Chemical vapour deposition of silicon carbide and its applications

Chemical vapour deposition of silicon carbide and its applications

Thm Sohd Fthns, 126 (1985)313-318 METALLURGICALAND PROTECTIVECOATINGS 313 C H E M I C A L V A P O U R D E P O S I T I O N OF S I L I C O N C A R B I...

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Thm Sohd Fthns, 126 (1985)313-318 METALLURGICALAND PROTECTIVECOATINGS

313

C H E M I C A L V A P O U R D E P O S I T I O N OF S I L I C O N C A R B I D E A N D ITS APPLICATIONS* R, BR(JTSCH Veremtgte Drahtwerke A G, CH-2501 Btel (Swttzerland)

(Recel~ed August 12, 1984,accepted October 19, 1984)

The coating of various substrate materials with thin layers of silicon carbide (SIC) and its applications were investigated. SIC was prepared by a chemical vapour deposition process using a volatile sllane derivative, hydrogen and nitrogen in the temperature range 1073-1473 K. By changing the parameters of the chemical reaction, the composition of the deposit as well as the mechanical properties such as hardness can be changed. The results and some applications of SIC coatings on hard metals are presented

1. INTRODUCTION Silicon carbide (SIC) Is a material with some outstanding properties such as high hardness, good oxidation resistance at high temperatures and inertness in contact with acids or other corrosive media. Investigations of this material, particularly as a coating, seem to be promising with regard to possible applications. The only techniques for the preparation of SiC coatings are chemical or physical vapour deposnlon and sintering processes A detailed survey of the various conditions for the deposition of SiC layers has been given by Schlichting 1 Another fact which enlarges the field of possible applications is the phenomenon that S~C deposits obtained by chemical vapour deposition (CVD) are often not stoIchiometrlc but contain excess carbon or silicon. Therefore S1C layers with various properties can be obtained by controlling the deposition parameters. Thus it is necessary to establish the deposition conditions and properties of the whole range of SIC compositions to select a suitable coating for a specific application. 2 EXPERIMENTALDETAILS SIC layers were prepared by a CVD process in a resistance-heated reactor in the temperature range 1073-1473 K at atmospheric pressure. The gas phase contained a * Paper presented at the Sixth International Conference on Thin Films, Stockholm, Sweden, August 13 17, 1984 0040-6090/85/'$3 30

~ , Elsevier Sequoia/Printed m The Netherlands

R BRLTTSCH

314

volatile alkyl or a r y l c h l o r o s d a n e a n d h y d r o g e n as the r e a c t a n t s with m t r o g e n as the carrier gas A h y d r o c a r b o n was a d d e d m s o m e e x p e r i m e n t s to increase the c a r b o n c o n t e n t of the gas phase. The layers were d e p o s i t e d o n t o various s u b s t r a t e m a t e r m l s such as W C - C o h a r d metal, graphite, steel, s a p p h i r e a n d sdlcon nitrlde S c a n n i n g electron m i c r o s c o p y was used to examine the m o r p h o l o g y of the d e p o s i t e d layers. The sdlcon a n d c a r b o n c o n t e n t s were d e t e r m i n e d in the same i n s t r u m e n t using wavelength-dispersive X-ray fluorescence analysis. A G u l n i e r focusing c a m e r a was used for X-ray diffraction e x a m i n a t i o n of the d e t a c h e d a n d p o w d e r e d films. The Vlckers m l c r o h a r d n e s s of the samples was m e a s u r e d after p o h s h l n g the SIC surface wtth d m m o n d paste 3

R E S U L T S A N D DISCUSSION

SIC layers were d e p o s i t e d at g r o w t h rates of 5 - 3 0 Bm h - ~: the thickness of the final layer was of the o r d e r of 5 - 5 0 pm. T h e g r o w t h rate is p l o t t e d against the d e p o s i t i o n t e m p e r a t u r e for t r l m e t h y l c h l o r o s d a n e as the sllane agent m F~g. 1. The c a r b o n c o n t e n t of the d e p o s i t d e p e n d s on the [ C ] / [ S 1 ] ratio in the gas phase. This can be c o n t r o l l e d by selecting a p p r o p r i a t e sdanes or by a d d i n g a h y d r o c a r b o n to the reaction mixture. The [ C ] / [ S 1 ] ratio of the various alkyl or aryl c h l o r o s d a n e s used v a n e d from 1 to 12 If [ C ] / [ S 1 ] was 1 or 2, layers with excess sthcon were obtained. W h e n sllanes with [ C ] / [ S i ] = 3 were used Stolchlometrlc d e p o s i t s were p r e p a r e d , whereas for [ C ] / [ S 1 ] > 3 the resulting p r o d u c t c o n t a i n e d excess c a r b o n (Fig. 2). The second m e t h o d of c o n t r o l l i n g the c a r b o n c o n t e n t of the layer ~s to a d d a h y d r o c a r b o n to the reaction m i x t u r e In this w a y StolChlometnc S~C d e p o s i t s can be o b t a m e d even if silane agents with [ C ] / [ S i ] < 3 are used C o r r e l a t i o n between the d e p o s i t i o n t e m p e r a t u r e a n d the c a r b o n c o n t e n t of the layer was absent or p o o r m the t e m p e r a t u r e range investigated All o u r StC deposits s h o w e d n o d u l a r g r o w t h which started with a l m o s t s i m u l t a n e o u s f o r m a t i o n of a d j a c e n t nuclei a n d resulted in an o r i e n t e d c o m p a c t layer with a n o d u l a r surface which is typical of chemically v a p o u r - d e p o s l t e d films The */.CI

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Fig 1 Growth rate t,~ temperature of compact SIC layers on a hard metal surface Q, I , different positions m the reactor The sdane agent is tnmethylchlorosdane used under different deposition conditions FLg 2 Carbon content (m weight per cent) of SIC deposits t's the [C],'[SI] rat]o of the sllane agent

315

CVD OF S I C AND ITS APPLICATIONS

rate of formation of the nuclei, the growth rate and the crystalhte size were influenced by varying the reaction parameters Ftgure 3 shows the surfaces of some SIC layers deposited under various conditions. The SIC deposit can be pohshed to obtain a c o m p a c t s m o o t h surface (Figs. 3(e) and 3(f)). Its colour depends on the c a r b o n content of the layer: it is light grey if excess slhcon is present and darkens to black with lncreasmg c a r b o n content. The hardness of the SIC layers ts a function of thetr c a r b o n content and the

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Fig 3 Surfaces of SIC layers deposzted under various con&tlons (a), (b) effect ofdeposntlon temperature ((a) 1223 K, (b) 1323 K); (c), (d) effect ofsdane agent ((c) methyl&chlorosllane, (d) &methyl&chlorosdane); (e), (f) pohshed surfaces of SnC layers deposited on hard metal ((e) fracture. (f) coated edge)

316

R BRUTSCH

d e p o s i t i o n t e m p e r a t u r e M a x t m u m h a r d n e s s was o b s e r v e d for deposits c o n t a i n i n g 30°oC which c o r r e s p o n d s to the s t o i c h l o m e t n c c o m p o s i t i o n D e c r e a s i n g or increasing the c a r b o n c o n t e n t causes a decrease in the hardness. In the o b s e r v e d t e m p e r a t u r e range higher d e p o s i t i o n t e m p e r a t u r e s result in layers with higher h a r d n e s s Therefore the m a x i m u m h a r d n e s s is o b t a i n e d for samples with stolchiometric c o m p o s i t i o n depos]ted at t e m p e r a t u r e s a b o v e 1273 K. The m e a s u r e d Vickers m l c r o h a r d n e s s of these s a m p l e s is a b o u t 6000 H V 0.05 The r e l a t i o n s h i p between the c a r b o n content, the d e p o s i t i o n t e m p e r a t u r e a n d the resulting h a r d n e s s is shown in Fig. 4. T h e value of the hardness d e p e n d s on the l o a d , m e a s u r e m e n t s of hardness t e r s u s l o a d are shown m Fig. 5 for two samples The slope of the straight lines m h g 5 gives the e x p o n e n t n in M e y e r ' s law P =

ad"

where P is the l o a d m g r a m s force, d is the d i a g o n a l of the Vlckers p y r a m i d in m i c r o n s a n d a IS a c o n s t a n t n expresses the d e p e n d e n c e of the h a r d n e s s on the load. The resulting value of n for SIC is 1 81

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Fig 4 Vickers mlcrohardness of S]C layers on hard metal ts the carbon content (m weight per cent) and the deposlUon temperature -~, below 1273 K, O, abo~e 1273 K Fig S Vlckers mlcrohardness of ~anous S1C layers t s the load (0, excess sdlcon, I , sto~chlometnc composition) d Is the dmgonal of the V]ckers pyramid m mtcrons, the numbers indicate the value of the measured hardness X-ray p o w d e r diffraction p a t t e r n s of samples with Stolchiometrlc SiC deposits show b r o a d e n e d lines of cubic 13-S1C indicating a fine-grained crystalline d e p o s i t (Fig. 6). [3-SIC has a cubic d l a m o n d - h k e structure in which half the c a r b o n a t o m s are r e p l a c e d by silicon. W e d o n o t k n o w whether one of the so-called high t e m p e r a t u r e m o & f i c a t i o n s of SIC, which are s u m m a r i z e d by the term s-SIC, ls present in o u r s a m p l e s since It is &fficult to detect small a m o u n t s of el-SiC in the presence of I3-StC. F o r a similar reason no a t t e m p t has been m a d e to d e t e r m i n e the structure of the SdlCOn- or c a r b o n - r i c h deposits. M o r e i n f o r m a t i o n on this subject is given by o t h e r workers 3 5 S1C c o a t i n g s can be used as wear-resistant m a t e r i a l s and as p r o t e c t i o n against chemical a t t a c k or high t e m p e r a t u r e oxidation. A fine-grained layer is highly

317

CVD OF S I C AND ITS APPLICATIONS

111

200I

220J

1222

F~g 6 X-ra~ powder diffraction pattern of cubic ~3-SICdeposited at 1323 K (indexed using ref 2) desirable for wear resistance. Its hardness or brittleness can be a d a p t e d to the i n t e n d e d a p p h c a t i o n by selecting the a p p r o p r i a t e d e p o s i t i o n parameters. SIC is chemically inert m c o n t a c t with acids o r o t h e r corrosive m e d i a and can only be dissolved by o x i d i z m g melts or fluorine at 573 K. It provides excellent p r o t e c t i o n

(a)

(b)

(c) Fig 7 Hard metal parts coated with S1C (a), (b) nozzles, (c) hard metal watchcase with a pohshed SIC coating (photograph by courtesy of Rado Watch Co Ltd )

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R BRUTSCH

a g a i n s t high t e m p e r a t u r e o x i d a t i o n because of the f o r m a t i o n of SIO 2 layers which p r o v i d e a very high b a r r i e r against oxygen diffusion 3. The latter s t a t e m e n t is valid for bulk SiC. In the case of SIC thin films p r o b l e m s of stress a n d a d h e s i o n to the s u b s t r a t e have to be t a k e n into a c c o u n t H a r d metal, graphite, steel, s a p p h i r e a n d silicon nltrlde c o m p o n e n t s have been c o a t e d w~th c o m p a c t a d h e r e n t SIC coatings. H a r d metal is very well suited for SIC d e p o s i t i o n because the t h e r m a l e x p a n s m n coefficients of the two solids are in the range from 5 × 10 6 to 6 × 10 6 K ~. Figures 7(a) a n d 7(b) show two examples of a p p l i c a t i o n s of an SIC layer 20 ~tm think on h a r d m e t a l c o m p o n e n t s to p r o v i d e a w e a r - r e s i s t a n t c o a t i n g in a hot corrosive m e d i u m . T o o l steel can also be c o a t e d with SIC if the thmkness of the a p p h e d c o a t i n g is less t h a n 10 Bin. Both h a r d metal and steel need an i n t e r m e d i a t e layer of a suitable h a r d m a t e r i a l s which acts as a diffusion b a r n e r a n d a v o i d s i n t e r a c t m n s between SIC a n d the s u b s t r a t e SIC a n d the i n t e r m e d i a t e layer can be a p p l i e d in the same C V D reactor in one run Polished chemically v a p o u r - d e p o s l t e d SIC has a b n l h a n t black a p p e a r a n c e and can be used as a d e c o r a n v e s c r a t c h p r o o f c o a t i n g for h a r d metal watchcases for e x a m p l e (Fig 7(c)} REFERENCES 1 J Schhchtmg, Powder Metall lnt, 12(1980) 141,196 2 Powder Diffraction Fde, Joint Committee on Powder Diffraction Standards, Swarthmore, PA, Card 29-1129, 1979 3 E F l t z e r a n d D Kehr, 7hmSohdFdms, 39(1976) 55 4 M B o n n k e a n d E Fxtzer, BerDts¢h Keram Ges ,43(1966)180 5 J Chm. P K G a n t z e l a n d R G Hud~on, ThmSohdbhlrn~.40(1977) 57