Y.H. Koh et al. Journal of the European Ceramic Society 24(2004)2339-2347 2345 propagation parameter R of FM sample is much lar- BN-rich cell boundaries. During thermal shock, the ger(>16 times) than that of monolithic Si3 N4, implying longitudinal thermal stress may fracture occasional that the crack propagation is more restricted for FM Si3 N4 cells, as shown in Fig. 10(B). The transverse ther sample, while the resistance to crack initiation is slightly mal stress most likely causes localized extension of the lower than that of monolithic Si3N4 pre-existing flaws in the BN-rich cell boundary and is Considering thermal shock parameters(R and R), unlikely to cause cracks within the Si3N4 cells. The FM sample is expected to show excellent thermal shock postulated BN-cell boundary cracks could not be resistance(see Fig. 4)because the resistance to crack observed because they would be observed by the pre- propagation(R")is much higher, while the resistance existing cracks in the BN and the rough topography of to crack initiation (R) is slightly lower compared to the cell boundary region. However, the extension of monolithic Si3 N4. Similar increased thermal shock BN-cell boundary cracks is believed to decreases the cell resistance have been observed for layered ceramic boundary fracture resistance(TBN), which is consisted structures where the cracks are deflected reliably at the with the observation of more extensive delamination interfaces. 16 after flexural testing of severely shocked sample The degree of delamination cracks is significantly 43. Pre-existing microcracks on BN cell boundaries the de of thermal stress due thermal shock that can extend the pre-existing cracks in We have previously observed the generation of cracks BN-rich cell boundary. The fraction of cell boundary within BN cell boundary layer after hot-pressing. The delamination was calculated by counting the ratio of CtE of BN in the basal plane is slightly negative from delaminated layers to the total number of cell boundary room temperature to 800 oC, about -2x10-6/oC, 24 layer from the SEM micrographs, as shown in Fig. 11 while, the Cte perpendicular to the basal plane is very Three cell boundaries(marked by arrows)were exten large and positive, about +40x10-/C. Therefore, sively delaminated and the rest one was remained ne bn contracts perpendicular to the basal plane (i.e, without delamination crack. The degree of delamination in the [0001] direction) during cooling. If the surround- cracks significantly increased after thermal shock, as ng Si3 n4 grains or glassy phase constrain the bn shown in Fig. 12. Before thermal shock, 40% cell platelets, large tensile stresses are developed perpend- boundaries were delaminated. However, after thermal cular to the basal plane, resulting in separating BN pla- shock with a temperature difference of 1400oC, almost telets into layers along the basal plane direction. Thus every BN cell boundary was delaminated. These results the as-fabricated specimens(before the thermal shock are attributed the decrease in cell boundary fracture treatment) had many pre-existing microcracks within resistance(TBN) through the extension of pre-existing he BN- rich cell boundary. The delaminated micro- cracks of the BN-rich cell boundaries, including the (a)Thermal Stress after Thermal Shock amount of the glass and the extent of pre-existing microcracks. determine the fracture resistance of the cell boundary (TBN) and the tendency for the crack deflec- tion and delamination Furthermore, shear stresses developed parallel to the basal plane made the surface of the platelets slide rela t ve to each other. Similarly, pre-existing mi were extended due to the anisotropy in CtE after ther- (b) Cracking due to Thermal Stress mal shock, dissipating the thermal stress; therefore, arrange 40.55 om crack propagations through bn cell boundaries became more favorable, resulting in high WOF (see Figs. 5 and Cell 7). Some researches have observed that pre-existing microcracks in BN platelets of Si3N-BN and Al_O3- t Cell Cracking Bn composites are beneficial to thermal shock cLonal & 0.80o SiN, Cell N-rich 4.4. Thermal shock induced cracks and crack Cell Boundary propagation during subsequent flexural testing After thermal shock, the Fm sample is placed under Fig. 10. (A)Therma after thermal shock in transverse and transverse and longitudinal stress depending on the fiber longitudinal direction Si3 Na cell between two BN-rich cell bo ting cracks in cell alignment, as sho wn In Fig. 10(A). The flaw tolerant boundaries(-.) and cell boundary cracks by thermal shock nature of the FM is related to crack deflection at the (),(ii) possible transverse cracks in Si, cellpropagation parameter R0000 of FM sample is much lar￾ger (>16 times) than that of monolithic Si3N4, implying that the crack propagation is more restricted for FM sample, while the resistance to crack initiation is slightly lower than that of monolithic Si3N4. Considering thermal shock parameters (R0 and R0000), FM sample is expected to show excellent thermal shock resistance (see Fig. 4) because the resistance to crack propagation (R0000) is much higher, while the resistance to crack initiation (R0 ) is slightly lower compared to monolithic Si3N4. Similar increased thermal shock resistance have been observed for layered ceramic structures where the cracks are deflected reliably at the interfaces.16 4.3. Pre-existing microcracks on BN cell boundaries We have previously observed the generation of cracks within BN cell boundary layer after hot-pressing.1 The CTE of BN in the basal plane is slightly negative from room temperature to 800
C, about 2106 /
C,24 while, the CTE perpendicular to the basal plane is very large and positive, about +40106 /
C.25 Therefore, the BN contracts perpendicular to the basal plane (i.e., in the [0001] direction) during cooling. If the surround￾ing Si3N4 grains or glassy phase constrain the BN platelets, large tensile stresses are developed perpendi￾cular to the basal plane, resulting in separating BN pla￾telets into layers along the basal plane direction. Thus the as-fabricated specimens (before the thermal shock treatment) had many pre-existing microcracks within the BN-rich cell boundary. The delaminated micro￾cracks of the BN-rich cell boundaries, including the amount of the glass and the extent of pre-existing microcracks, determine the fracture resistance of the cell boundary (GBN) and the tendency for the crack deflec￾tion and delamination.810 Furthermore, shear stresses developed parallel to the basal plane made the surface of the platelets slide rela￾tive to each other. Similarly, pre-existing microcracks were extended due to the anisotropy in CTE after ther￾mal shock, dissipating the thermal stress; therefore, crack propagations through BN cell boundaries became more favorable, resulting in high WOF (see Figs. 5 and 7). Some researches have observed that pre-existing microcracks in BN platelets of Si3N4–BN and Al2O3– BN composites are beneficial to thermal shock resistance.26,27 4.4. Thermal shock induced cracks and crack propagation during subsequent flexural testing After thermal shock, the FM sample is placed under transverse and longitudinal stress depending on the fiber alignment, as shown in Fig. 10(A). The flaw tolerant nature of the FM is related to crack deflection at the BN-rich cell boundaries. During thermal shock, the longitudinal thermal stress may fracture occasional Si3N4 cells, as shown in Fig. 10(B). The transverse ther￾mal stress most likely causes localized extension of the pre-existing flaws in the BN-rich cell boundary and is unlikely to cause cracks within the Si3N4 cells. The postulated BN-cell boundary cracks could not be observed because they would be observed by the pre￾existing cracks in the BN and the rough topography of the cell boundary region. However, the extension of BN-cell boundary cracks is believed to decreases the cell boundary fracture resistance (GBN), which is consisted with the observation of more extensive delamination after flexural testing of severely shocked sample. The degree of delamination cracks is significantly dependent on the magnitude of thermal stress due to thermal shock that can extend the pre-existing cracks in BN-rich cell boundary. The fraction of cell boundary delamination was calculated by counting the ratio of delaminated layers to the total number of cell boundary layer from the SEM micrographs, as shown in Fig. 11. Three cell boundaries (marked by arrows) were exten￾sively delaminated and the rest one was remained without delamination crack. The degree of delamination cracks significantly increased after thermal shock, as shown in Fig. 12. Before thermal shock, 40% cell boundaries were delaminated. However, after thermal shock with a temperature difference of 1400
C, almost every BN cell boundary was delaminated. These results are attributed the decrease in cell boundary fracture resistance (GBN) through the extension of pre-existing Fig. 10. (A) Thermal stress after thermal shock in transverse and longitudinal direction and (B) A schematic of single Si3N4 cell between two BN-rich cell boundaries, illustrating (i) pre-existing cracks in cell boundaries (- - -) and cell boundary cracks extended by thermal shock (—), (ii) possible transverse cracks in Si3N4 cell. Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 2339–2347 2345