Journal of the American Ceramic Society-Kovar et al. Vol. 80. No. 10 Fig. 2. SEM micrograph of a fracture surface showing a BN cell boundary between two Si,N, cells viewed edge on. Glassy Phase 20m Fig. 1. Low-magnification SEM composites illustrating three sec- (Si,N, cells run continuously down the length of 5um separated by BN cell boundaries)and a(b)[/90] architectu of cells are stacked with a 90 rotation between the fractue surface prateletike mor phology of the bn irais as wetl The BN grain alignment is obvious by visual examination and as the discontinuous glassy phase are visible confirmed by X-ray diffractometry (XRD). 0 It is crucial that the(0001)cleavage planes be oriented parallel to the Si3N4 interface; otherwise, cracks do not deflect in the BN inter- boundary, looking down onto the fracture surface. The platey hase. This grain alignment occurs during the coextrusion step of green fabrication, during which the BN platelets are plane oriented parallel to the cell boundary. In this secondary- (2) Microstructure at Scale of the grains darker regions are BN platelets and the brighter areas are yttria aluminosilicate glass The microstructure within the Si3N4 cells is quite conven- lon-milled samples of fibrous monoliths were prepared for tional for this particular grade of Si, N4 densified with 6 wt% ansmission electron microscopy(TEM)using techniques de- ular, grains within a matrix of a glassy, grain-boundary revealed by TE Sewhere. 12 The major features of the BN rocracks between the (0001) basal planes of BN platelets. These are shown in Fig. 4 ditions, we find B-Si Na grains 0.2-1.5 um wide, with aspect The inset diffraction pattern indicates the foil plane to be Junctions. Figure 2 is an SEM micrograph of a fracture s efie, Som G,, te how each BN grain has exfoliated along its basal ratios of 2-10. The grain-boundary phase is glassy, present as (20).No the usual thin film between grains and in pockets at Si3 N4 grain planes into many layers. A higher magnification view of a BN in shown in Fig. 5 reveals a finer pattern of microcrack showing a Bn cell boundary between two Si3 N4 cells. Visual Some layers are divided as fine as 50 nm. (A unit graphi inspection suggests that many of the B-Si3N4 grains in the cell layer in the BN crystal structure has a thickness of are oriented with their [0001] long axes aligned along the cell nm. direction. This texture has been confirmed by XRD. o Note A similar microcrack structure has been described by also the obvious orientation of the Bn platelets in the cell Mrozowski 3 in graphite that has a crystalline structure similar boundary to BN 14 Sinclair and Simmons is have attributed these basal Figure 3 is an SEM micrograph of the fractured BN-rich cell plane cracks that they observed using TEM to the thermalThe BN grain alignment is obvious by visual examination and confirmed by X-ray diffractometry (XRD).10 It is crucial that the (0001) cleavage planes be oriented parallel to the Si3N4 interface; otherwise, cracks do not deflect in the BN inter￾phase.11 This grain alignment occurs during the coextrusion step of green fabrication, during which the BN platelets are oriented by the flow field in the extrusion die. (2) Microstructure at Scale of the Grains The microstructure within the Si3N4 cells is quite conven￾tional for this particular grade of Si3N4 densified with 6 wt% Y2O3 and 2 wt% Al2O3. This grade of Si3N4 consists of acic￾ular b-Si3N4 grains within a matrix of a glassy, grain-boundary phase. For our particular raw materials and densification con￾ditions, we find b-Si3N4 grains 0.2–1.5 mm wide, with aspect ratios of 2–10. The grain-boundary phase is glassy, present as the usual thin film between grains and in pockets at Si3N4 grain junctions. Figure 2 is an SEM micrograph of a fracture surface, showing a BN cell boundary between two Si3N4 cells. Visual inspection suggests that many of the b-Si3N4 grains in the cell are oriented with their [0001] long axes aligned along the cell direction. This texture has been confirmed by XRD.10 Note also the obvious orientation of the BN platelets in the cell boundary. Figure 3 is an SEM micrograph of the fractured BN-rich cell boundary, looking down onto the fracture surface. The platey features are the BN grains, which lie with their (0001) basal plane oriented parallel to the cell boundary. In this secondary￾electron micrograph, there are two distinct contrast areas. The darker regions are BN platelets and the brighter areas are yttria aluminosilicate glass. Ion-milled samples of fibrous monoliths were prepared for transmission electron microscopy (TEM) using techniques de￾scribed in detail elsewhere.12 The major features of the BN revealed by TEM are extensive microcracks between the (0001) basal planes of BN platelets. These are shown in Fig. 4. The inset diffraction pattern indicates the foil plane to be (2110). Note how each BN grain has exfoliated along its basal planes into many layers. A higher magnification view of a BN grain shown in Fig. 5 reveals a finer pattern of microcracking. Some layers are divided as fine as 50 nm. (A unit ‘‘graphine’’ layer in the BN crystal structure has a thickness of c0 4 0.66 nm.) A similar microcrack structure has been described by Mrozowski13 in graphite that has a crystalline structure similar to BN.14 Sinclair and Simmons15 have attributed these basal plane cracks that they observed using TEM to the thermal Fig. 3. SEM micrograph of the BN cell boundary looking down onto the fracture surface. Plateletlike morphology of the BN grains as well as the discontinuous glassy phase are visible. Fig. 1. Low-magnification SEM composites illustrating three sec￾tions of a fibrous monolith with a (a) uniaxially aligned architecture (Si3N4 cells run continuously down the length of the specimen and are separated by BN cell boundaries) and a (b) [0/90] architecture (layers of cells are stacked with a 90° rotation between lamina). Fig. 2. SEM micrograph of a fracture surface showing a BN cell boundary between two Si3N4 cells viewed edge on. 2472 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10