October 1997 Fibrous Monolithic Ceramics 2473 Boron Nitride Boron Nitride CTO00IDirection Glassy Phase Cracks 250nm Fig. 4. TEM micrograph showing extensive microcracks between the(0001)basal planes of the BN platelets. Also note the presence of Fig. 5. Higher-magnification view of a single bN platelet showing a glassy phase between the bn platelets expansion anisotropy between the a-axis and c-axis of graph- te.5In the basal plane, the coefficient of thermal expansion (CTE)of BN is slightly negative through 800C, about -2 10-6rC. 16 Perpendicular to the basal plane, the CTE is very large and positive, about +40 x 10-b/oC. As the composite Glassy Phase Silicon Nitride Doled from the hot-pressing ter ature(1750°C, the BN contracts perpendicular to the basal plane (i.e, in the [0001] direction), while there is a small expansion within the plane. Hf the surrounding Si3 Na grains or glassy phase constrain the BN platelets, large tensile stresses are developed perpendicular to the basal plane upon cooling. This acts to separate the BN platelet into layers along the basal plane direction. Further- more, shear stresses developed parallel to the basal plane shea the surfaces of the platelets relative to each other. The BN platelets labeled A and B in Fig. 5 clearly once existed as a single platelet before they were split and translated relative to one another during cooling A representative TEM micrograph of a Sia NBN interface shown in Fig. 6. There is no cracking between the Si3N4 and Boron Nitride the BN Rather there seems to be excellent adhesion between the two phases. a thin layer of glass is observed between the two phases in some places a glassy phase also is found residing in pockets in the BN cell boundary. No glass-forming compounds were added to the Bn powders; therefore, this glass must be residual liquid in- truded into the cell boundary from the neighboring Si N, cells 250nm during hot pressing. Figure 4 shows a large pocket of glass between exfoliated layers of BN. The selected-area electron diffraction pattern in Fig. 4 shows amorphous rings from the Fig. 6. Bright-field TEM image of a typical interface between the hase exists in pockets between booklets of bn grains. The composition of the glass in Si3N4 cells and BN cell-boundary glassy phases was determined with energy dispersive spectros- borate. The Y: Al ratio of the glass in the Bn cell boundaries copy(EDS). EDS spectra of the glassy phase between the bn is similar to the composition of the glass between Si3N4 grains platelets show the presence of yttrium, aluminum, silicon, oxy- Because of the presence of silicon, aluminum, and yttrium, it is gen, and nitrogen. Boron could not be detected by this EDS clear that the sintering-aid glass is being either drawn or forced spectrometer; therefore, we do not know if the glass contains into the bn during hot pressingexpansion anisotropy between the a-axis and c-axis of graph￾ite.15 In the basal plane, the coefficient of thermal expansion (CTE) of BN is slightly negative through 800°C, about −2 × 10−6/°C.16 Perpendicular to the basal plane, the CTE is very large and positive, about +40 × 10−6/°C.17 As the composite is cooled from the hot-pressing temperature (1750°C), the BN contracts perpendicular to the basal plane (i.e., in the [0001] direction), while there is a small expansion within the plane. If the surrounding Si3N4 grains or glassy phase constrain the BN platelets, large tensile stresses are developed perpendicular to the basal plane upon cooling. This acts to separate the BN platelet into layers along the basal plane direction. Further￾more, shear stresses developed parallel to the basal plane shear the surfaces of the platelets relative to each other. The BN platelets labeled A and B in Fig. 5 clearly once existed as a single platelet before they were split and translated relative to one another during cooling. A representative TEM micrograph of a Si3N4–BN interface is shown in Fig. 6. There is no cracking between the Si3N4 and the BN. Rather, there seems to be excellent adhesion between the two phases. A thin layer of glass is observed between the two phases in some places. A glassy phase also is found residing in pockets in the BN cell boundary. No glass-forming compounds were added to the BN powders; therefore, this glass must be residual liquid in￾truded into the cell boundary from the neighboring Si3N4 cells during hot pressing. Figure 4 shows a large pocket of glass between exfoliated layers of BN. The selected-area electron diffraction pattern in Fig. 4 shows amorphous rings from the glass with diffraction spots identified with BN. The glassy phase exists in pockets between booklets of BN grains. The composition of the glass in Si3N4 cells and BN cell-boundary glassy phases was determined with energy dispersive spectros￾copy (EDS). EDS spectra of the glassy phase between the BN platelets show the presence of yttrium, aluminum, silicon, oxy￾gen, and nitrogen. Boron could not be detected by this EDS spectrometer; therefore, we do not know if the glass contains borate. The Y:A1 ratio of the glass in the BN cell boundaries is similar to the composition of the glass between Si3N4 grains. Because of the presence of silicon, aluminum, and yttrium, it is clear that the sintering-aid glass is being either drawn or forced into the BN during hot pressing. Fig. 4. TEM micrograph showing extensive microcracks between the (0001) basal planes of the BN platelets. Also note the presence of a glassy phase between the BN platelets. Fig. 5. Higher-magnification view of a single BN platelet showing fine-scale pattern of microcracking. Fig. 6. Bright-field TEM image of a typical interface between the Si3N4 and the BN. October 1997 Fibrous Monolithic Ceramics 2473