Z Krstic, V.D. Krstic/Joumal of the European Ceramic Sociery 28(2008)1723-1730 1729 The effect of the number of Si3 Na layers on mechanical properties of the self-sealed Si3N4/BN laminated structures Number of Si3 N4 layers Materials S Work of Apparent fracture Strength toughness(MPam )(MPa) fracture(kJ/m) toughness(MPam2) (MPa) fracture(kJ/m) 10 310 Monolithic si N4~92 92 In the both laminates, there is an increase in the apparent fracture toughness and strength with number of Si3N4 layers followed by a decrease in the both by further increase in the ayers numbers. It is believed that this increase of apparent fracture toughness and strength is attributed to the crack interac tion(crack deflections)with the BN interphase. After deflection along the weak bn interphase(Fig. 13), crack propagation occurred in both radial and axial directions( Fig. 14). This led to an increase in apparent fracture toughness and fracture strength Also, Table 1 reveals the effects of the number of Si3N4 layers on the work of fracture. the highest work of fracture of 230 and 320kJ/m, respectively, are observed in SN-(BN-Al203)and in SN-(Bn+SN) laminates 4. Conclusion Fig. 13. A crack propagation through the Si3 N4 layer and deflection at BN Self-sealed Si3 N4/BN-based laminated structures with uare cross-section having different number of with different thickness have been fabricated for the fir using modified slip-casting method and densified by pressure- less sintering process either delamination nor peeling during sintering or cool ing from sintering temperature were observed in either SN-(BN+ Al2O3)or SN-(BN +SN) laminated structures. The highest apparent density of over 3.22 g/em' was achieved in SN-(BN+ Al2O3) with 5 Si3N4 layers, and the highest apparent density of 3.20 g/cm was achieved in SN-(BN+SN) both I SN-(BN+AlO3)and sn-(Bn+SN), it was found that the samples with highest apparent density were also samples with highest Youngs modulus with values of over 310 and 315 GPa. The microstructure of Si3 N4 layers in both laminates con- sists of B-Si3N4 phase, YAG phase and smaller amount of B-phase. The microstructure of the BN-based interface in ly of YAG phase with BN and Si3N4 as minor phases. Low level of porosity w observed in this interfaces The microstructure of the bn-based interfaceinSN-(BN+SN)laminates consists of BN and Si3N4 Fig. 14. Radial and axial direction of a crack propagatio as major phases without the presence of YAG phase. A muchZ. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 1729 Table 1 The effect of the number of Si3N4 layers on mechanical properties of the self-sealed Si3N4/BN laminated structures Number of Si3N4 layers Materials SN − (BN + Al2O3) SN − (BN + SN) Apparent fracture toughness (MPa m1/2) Strength (MPa) Work of fracture (kJ/m3) Apparent fracture toughness (MPa m1/2) Strength (MPa) Work of fracture (kJ/m3) 3 – – – 17.2 337 150 4 10.4 100 100 19.5 515 225 5 11.2 290 125 16 412 279 7 22.3 470 200 16.3 375 300 9 16 367 230 13.2 300 320 11 12.8 322 150 13 220 246 13 10 310 140 – – – Monolithic Si3N4 ∼9.2 ∼790 80 ∼9.2 ∼790 80 Fig. 13. A crack propagation through the Si3N4 layer and deflection at BN interface. Fig. 14. Radial and axial direction of a crack propagation22. In the both laminates, there is an increase in the apparent fracture toughness and strength with number of Si3N4 layers followed by a decrease in the both by further increase in the layers numbers. It is believed that this increase of apparent fracture toughness and strength is attributed to the crack interac￾tion (crack deflections) with the BN interphase. After deflection along the weak BN interphase (Fig. 13), crack propagation occurred in both radial and axial directions (Fig. 14). This led to an increase in apparent fracture toughness and fracture strength. Also, Table 1 reveals the effects of the number of Si3N4 layers on the work of fracture. The highest work of fracture of 230 and 320 kJ/m3, respectively, are observed in SN − (BN-Al2O3) and in SN − (BN + SN) laminates. 4. Conclusion Self-sealed Si3N4/BN-based laminated structures with square cross-section having different number of Si3N4 layers with different thickness have been fabricated for the first time using modified slip-casting method and densified by pressure￾less sintering process. Neither delamination nor peeling during sintering or cool￾ing from sintering temperature were observed in either SN − (BN + Al2O3) or SN − (BN + SN) laminated structures. The highest apparent density of over 3.22 g/cm3 was achieved in SN − (BN + Al2O3) with 5 Si3N4 layers, and the highest apparent density of 3.20 g/cm3 was achieved in SN − (BN + SN) laminates in samples having 3 Si3N4 layers. In both laminates, SN − (BN + Al2O3) and SN − (BN + SN), it was found that the samples with highest apparent density were also samples with highest Young’s modulus with values of over 310 and 315 GPa, respectively. The microstructure of Si3N4 layers in both laminates con￾sists of -Si3N4 phase, YAG phase and smaller amount of B-phase. The microstructure of the BN-based interface in SN − (BN + Al2O3) laminates consists mainly of YAG phase with BN and Si3N4 as minor phases. Low level of porosity was observed in this interfaces. The microstructure of the BN-based interface in SN − (BN + SN) laminates consists of BN and Si3N4 as major phases without the presence of YAG phase. A much