Y.H. Koh et al. Journal of the European Ceramic Society 24(2004)2339-2347 0.8 06 Pull-out Length 204 200um v”△T=1400° Fig. Il. SEM micrograph of the thermally shocked sample after flex. 0 ural testing, illustrating the delamination cracks(marked by arrow Pull-out Length mm and pull-out length Fig 13. Cumulative distribution function versus pull-out length of the samples before and after thermal shock with a various temperature difference△T) fracture resistance(T BN). The FM samples after thermal shock with the temperature difference of 1200C, 70% of the delamination cracks showed the long pull-out length(>1000 um). However, after thermal shock with the temperature difference of 1400C, the pull-out length was decreased again even tough almost every BN- rich cell boundary was delaminated (see Fig. 12) This result is attributed to the damage of both Si3N4 40 cells and bn cell boundaries due to the oxidation in other words, when the Si3 N4 cell is strong (i.e, few flaws on the surface), the delamination cracks extends to long 200400600800100012001400 distance before kinking out of BN-rich cell boundary. Temperature Difference [Cl However, when the Si3 N4 cell is damaged due to the oxidation (i.e, many flaws on the surface, see Fig. 8) Fig.12.Fraction of crack delamination in the thermally shocked amounts of the delamination cracks kink out of the BN sample after flexural testing as a function of temperature difierence rich cell boundary after propagating only a short dis (An. Temperature difference(An) of 0 oC represents the sample before thermal shoc tance, in which the flaws are present on Si3N4 cell, while almost every cell boundary is delaminated due to the cracks on BN-rich cell boundaries, consequently pro- decrease in cell boundary fracture resistance moting the delamination cracks The magnitude of thermal stress is expected to change the length of the delamination cracks however it is 5. Conclusions difficult to quantify the length of delamination cracks, because it is not easy to discern the crack tip in the BN Excellent thermal shock resistance was observed for rich cell boundary. Therefore, the delamination dis- fibrous monolithic Si3 N4/BN ceramics. Monolithic tance, defined as pull-out length(see Fig. 11), can be Si3 n4 showed a catastrophic drop in flexural strength measured from the distance between through-thickness with temperature difference of 1000C, meaning that cracks in adjacent Si3N4 layers. A cumulative distribu- the tensile stress was developed on the surface exceeding tion plot of pull-out lengths is shown in Fig 13 for each its fracture strength, and thus cracking the surface. of the samples before and after thermal shock. Before However, fibrous monolithic showed negligible reduc- thermal shock, the FM sample showed the amounts of tion in flexural strength, and remarkable increase in short pull-out length(<100 um). Almost half of the work-of-fracture(WOF). Such excellent thermal shock delamination cracks kinked out of the BN cell boundary resistance was attributed to high resistance to crack after propagating only a short distance. However, after propagation(rm) through crack interactions with weak thermal shock, the pull-out length was significantly cell boundaries. The remarkable increase in WOF increased, implying that the decrease in cell boundary delamination cracks were attributed to the reductioncracks on BN-rich cell boundaries, consequently pro￾moting the delamination cracks. The magnitude of thermal stress is expected to change the length of the delamnination cracks; however, it is difficult to quantify the length of delamination cracks, because it is not easy to discern the crack tip in the BN￾rich cell boundary. Therefore, the delamination dis￾tance, defined as pull-out length (see Fig. 11), can be measured from the distance between through-thickness cracks in adjacent Si3N4 layers. A cumulative distribu￾tion plot of pull-out lengths is shown in Fig. 13 for each of the samples before and after thermal shock. Before thermal shock, the FM sample showed the amounts of short pull-out length (<100 mm). Almost half of the delamination cracks kinked out of the BN cell boundary after propagating only a short distance. However, after thermal shock, the pull-out length was significantly increased, implying that the decrease in cell boundary fracture resistance (GBN). The FM samples after thermal shock with the temperature difference of 1200
C, 70% of the delamination cracks showed the long pull-out length (>1000 mm). However, after thermal shock with the temperature difference of 1400
C, the pull-out length was decreased again even tough almost every BN-rich cell boundary was delaminated (see Fig. 12). This result is attributed to the damage of both Si3N4 cells and BN cell boundaries due to the oxidation. In other words, when the Si3N4 cell is strong (i.e., few flaws on the surface), the delamination cracks extends to long distance before kinking out of BN-rich cell boundary. However, when the Si3N4 cell is damaged due to the oxidation (i.e., many flaws on the surface, see Fig. 8), amounts of the delamination cracks kink out of the BN￾rich cell boundary after propagating only a short dis￾tance, in which the flaws are present on Si3N4 cell, while almost every cell boundary is delaminated due to the decrease in cell boundary fracture resistance. 5. Conclusions Excellent thermal shock resistance was observed for fibrous monolithic Si3N4/BN ceramics. Monolithic Si3N4 showed a catastrophic drop in flexural strength with temperature difference of 1000
C, meaning that the tensile stress was developed on the surface exceeding its fracture strength, and thus cracking the surface. However, fibrous monolithic showed negligible reduc￾tion in flexural strength, and remarkable increase in work-of-fracture (WOF). Such excellent thermal shock resistance was attributed to high resistance to crack propagation (R0000) through crack interactions with weak cell boundaries. The remarkable increase in WOF and delamination cracks were attributed to the reduction in Fig. 11. SEM micrograph of the thermally shocked sample after flex￾ural testing, illustrating the delamination cracks (marked by arrow) and pull-out length. Fig. 12. Fraction of crack delamination in the thermally shocked sample after flexural testing as a function of temperature difference (
T). Temperature difference (
T) of 0
C represents the sample before thermal shock. Fig. 13. Cumulative distribution function versus pull-out length of the samples before and after thermal shock with a various temperature difference (
T). 2346 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 2339–2347