i s e e

a

cause de l ' e n v i r o n n e m e n t , des rev6tements s p e c i a u x r e s i s t a n t

a

l l u s u r e , e t

a

basse f r i c t i o n peuvent 6 t r e a p p l i q u e s s u r l e s s u r f a c e s f o n c t i o n n e l l e s . Les rev6tements q u i s o n t s o n s i d e r e s i c i

,

s o n t du t y p e d u r t e l s que T i C, TiN e t Ti(C,N), e t du t y p e t e n d r e , a u t o - l u b r i f i a n t t e l que KoSz; l e s g p a i s s e u r s s o n t e n t r e 0.5 e t 5 vm. Les procedes d ' o b t e n t i o n de ces r e v 6 t e - ments s o n t l e depdt chimique en phase vapeur (C.V.D.), e t l e depdt p h y s i - que en phase vapeur (P.V.D. ). L ' e x p e r i e n c e a montre que l e s rev6tements mentionnes ci-dessus, peuvent dans c e r t a i n s cas augmenter notablement l a duree de v i e de c o n t a c t s mecaniques, l u b r i f i e s

a

l ' h u i l e e t / o u g r a i s s e . A b s t r a c t

-

The f u n c t i o n a l l i f e t i m e o f mechanical components, which a r e i n s l i d i n g , r o l l i n g and v i b r a t i n g c o n t a c t , i s o f t e n determined by t h e i r wear r a t e s . The mostcommon means o f f i g h t i n g wear i s l u b r i c a t i o n . When con- v e n t i o n a l 1 u b r i c a t i o n cannot be used because o f t h e e n v i ronment, s p e c i a1 purpose, w e a r - r e s i s t a n t , l o w - f r i c t i on c o a t i n g s can be a p p l i e d on t h e f u n c t i o n a l s u r f a c e s . The c o a t i n g s which a r e considered, a r e o f t h e h a r d t y p e l i k e T i c , T i N and Ti(C,N), and o f t h e s o f t , s e l f - l u b r i c a t i n g t y p e l i k e KoS2; t h e c o a t i n g t h i c k n e s s e s a r e t y p i c a l l y between 0.3 and 5 pm. The processes used t o d e p o s i t these c o a t i n g s a r e Chemical Vapor D e p o s i t i o n

(C.V.D.) and P h y s i c a l Vapor D e p o s i t i o n (P.V.D.). P r a c t i c e has proven t h a t t h e mentioned c o a t i n g s can d r a m a t i c a l l y i n c r e a s e t h e performance of c e r t a i n o i 1 /grease 1 u b r i c a t e d c o n t a c t s .

I

-

INTRODUCTION

Many s u r f a c e treatments, e s p e c i a l l y t h e a t o m i s t i c d e o o s i t i o n processes, have t h e p o t e n t i a l o f d e p o s i t i n g m a t e r i a l s which v a r y s i g n i f i c a n t l y f r o m t h e c o n v e n t i o n a l m e t a l l u r g i c a l l y processed m a t e r i a l s . The c o a t i n g s may have h i g h i n t r i n s i c s t r e s s e s , h i g h p o i n t d e f e c t c o n c e n t r a t i o n s , e x t r e m e l y f i n e g r a i n s i z e , o r i e n t e d m i c r o s t r u c t u - r e s , p e t a s t a b l e phases, m i c r o - and m a c r o p o r o s i t y , a h i g h degree o f p u r i t y o r may on t h e c o n t r a r y c o n t a i n i n c o r p o r a t e d i m p u r i t i e s . These p r o o e r t i e s n a t u r a l l y i n - f l uence t h e b e h a v i o u r o f t h e s e m a t e r i a l s under mechanical load, chemical e n v i ronment, thermal chock o r i n p a c t l o a d i n g . Metal 1 u r g i c a l p r o r l e r t i e s which lnay b e a f f e c t e d , i n - c l u d e t h e f o l l o w i n g : e l a s t i c modulus, d i f f u s i o n r a t e s , t e n s i l e s t r e n g t h , f r i c t i o n and wear p r o p e r t i e s , f r a c t u r e toughness, c o r r o s i o n r e s i s t a n c e , adhesion and hard- ness.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986117

c1-112 J O U R N A L DE PHYSIQUE

A t e l e v a t e d temperatures, i n n u c l e a r r e a c t o r s , i n vacuum a p p l i c a t i o n s , i n space, and sometii:les i n t e x t i l e and f o o d t r e a t i n g machinery, however, c o n v e n t i o n a l l u b r i c a n t s

l i k e o i l and grease can o f t e n n o t b e used. To a s s u r e p r o p e r o p e r a t i o n o f t h e me- chanisms, i n such cases, s p e c i a l purpose w e a r - r e s i s t a n t and l o w - f r i c t i o n c o a t i n g s must be a p p l i e d .

Such c o a t i n g s a r e a l s o v e r y u s e f u l f o r c e r t a i n t r i b o l o g i c a l c o n t a c t s , where o i l and/

o r grease i s b e i n g used. I t has i n d e e d been shown, t h a t i n t h i n f i l m l u b r i c a t e d con- t a c t s , t h e l o a d b e a r i n g c a p a c i t y o f coniponents can be g r e a t l y i n c r e a s e d , if t h e r i g h t c o a t i n g s a r e a p p l i e d .

Today c h e m i c a l l y vapor d e p o s i t e d (CVD) and p h y s i c a l l y vapor d e p o s i t e d (PVD) me- t a l l u r g i c a l c o a t i n g s (e.g. T i c , TiN, T i (C,N), 1 1 0 s ~ ) a r e e x p l o i t e d i n d u s t r i a l l y t o p r o v i d e t o o l s and machine elements w i t h an extended l i f e t i m e , an i n c r e a s e d e f f i c i e n c y and a n enhanced r e 1 i a b i l i t y .

The CVD t e c h n i q u e i s p r e f e r r e d , when t h e s u b s t r a t e s t o be c o a t e d a r e unaffected by h i g h temperatures and when v e r y h a r d c o a t i n g s w i t h maxinlum adhesion a r e r e q u i r e d , For c o a t i n g s u b s t r a t e s t h a t cannot s t a n d u p t o h i g h temperatures, however, p r e f e - r e n c e i s g i v e n t o PVD methods such as i o n p l a t i n g and s p u t t e r i n g .

I n t h i s paper t h e processes and t h e c h o i c e o f t h e s u b s t r a t e s a r e discussed. S p e c i a l c o n s i d e r a t i o n w i l l be g i v e n t o t h e f r i c t i o n and wear p r o p e r t i e s o f t h e CVD and PVD c o a t i n g s .

I 1

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PROCESSES

Chemical Vapor D e p o s i t i o n (CVD)

CVD i s a c h i e v e d by gas-phase chemical r e a c t i o n s o c u r r i n g under c o n t r o l l e d tempera- t u r e and p r e s s u r e , e n e r g i z e d by heat, t o f o r m s o l i d p r o d u c t s and v o l a t i l e by-pro- d u c t s . The p r i n c i p l e of t h i s d e p o s i t i o n t e c h n i q u e i s shown i n F i g , 1. On t h e l e f t a r e t h e gas d i s t r i b u t i o n and m i x i n g devices. I n t h e m i d d l e a r e t h e r e a c t o r and t h e r e s i s t a n c e - h e a t e d f u r n a c e . The used gases a r e evacuated by a pumping system / I , ? / . I n most cases, m e t a l - h a l i d e s r e d u c t i o n , o p e r a t i n g a t t e m p e r a t u r e s between 700 and 1100° C, a r e used. A t t h e s e t e m p e r a t u r e s and a t d e p o s i t i o n times, which can go up t o s e v e r a l hours, changes i n s t r u c t u r e and c o m p o s i t i o n o f t h e s u b s t r a t e - b u l k , of t h e c o a t i n g and o f t h e i n t e r f a c e can o c c u r and w i l l i n f l u e n c e s t r o n g l y t h e mechanical and c o r r o s i o n b e h a v i o u r o f t h e s e composite m a t e r i a l s .

-

_FILTER

a A S

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EVAPORATOR

F i g . 1

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Schematic o f a CVD apparatus

I o n P l a t i n g (PVD)

I o n p l a t i n g i s a g e n e r i c t e r m a p p l i e d t o f i l m d e p o s i t i o n processes, i n w h i c h t h e s u b s t r a t e s u r f a c e and/or t h e d e p o s i t i n g f i l m i s s u b j e c t e d t o a f l u x o f h i g h energy p a r t i c l e s , s u f f i c i e n t t o cause changes i n t h e i n t e r f a c i a l r e g i o n , o r i n t h e f i l m p r o p e r t i e s compared t o non bombarded d e p o s i t i o n , Such changes may be r e f l e c t e d i n t h e adhesion, t h e f i l m morphology, t h e f i l m s t r e s s , t h e f i l m d e n s i t y o r t h e c o v e r - age o f t h e s u r f a c e b y t h e d e p o s i t i n g f i l m m a t e r i a l /3/.

I n t h i s method Argon i s a d m i t t e d i n t o a vacuum chamber a t a p r e s s u r e o f a b o u t 1 Pa, and a h i g h v o l t a g e source i s connected t o t h e s u b s t r a t e h o l d e r . A gas d i s c h a r g e i s s e t u p i n t h e vacuum chamber, so t h a t some o f t h e e v a p o r a t e d m a t e r i a l i s g i v e n a p o s i t i v e charge. Because of t h e s t r o n g n e g a t i v e s u b s t r a t e p o t e n t i a l , t h e i o n i z e d p a r t i c l e s s t r i k e t h e s u b s t r a t e a t h i g h v e l o c i t y . I n comparison w i t h o r d i n a r y vacuum e v a p o r a t i o n , t h i s process g i v e s b o t h b e t t e r adhesion and a more ~ l n i f o r m c o a t i n g o f n o n - f l a t s u b s t r a t e s . A schematic o f t h i s d e p o s i t i o n t e c h n i q u e i s shown i n F i g . 2 .

11

^{WATER COOLING}

PUMP

F i g . 2

-

Schematic of an i o n F i g . 3

-

Schenlatic o f a

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

S p u t t e r i n g (PVD)

In t h i s method, Argon i s a d m i t t e d i n t o t h e vacuum chamber a t a p r e s s u r e o f about 1 Pa, and a DC o r r a d i o - f r e q u e n c y h i g h v o l t a g e s o u r c e i s connected t o t h e t r a g e t , A gas d i s c h a r g e forms between t h e t a r g e t and t h e s u b s t r a t e h o l d e r , which i s e l e c t r i - c a l l y connected t o t h e vacuum chamber. A schematic o f t h e s p u t t e r i n g process i s shown i n Fig, 3. The p o s i t i v e Argon i o n s produced i n t h e d i s c h a r g e s t r i k e t h e t a r g e t w i t h so much energy, t h a t t h e y knock p a r t i c l e s o u t o f it. These p a r t i c l e s t r a v e l t h r o u g h t h e r e a c t o r and s e t t l e on t h e s u b s t r a t e t o f o r m a t h i n c o a t i n g . Since t h e t a r g e t i s c o o l e d d u r i n g t h e s p u t t e r i n g , t h e r e i s no d i f f u s i o n i n i t , so t h a t t a r g e t s made of a l l o y s and chemical compounds can be s p u t t e r e d , Even non-conducting t a r g e t s can be s p u t t e r e d w i t h s a t i s f a c t i o n . The scope o f t h e method can be widened b y a d m i t t i n g a r e a c t i v e gas d u r i n g t h e s p u t t e r i n g .

J O U R N A L DE PHYSIQUE

T h i s i s used f o r example i n t h e p r o d u c t i o n o f TiN; t h e t a r g e t m a t e r i a l i s o f t i - t a n i u m metal and n i t r o g e n i s i n t r o d u c e d a t l o w p r e s s u r e i n t o t h e r e a c t o r /4/.

I 1 1

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SUBSTRATES

One o f t h e most i m p o r t a n t d i f f e r e n c e s between t h e CVD and PVD t e c h n i q u q s i s t h e f a c t t h a t CVD o c c u r s m o s t l y between 700

-

11000 C, w h i l e good q u a l i t y PVD-coatings can be o b t a i n e d below 550' C; t h i s t e m p e r a t u r e f a c t o r p l a y s a dominant r o l e i n t h e c h o i c e o f s u b s t r a t e s . Another i m p o r t a n t d i f f e r e n c e i s t h e d e p o s i t i o n t r e a t m e n t d u r a t i o n ; t h e average CVD d e p o s i t i o n - r a t e i s a b o u t lpm/hour, w h i l e t h e PVD depo- s i t i o n - r a t e i s about 5 t i m e s l a r g e r . T h i s means t h a t , due t o d i f f u s i o n t a k i n g p l a c e d u r i n g t h e CVD p r o c e s s , t h e s u b s t r a t e p a r t i c i p a t e s a c t i v e l y t o t h e p r o d u c t i o n o f t h e c o a t i n g / s u b s t r a t e composite /5/.

I t i s v e r y important,when h a r d c o a t i n g s a r e used f o r mechanical purposes, t h a t t h e s u b s t r a t e has a l s o s u f f i c i e n t hardness t o a v o i d b e i n q p r e s s e d - i n under l o c a l loads.

When t h e c o a t i n g i s a p p l i e d f o r c o r r o s i o n r e s i s t a n c e o r e s t h e t i c s , t h e hardness o f t h e s u b s t r a t e i s l e s s i m p o r t a n t . F o r mechanical a p p l i c a t i o n s i t i s o f t e n so, t h a t t h e h a r d e r and t h e t o u g h e r t h e s u b s t r a t e i s t h e b e t t e r i t i s an i m p o r t a n t a s p e c t f o r t h e s u b s t r a t e c h o i c e , v a l i d f o r b o t h CVD and PVD, i s t h e f a c t t h a t t h e components must be e a s i l y shaped by c o n v e n t i o n a l m a c h i n i n g methods such as c u t t i n g , m i l l i n g , g r i n d i n g , e t c . As f a r as c a r b i d e and n i t r i d e PVD and CVD c o a t i n g s a r e concerned, t h e most common s u b s t r a t e m a t e r i a l s a r e s t e e l and cemented c a r b i d e .

Carbon s t e e l s , carbon-chromium t y p e t o o l s t e e l s , m a r t e n s i t i c t y p e s t a i n l e s s s t e e l s ( A I S I 440C) have been e x t e n s i v e l y used as s u b s t r a t e m a t e r i a l s f o r CVD c o a t i n g s . D u r i n g r e c e n t years, h i g h speed s t e e l s have a1 so become i n c r e a s i n g 1 y i m p o r t a n t as a s u b s t r a t e m a t e r i a l f o r CVD and PVD c o a t i n g s , e s p e c i a l l y i n t h e f i e l d o f t o o l s and b e a r i n g s ,

F i g . 4 - T y p i c a l t e m p e r a t u r e c y c l e s en- countered w i t h CVD and PVD t r e a t m e n t s .

SUyr

TEMPERATURE CYCLE

"'t

F i g . 4 shows d i f f e r e n t t y p i c a l t e m p e r a t u r e c y c l e s encountered w i t h CVD and PVD p r o - cesses, and r e l a t e d t o t h e s u b s t r a t e m a t e r i a l s . From t h e diagrams i t can be seen t h a t t h e t e m p e r a t u r e c y c l e s a r e b y f a r t h e s i m p l i e s t when cemented c a r b i d e s ~ ! b s t r a t e s a r e used, No s i z e n o r shape d i s t o r t i o n s o c c u r w i t h t h e s e m a t e r i a l s , and i n a d d i t i o n , t h e hardness b e f o r e and a f t e r t h e t r e a t m e n t i s i d e n t i c a l . When s t e e l s u b s t r a t e s a r e coated, t h e t e m p e r a t u r e c y c l e s a r e d e f i n i t e l y more c o m p l i c a t e d , s i n c e t h e p a r t s might have t o be quenched t o r e c o v e r t h e i r hardness. Very i m p o r i a n t , f o r t h e s t e e l c h o i c e i n t h e m a n u f a c t u r i n g o f h i g h p r e c i s i o n c o a t e d p a r t s , a r e t h e phase changes: p e a r l i t e t o a u s t e n i t e and a u s t e n i t e t o m a r t e n s i t e . These phase changes a r e accompanied by volume changes and i f t h e h e a t i n g and c o o l i n g a r e n o t done a c c o r d i n g t o p r e s c r i b e d c y c l e s , i m p o r t a n t d i s t o r t i o n m i g h t o c c u r /6/. The diagrams c o r r e s p o n d i n g t o t h e CVD on s t e e l s u b s t r a t e s show t h a t a thermal t r e a t m e n t i s performed before c o a t i n g . T h i s i s needed t o a l l o w p r e c i s i o n machining t o narrow t o l e r a n c e s and w i t h a good s u r f a c e q u a l i t y . To r e s t o r e t h e mechanical p r o p e r t i e s t o t h e s u b s t r a t e m a t e r i a l a new quenching t r e a t m e n t i s necessary a f t e r t h e c o a t i n g has been a p p l i e d . S i n c e PVD needs a t e m p e r a t u r e o f about 550°cto o b t a i n good q u a l i t y c o a t i n g s , t h i s method can be a p p l i e d t o h i g h speed s t e e l p a r t s a f t e r quenching, w i t h o u t d e s t r u c t i o n of t h e mar- t e n s i t i c s t r u c t u r e . I n t h i s case t h e r e i s no need f o r a second t h e r m a l t r e a t m e n t a f t e r t h e c o a t i n g a p p l i c a t i o n and t h e r i s k s o f d i s t o r t i o n a r e minimised.

I V

-

ADHESION

I n mechanical a p p l i c a t i o n s a c o a t i n g i s o n l y as good and u s e f u l as i t s a d h e s i o n t o t h e s u r f a c e o f t h e s u b s t r a t e . There i s u n f o r t u n a t e l y 1 i t t l e a v a i l a b l e i n methods and i n s t r u m e n t a t i o n which p r o v i d e s a m e a n i n g f u l u s e f u l c h a r a c t e r i z a t i o n o f t h e mechanical s t r e n g t h o f w e a r - r e s i s t a n t t h i n c o a t i n g s , i n p a r t i c u l a r t h e i n t e r f a c i a l bond s t r e n g t h , i .e., t h e adhesion t o t h e s u b s t r a t e . Among t h e v a r i o u s t e c h n i q u e s proposed so f a r f o r t e s t i n g adhesion t o t h e s u b s t r a t e o f t h i n , h a r d f i l m s , one which has l e d t o m e a n i n g f u l r e s u l t s and w h i c h can b e used f o r q u a l i t y c o n t r o l i n p r d u c t i o n i s t h e s o - c a l l e d " s c r a t c h t e s t " . T h i s t e s t i s s u i t e d f o r t h e i n v e s t i g a t i o n of t h e mechanical s t r e n g t h i n c o a t i n g / s u b s t r a t e i n t e r f a c e s and i s o f t e n used today i n t h e t o o l m a k i n g i n d u s t r y / 7 / . The s c r a t c h - t e s t c o n s i s t s o f i n t r o d u c i n q s t r e s s e s a t t h e i n t e r f a c e by d e f o r m i n g t h e s u r f a c e by means o f t h e i n d e n t a t i o n o f a moving diamond t i p , The a p p l i e d l o a d i s i n c r e a s e d s t e p w i se o r c o n t i n u o u s l y u n t i l t h e d e f o r m a t i o n causes s t r e s s e s which r e s u l t i n f l a k i n g o r c h i p p i n g o f t h e c o a t i n g . The s m a l l e s t l o a d a t which t h e c o a t i n g i s damaged, i s c a l l e d t h e c r i t i c a l l o a d and i s determined by o p t i c a l - o r e l e c t r o n m i c r o s c o p y a n d / o r b y a c o u s t i c emission, Several r e s e a r c h e r s a r e p r e s e n t l y engaged i n comparisons o f t h e c r i t i c a l l o a d s o f c o a t i n g s o b t a i n e d by CVD and PVD techniques. One such s t u d y showed r e c e n t l y t h a t t h e presence of t u n g s t e n a t t h e i n t e r f a c e between CVD-Tic and a s t e e l s u b s t r a t e , can n o t a b l y i n - crease t h e c r i t i c a l l o a d /8/,

V

-

FRICTION AND WEAR BEHAVIOR

I t i s common knowledge now, t h a t wear problenls encountered i n d r y - f r i c t i o n a r e s o l v e d b y means o f c o a t i n g s which m o d i f y t h e s u r f a c e p r o p e r t i e s o f mechanical con~po- n e n t s so t h a t t h e i r wear r a t e can be i n f l u e n c e d , A l t h o u g h t h i s i s i n d e e d so, one cannot s e l e c t any c o a t i n g on any s u b s t r a t e t o s o l v e a t r i b o l o g i c a l problem; t h e r e i s a n e c e s s i t y f o r some u n d e r s t a n d i n g o f what i s g o i n g on between t h e c o n t a c t i n g p a r t n e r s .

The d r y - f r i c t i o n p r o p e r t i e s o f t h e c o a t i n g s have t o be examined i n d e t a i l . F i g . 5 p r e s e n t s , f o r a s e r i e s o f coating-materials,friction c o e f f i c i e n t s , o b t a i n e d on a s t a n d a r d p i n d - d i s k t e s t r i q i n a m b i e n t a i r , It i s i n t e r e s t i n g , t h a t c e r t a i n c o a t i n g systems g i v e v e r y l o w f r i c t i o n c o e f f i c i e n t s w h i l e o t h e r s have f r i c t i o n c o e f f i c i e n t s s i m i l a r t o uncoated s t e e l t o s t e e l c o n t a c t s . The f r i c t i o n c o u p l e chromium-carbide

J O U R N A L DE _PHYSIQUE

a g a i n s t s t e e l , c h a r a c t e r i s e d by a h i g h f r i c t i o n - c o e f f i c i e n t , i s v e r y i n t e r e s t i n g , and f i n d s s u c c e s f u l a p p l i c a t i o n s .

F i g . 5

-

D r y - f r i c t i o n c o e f f i c i e n t s determined i n ambient a i r .

The f r i c t i o n c o e f f i c i e n t s g i v e n i n f i q u r e 5 a r e t h e r e s u l t s o f measurements p e r - formed i n ambient a i r , w i t h a 8.H. o f a b o u t 50 %. F o r environments l i k e vacuum o r He a t e l e v a t e d temperatures, t h e v a l u e s o f a l l t h e s e c o e f f i c i e n t s a r e n o t a v a i l a b l e . The e x i s t i n g d a t a show,however, t h a t t h e c o e f f i c i e n t s i n c r e a s e , b u t t h a t t h e f r i c t i o n

remains much below t h a t o f uncoated s t e e l ; t h i s i s p r e c i s e l y what makes t h e s e CVD and PVD c o a t i n g s a t t r a c t i v e t o s o l v e t r i b o l o g i c a l problems i n extreme environments, when c o n v e n t i o n a l l u b r i c a n t s can n o t be used.

S e v e r a l t e s t s have shown t h a t when o i l and grease l u b r i c a t i o n i s b e i n g used, t h e t r i b o l o g i c a l performances o f mechanical c o n t a c t s can be i n c r e a s e d a p p r e c i a b l y . I t was i n d e e d found t h a t t h e l o a d b e a r i n g c a p a c i t y o f l u b r i c a t e d s t e e l c o n t a c t s (super-

r e f i n e d m i n e r a l o i l ) i s a p p r e c i a b l y i n c r e a s e d when t h e p a r t s a r e CVD c o a t e d w i t h T i c , T i N and Fe/Cr c a r b i d e . The graph shown i n F i g . 6 r e p r e s e n t s t h e t r a n s i t i o n d i a - gram f o r l u b r i c a t e d p o i n t c o n t a c t s ; t h e d a t a were o b t a i n e d b y u s i n g a s t e e l r i n g t u r n i n g a t a speed v a g a i n s t a f i x e d s t e e l b a l l , p r e s s i n g a g a i n s t t h e r i n g w i t h a f o r c e F. The graph, c o r r e s p o n d i n g t o uncoated r i n g s and b a l l s , shows c l e a r l y t h e 3 d i f f e r e n t l u b r i c a t i o n regimes. I n r e g i o n I , t h e r e i s a v e r y n a r r o w f r i c t i o n peak r e - corded a t t h e s t a r t o f t h e experiment. Very r a p i d l y , however, t h e c o e f f i c i e n t o f f r i c t i o n decreases t o a v a l u e o f 0.07 o r l e s s and wear becomes n e g l i g a b l e . I n r e g i o n

11, t h e system operates i n a region of boundary l u b r i c a t i o n .

TRANSITION DIAGRAM FOR LUBRICATED POINT CONTACTS

F I RING

I E H 0 LUBRICATION REGIME 11 BOUNDARY LUBRICATION REGIME I11 UNLUBRICATEO "SCUFFING " REGIME

Fig. 6 - T r a n s i t i o n diagram f o r l u b r i c a t e d point c o n t a c t s .

RING STEEL STEEL1 TIC - -

STEEL/ TIC STEEL/ TtC STEEL/ TIN STEEL/ TIN

Now t h e c o e f f i c i e n t o f f r i c t i o n reaches i n i t i a l l y a value of approximately 0.35.

Consequently, t h e c o e f f i c i e n t of f r i c t i o n s t a r t s t o decrease appreciably a f t e r 30 t o 40 seconds u n t i l a l e v e l of 0.12 i s reached. From t h a t moment on, a slow gradual decrease i n t h e f r i c t i o n c o e f f i c i e n t and i n t h e wear s e t s i n a s a r e s u l t of con- tinued running-in of t h e system. In region 111, t h e c o e f f i c i e n t of f r i c t i o n reaches a value of about 0.4 immediately a f t e r a p p l i c a t i o n of t h e normal f o r c e ; important f r i c t i o n s p i k e s occur and wear remains severe / 9 / . For a s p e c i f i c speed, t h e p o i n t s A and B r e p r e s e n t resp. t h e t r a n s i t i o n points from l u b r i c a t i o n reginle I t o I 1 and I 1 t o 111; t h e values f o r s t e e l t o s t e e l c o n t a c t s a r e resp. 175 and 425 N. The data obtained with coated rings and b a l l s a r e given i n Fig. 6. In a l l cases where coated components a r e t e s t e d , t h e data p o i n t s A and B a r e increased; t h e most s p e c t a c u l a r i n c r e a s e i s when t h e r i n g i s coated with TiN and t h e b a l l i s uncoated. Uhen both r i n g and b a l l a r e coated, t h e A and B values coincide; t h i s i s due t o t h e f a c t t h a t i n region I , t h e coatings a r e p r e s e n t and t h a t a s soon a s t h e coatings a r e removed, one i s i n l u b r i c a t i o n regime 111.

VI

-

INDUSTRIAL APPLICATIONS

BALL STEEL STEEL STEEL/ TIC STEEL/ T f N

STEEL STEELITIN

There i s unfortunately no room i n t h e scope of t h i s paper t o d e s c r i b e i n d e t a i l di! . r e n t i n d u s t r i a l a p p l i c a t i o n s where CVD and PVD coatings a r e apnlied succes- f u l l y .

A IN1 175 1200 850 650

> LO00

1000

Therefore several references a r e given, where t h e i n t e r e s t p d readers w i l l f i n d more information about p r a t i c a l case s t u d i e s :

B ( N l 425 1 6 5 0 850 1300 4 0 0 0 1 0 0 0

-

Cutting t o o l s : r e f e r e n c e s /10,11,12 and 13/

-

Metal working t o o l s : references / 5 and 14/

-

Ball bearings: r e f e r e n c e s /15 and 16/

-

Nuclear technology: reference /17/

JOURNAL DE PHYSIQUE

V I I

-

CONCLUSIONS

Hard w e a r - r e s i s t a n t c a r b i d e , n i t r i d e , o x i d e and b o r i d e c o a t i n g s o b t a i n e d b y CVD and PVD t e c h n i q u e s on t o o l s , b e a r i n g - and machine elements, can e x t e n d t h e i r l i f e t i m e , i n c r e a s e t h e i r performance and enhance t h e i r r e l i a b i l i t y .

The c a r b i d e and n i t r i d e o f T i have good t r i b o l o g i c a l p r o p e r t i e s a g a i n s t s t e e l and t h e ~ ~ s e l v e s . Therefore, t h e y a r e a p p l i e d s u c c e s s f u l l y f o r l u h r i c a t i n ? purpose under extreme environments, where no o i l n o r grease can be used. When such c o n v e n t i o n a l l u b r i c a n t s can be used, t h e c a r b i d e and n i t r i d e t y p e c o a t i n g s a p p r e c i a b l y i n c r e a s e t h e l o a d - b e a r i n g c a p a c i t i e s o f mechanical components.

ACKNOW LEDGEI-IENT

F o r t h e i n v i t a t i o n t o p r e s e n t t h i s paper a t t h i s 1 3 t h I n t e r n a t i o n a l Conference on

"Science o f Ceramics", t h e authors- g r a t e f u l l y acknowledge Prof. F. Cabannes and t h e o r g a n i z i n g Committee.

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