GRAIN BOUNDARY STUDIES IN α SiC

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GRAIN BOUNDARY STUDIES IN α SiC

E. Laurent-Pinson, G. Nouet, J. Vicens

To cite this version:

E. Laurent-Pinson, G. Nouet, J. Vicens. GRAIN BOUNDARY STUDIES IN α SiC. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-221-C1-226. �10.1051/jphyscol:1990134�. �jpa-00230292�

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COLLOQUE DE PHYSIQUE

Colloque Cl, suppl6ment au nol, Tome 51, j a n v i e r 1990

GRAIN BOUNDARY STUDIES IN ct Sic

E. LAURENT-PINSON, G. NOUET and J. VICENS

L a b o r a t o i r e d l E t u d e s e t d e R e c h e r c h e s sur l e s M a t e r i a u x , CNRS URA 1 3 1 7 , I n s t i t u t d e s S c i e n c e s d e l a M a t i e r e e t d u R a y o m e m e n t , B o u l e v a r d d u Marechal J u i n , F-14032 Caen C e d e x , F r a n c e

Resume

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Une description des joints de grains dans cc Sic basee sur les modeles de coincidence est proposee pour les deux polytypes 4H, 6H et egalement pour les interfaces entre deux polytypes differents. Les listes de coincidence etablies pour les differents cas sont comparees aux resultats des relations dforientation entre cristaux dans differents ma,teriaux Sic.

Une deviation assez importante par rapport aux coincidences exactes a ete notee. La determination du plan des joints et de la relation d'orientation ont permis de mettre en evidence quelques caracteristiques particulieres a a Sic.

Abstract

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A description of grain boundaries in a Sic based on coincidence orientations is proposed for the two 4H and 6H polytypes and grain boundaries between two different polytypes. The lists of coincidence orientations established for the different cases are compared to experimental orientations performed in different Sic materials. A deviation from the coincidence orientations rather high has been found. The determination of the grain boundary plane and the relationship between the two crystals lead to some conclusions characteri- zing a Sic materials.

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INTRODUCTION

Tridimensional coincidence orientations can be calculated in the hexagonal symmetry by assuming a ( ~ / a ) ~ = p/v rational value /1,2,3/.

A real hexagonal cr stal is always described by a c/a parametric ratio for which (c/aIY has not a rational value. On a first approxi- mation the relationship between grains are calculated with one or several p/v values close to the real value. This model has been pre- viously applied to ceramic materials A1203 /4,5/, Si3N4 /6/ and com- posite materials such as WC-CO /7/. Experimental coincidence descriptions could be obtained between crystals in rhombohedral A1403 /5/ and between hexagonal tungsten carbide crystals /7/. A deviation from coincidence orientations has been observed between hexagonal Si,N, crystals 6 The aim of this work is to obtain the list of theoretical coincidence orientations for the two main polytypes 4H and 6H observed in a Sic and to compare with experimental orientations.

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^MATERIALS

Silicon carbide has been supplied by the Elektroschmelzwerk Company.

Hot pressed Sic has been sintered with amounts of 0.3% or 1.5% A1 based additives at about 2000°C. As a consequence a glassy phase

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990134

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Cl-222 COLLOQUE DE PHYSIQUE

(aluminosilicate) is formed and retained mainly at the triple junc- tions of the grain boundaries. Thin foils have been prepared by ion milling (~r', 6 KV) for TEM studies (Jeol 120 CX)

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THEORETICAL COINCIDENCE DESCRIPTIONS IN a Sic POLYTYPES

Descriptions of grain boundaries on the basis of coincidence orientations involve a choice of a (c/a12 = p/" rational value. The different p/v values close to the (c/a)' value have been calculated with a maximum deviation of 2% for the two 4H (c/a = 3.271) and 6H

(c/a = 4.907) polytypes.

Results have shown that three ratios give a good description of coincidence orientations in the 4H polytype : p/v = 21/2, 32/3 and 54/5. In fact they give rise to a high number of coincidence des- cri tions and are very close to the real c/a value. The deviation (

P

^p/v

^-

^c/a)^/^c/a^% with respect to the c/a value has been plotted in Fig. 1.

Only one ratio p/v = 24/1 has been used to obtain coincidence descriptions in the 6H polytype. The deviation with respect to the exact c/a ratio is only 0.2% (4.895 instead of 4.907). Moreover this ratio generates a high number of coincidence descriptions.

A common cell between the 4H and 6H polytypes can be defined with a * = a,, = a6, and c = 3~~~ = 2c6". A list of coincidence orienta- tlons between the two different polytypes has been obtained by using this new cell a and c. In this case the p/v ratio is equal to 96/1 Fig. 2 illustrates for a same orientation (7O053 around the <1120>

rotation axis) the CSL formed between two 4H crystals (Z36), two 6H crystals (Z6) and between 4H and 6H crystals (Z12) both representa- ted in the common cell a and c ^(p/v = 96/1). If the nodes of the two hexagonal cells 4H and 6H are taken into account for this last case a new CSL is created (Fig. 3). It is defined with respect to the 4H and 6H polytypes by two Z values Z t b H = 18 and ,XI6, = 12.

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EXPERIMENTAL ORIENTATIONS

About 60 experimental orientations have been determined for grain boundaries between two 4H, two 6H or 4H and 6H grains in different Sic materials. Table 1 gives some examples with the corresponding Z values. The deviation from the exact coincidence orientation A0 has been indicated in table 1 as well as the nature of the grain boundary plane. Even if the deviation from the coincidence orientation rather high in many cases, examination of the grain boundary plane and the relationship between the two crystals lead to some comments.

Two types of grain boundaries will be described in this work.

4 . 1

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( O O O l ) , , g r a i n boundarv plane : Some orientations have a common grain boundary plane. Two cases Z19 (n02) and Z7 (n03) reported in table 1 correspond to a twist orientation with a (0001) grain boundary plane. The stereographic projection (Fig. 4) is given for the 219 (n02) with a micrograph illustrating the facetted 219 grain boundary. One facet a is parallel to the basal plane and viewed parallel to the electron beam in Fig. 5. The orientation of the two crystals reveals a high tilt deviation (3") from the exact Z19 coincidence orientation (p/v=96/1). On the contrary there is a very low twist deviation (<lo)

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4.2

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^(10i4Il2srain boundary plane : The (10i4),~ common plane (n013) appears likely as a low energy configuration close to a 26 orientation (6H polytype). The grain boundary plane has been found perpendicular to the [4401] rotation axis. Previous results performed in Sic bicrystals obtained by sublimation have revealed the same Z6 orientation with an asymmetric configuration (OOOl)l//(10~2)z

/a/.

The symmetric (10i4),~ grain boundary plane found in sintered a Sic can be favorised by the dense atomic arrangement of this plane.

The equivalent Z36 orientation has not been observed in the 4H polytype. From previous observations, three different configurations can arlse in a 4H-6H grain boundary between two crystals rotated by 70°53 around the <1120> axis.

Fig. 6 gives the three possibilities. The first one (i) parallel to a dense CSL plane appears as the favoured configuration. A grain boundary close to this configuration has been experimentally observed (n020) 212, p / v = 96/1.

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CONCLUSIONS

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Results show that the experimental orientations deviate greatly from the exact coincidence orientations in a Sic.

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However in many cases the boundary plane is found parallel to the densest plane (0001) which is the main facet formed in this material after sintering. The most common observation consists of a boundary plane parallel to the basal plane for one crystal and parallel to higher indice planes for the other crystal. Some dense planes have also been found parallel to the basal plane. They have not been discussed in the present work.

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Two special twist grain boundaries have been observed. One has a common basal plane (219). The other grain boundary is parallel to the (1074) plane (Z6) which appears as a low energy configuration for the 6H polytype. Conversely for the same orientation an asymmetric grain boundary (Z12) seems to be favoured.

REFERENCES

Warrington, D.H., J. Phys. 36-C4 (1975) 87.

Bleris, G.L., Nouet, G., Hagege, S. and Delavignette, P., Acta Cryst. A38 (1982) 550.

Grimmer, H. and Warrington, D.H., Acta Cryst.

A43

(1987) 232.

Morrissey, K.J. and Carter, C.B., J. Am. Ceram. Soc. 67 (1984) 297.

Lartigue, S., These de Doctorat dfEtat, Orsay 1988.

Lay, S., Nouet, G. and Vicens, J., to be published.

Vicens, J., Laurent-Pinson, E., Chermant, J.L. and Nouet, G., J. Phys. =-C5 (1988) 271.

Ichinose, H., Inomata, Y. and Ishida, Y., ceramic microstruc- ture, Role of interfaces, Mat. Sci. Res., edited by Pask, J.A., Evans, A.G., 21 (1986) 255.

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COLLOQUE DE PHYSIQUE

Fig. l : Deviation (\lrr/y - c/a)/c/a ( % ) with respect t o t h e c/a value f o r the 4 H polytype. Different ~ L / L , values have been plotted on the figure.

(

^#

I 1

^A

1

Grain boundary plane Grains rotated 1

around a common 2 [0001] a x i s I

,

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Table 1 : Experimental orientations.

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Fig. 2 : CSL between two 4H crystals ( Z 3 6 ) , two 6H crystals ( Z 6 ) and 4H and 6H crystals ( f 1 2 ) ( ~ / v = 9 6 / 1 ) for the same rotation 70'53 around the < 1 1 2 0 > axis (perpendicular to the drawing.

Coincidence nodes Q ; crystal l + ; crystal 2 0 0 ; dis- tance from the drawing plane : 0

+ 8

^:a / 2 *0.

Fig. 3 : CSL found between one 4H and one 6H crystal in the same orientation of Fig. 2. All the nodes of the two hexagonal cells have been denoted. Thus two Z values with respect to each polytype have to be given : = 1 8 and E r , # = 12.

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Cl-226 COLLOQUE DE PHYSIQUE

[0001] nearly common axis. The pole of the grain boundary plane a has been indicated.

Fig. 5 : Grain boundary between 4H and 6H grains corresponding to the orientation shown in Fig. 4. The grain boundary plane is parallel to the electron bean.

Fig. 6 : Different configurations proposed between 4H and 6H crystals in a X = 12 coincidence orientation ( p / v = 96/11 (0001), , Q ( 1 0 i 2 ) ~ , appears as the most favoured configura- t ion.