MICROSTRUCTURE OF Si3N4 FILMS DEPOSITED ON VARIOUS SUBSTRATES BY CVD

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MICROSTRUCTURE OF Si3N4 FILMS DEPOSITED ON VARIOUS SUBSTRATES BY CVD

F. Anxionnaz, M. Parlier, A. Riviere, M. Lancin

To cite this version:

F. Anxionnaz, M. Parlier, A. Riviere, M. Lancin. MICROSTRUCTURE OF Si3N4 FILMS DE- POSITED ON VARIOUS SUBSTRATES BY CVD. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-303-C1-308. �10.1051/jphyscol:1986144�. �jpa-00225574�

JOURNAL DE PHYSIQUE

Colloque C1, supp16rnent au n02, Tome 47, f6vrier 1986 page c1-303

MICROSTRUCTURE OF Si,N, FILMS DEPOSITED ON VARIOUS SUBSTRATES BY CVD

F. ANXIONNAZ, M. PARLIER*, A. RIVIERE and M. LANCIN

Laboratoire de Physique des MatBriaux, C.N.R.S., 1 , place Aristide Briand, F-92195 Meudon Cedex, France

'service O M , ONERA, 29, Avenue de l a Division Leclerc, F-92320 Chdtillon, France

Resume

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Nous avons prepare des bicouches afin d16tudier les relations qui GZG'Ent entre les proprietCs mecaniques et la microstructure de composites Sic-Si3N4. Des couches de Si 3N4, obtenues ?I basse pression, par decomposition de Si(CH3)4 dans une chambre isotherme

a

1300°C, ont ete deposees sur du Sic monocristallin ou fritte. Nous avons determine leurs structures cristallines par diffraction des rayons X. L1influence du substrat sur leur microstructure a &t& etudiee par microscopie electronique ?I balayage ou

a

transmission.

Abstract

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To study the relationships between the mechanical properties and t h e r o s t r u c t u r e of Sic-SigNq composites, bi 1 ayers were prepared. By decomposition under low pressure of Si(CH3)4, in an isothermal chamber at

1300°C, layers of Si3N4 were deposited on single crystals of Sic and on sintered Sic. Their crystalline structures were determined by X-rays diffraction. The influence of the substrate on their microstructure was studied using scanning or transmission electron microscopy.

I

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INTRODUCTION

Among the few obstacles which still prevent ceramics to be used in real systems, brittleness is the most critical. Attempts are made to reinforce these materials with refractory fibers in order to increase the energy of failure. The chemical vapour deposition (CVD) is one of the most efficient tools to infiltrate complex fibers structures. The performances of the composites depend on the interface fiber-matrix and on the microstructure of the matrix. The study of pyrolytic deposits performed on flat substrates, is a necessary step prior to investigate more complex materials. This communication deals with the characterization of pyrolytic Si3N4 deposited on various substrates. After a short description of the experimental procedure, it describes the microstructural investigations.

I 1 - EXPERIMENTAL PROCEDURE FOR CHEMICAL VAPOUR DEPOSITION

The experimental set up has been designed to prepare ceramic matrix composites by infiltration of fibers structures : To obtain a well stabilized (?l0C) and homogeneous temperature in the reaction chamber, a graphite succeptor is heated using a radio frequency generator; a constant growth rate is achieved by monitoring the gases flow rates thanks to three mass flowmeters.

The best conditions for coating complex shapes or infiltrating fibers structures have been determined : A mixture of Si(CH3)4 and NH3, with an atomic ratio N/Si=5,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986144

c I - 3 0 4 JOURNAL DE PHYSIQUE

was used as a precursor; the pressure was equal to 66 Pa and the deposition rate to 10 prnh'l C1,21.

Pyrolytic Si3N4 has been deposited on various flat substrates : Graphite has been used for the initial characterizations, aSiC single crystals and a sintered Sic (grains size ^% 100 vm) for the microstructural studies.

I11

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RESULTS

I11

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¹

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Purity and some physical properties

The following characteristics have been obtained on Si3N4 deposited on graphite : (i) ESCA studies show that pyrolytic Si3N4 is stoichiometric : The ratio of the pic heights, corresponding to the binding energies of the electrons 2s in Si and Is in N, is equal to 0.76. Chemical analysises agree with the result.

(ii) The density of the deposits is greater than 0.97dth when the deposition rate is inferior to 30 wnh-1.

(iii) The mean dilatation coefficient, measured parallely to the growth surface is equal to 4.10-61"~.

I11 - 2 - Microstructure

The microstructure of the deposits is independent of the crystallinity of the Sic substrates. On the contrary, it depends strongly on P (pressure), T (temperature), t (time) and on v (speed).

11 1

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2.1

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Mqcygskyustuyu-gf -$Qg-d,gegs_i$s_-eygpa_ygi-;~ig[-$Qg-~~;i$i~gi-~~~ii;~g~~

Under such conditions (T=1300°C, P=66 Pa, t=8h, v=10 vm h-l), the deposits always consist of two layers. Both layers are constituted of a S i 3 ~ 6 but they differ by their microstructure :

Figure 1

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Scanning electron micrographs of Sic-Si3N4 prepared as described in 111-2.1. The cross section has been obtained by fracture of the material. Numbers 1 and 2 refer to the a-SigN4 layers and 3 to the Sic substrate (here sintered Sic).

(i) Layer 1 :

(a) It is dense and homogeneous and covers the whole substrate; no cavities or flaws are visible (fig.1).

( 6) Its surface, rather smooth, may be described .as the intersection of

portions of sphere (fig.2). ^o

(Y) It consists of very mall grains; the grains size may reach 500 A but, it

i

generally does not exceed 100 (f ig.3).

( 6 ) The binding between Sic and Si3N4 resists to the thermal stresses which occur during the cooling stage, after the CVD process. On the contrary, it may break when strong mechanical stresses are applied on the composite (eg.: during the mechanical polishing, fig.3).

Figure 2

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Scanning electron Figure 3

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Cross section of the material micrograph of the surface of a Si3N4 prepared as described in 111-2.1. The deposit : it shows the layer 1 interface Sic-Si3N4 is perpendicular to partly recovered by the layer 2. The the transmission electron micrograph. The surface of the layer 1 consists of diffraction diagram corresponds to an area bubbles looking like hyperboloids of of 0.3 ~ m 2 and it is typical of a grains

revolution. size < 100 1.

(ii) Layer 2 :

( a) The layer consists of irregularly shaped columns, the boundaries of which are revealed by the fracture (fig.1). The columns, first distinct from one another, enclose cavities near their feet. At 10 to 20 um from the interface, they join one another and form a dense material. Their diameter varies from 20 to 50 pm.

( 8 ) The surface is constituted of polyedric crystals, with sharp angles, the size of

which is roughly equal to 10 pm (fig.4). Some hexagonal features are visible. The surface does not exhibit porosity; this confirms the above observation.

( r ) The columns are constituted of grains, the size of which varies from a few microns to 10 microns (fig.3). The orientation relationships between the grains will be described elsewhere.

( 6 ) Due to the cavities, the adherence of the layers 1 and 2 is not excellent : During the cooling stage, the composite sometimes breaks up. X-rays diagrams show that the fracture develops in between layers 1 and 2. Scanning electron micrographs confirm the X-rays analysis. They also clearly demonstrate the morphology of the layer 2 at the interface with the layer 1 (fig.5).

( E ) A preferred crystal growth direction is suggested by the X-rays diagrams : The

relative peak heights have not the values characteristic of an equiaxe material. The diagrams obtained on the surface of the layer 2 show that the peaks {I071 ) and (3032) are higher than they should be. The diagram performed on the opposite side when the layer 2 is separated from the substrate, demonstrates that the same peaks but also the peak (0001 1 are reinforced.

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Figure 4

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Micrograph of the surface Figure 5

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Micrograph of the bottom of the of Si3N4 prepared as described in layer 2 in contact with the layer 1 : 2 111-2.1. The layer 2 represented is separated from 1 during cooling.

90 um thick.

Some experiments have been carried out to observe the growth of the layer 2.

Deposits have been prepared under the same conditions than in 111-2.1, but during shorter times. As shown in figure 6, overlapping crystals are constituting blocks, the diameter of which can reach 50 vm. The crystals are delimited by {1120), .(I1011 and f0001) faces.

Figure 6 - Micrographs representing the growth of the layer 2 on the layer 1.

Deposit has been performed at ~=1300"C, P=66 Pa, v=10 pn.h-1 and t=4h.

a) part of the layer the most dense;

b) the less dense;

c) magnification of the crystals constitu- ting the layer.

Figure 7

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Micrograph of Si3N4 deposit obtained when turbulances occur. The thickness of the layer 2 is equal to 120

m.

11 1

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²

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³

- S13N4-deeoslts-obtai!!ed-w,he!!-gas-f lowwr!Ist~_rba~sesS_ass!~

In the part of the chamber directly exposed to the entrant gas flow, the substrate is in contact with the gases the richest In initial species. Besides, important turbulences occur there. In such an area, the deposit consists also of two layers of a Si3N4 but the microstructure of the layer 2 is totally different from that previously described (fig.7) :

(i) It is constituted by honeycombed blocks.

(ii) Its growth seems to involve screw dislocations.

(i i i) Few hexagonal crystals are growing competitively.

IV

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DISCUSSION

The microstructure is independent of the crystalline structure of the Sic substrates.

On the contrary, it depends strongly on the location and size of the substrates.

Deposits obtained on materials centred in the chamber exhibit the microstructure described in 111-2.1. However, their lower part, more exposed to the entrant gas flow, may have the feature showed in 111-2.3. Such a microstructure is also obtained in the whole deposit when the substrate is too large, that is to say when the substrate introducesturbulences in the gas flow.

When a substrate, of the right dimension, is centred in the chamber, the micro- structure of the deposit is reproducible. The process occurs as follow :

(i) Deposition of a layer of 1 or 2.pm, Constituted with tiny little crystals (0 ^Q,

100

8 ) .

The surface consists of bubbles which intersect one another. These bubbles

look like hyperboloids of revolution. It is known that hyperboloids develop when the grains size is smaller than a critical value. Therefore, the feature of the surface indicates that the grainssize is inferior to that critical value. Such a growth has been observed by one of the authors when' Sic was prepared in the same chamber, starting from the same precursor C11. The layer 1 should be formed before the CVD steady state. This would mean that the kinetic for the deposition is much quicker during this period of time than during the steady state.

(ii) Deposition of a layer constituted of grains the size of which is roughly equal to a few microns : such a deposit should be formed in the centre of the chamber, when the CVD process is stabilized. First, small hexagonal crystals develop on the layer 1. The C axes point in the growth direction of the deposit (fig.6). Then, the small crystals act as preferential sites for the Si3N4 deposition. Blocks of crystals are formed and soon, join one another. On this uneven surface, grows a dense deposit, which is more and more equiaxial. (When the whole layer reacher90 pm, a texture is still visible, but on the X-rays diagrams the C axis is no more reinforced). A detailed analysis of the texture wi 1 1 be pub1 ished elsewhere.

For deposits thicker than 40 to 50 um, the surface consists of an almost flat pavement (fig.4). The same feature is observed with Sic or graphite C21 substrates.

Pyrolytic crystallised ^ct -Si3N4 prepared by Niihara and Hirai 131 or Galasso C41 exhibit similar aspect. The deposits prepared by Niihara and Hirai are very thick (4.6 mm); this certainly explains why hexagonal crystals are no more visible.

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Precursors and experimental conditions being different in these works, it is likely that this feature is characteristic of pyrolytic crystallised Si3N4.

The dilatation coefficients of pyrolytlc Si3N4 [4.10-6/~~1 and of Sic [~.Io-~/"cI are similar. This provides a good thermal stability to the composite. The weak points which possibly lead to the fracture of the Si3Nq deposit are related to the cavities. Experiments are developed to eliminate these defects. The fracture which sometimes occurs between Sic and Si3N4 shows that the binding between the two materials is rather weak. Such a weak binding is a quality required in ceramic compounds in which fibers and matrix are brittle.

REFERENCES

[I1 PARLIER M., These de Docteur Inggnieur, PARIS VI, Septembre 1984

C21 PARLIER M., BIND J.M., Euro CVD five, proceeding of the f ith european conference on CVD, Uppsula, 18-20/6/1985, Sweden, Ed. J.O. Carlson and J. Lindstram

C31 NIIHARA K . , HIRAI T., J. Mat. Science 1 1 (1976), 593-603 C41 GALASSO F.F., Powder metal lurgy international, 1 1 ( 1979) , 7-9