High Resolution Transmission Electron Microscopy of Grain Boundaries between Hexagonal Boron Nitride Grains in Si 3. SiC Particulate Composites

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1 Cryst. Res. Technol. K. M. KNOWLES, S. TURAN* Department of Materials Science and Metallurgy, Cambridge, U.K. *Department of Ceramic Engineering, Eskisehir, Turkey High Resolution Transmission Electron Microscopy of Grain Boundaries between Hexagonal Boron Nitride Grains in Si N SiC Particulate Composites Dedicated to Prof. Dr. J. Heydenreich on the occasion of his th birthday High resolution transmission electron microscope observations of grain boundaries in hexagonal boron nitride in which adjacent grains are rotated with respect to one another about either <> or <> directions are shown to be free of intergranular glass, in agreement with previous work. The implication of these observations is that the solid-solid boundary energies of such grain boundaries in hexagonal boron nitride are relatively small in magnitude. Keywords: hexagonal boron nitride, transmission electron microscopy, grain boundaries (Received June 9, ; Accepted July, ) Hexagonal boron nitride powder (h-bn) is an important industrial material because of its attractive lubricant properties and its high thermal conductivity. It has a graphite-like layered structure in which the bonding is covalent (PEASE 9). Hence, it is difficult to sinter h-bn to full density without using both high pressures and sintering aids. Thus, for example, dense h-bn samples prepared for a recent internal friction and microstructural study were obtained by hot pressing h-bn powder compacts with additions of. wt% B O and. wt% CaO at 9 C and MPa pressure (PEZZOTTI et al. 99). In this case, the boron oxide and calcium oxide additions form a low-melting point Ca/B-containing glass which aids densification at high temperatures, but which is retained as an unwanted glass after sintering. An alternative process route, in which h-bn powders are ground and purified by being treated in methanol prior to pressureless sintering, has also been shown to produce h-bn compacts with relatively good bending strengths, despite their low density (MIYAZAKI et al. 99; HAGIO and YOSHIDA 99). In this latter process route, boron oxide residing as an impurity in the powders is thought to play an important rôle in both densification and grain growth (HAGIO and YOSHIDA 99). There is very little literature on transmission electron microscopy (TEM) observations of boron nitride grain boundaries. Room temperature TEM observations by PEZZOTTI et al. (99) of their dense h-bn samples are reported to show that the plate-like h-bn grains were not wet by the Ca-containing glass phase, which was confined to triple junctions. However, their internal friction measurements at elevated temperatures showed evidence for grain boundary sliding, consistent with the remanent intergranular glass melting at a relatively low temperature and wetting the h-bn grain boundaries. MIYAZAKI et al. (99) have also reported transmission electron microscopy observations of grain boundaries in h-bn and concluded on the basis of their bright field TEM observations that h-bn grains were in contact with one another. However, since they did not use other, more sensitive, TEM techniques, such as dark field imaging or high resolution transmission electron microscopy

2 K. M. KNOWLES, S. TURAN: Grain Boundaries between Boron Nitride Grains (HRTEM) to examine their h-bn grain boundaries, their work cannot be regarded as giving unequivocal evidence for the absence of thin intergranular glassy films ~ nm or so in thickness at the boundaries. In a previous article (TURAN and KNOWLES 99a), we have reported the existence of sub-micron sized h-bn inclusions in Si N -SiC particulate composites prepared by hot isostatic pressing at K under MPa pressure. The inclusions arose indirectly during the densification process from boron oxide present on the surface of particles of h-bn. These h-bn particles were sprayed onto the internal surface of the tantalum can used to contain the ceramic powders during the hot isostatic pressing procedure in order to prevent an unwanted chemical reaction between SiC and the tantalum can. Although the h-bn inclusions were mostly isolated from each other, a few examples were found in which we were able to examine grain boundaries between two h-bn inclusions by HRTEM in a JEOL EX-II TEM. This microscope has a spherical aberration coefficient, C s, of mm and a point-to-point resolution of. Å. In each case examined, the boundaries were high angle tilt grain boundaries with the axis of rotation parallel to, or almost parallel to, a low index direction in the h-bn () basal plane. As a consequence, HRTEM images could be readily obtained with the () planes from each grain parallel to the electron beam. An example of a boundary where the c-axes of two h-bn inclusions were aligned almost perpendicular to one another is shown in Fig.. These two inclusions were inside a C SiC grain. Each h-bn inclusion was oriented with respect to the C SiC so that the () basal plane was parallel to a {} C SiC plane, with a {} h-bn plane almost parallel to a second {} SiC plane. In this figure the beam direction is approximately parallel to a common [] h-bn direction, so that the only () planes and () planes can be resolved when they are both accurately parallel to the electron beam (TURAN and KNOWLES 99b). The detail in the image is sensitive to small rotations: a rotation of about [] h-bn of the electron beam away from [] is sufficient to change the image into a onedimensional image with only the () planes resolved (TURAN and KNOWLES 99b). It is apparent from Fig. that these two h-bn inclusions do not have a thin amorphous intergranular film between them, despite the presence of amorphous film in the vicinity of the particles evident in the far right and left hand side of the figure. The clear implication is that despite the orientation relationship forced on the particles by virtue of their individual orientation relationships with the surrounding SiC matrix, it has been favourable energetically on cooling the ceramic from the processing temperature to expel liquid from this boundary, on the grounds that the solid-solid interfacial energy, γ b, is less than that of the wetted boundary, γ, where γ is the liquid-solid interfacial energy. This initial observation is also consistent with the report by PEZZOTTI et al. (99) of h-bn grain boundaries free of glassy films. A further interesting aspect of the grain boundary in Fig. between the two inclusions is the faceted nature of the boundary. The longer facets are parallel to () of grain h-bn and () of grain h-bn. The shorter facets make an angle of ~ 9 with () of grain and ~ with () of grain. These angles compare well with calculated angles of () : ( ) =.9 and () : ( ) =. for assumed h-bn lattice parameters of a =. Å and c =. Å. A second example of a h-bn grain boundary between two inclusions arising from an orientation relationship forced on the particles by virtue of their individual orientation relationships with the surrounding C SiC matrix is shown in Fig.. In this figure the common zone axis parallel to the electron beam is again [] h-bn. Here, each inclusion is oriented with respect to the matrix so that for each particle [] h-bn is parallel to []

3 Cryst. Res. Technol. () - C SiC, and the () basal planes of each particle are parallel to the two {} C SiC planes in the [] zone. Thus, the angle between the () h-bn planes of each grain is cos - ( /) = 9.. As in Fig., it is evident that the grain boundary between the inclusions is glass-free. Trace analysis of the boundary orientation shows that it make an angle of. with the () planes of grain h-bn and with the () planes of grain h-bn. Thus, to a fair approximation, the boundary plane is ( ) with respect to inclusion and () with respect to inclusion, since for h-bn ( ) : () =. and () : () =.. Fig. : HRTEM image of a grain boundary between two h-bn grains labelled h-bn and h-bn rotated ~ 9 with respect to one another about a common [] direction parallel to the electron beam. Areas such as those boxed in the two grains have strong local changes in contrast indicative of local changes in the stacking of () planes. In contrast to Figs. and, in which the orientations between h-bn inclusions are forced as a consequence of preferred orientation relationships with the surrounding C SiC matrix, the grain boundaries in Figs. and arise in single particles. In these two figures the electron beam is parallel to a common [] zone. The angles between the () planes either side of the grain boundaries are ~ and respectively. In both cases the grain boundaries are close to a common () plane, and the orientation relationship is close to the reported twinning mode of h-bn (KURDYMOV et al. 9; HIRAGA et al. 99; TURAN and KNOWLES 99b), in which () is the twin plane, so that there is an angle of between the () planes of twinned grains. As in Figs. and, there is no evidence for glassy films at either of the boundaries. The contrast in the bright field micrograph in Fig. highlights the most commonly observed boundaries seen in h-bn: very low angle () twist boundaries parallel to the () planes bounding the grains. We have discussed these twist boundaries elsewhere (TURAN and KNOWLES 99b). Significantly, these are also free of intergranular glassy films. In each of these examples, additional contrast in the HRTEM images from the h-bn grains is generated from severe deformation of the grains arising from the application of shear stresses and compressive stresses acting on these particles either during growth at the

4 K. M. KNOWLES, S. TURAN: Grain Boundaries between Boron Nitride Grains processing temperature or during cooling of the composite ceramic to room temperature from its processing temperature (TURAN and KNOWLES 99b). Similar microstructural complexity has also been reported recently in ball milled h-bn (HUANG et al. 999, ). Areas such as the small rectangular regions highlighted in Fig. have very local changes in contrast indicative of local changes in the stacking of () planes, and elsewhere there is evidence for dislocation loops with ½ [] Burgers vectors. Fig. : HRTEM image of a grain boundary between two h-bn grains rotated ~ with respect to one another about a common [ ] direction parallel to the electron beam. The grain boundary has faceted onto low index planes of the two grains. Fig. : [ ] HRTEM image from a h-bn inclusion where the () planes make an angle of ~ across the boundary between the two parts of the inclusion. The geometry here is similar to that of kink bands formed in these materials. The observation of these glass-free grain boundaries in h-bn is consistent with the conclusions of previous, less comprehensive, work (MIYAZAKI et al. 99; PEZZOTTI et al. 99). These observations are in sharp contrast to boundaries between grains of silicon nitride, in which it is common to observe thin intergranular films at high angle grain boundaries.

5 Cryst. Res. Technol. () - uvw θ d () Σ [U] F ( Σ) ε, ε, ε M M Table : Selected near-coincident cells for h-bn. Computations performed for rotations about the three index vectors [] and [], corresponding to the four-vectors [] and [] respectively. The calculation variables used were Σ =, L =. Å, S max max =., u =. and θ =. rad in the nearcoincidence site lattice notation used by BONNET and COUSINEAU (9). The lattice parameters for h-bn were taken to be a =. Å and c =. Å. The columns of M and M define base vectors of possible nearcoincident cells for h-bn grains and respectively. ε, ε, and ε are the principal strains which when applied to the near-coincident cell for grain produce the shape of the near-coincident cell for grain. [U] F relates the components of vectors referred to the lattice of grain to components referred to the lattice of grain through the equation v = [U] v. [uvw] and θ d are the pairs of disorientation axis and angle for which a cell M is near coincident to a cell M (BONNET and COUSINEAU 9). For each disorientation θ d in the table, a symmetrically equivalent NCSL description occurs about the axis specified at a disorientation of θ d.

6 K. M. KNOWLES, S. TURAN: Grain Boundaries between Boron Nitride Grains Fig. : (a) Bright field TEM image of variants h-bn and h-bn. Each variant is bounded by () planes. The angle between the basal planes in the variants is ~. (b) A schematic of the [] diffraction pattern from the inclusions in (a). This shows that the () planes from these variants are almost parallel, so that at this orientation, the variants are close to the deformation twinning relationship reported for h-bn.

7 Cryst. Res. Technol. () - Apart from the interface in Fig. (and possibly Fig. ), where it can be argued qualitatively that the boundary energy can be expected to be low because of the twinning relationship between the two grains, there is no reason to believe that the boundaries in the other figures are particularly special. Although both boundaries in Figs. and are faceted, consistent with a lowering of the boundary energy, neither boundary arises at an orientation relationship between h-bn which has any three dimensional near-coincidence of supercells of the h-bn crystal structure: near-coincident site lattice calculations shown in Table that we have undertaken using the algorithm of BONNET and COUSINEAU (9) do not produce low Σ orientations close to the orientations in Figs. and. However, it is evident from Table that there is a Σ = description relevant to the twin orientation of Fig., which we have discussed elsewhere (TURAN and KNOWLES 99b). The clear implication of our observations is that despite the lack of any obviously special orientation relationship, the solid-solid boundary energies of the grain boundaries in hexagonal boron nitride we have observed are relatively small in magnitude. This is certainly consistent with the recent reports on boron nitride nanotubes (see, for example, LOISEAU et al. 99, 99) in such nanotubes the sp -bonded () planes are able to bend around the tube axis, and can form flat ends of nanotubes by linking together even-numbered rings containing B-N bonds (BLASE et al. 99), rather than having odd-numbered rings which would necessitate the formation of unfavourable B-B and N-N bonds. Thus, we might expect that h-bn grains related to one another by either a rotation about [], or by a rotation vector in the () plane, might give rise to grain boundaries in which the energy penalty of producing a grain boundary is small of the order of a few mj m - at most, and necessarily significantly smaller than the energy penalty for producing high angle grain boundaries in silicon nitride. This is not an unreasonable inference. Such an estimate of grain boundary energies compares with a prediction for h-bn average surface energies of ~ mj m - from a simple calculation of surface energies from the Hamaker constant of identical phases interacting across vacuum using equations (.) and (.) of ISRAELACHVILI (99) with averaged values of static dielectric constant and refractive index for h-bn of.9 and. respectively. Finally, in the light of our own observations and those of others, we note that it would be obviously be worthwhile examining in more detail evidence for the presence or absence of intergranular films at general grain boundaries in h-bn compacts in which grains are not related by relatively simple rotations. References BONNET, R. and COUSINEAU, E.: Acta Cryst A (9) BLASE, X., DE VITA, A., CHARLIER, J.-C. and CAR, R.: Phys. Rev. Lett. (99) HAGIO, T. and YOSHIDA, H.: J. Mater. Sci. Lett. (99) HIRAGA, K., OKU, T., HIRABAYASHI, M., MATSUDA, T. and HIRAI, T.: J. Mater. Sci. Lett. (99) HUANG, J.Y., JIA, X.B., YASUDA, H. and MORI, H.: Phil. Mag. Lett. 9 (999) HUANG, J.Y., YASUDA, H. and MORI, H.: J. Am. Ceram. Soc. () KURDYMOV, A.V., OSTROVSKAYA, N.F. and PILYANKEVICH, A.N.: Soviet Phys. Cryst. (9) 9 LOISEAU, A., WILLAIME, F., DEMONCY, N., HUG, G., and PASCARD, H.: Phys. Rev. Lett. (99) LOISEAU, A., WILLAIME, F., DEMONCY, N., SCHRAMCHENKO, N., HUG, G., COLLIEX, C. and PASCARD, H.: Carbon (99) MIYAZAKI, Y., HARADA, H., SAKAMAKI, S. and HAGIO, T.: J. Ceram. Soc. Jap. Int. Ed., 99 (99) PEASE, R.S.: Acta Cryst. (9) PEZZOTTI, G., KLEEBE, H.-J., OTA, K. and NISHIDA, T.: Acta Materialia, (99) TURAN, S. and KNOWLES, K.M.: J. Am. Ceram. Soc. (99a) TURAN, S., and KNOWLES, K.M.: phys. stat. sol. (a), (99b)

8 K. M. KNOWLES, S. TURAN: Grain Boundaries between Boron Nitride Grains Contact information: Dr. Kevin M. KNOWLES* University of Cambridge Department of Materials Science and Metallurgy Pembroke Street Cambridge CB QZ U.K. Dr. Servet TURAN Anadolu University Department of Ceramic Engineering Iki Eylül Kampüsü Eskisehir Turkey *corresponding author