ues are higher than the tensile strength of the material, zones on the fracture surface. The first zone near the and therefore there is much cracking and a decrease in notch tip has a rough surface and corresponds to a slow all mechanical properties crack growth. The second zone has a rather smooth surface with distinct steps only at the interfaces be tween layers. This zone corresponds to a fast crack 4.2. Fracture surfaces growth(Fig. 2B). No crack bifurcation occurred and The typical fracture surfaces of pure Si3 N4 layer and two equal parts of the sample could be found after fail Si3N4-20 wt %TiN layer are shown in Fig. 1. The bi- ure. The Si3N4/2(Si3N4-20 wt %TiN) laminates failed modal grain size distribution exists with a number of after crack bifurcation. The part of the fracture surface elongated grains being surrounded by small rounded near the notch tip was the same as the ones shown in grains of Si3N4(Fig. 1a). The average grain size in the Fig. IA and B. At the moment when the crack bifur- 13n4 layer was 0. 4-0.5 um. The micrograph of the cated, an unusually smooth fracture surface was ob- Si3N4-20%TiN fracture surface revealed that a major- served(Fig. 2C). When the value of residual tensile ity of the grain sizes were in the range of 1-2 um, with stresses approaches the value of the tensile strength of some grains of a size less than I um(Fig. Ib). It was the layer, cracks in the layers are generated, as was the shown that the tin has a homogeneous distribution in case of the Si3 Na/Si3N4-50 wt %TiN and Si3N4/TiN the Si3N4 matrix and no solid solution was detected laminates. The cracks originated during the cooling between Si3n4 and Tin particles [22] stage after the hot pressing of the laminates and ap Fracture surfaces of Si3 N/Si3 N4, Si N4/Si3N4- peared due to mismatching of CTEs and elastic mod- 20 wt%TiN, Si3N4/2(Si3N4-20 wt %TiN) and Si3N4/ uli of two different layers. Channel cracks were ob TiN laminates are shown in Fig. 2. 2(Si3N4-20 served in the laminates with a difference in composi- wt%TiN)means that thickness of Si3N4-20 wt%TiN) tion between layers, starting with 50 wt %TiN content layer is twice than that of Si3 N4 layer. The fracture and higher. Si3 N4/TiN laminates demonstrate channel surface of the Si3 N4/Si3 N4 laminate, where no resid- cracking(Fig. 2D) similar to the cracks described in stresses were generated during cooling, is flat and [23]. These cracks are responsible for the dramatic de smooth(Fig. 2A). As layers of different composition crease in the mechanical properties of Si]N4 based lam- tre used. the fracture surface becomes rougher. For inates. To reduce or eliminate cracking, it is necessary the Si3N/Si3N4-20 wt %TIN laminate, there are two to make composites with characteristics more close between layers, especially the CtE and elastic moduli The extent of channel cracking was decreased in lam- inates with Si3N4-50 wt. TiN layers in comparison to composites where one of the layers was pu Channel cracking was fully eliminated for composites with a Si3N4-20 wt %TiN layer composition. An ab- sence of pre-existent cracks resulted in an increase of the strength and fracture toughness The fracture surface of Si3 N4/Si3 N4- 50 wt% TIN is shown in Fig. 3. As one can see, there is a high roughness of the surface, and bifurcation of the mov ing crack occurred when it was inside the Si3N4 layer with residual compressive stresses. There are fracture steps and channel cracks at the Si3N4-50 wt % TIN lay ers which are perpendicular to the interfaces of com- ite. The fracture step ed only at layers wit tensile stresses. Such fracture steps and other defects are responsible for a decrease in mechanical proper B ties. Multiple bifurcations occur for preexisting cracks inside the layers with residual compressive stresses, and in addition, the moving crack bifurcates during e sa ding. The schematic presentation and an optical image of the crack bifurcation during the failure of this laminate are shown in Fig 4 4.3. Raman shift measurements A measurement of residual stresses is an important he development of the laminates ber of works have been published which use Raman spectroscopy for the determination of residual stresses. from the residual stresses in Figure/ Micrographs of fracture surfaces of Si3N4 layer(A)and Al2O3/ZrO2 composites has been evaluated [24]. The Si3N4-20 wt %oTiN layer(B)in Si3 N4/Si3 N4-20 wt. TiN laminate magnitude of bridging stresses in Si3 N4 and AlOues are higher than the tensile strength of the material, and therefore there is much cracking and a decrease in all mechanical properties. 4.2. Fracture surfaces The typical fracture surfaces of pure Si3N4 layer and Si3N4-20 wt.%TiN layer are shown in Fig. 1. The bi￾modal grain size distribution exists with a number of elongated grains being surrounded by small rounded grains of Si3N4 (Fig. 1a). The average grain size in the Si3N4 layer was 0.4–0.5 µm. The micrograph of the Si3N4-20%TiN fracture surface revealed that a major￾ity of the grain sizes were in the range of 1–2 µm, with some grains of a size less than 1 µm (Fig. 1b). It was shown that the TiN has a homogeneous distribution in the Si3N4 matrix and no solid solution was detected between Si3N4 and TiN particles [22]. Fracture surfaces of Si3N4/Si3N4, Si3N4/Si3N4- 20 wt.%TiN, Si3N4/2(Si3N4-20 wt.%TiN) and Si3N4/ TiN laminates are shown in Fig. 2. 2(Si3N4-20 wt.%TiN) means that thickness of (Si3N4-20 wt.%TiN) layer is twice than that of Si3N4 layer. The fracture surface of the Si3N4/Si3N4 laminate, where no resid￾ual stresses were generated during cooling, is flat and smooth (Fig. 2A). As layers of different composition are used, the fracture surface becomes rougher. For the Si3N4/Si3N4-20 wt.%TiN laminate, there are two Figure 1 Micrographs of fracture surfaces of Si3N4 layer (A) and Si3N4-20 wt.%TiN layer (B) in Si3N4/Si3N4-20 wt.%TiN laminate. zones on the fracture surface. The first zone near the notch tip has a rough surface and corresponds to a slow crack growth. The second zone has a rather smooth surface with distinct steps only at the interfaces be￾tween layers. This zone corresponds to a fast crack growth (Fig. 2B). No crack bifurcation occurred and two equal parts of the sample could be found after fail￾ure. The Si3N4/2(Si3N4-20 wt.%TiN) laminates failed after crack bifurcation. The part of the fracture surface near the notch tip was the same as the ones shown in Fig. 1A and B. At the moment when the crack bifur￾cated, an unusually smooth fracture surface was ob￾served (Fig. 2C). When the value of residual tensile stresses approaches the value of the tensile strength of the layer, cracks in the layers are generated, as was the case of the Si3N4/Si3N4-50 wt.%TiN and Si3N4/TiN laminates. The cracks originated during the cooling stage after the hot pressing of the laminates and ap￾peared due to mismatching of CTEs and elastic mod￾uli of two different layers. Channel cracks were ob￾served in the laminates with a difference in composi￾tion between layers, starting with 50 wt.%TiN content and higher. Si3N4/TiN laminates demonstrate channel cracking (Fig. 2D) similar to the cracks described in [23]. These cracks are responsible for the dramatic de￾crease in the mechanical properties of Si3N4 based lam￾inates. To reduce or eliminate cracking, it is necessary to make composites with characteristics more close between layers, especially the CTE and elastic moduli. The extent of channel cracking was decreased in lam￾inates with Si3N4-50 wt.%TiN layers in comparison to composites where one of the layers was pure TiN. Channel cracking was fully eliminated for composites with a Si3N4-20 wt.%TiN layer composition. An ab￾sence of pre-existent cracks resulted in an increase of the strength and fracture toughness. The fracture surface of Si3N4/Si3N4-50 wt.% TiN is shown in Fig. 3. As one can see, there is a high roughness of the surface, and bifurcation of the mov￾ing crack occurred when it was inside the Si3N4 layer with residual compressive stresses. There are fracture steps and channel cracks at the Si3N4-50 wt.%TiN lay￾ers which are perpendicular to the interfaces of com￾posite. The fracture steps appeared only at layers with tensile stresses. Such fracture steps and other defects are responsible for a decrease in mechanical proper￾ties. Multiple bifurcations occur for preexisting cracks inside the layers with residual compressive stresses, and, in addition, the moving crack bifurcates during the sample loading. The schematic presentation and an optical image of the crack bifurcation during the failure of this laminate are shown in Fig. 4. 4.3. Raman shift measurements A measurement of residual stresses is an important issue in the development of the laminates. A num￾ber of works have been published which use Raman spectroscopy for the determination of residual stresses. Strengthening arising from the residual stresses in Al2O3/ZrO2 composites has been evaluated [24]. The magnitude of bridging stresses in Si3N4 and Al2O3 5446