Z Krstic, V.D. Krstic /Jounal of the European Ceramic Sociery 29(2009)1825-1829 etched surface showing the direction of the crack propagation(pointed by the black arrow)and the bridging grain pull out at of the crack at the weak interface(the white arrow ). (b) A weak/porous interf M450 um. The evaluation of the relationship between Si3N4 and BN layer thickness reveals that there is an optimum thick- ness ratio for Si3 N4/BN of 30 for the SN-(Bn SN) laminate and 20 for SN-(BN+Al2O3)laminates. The lowest apparent fracture toughness of 8 MPamis observed with the samples having the ratio of Si3N4 layers to BN layers thickness of 6 which is almost identical to that of SN-(bn Al2O3) laminates. It is worth noting that difficulties were experienced in keeping the thickness of the Si3 N4 layers constant as the number of the layers increased. This difficulty stems from the fact that number of the layers increases, so does the wall thickness leading SiN Layer Thickness [uml to reduced rate of the particle deposition. In order to eliminate Fig. 7. Effect of Sig Na layers thickness on fracture toughness of SN. the effect of number of the layers on the fracture toughness, a series of tests were conducted where the number of the si3na layers was kept constant. The results are shown in Fig. 9 for The decrease in the fracture toughness at higher Si3 N4 layer the SN-(BN+Al2O3)laminates. A strong effect of the layer thickness is considered to be caused by a decrease in the number thickness on the apparent fracture toughness was observed for of interfaces available for crack deflection and responsible for all thickness up to -230 um, followed by a slight decrease in toughening and strengthening he apparent fracture toughness(Fig. 9). This decrease in the Fig. 8 depicts the effect of the Si3N4 layers thickness on apparent fracture toughness above -230 um is not clear at this the apparent fracture toughness of the SN-(BN + SN) laminates. point. One possible explanation could be the decrease in strength Unlike the SN-(BN +Al2O3) laminates, which show maximum of the interface as its thickness becomes smaller compared to the in the apparent fracture toughness at Si3 N4 layer thickness of thickness of the Si3 N4 layers 230 Hm, the SN-(BN+ SN) laminates exhibit maximum in the apparent fracture toughness of 19.5 MPam at thickness of SigNa Layer Thickness [um SisN, Layer Thickness [H Fig8. Effect of Si3 N4 layers thickness on fracture toughness of SN-(BN +SN) Fig 9. Apparent fracture toughness vs SiaN4 layer thickness in(BN+Al2O3)1828 Z. Krstic, V.D. Krstic / Journal of the European Ceramic Society 29 (2009) 1825–1829 Fig. 6. (a) Micrograph of an etched surface showing the direction of the crack propagation (pointed by the black arrow) and the bridging grain pull out at the surface of the crack at the weak interface (the white arrow). (b) A weak/porous interfaces between dense and strong Si3N4 layers. Fig. 7. Effect of Si3N4 layers thickness on fracture toughness of SN- (BN + Al2O3) laminated structure. The decrease in the fracture toughness at higher Si3N4 layer thickness is considered to be caused by a decrease in the number of interfaces available for crack deflection and responsible for toughening and strengthening. Fig. 8 depicts the effect of the Si3N4 layers thickness on the apparent fracture toughness of the SN-(BN + SN) laminates. Unlike the SN-(BN + Al2O3) laminates, which show maximum in the apparent fracture toughness at Si3N4 layer thickness of ∼230m, the SN-(BN + SN) laminates exhibit maximum in the apparent fracture toughness of 19.5 MPa m1/2 at thickness of Fig. 8. Effect of Si3N4 layers thickness on fracture toughness of SN-(BN + SN) laminated structure. ∼450m. The evaluation of the relationship between Si3N4 and BN layer thickness reveals that there is an optimum thick￾ness ratio for Si3N4/BN of ∼30 for the SN-(BN + SN) laminates and ∼20 for SN-(BN + Al2O3) laminates. The lowest apparent fracture toughness of ∼8 MPa m1/2 is observed with the samples having the ratio of Si3N4 layers to BN layers thickness of ∼6 which is almost identical to that of SN-(BN + Al2O3) laminates. It is worth noting that difficulties were experienced in keeping the thickness of the Si3N4 layers constant as the number of the layers increased. This difficulty stems from the fact that, as the number of the layers increases, so does the wall thickness leading to reduced rate of the particle deposition. In order to eliminate the effect of number of the layers on the fracture toughness, a series of tests were conducted where the number of the Si3N4 layers was kept constant. The results are shown in Fig. 9 for the SN-(BN + Al2O3) laminates. A strong effect of the layer thickness on the apparent fracture toughness was observed for all thickness up to ∼230m, followed by a slight decrease in the apparent fracture toughness (Fig. 9). This decrease in the apparent fracture toughness above ∼230m is not clear at this point. One possible explanation could be the decrease in strength of the interface as its thickness becomes smaller compared to the thickness of the Si3N4 layers. Fig. 9. Apparent fracture toughness vs. Si3N4 layer thickness in (BN + Al2O3) laminates with 7 layers.