CVD OF TITANIUM CARBIDE AT MODERATE TEMPERATURE FROM TITANIUM SUBCHLORIDES

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CVD OF TITANIUM CARBIDE AT MODERATE TEMPERATURE FROM TITANIUM

SUBCHLORIDES

B. Drouin-Ladouce, J. Piton, L. Vandenbulcke

To cite this version:

B. Drouin-Ladouce, J. Piton, L. Vandenbulcke. CVD OF TITANIUM CARBIDE AT MODERATE

TEMPERATURE FROM TITANIUM SUBCHLORIDES. Journal de Physique Colloques, 1989, 50

(C5), pp.C5-367-C5-376. �10.1051/jphyscol:1989544�. �jpa-00229567�

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JOURNAL DE PHYSIQUE

Colloque C5, supplement au n05, Tome 50, mai 1989

CVD OF TITANIUM CARBIDE AT MODERATE TEMPERATURE FROM TITANIUM SUBCHLORIDES

B. DROUIN-LADOUCE, J.P. PITON* and L. VANDENBULCKE

Centre de Recherches sur la Chimie de la Combustion et des Hautes Temperatures, CNRS, F-45071 Orleans Cedex 2 , France

" ~ n i v e r s i t 6 dlOrleans, U.F.R. Facult6 des Sciences, BP. 6759, F-45067 Orleans Cedex 2, France

Resume : La f o r m a t i o n i n - s i t u des sous-haloggnures de t i t a n e par r e d u c t i o n du tetrachforure de t i t a n e par l e t i tane metal lique e s t u t i 1 is & conjointement avec du butane pour deposer des couches de carbure de t i t a n e I temperature modgree, de 1 'ordre de 850°C.

La vitesse de depdt, l a composition en C/Ti e t en chlore des revetements ainsi que l e u r m i c r o d u r e t e s o n t e t u d i e e s en f o n c t i o n des parametres experimentaux, specialement l a composition i n i t i a l e de l a phase gazeuse

.

^Enl e s comparant aux r e s u l t a t s de calculs thermodynamiques, l e s variations de l a vitesse de depdt e t de 1 a composition du sol ide permettent de di scuter l ' i n f 1 uence de d i f f e r e n t e s 1 imi t a t i o n s cinetiques du processus qui interviennent en fonction des conditions de dCipdt

.

Dans t o u s l e s c a s un grand & a r t i l ' e q u i l i b r e e s t observe

.

^On

montre que ces conditions de temperature de depdt moderee e t de sursaturation elevee condui sent I une structure I grains tr6s fins.

La m i c r o d u r e t e des d e p - t s v a r i e dans un l a r g e domaine

.

Oes v a l e u r s a u s s i f a i b l e s que 1000 kg.mm-! s o n t obtenues quand 1 a q u a n t i t e de c h l o r e incorporge e s t @levee. Dans l e s m e i l l e u r e s c o n d i t i o n s de depdt, l e s couches s o n t bien c r i s t a l l i s 6 e s avec une concentration en chlore f a i b l e e t un rapport C/Ti proche de 1

.

Dans ce cas l a t a i l l e des grains e s t particulierement plus f a i b l e compa- ree 5 c e l l e observee sur l e s couches deposees de facon classique 1 temperature plus @lev@e, comprise e n t r e 1000 e t 1050°C, l a morphologie e s t tr6s dense e t l a microdurete a t t e i n t d e s v a l e u r s e n t r e 4000 e t 5000 kg.mm" sous une charge de 50g.

Abstract :The in-situ formation of the titanium subchlorides.by t h e reduction of titanium t e t r a c h l o r i d e by titanium metal i s used together with butane t o deposit titanium carbide layers a t moderate temperature, in t h e order of 850°C.

The deposition r a t e

,

t h e C/Ti composition of t h e coatings, the chlorine incorporation and the microhardness are studied as functions of t h e i n i t i a l gaseous composition. When compared w i t h t h e thermodynamical calculations, t h e variations of t h e deposition r a t e and the solid composition allow t o discuss the influence of d i f f e r e n t kinetic l i m i t a t i o n s of the process which a r i s e as a function of t h e deposition c o n d i t i o n s . In any c a s e a g r e a t d e p a r t u r e from t h e e q u i l i b r i u m i s observed. I t i s shown t h a t these conditions of moderate temperature and supersa- t u r a t i o n lead t o a very fine-grained structure.

The m i rohardness of the deposits varies in a large range. Values as tow as 1000 kr~.mm-~ can be observed when the chlorine content i s high. With t h e best deposition conditions, the deposits are we1 1-crystal 1 ized with a low chlorine content and a C/Ti r a t i o approaching 1. In t h a t c a s e i t i s shown t h a t t h e g r a i n s i z e i s p a r t i c u l a r l y lower than those observed in layers deposited a t c l a s s i c a l higher temperatures of 1000-1050°C, the morphology i s very dense and the microhardness i s in the range 4000-5000 kg.mm-2 under 509 load.

I Introduction :

In a previous s t u d y (1 ) a comparison between thermodynamic c a l c u l a t i o n s a p p l i e d t o s t o i - chiometric titanium carbide and some preliminary experiments f o r two systems TiC14

-

^C4H10

-

H p

-

Zn vapor and TiClx

-

C4H1

-

^Hz ⁽ ^{w i t h}^{x < 4} allowed t o reveal the conditions necessary t o overcome the k i n e t i c limrta%ions of t h e actual TiC14

-

^CH4

-

^Hz reactant gas mixture when Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989544

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employed a t moderate temperature. The role of Tic12 through i t s reduction or i t s disproportionation was especially emphasized and i t was shown t h a t a prereducing step of TiC14 t o i t s subchlorides by e l e m e n t a l t i t a n i u m coupled with t h e use of a l e s s s t a b 1 e hydrocarbon 1 ik e butane was an i n t e r e s t i n g way t o decrease t h e deposition temperature of titanium carbide.

Thermodynamic c a l c u l a t i o n s a r e extended here t o n o n - s t o i c h i o m e t r i c t i t a n i u m c a r b i d e and a more extensive experimental work i s reported with t h e TiCl.,

-

^C4H10

-

^H system a t 850°C. As a r e s u l t from a comparison, the deposition mechanism i s discussed and t%e deposition process optimized.

I1 Experimental techniques :

The deposition apparatus presents some d i s t i n c t i v e c h a r a c t e r i s t i c s necessary t o implement the process. I t i s composed of a hot-wall r e a c t o r ( of 53 m m ID ) surrounded by a two zones furnace. The f i r s t zone p e r m i t s reducing TiC14 by T i c u r l e d c h i p s a t a h i g h e r t e m p e r a t u r e than d e p o s i t i o n . Most of t h e hydrogen flow and t h e butane a r e introduced downstream i n a colder part of the reactor while the TiC14 i s passing through the titanium chips in order t o make up t h e whole m i x t u r e of hydrogen, butane and t i t a n i u m c h l o r i d e s which r e a c t s i n t h e deposition zone.

All t h e o t h e r p a r t s a r e c l a s s i c a l w i t h a c o n t r o l u n i t of g a s f l o w , Tic1 evaporation and regulation of pressure. A pumping system c a r r i e s t h e g a s e s through a f i j t e r and v a r i o u s traps.

The following deposition conditions were used : deposition temperature = 850+ 10°C, T i chips temperature evaporator = = 37OC, butane flow r a t e 1050°C, t o t a l p r e s s u r e = 5-50 cm /mn3 Tic14 flow r a t e = 40 t r r s (5.33 l o 3 Pa), temperatuF5 of t h e TiCll

S

= 5-50 cm / m n , hydrogen f l w r a t e through t h e Tic1 evaporator = 5-100 cm /mn, t o t a l hydrogen flow r a t e = 1000-4000 cm

9

/mn, deposition time

,(j

hours.

Most of t h e deposition experiments were carried out on s t e e l s samples, especially on XC 38 and 2220 CW 14, but cemented carbides were also t e s t e d as substrates. The growth r a t e s of t h e coating were deduced from the mass increase per unit surface, generally during two hours, and t h e coating thicknesses were a l s o measured on polished cross sections of t h e samples.

Fractured cross-sections were observed. by S.E.M. and the chemical composition of t h e deposits were measured by quantitative microprobe analysis. Vickers microhardnesses of s u f f i c i e n t l y thick coatings were measured under a 509 load.

I11 Thermodynamical study :

Generalities on the calculation procedure :

The chemical equilibrium calculations involve the minimization of the t o t a l Gi bbs f r e e energy taking i n t o account the conservation of the mass of each element, a constant temperature and a constant t o t a l pressure. A s l i g h t l y modified version of SOLGASMIX program elaborated by G.

ERIKSSON ( 2 ) has been employed t o perform t h e c a l c u l a t i o n s of t h i s paper. In t h i s program, non-stoichiometric s o l i d s can be considered.

As reported previously (1) these calculations are common f o r a l l a l i p h a t i c saturated hydro- carbons a s t h e C/H r a t i o i s c o n s t a n t whatever n i n t h e C n H 2 n + L

+

HL m i x t u r e f o r a c o n s t a n t t o t a l pressure, a given titanium chloride concentration and an i n i t i a l carbon concentration : n X°CnH2n+2.

The f o l l o w i n g s p e c i e s were considered : H , CH

,

TiC14, TiC13, Tic1 , TiC1, HC1, C12, C 2 H 2 and CC14 as constituants of t h e cJaseous mix?ure;?i,c and Ticx as solic?s ( pure Ti was considered a s t h e s o l u b i l i t y of C i n T i i s low ( 3 ) ) . Tic12 and Tic1 s o l i d s were not t a k e n i n t o account as stoichiometric thermodynamical calculations showed h a t they were never produced a t t h e c o n d i t i o n s s t u d i e d , e s p e c i a l l y when t h e p a r t i a l p r e s s u r e of t h e s e s p e c i e s were maximum, when aTi = 1.

All the thermodynamical properties of t h e gaseous species were taken in t h e JANAF tables. For non-stoichiometric sol i d s , . o t h e r d a t a sources must be used. Several s t u d i e s have r e p o r t e d thermodynamic properties of the Tic over i t s homogeneity range a t temperature between 850 and 1900 K (3-5). The p a r t i a l molar f r e e energies of titanium were calculated a t 1100 K from these r e s u l t s taking i n t o account the f r e e energy of stoichiometric Tic formation given by JANAF t a b l e s and the extent of t h e homogeneity range of t h e titanium carbide phase a t 1100 K.

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The carbon p a r t i a l thermodynamic prgperties were deduced from t h e Gibbs-Duhem equation and the values of t h e titanium p a r t i a l thermadynamic properties a t 1100 K. The r e s u l t s have been compiled i n a polynom form d i s c l o s i n g t h e l o g a r i t h m of t h e a c t i v i t y c o e f f i c i e n t s a s a function of t h e composition.

A t 1100 K t h e homogeneity range extends from C / T i = 0.59 t o C/Ti = 0.98 (3-6). For C/ti between 0.59 and 0.98 t h e following re1 ations were used :

were

Yi

is the a c t i v i t y coefficient and Xi the molar fractions.

In t h e Tico 98

+

C two phases domain ( C / T i > , 0.98) t h e C a c t i v i t y i s equal t o 1 and t h e T i a c t i v i t y is'constant and equal t o i t s value in Ticoag8 t h a t i s :

a ~ i = 6.9427 10" ;

YTi

= 6.9427 10"/xTi

In t h e TiC0.59 + T i two phases domain ( C/Ti

<

0.59 ) t h e a c t i v i t y of T i i s supposed t o be equal t o 1 as t h e s o l u b i l i t y of C i n Ti i s low (3) and t h e a c t i v i t y of carbon i s constant and equal t o i t s a c t i v i t y i n t h a t i s :

Calculation r e s u l t s :

From thermodynamic calculations of the interaction of TiC14 with pure Ti a t 1300 K, t h e Cl/Ti r a t i o was fixed a t 3.0071. Then t h e equilibrium i n t h e Ti

-

^C

-

^H

-

C1 system was calculated f o r d i f f e r e n t i n i t i a l compositions in one mole of t h e Tic1

-

^C^H

-

^H gaseous i x t u r e and c o n s t a n t v a l u e s of t h e t e m p e r a t u r e and t h e pressure3('0f7=1 110% J0and

?

⁼^{5.33 10}

^"3

^Pa ^).

In t h e following of t h i s paper t h i s TiC13.0071 mixture of titanium chloride w i l l be written TiC13.0.

Thermodynamic calculations a r e often used t o specify t h e equi 1 i brium domain of t h e d i f f e r e n t phases ( Ticx, T i and C ) versus t h e i n i t i a l gaseous mixture composition. When compared with t h e deposition domains calculated with TiC14 (1 1, i t i s c l e a r t h a t t h e + Ti deposition range i s considerably enlarged especially a t high XO TiC13.0. On t h e other hand, the two phases TiC0.98 + C domain remains almost unchanged.

The v a r i a t i o n s i n t h e production of t h e d i f f e r e n t gaseous s p e c i e s and s o l i d phases a r e presented on fig.1 a s a f u n c t i o n of t h e i n i t i a l gaseous composition. The t h e o r e t i c a l e f f i - ciencies of T i - c o n t a i n i n g s p e c i e s ( f u l l l i n e s and C-containing$pecies ( d o t t e d l i n e s ) are p l o t t e d a g a i n s t 4 X 0 C H f o r a c o n s t a n t X O Tic1

-

2.8 10'

.

The y i e l d s a r e defined with respect t o X0 TiC13 joo? Pi-containing species and.!

i0

^C4H2P f o r C-containing species.

As previously shown i n d e stoichiometric case ( I ) , TiC13 and 1 2 have a high y i e l d i n the Ti-rich region despite the low temperature employed because TiC13.0 chlorides were used as Ti-containing c a r r i e r s in t h e i n i t i a l mixture.

TiC14,TiC1

,

Tic1 e f f i c i e n c i e s remains constant i n t h e + Ti domain as t h e Ti a c t i v i - t y i s equa? t o 1. t h e n TiC13 and TiC12 d e c r e a s e q u i c k l y

-

e s p e c i a l l y TiC12

-

i n t h e s i n g l e phase region while the TiC14 y i e l d increases ( before i t decreases also near t h e l i m i t of t h e single-phase domain ). Thus a s d i s c u s s e d p r e v i o u s l y from e q u i l i b r i u m c a l c u l a t i o n s which involve stoichiometric Tic ( 1 ) t h i s important amount of TiC14 i s produced by t h e disproportionation of t h e s u b c h l o r i d e s . Accordingly, t h e s u b c h l o r i d e s formed a t 1300 K allow t o l i b e r a t e and incorporate Ti i n a titanum carbide deposit a t 1100 K.

The CH4 y i e l d which i s lower t h a n 1 0 ' ~ i n t h e two phases T i c O 59 + Ti domain and i n t h e Ti- rich part of t h e single phase region increases quickly u n t i l 1

lt

reaches a maximum value when the f r e e carbon production begins. Then C H 4 e f f i c i e n c y d e c r e a s e s a s t h e f r e e carbon y i e l d r i s e s .

The C/Ti v a r i a t i o n s a r e shown on fig.2 which p r e s e n t s t h e C.V.D. phase diagram f o r Ti-C s o l i d s deposited from TiC13.0

-

^C4H10

-

Hz mixtures. While titanium carbide can be deposited

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a t e q u i l i b r i u m i n a wide range o f concentrations o f t h e i n p u t reactants, t h i s diagram shows t h e s e n s i t i v i t y o f t h e composition o f t h e n o n - s t o i c h i o m e t r i c t i t a n i u m c a r b i d e phase t o t h e i n i t i a l m i x t u r e composition.

F i g u r e 1 : V a r i a t i o n s o f t h e e- q u i l i b r i u m y i e l d s o f t h e p r i n c i p a l species as a f u n c t i o n o f t h e i n i - t i a l h y d r o c a r b o n c o n t e n t a t X 0 T i c 1 - = 2 8 T = 1100 K and P =

2% l o 3

Pa.

F i g u r e 2 : Phase f i e l d s f o r s o l i d spec? es and i s o - c o n c e n t r a t i o n curves o f t i t a n i u m c a r b i d e i n i t s s i n g l e ehase d o m a i n a t e a u i 1 i b r i u m f o r i i c 1 3

-

^C ^H

-

H ' m 0 x t u r e s a t T = 1100 R and

8

lO5.33210~ Pa.

I V Comparison w i t h t h e e x p e r i a e n t a l r e s u l t s and d i s c u s s i o n on t h e d e p o s i t i o n mecanism : The use o f b o t h a l e s s s t a b l e h y d r o c a r b o n t h a n methane and a p r e r e d u c i n g s t e p o f t i t a n i u m t e t r a c h l o r i d e by t i t a n i u m metal a t a low pressure improves t h e chemical k i n e t i c s enough t o a l l o w t h e d e p o s i t i o n o f t i t a n i u m c a r b i d e a t a non-negligeable r a t e a t moderate temperature.

However some k i n e t i c s contr.01 o f t h e p r o c e s s can o c c u r a g a i n a t t h e s e l o w e r t e m p e r a t u r e s , producing some departure f r o m t h e e q u i l i b r i u m . We remember t h a t such k i n e t i c s c o n t r o l i s i n f a c t necessary t o o b t a i n n e a r l y constant d e p o s i t i o n c o n d i t i o n s i n l a r g e s c a l e reactors.Two k i n d s o f k i n e t i c s l i m i t a t i o n s can be considered, caused e i t h e r by t h e r e d u c t i o n o r (and) t h e d i s p r o p o r t i o n a t i o n o f t h e t i t a n i u m subchlorides o r by t h e p y r o l y s i s o f butane.

A comparison b e t w e y t h e t h e o r e t i c a l and t h e experimental C/Ti r a t i o s versus 4 XO C4H10, a t X0TiC13 = 2.8 10'

,

i s presented i n f i g u r e 3. The corresponding v a r i a t i o n s o f t h e t h e o r e t i - c a l y i e l d and e x p e r i m e n t a l g r o w t h r a t e a r e r e p o r t e d i n f i g u r e 4 and a l l t h e e x p e r i m e n t a l r e s u l t s a r e p u t t o g e t h e r i n f i g u r e 5, t h a t i s t h e v a r i a t i o n s o f C/Ti, t h e g r o w t h r a t e , t h e c h l o r i n e i n c o r p o r a t i o n and t h e microhardness o f t h e deposits.

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.-

^{C I T i}

.-.-Deposition rate

1.5

i

-

10-2 10.'

4X~C4H10

F i g u r e 3 : Comparison o f t h e va- 0.9

r i a t i o n s o f t h e C/Ti r a t i o b e t - ween t h e e q u i l i b r i u m a t T = 1100 k and t h e experimental r e s u l t s a t

T = 1123 K as a f u n c t i o n o f t h e o 0.02 0.04 0.06 4 x0 C ~ H , ~

i n i t i a l h y d r o c a r b o n c t e n t f o r X O T i C b 3 0 = 2.8 10-'and P = 5.33 10 Pa.

F i g u r e 5 : V a r i a t i o n s o f t h e C/T7 r a t i o , t h e d e p o s i t i o n r a t e ,

.E 9 6

N

B Y m

t h e c h l o r i n e c o n t e n t a n d t h e Vickers microhardness o f t h e coa- t i n g s d 5 p o s i t e d a t X 0 TiC13 = 2.g 10-

,

T = 1123 K and P = 5.33 10 Pa v e r s u s t h e i n i t i a l b u t a n e concentration.

3 ^e--o. "

8

>

1500

-

2 1

.-

Microhardness .

. - _ . A t . % Chlorine

___-

-

. .

F i g u r e 4 : Comparison b e t w e e n t h e y i e l d o f t h e s o l i d species a t

e q u i l i b r i u m f o r one m o l e o f i n i - F i g u r e 6 : V a r i a t i o n s o f t h e C/Ti r a t i o t i a l m i x t u r e a t T = I 1 0 0 K and versus e i n i t i a hydrocarb n conten f o r X 0 t h e d e p o s i t i o n r a t e o f t h e coa- TiC13

Otl

^2.8

^lo-',

^1.3

^lo-',

^6.^10-

5

^{a t T}⁼

t i n g s d e p o s i t e d a t T = 1123 K 1123R and P = 5.33 10 Pa.

versus t h e i n i t i a l h y d r o c a r b o c o n t e n t f o r X 0 -$iC13.~ = 2.8 1 0 -

!!

and P = 5.33 10 Pa.

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For 4 X O C H1

o,(

^2.8 t h e experimental C/Ti r a t i o i s f a r g r e a t e r than t h e t h e o r e t i c a l one ( f i g .

4)

and f r e e carbon has been d e t e c t e d by X-ray d i f f r a c t i o n . In t h e s e c o n d i t i o n s of high titanium subchlorides concentrations ( XO Tic1 ),4 X O C4HI0 ) titanium is not proper- l y incorporated i n t o the titanium carbide. ~ c c o r d i n ? j f ~ t h e chlorine content i s very high i n t h e corresponding region of f i g u r e 5. The whole r e s u l t s show some l i m i t a t i o n of t h e process by t h e r e d u c t i o n and ( o r ) d i s p r o p o r t i o n a t i o n of t h e t i t a n i u m s u b c h l o r i d e s . This d e p a r t u r e from e q u i l i b r i u m i s apparently reduced f o r X O TiC13.0.= 4 X" C4Hb0 ( f i g . 3). How v e r t h e similar theoretical and experimental values of C/Ti o b t a ~ n e d fo r 4 X C4H10 = 2.8 10" do not permit t o conclude t h a t near e q u i l i b r i u m c o n d i t i o n s a r e reached because t h e experimental values afterwards vary only s l i g h t l y up t o about 1 when 4X0 C4H10), XO TiC13.0.

Moreover microprobe analysis showed a C/Ti r a t i o near t h e stoichiometry and no f r e e carbon has been detected by X-ray diffraction. The whole r e s u l t s obtained in t h i s l a s t range of t h e i n l e t composition d e p a r t a l o t from t h e equi 1 ibrium r e s u l t s which p r e d i c t an i m p o r t a n t deposition of f r e e carbon ( f i g . 3-4). I t i s c l e a r t h a t , i n t h i s d e p o s i t i o n range, t h e process i s principally control led by t h e decomposition of butane. Accordingly, the chlorine incorporation in the solid i s lower as shown in f i g u r e 5.

A comparison, a t X0 TiC13-0 .= 2.8 between the solid phases e f f i c i e n c i e s a t equilibrium and t h e e x p e r i m e n t a l deposition r a t e s ( f i g . 4) c o n f i r m s t h e e x i s t e n c e of t h e two k i n e t i c controls. The coating r a t e variation exhibits two levels which can be linked respectively t o t h e kinetics l i m i t a t i o n of the titanium deposition and then t h e carbon incorporation when t h e r a t i o 4 X O C4H10/ XO TiC13.0 increases.

I t can be supposed t h a t the kinetics l i m i t a t i o n by t h e titanium incorporation r a t e i s overcome because a t r a n s i t i o n occurs from a reduction reaction t o a disproportionation one when 4 XO C4H10 increases, as t h i s second reaction type becomes thermodynamically enhanced in the single-phase deposition domain (fig. 1 ). Accordingly, t h e deposition r a t e increases f o r 4 X"

C4HIO i n t h e range 0.04

-

0.05 u n t i l i t i s l i m i t e d by t h e carbon i n c o r p o r a t i o n ( f i g . 4). I t i s i n t e r e s t i n g t o note t h a t i n t h a t case t h e C/Ti r a t i o remains near t h e s t o i c h i o m e t r i c composition ; when t h i s l i m i t a t i o n of t h e deposition r a t e i s compared t o t h e titanium carbide and f r e e carbon y i e l d s p r e d i c t e d a t e q u i l i b r i u m , i t appears t h a t high s u p e r s a t u r a t i o n i n butane can be employed w i t h o u t d e p o s i t i o n of f r e e carbon. Such a r e s u l t was p r e v i o u s l y reported also f o r t h e deposition of titanium carbide and boron carbide a t higher temperature, respectively a t about 1300 K and 1400 K when C H 4 i s employed a s carbon c o n t a i n i n g s p e c i e

(7,8). The i n h i b i t i n g e f f e c t of C H 4 r e p o r t e d p r e v i o u s l y ( 8 ) seems t o be found again f o r C4H10, as the deposition r a t e s l i g h t l y decreases f o r high values of 4 X O C4H10.

The f i g u r e 6 shows t h e varlation of t h e experimental C/Ti r a t i o in the solid as a function of 4 XO C H f o r three values of t h e i n l e t concentration of TiC13

.

When t h e concentration of TiC13

:

ieO;reases, t h e C/Ti r a t i o increases s l i g h t l y f o r a conitant concentration of C4H10, especially f o r 4 X°C4H 0 between 0.02 and 0.03. This r e s u l t can be.explained e a s i l y because t h e boundary between t k e single and two-phases domains occurs f o r decreasing values of t h e C4HI0 c o n c e n t r a t i o n when t h e Tic1 c o n c e n t r a t i o n d e c r e a s e s ( v a l u e s of t h i s l i m i t a r e indicated by arrows i n f i g . 6 ). h O a n y c a s e , t h e i m p o r t a n t d e p o s i t i o n of f r e e carbon, predicted a t equilibrium f o r C4H10 concentrations higher than t h i s l i m i t , does not occur and an important departure from the equilibrium remains.

The f i g u r e 7 p u t s t o g e t h e r t h e 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 , t h e composition of t h e solid ( C/Ti and C1 content ) and the microhardness as function of t h e Tic1 concentration, f o r a c o n s t a n t value of 4 X O C 4 H equal t o 3. According t o f i g . 6, 2f?e C / T i r a t i o i n t h e s o l i d i n c r e a s e s s l i g h t l y a s ? i ~ 1 decreases. For X O TiC13 v a l u e s lower t h a n 0.01 i t appears t h a t t h e deposition r a t e i s ?imited by t h e concentratiofi of t h e titanium chlorides, and t h e c h l o r i n e c o n t e n t i n t h e s o l i d i s low. As X 0 Tic13 0 i n c r e a s e s , t h e d e p o s i t i o n r a t e reaches a maximum and t h e n s l o w l y d e c r e a s e s : t h i s r e j u l t confirms t h e f i r s t l e v e l of f i g u r e s 4 and 5 f o r t h e d e p o s i t i o n r a t e , and corresponds t o t h e k i n e t i c 1 im i t a t i o n by t h e deposition r a t e of t h e titanium attributed t o the reduction reaction. A t t h e moderate tempe- r a t u r e employed i t i s obvious t h a t t h i s l e a d s t o a higher i n c o r p o r a t i o n of c h l o r i n e i n t h e solid a s shown on f i g u r e 7.

The influence of t h e i n i t i a l conditions on the deposition mechanism will be more developped e l sewhere ( 9 ) .

(8)

.

^-^ClTi

.-.-

Deposition rate

Figure 7 : V a r i a t i o n s of t h e C/Ti r a t i o , t h e d e p o s i t i o n r a t e , t h e c h l o - r i n e content and t h e Vickers microhardness of the co t i n g s deposited a t

h

4 X0 C H = 3. 10'

,

T = 1123 K and P = 5 ? 3 J 0 ~ ~ 3 Pa versus t h e i n i t i a l subchlo- r i d e s mixture concentration.

V Optimization of the deposition process :

In o r d e r t o improve t h e c o a t i n g q u a l i t y , t h e v a r i a t i o n s of t h e C/Ti r a t i o v e r s u s t h e i n l e t gaseous composition and t h e c h l o r i n e conte'nt i n t h e s o l i d , t h e d e p o s i t i o n r a t e and t h e microhardness have t o be compared t o each other.

The existence of a certain chlorine amounts i n the titanium carbide leads t o a poor chemical s t a b i l i t y and t o poor coating properties. From f i g u r e 5, i t i s c l e a r t h a t t h e microhardness decreases when t h e C1 c o n t e n t i n c r e a s e s f o r a n e a r l y con t a n t value of C/Ti, i n t h e range 0.98

-

^{1 when 4}^{X O}^C ^H10 v a r i e s between 3. and 8. 10-I. The i n f l u e n c e of t h e C1 c o n t e n t i s confirmed by t h e $ i g u r e 7, a t l e a s t f o r X 0 T i C 1 3 - 0 5 0.06

.

When i t s value i s lower t h a n 0.5 at.%, t h e microhardness does n o t depend a l o t on t h e c h l o r i n e i n c o r p o r a t e d a s shown on fig. 7 f o r X O Tic1 G 0 . 0 6 . Hard t i t a n i u m c a r b i d e d e p o s i t s can t h e r e f o r e be d e p o s i t e d , a s expected, i f t h e &'content i s low and the composition near the stoichiometry. Such a r e s u l t i s o b t a i n e d f o r X 0 Tic1 O / X O C 4 H k Q < 1, t h a t i s a r a t i o Ti/C <1/4 i n t h e i n l e t gaseous mixture. Moreover the in?et Cl/H r a l o must be s u f f i c i e n t l y low t o l i m i t t h e chlorine incorporation b u t the deposition r a t e decreases a t low reactant concentrations as shown on f i g u r e

-

A compromise s o l u t i o n f o r t h e d e p o s i t i o n a t higher r a t e of a good d e p o s i t was found by increasing the titanium chloride and butane concentrations, the i n l e t r a t i o Ti/C being main- tained equal or lower than 1/4. As the C1 content increases w i t h X O Tic1 t h e threshold of 0.5 at.% C1 incorporated in t h e solid determines t h e upper l i m i t of the3.Pic1 concentration. Table I gives t h e r e s u l t s of some experiments carried out i n t h i s way, w%?ch leads t o very high microhardnesses.

(9)

Figure 8 : S.E.M. micrograph of a fractured c r o s s - s e c t i o n of a titanium carbide coating deposited on steel at moderate temperature a/ 1D23, b/ ID39

Figure 9 : S.E.M. micrograph of a fractured c r o s s - s e c t i o n of a titanium carbide coating aeposited on cemented carbide by a/ the moderate temperature process, b/ the classical C.V.D process.

(10)

TABLE I :

Values i n t h e range 4700

-

⁵⁰⁰⁰^kg.mm-2 were measured i n samples coated a t t h e s e s p e c i a l conditions (a1 so f o r one experiment r e p o r t e d on f i g u r e 8 ) ; t ey a r e s i g n i f i c a n t l y higher than t h e c l a s s i c a l r e orted microhardness of about 3200 k g . m i p f o r Tic. Values i n the range of 2600

-

3200 kg...-' were verified, with t h e same measurement conditions under 509 load, f o r l a y e r s d e p o s i t e d by t h e c l a s s i c a l C.V.0 process, from TiC14

-

^{C H 4}

-

^H2a t T >100O0C on cemented carbi de.

Sample number 1039 ? 20109 201 10

)r

TABLE I 1 :

These high values of the microhardness can be attribuated t o the grain refinement obtained with t h i s process a t moderate temperature, especially f o r the best conditions l i s t e d in t h e t a b l e I. The fractured cross-sections of two coatings deposited on s t e e l in t h e conditions of t a b l e I I a r e given on f i g u r e 8.

*

r = X0 TiC13.0 / X o C4H10

C1 a t .

0.3

+

^0.02

0.25

+

^0.03

0.16

+

^0.10

Sample N o 23 e x h i b i t s a q u i t e f i n e g r a i n b u t t h e C1 c o n t e n t i s very high. Sample ID39 was obtained w i t h i n the range of conditions which optimize t h e composition of t h e solid and t h e grain refinement. I t i s interesting t o note that the f i n e s t grain s i z e which can be obtained depends on t h e s u b s t r a t e . As t h e r e i s no d i f f e r e n c e i n t h e growth r a t e of t h e T i c l a y e r between s t e e l and cemented c a r b i d e coated i n t h e same c o n d i t i o n s and a s a d e c a r b u r i z a t i o n i n t e r l a y e r has never been observed neither on s t e e l nor on cemented carbide, t h e grain s i z e variations could not be l i n k e d t o t h e carbon d i f f u s i o n from t h e s u b s t r a t e . On cemented carbides t h e g r a i n s i z e does not change a l o t from t h e s u b s t r a t e t o t h e s u r f a c e ( f i g 9 a ) , but i t i s g e a t e r than on s t e e l , and the best microhardnesses are lower, in t h e range 4000

-

4300 kg.mm-', however t h e e f f e c t of c o n s t r a i n t s lower t h a n on s t e e l cannot be neglected.

Figure 9 b p r e s e n t s f o r comparison a f r a c t u r e of a Tic c o a t i n g d e p o s i t e d on a cemented carbide by t h e c l a s s i c a l C.V.D process.

X" TiC13.0

1.029 1.045 loa2 1.646

Microh rdness kg.,-

h

1160

+

⁶⁰

Sample number ID23

Finer grain s t r u c t u r e s obtained i n a single layer from t h i s process a t moderate temperature can be connected t o t h e f i n e - g r a i n e d s t r u c t u r e achieved by t h e d e p o s i t i o n of a m u l t i l a y e r coating and ( o r ) t h e use of extraneous dopants in the deposition process a t high temperature (10).

C/Ti

0.96

+

^0.02

0.98

-

+ 0.02 0.99

+

^0.02

V I Conclusion :

4 X O C4H10

5.791 5.795 lom2

8.522

X O TiC13.0

2.787

Titanium c a r b i d e c o a t i n g s can be d e p o s i t e d a t high r a t e , i n t h e range 5

-

10 microns p e r hour, a t a moderate temperature of about 850°C from butane instead of methane and a mixture of titanium chlorides prepared in-situ by the prereduction of the TiCT4 by Ti metal.

r*

0.711 0.721 0.773

Grow: r a e g.cm

'.,-I

0 . 8 1.01 1.33

From a comparison with a thermodynamic study, i t can be shown t h a t t h e deposition conditions and r e s u l t s d e v i a t e from e q u i l i b r i u m . The s u p e r s a t u r a t i o n a t the deposit surface together with the moderate temperature lead t o grain refinement. However, t h e f i n a l grain s i z e depends t o a c e r t a i n extent on the s u b s t r a t e and i t s preparation. When associated w i t h a low chlorine incorporation and a composition c ose t o stoichiometry, t h e f i n e s t s t r u c t u r e leads t o a very

h

high microhardness of 5000 kg.mm-

.

Microh rdness kg.,-

9

4950

+

²⁷⁰

4800

+

³⁵⁰

4800

+

²⁵⁰

4 X0 C4HI0

7.431

r*

1.5

C1 a t .

2.4

+

^0.27

C/Ti

0.99

+

^0.01

Grow! r a e g c

.

1.51

(11)

REFERENCES :

1 1 J. P. P i t o n , B. Ladouce, L. Vandenbulcke, Proceedings of t h e 6th European Conference on C.V.D., R. P o r a t , E d i t o r . I s c a r Ltd, Nahariya, I s r a e l (1987) p. 120.

2/ G. Eriksson, Chemica Scripta, 8 (1975) p.100

3/ E. K. Storms, " The r e f r a c t o r y c a r b i d e s

",

J. L. Margrave, E d i t o r . Academic P r e s s , New York and London (1967)

4/ K. Koyama and Y. Hashimoto, Nippon Kinzoku Gakkaishi, 37-4 (1973) p. 406.

5/ V. I. Malkin and V. V. Pokidyshev, Russian J. Phys. Chem., 45-8 (1971) p.1159.

6/ M. Hansen " C o n s t i t u t i o n of b i n a r y a l l o y s

",

Mc. Graw-Hill book Company Inc., New York, Toronto and London (1958).

7/ L. Vandenbulcke, Proceedings of t h e 8th International Conference on C.V.D., J. M. Blocher, Jr., G. E. V u i l l a r d and G. E. Wahl, E d i t o r s , The E l e c t r o c h e m i c a l S o c i e t y . Softbound S e r i e s , Pennington N. J. ( 1 981 ) p.32

8/ L. Vandenbulcke and G. V u i l l a r d , Proceedings of t h e 8th I n t e r n a t i o n a l Conference on C.V.D., J. M. Blocher, Jr., G. E. V u i l l a r d and G. E. Wahl, E d i t o r s , The E l e c t r o c h e m i c a l Society. Softbound S e r i e s , Pennington N. 3. ( 1 981 3 p.95

9/ B. Drouin-Ladouce, J. P. Piton and L. Vandenbulcke, t o be published.

10/ H. Van Den Berg, U. K6nig and N. R e i t e r , Proceedings of t h e 6th European Conference on C.V.D., R. P o r a t , E d i t o r . I s c a r Ltd, Nahariya I s r a e l (19871 p.114.