2484 Joumal of the American Ceramic Sociery-Kovar et al. Vol. 80. No. 10 600 500 be 200°C 300°C 200 1400°C Crosshead Deflection, d(mm) Fig. 20. Stress-deflection curves are shown for specimens tested at room temperature and at elevated temperatures features above the fracture surface are bn platelets that have been peeled or sheared during deformation. Close observation also reveals the presence of several small globules -0.5-1.0 les increase substantially. EDS experiments on these globules (a) are present. Boron also is detected; however, because or sen indicate that yttrium, aluminum, silicon, nitrogen, and ox small globule size, the surrounding bn grains may be influ 1300 C are believed to be a result of increased oxidation of the BN grains, exposing more of the glassy phase underneath. The high contact angle between the glassy phase and the bn plate lets indicates that the glassy phase does not readily wet the Preliminary measurements indicate that the interfacial frac- sistance) decreases above 1100 C. This is corrobo- at 1100-1300%C. When the interfacial fracture resis tance is decreased, the interface is weakened to the point where tBNPlatele shear failure occurs. The plateau in the apparent failure stress 101m or specimens tested at 1100-1300oC is an indication that, at nese temperatures, the interfacial fracture resistance is less an this critical value. Research is currently under way to determine the cause of the decrease in interfacial fracture re- sistance (b)2 At 1400oC, the high-temperature creep properties of the lass Si3N4 dominate behavior of the fibrous monolithic ceramic For example, nonlinearities are observed during loading at stresses as low as 65 MPa. No shear cracking and minimal crack deflection are observed simens tested at this tem- perature. This phenomenon is consistent with the flow of the grain-boundary glassy phase present in the Si3 N, cells The high-temperature flexural strength of fibrous monolithic ceramics and monolithic Si, N, are compared in Fig 22. The composition of sintering aids that were added to the Si3N4 each was 6 wt%Y2O3 and 2 wt%Al,O3. A larger decrease in flexural strength occurs in the monolithic Si3N4 from room temperature to 1000oC as compared to the fibrous monolithic ceramic In the 11000-1300oC regime, the monolithic Si3N a bars demonstrate superior strength. This is a result of the BA Paelet change in failure mechanism from tensile failure to shear fail re for the fibrous monolith that was described earli (5) Comparison of Properties with laminates Fig. 21. SEM micros e surfaces along a cell Although fibrous monoliths are a type of laminate, they have 0and(b)1300°C of the bn platelets a three-dimensional structure that gives them unique proper- specimen tested at 1100 ties. Ho in many ways thesestructured laminates he test temperaturefeatures above the fracture surface are BN platelets that have been peeled or sheared during deformation. Close observation also reveals the presence of several small globules ∼0.5–1.0 mm in diameter. At 1300°C, the size and quantity of the glob￾ules increase substantially. EDS experiments on these globules indicate that yttrium, aluminum, silicon, nitrogen, and oxygen are present. Boron also is detected; however, because of the small globule size, the surrounding BN grains may be influ￾encing this measurement. The larger globules observed at 1300°C are believed to be a result of increased oxidation of the BN grains, exposing more of the glassy phase underneath. The high contact angle between the glassy phase and the BN plate￾lets indicates that the glassy phase does not readily wet the surface of the BN. Preliminary measurements indicate that the interfacial frac￾ture resistance (Gi ) decreases above 1100°C. This is corrobo￾rated by the change in fracture morphology for specimens tested at 1100°–1300°C. When the interfacial fracture resis￾tance is decreased, the interface is weakened to the point where shear failure occurs. The plateau in the apparent failure stress for specimens tested at 1100°–1300°C is an indication that, at these temperatures, the interfacial fracture resistance is less than this critical value. Research is currently under way to determine the cause of the decrease in interfacial fracture re￾sistance. At 1400°C, the high-temperature creep properties of the Si3N4 dominate behavior of the fibrous monolithic ceramic. For example, nonlinearities are observed during loading at stresses as low as 65 MPa. No shear cracking and minimal crack deflection are observed in specimens tested at this tem￾perature. This phenomenon is consistent with the flow of the grain-boundary glassy phase present in the Si3N4 cells. The high-temperature flexural strength of fibrous monolithic ceramics and monolithic Si3N4 are compared in Fig. 22. The composition of sintering aids that were added to the Si3N4 in each was 6 wt% Y2O3 and 2 wt% Al2O3. A larger decrease in flexural strength occurs in the monolithic Si3N4 from room temperature to 1000°C as compared to the fibrous monolithic ceramic. In the 1100°–1300°C regime, the monolithic Si3N4 bars demonstrate superior strength. This is a result of the change in failure mechanism from tensile failure to shear fail￾ure for the fibrous monolith that was described earlier. (5) Comparison of Properties with Laminates Although fibrous monoliths are a type of laminate, they have a three-dimensional structure that gives them unique proper￾ties. However, in many ways these ‘‘structured laminates’’ Fig. 21. SEM micrographs of the fracture surfaces along a cell boundary are shown after testing at (a) 1100° and (b) 1300°C. Small globules of a glass are visible on the surface of the BN platelets of the specimen tested at 1100°C. Size of the glass globules increases with the test temperature. Fig. 20. Stress–deflection curves are shown for specimens tested at room temperature and at elevated temperatures. 2484 Journal of the American Ceramic Society—Kovar et al. Vol. 80, No. 10