Z Krstic, V.D. Krstic/Jounal of the European Ceramic Sociery 28 (2008)1723-1730 13 Si N, laye wvw B-4 BsiN sIn, layers 6 SiN, layers B-Phase-Y. SiAJON YAG3Yo5A°3 5001 BNB-SiN, ss,N B-SiN, B-SI B-Si 50 2 Theta(degree) 2 Theta(degree) Fig. 11. X-ray diffraction patterns of BN interface with a different number of Fig. 12. X-ray diffraction patterns of BN-based interface with different number Si3N4 layers for SN-(BN+Al2O3) laminated structures sintered at 1760C. for Si3 N4 layers of SN-(BN+SN)laminated structures sintered at 1760C eacti small undetected amount of YAG may be present in the inter 10Y2 SiAIOSN→6Y2O3+3￥2O35Al2O3+Y2SigO6 face. The absence of YAG phase makes BN-based interfaces in SN-(BN+SN) laminated structures more porous than in +Si2N2O2↑+4N2↑ () SN-(BN+ Al203)laminates. Considering that the bN phase In turn, Y2O3 formed by Reaction(1)diffuses into the BN- has a low diffusivity, its sintering will be limited and it is xpected that a much larger level of porosity will remain in based interface layer and reacts with Al2 O3 to form YAG phase these interfacial layers than in the BN+ Al2O3 interfacial lay- according to the reaction: ers.Support for this statement is found from the results obtained 3Y203+5Al2O3=3Y203. 5Al20 (2) from on Youngs modulus measurement presented in Figs 5-7 which show a much larger drop in Youngs modulus for sam- The support for this reaction mechanism is found in Fig. 11 ples with BN+ Si3 N4 interfacial layers than for samples with which shows a continuous decrease of B-phase peak with the BN+AlO3 interfacial layers. The results are in line with pre- increase in the number of Si3N4 layers along with the increase diction that a sharp reduction in Youngs modulus is associated of the intensity of X-ray peaks generated by the YAG phase with porosity Quite different X-ray diffraction patterns were produced in Initial mechanical properties measurements revealed a sig- the laminates containing BN+ Si3N4 interfacial layers(Fig. 12). nificant increase in apparent fracture toughness compared to Besides Si3N4 phase, only bn phase is detected and its amount the monolithic terpart. The measurements also showed increases with an increase of the number of Si3N4 layers. the effect of the Si3N4 layers number on the apparent frac When the number of Si3N4 layers exceeds 10, the bn phase ture toughness, fracture strength and work of fracture(Table 1) becomes the major phase in the system. Expectedly, no YAG The highest apparent fracture toughness of 22 MPa mand phase was observed in any of the laminates. Since the inter- fracture strength of 470 MPa, respectively, were found for 7 face in the SN-(Bn+ SN)laminates consists of 90wt. Bn Si3 N4 layers in SN-(BN+Al2O3) laminates. Somewhat lower and 10 wt% Si N, there is no Y2O3 or Al2O3 present to form apparent fracture toughness of 19.5 MPa/2 but higher frac YAG phase. Although the X-ray diffraction did not detect the ture strength of 515 MPa were found for 4 Si3N4 layers in the presence of YAG phase it should be emphasized that some SN-(BN+SN) laminates1728 Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 28 (2008) 1723–1730 Fig. 11. X-ray diffraction patterns of BN interface with a different number of Si3N4 layers for SN − (BN + Al2O3) laminated structures sintered at 1760 ◦C. reaction: 10Y2SiAlO5N → 6Y2O3 + 3Y2O3·5Al2O3 + Y2Si8O6 + Si2N2O2↑ + 4N2↑ . (1) In turn, Y2O3 formed by Reaction (1) diffuses into the BN￾based interface layer and reacts with Al2O3 to form YAG phase according to the reaction: 3Y2O3 + 5Al2O3 = 3Y2O3·5Al2O3. (2) The support for this reaction mechanism is found in Fig. 11 which shows a continuous decrease of B-phase peak with the increase in the number of Si3N4 layers along with the increase of the intensity of X-ray peaks generated by the YAG phase. Quite different X-ray diffraction patterns were produced in the laminates containing BN + Si3N4 interfacial layers (Fig. 12). Besides Si3N4 phase, only BN phase is detected and its amount increases with an increase of the number of Si3N4 layers. When the number of Si3N4 layers exceeds ∼10, the BN phase becomes the major phase in the system. Expectedly, no YAG phase was observed in any of the laminates. Since the inter￾face in the SN − (BN + SN) laminates consists of 90 wt.% BN and 10 wt.% Si3N4 there is no Y2O3 or Al2O3 present to form YAG phase. Although the X-ray diffraction did not detect the presence of YAG phase it should be emphasized that some Fig. 12. X-ray diffraction patterns of BN-based interface with different number for Si3N4 layers of SN − (BN + SN) laminated structures sintered at 1760 ◦C. small undetected amount of YAG may be present in the inter￾face. The absence of YAG phase makes BN-based interfaces in SN − (BN + SN) laminated structures more porous than in SN − (BN + Al2O3) laminates. Considering that the BN phase has a low diffusivity, its sintering will be limited and it is expected that a much larger level of porosity will remain in these interfacial layers than in the BN + Al2O3 interfacial lay￾ers. Support for this statement is found from the results obtained from on Young’s modulus measurement presented in Figs. 5–7 which show a much larger drop in Young’s modulus for sam￾ples with BN + Si3N4 interfacial layers than for samples with BN + Al2O3 interfacial layers. The results are in line with pre￾diction that a sharp reduction in Young’s modulus is associated with porosity. Initial mechanical properties measurements revealed a sig￾nificant increase in apparent fracture toughness compared to the monolithic counterpart. The measurements also showed the effect of the Si3N4 layers number on the apparent frac￾ture toughness, fracture strength and work of fracture (Table 1). The highest apparent fracture toughness of ∼22 MPa m1/2 and fracture strength of 470 MPa, respectively, were found for 7 Si3N4 layers in SN − (BN + Al2O3) laminates. Somewhat lower apparent fracture toughness of 19.5 MPa m1/2 but higher frac￾ture strength of 515 MPa were found for 4 Si3N4 layers in the SN − (BN + SN) laminates