Jounal of the American Ceramic Society-Wang et al. (a) Table II. Mechanical Properties of the Laminated Si, N,/BN Composites with Al,O, as Separating Layer Modifier ALO 18.7±2.04659±43.3 24.9±1.6623.1±27 28.9±4.17095±89 23.4±2.0651.5±74.9 AL,O 100 7.7±1.8666.8±37.9 toughness of the composites are the highest. The above expe ments show that this optimal interface composition is 50 wt% BN +50 wt% AlO DO um (B) Adding SiNN, as a Separating Layer Modifier: Table Ill modifier on the bulk mechanical properties of laminated Si, N,/BN (b) composites. The results show that Si,N, as a separating layer modifier appears a little bit different from Al,O,. With an increase of Si N, content in the BN layers, the bending strength of the composites increases and reaches a maximum at 75 wt% Si, Nas while the apparent fracture toughness of the composites reaches a maximum at 10 wt% SiN, and then decreases The effect of Si Na doping in the BN interfacial layer is different from that of Al,O, doping. During sintering of the laminated Si N,/BN composites. the sintering aids in the Si, N matrix layer diffuse into the BN interfacial layer and assist with sintering the Si N4 doped in the BN interfacial layer. With an increase of Si Na content in the BN interfacial layer, the sintering improves and thereby the bonding strength of the interfacial layer increases gradually. When the content of Si,N, in the BN interfacial layer is close to 100 wt%, the interfacial layer is almost the same as the matrix layer, and the laminated composite behaves as a monolithic brittle Si,Na ceramic. Figure 7 shows the effect of Si,N, content in the BN interfacial layer on the shear strength of 80 um he interfacial layer. It can be found that the shear strength of the interfacial layer linearly increases with respect to the content of Si,Na in the BN interfacial layer. In the case of this interfacial state, the stronger the interfacial layer, the higher the strength of the composites, but at the expense of the apparent fracture toughness of the composite. As a compromise, about 10 wt% E600 180pm Fig. 4. SEM photographs of crack propagating paths in laminated SiN/BN composites showing (a) crack deflection. (b) crack bifurcate and (e)matrix layer pull-out. 20-1000oC)and Si, N, (3.2 X 10"/K 20-1000 C), the interfa- cial layer is under tensile stress after cooling from the sintering AL,O, content in BN separating layer(wt%) state, which is unfavorable for strength and toughness of the composite. Therefore, there exists an optimal moderate amount of Al O, doped in the BN interfacial layer, at which the strength and with AL, O, as a separating layer modifierhinated Si,N,/BN Fig. 5. Mechanical properties of the lar2460 Journal of the American Ceramic Societ}-—Wang et al. Vol. 85, No. 10 Table II. Mechanical Pniperties of the Laminated Si^N/BN Composites with AI,O, as Separatin};; Layer Modifier Fig. 4. SEM photographs of crack propagiiiing paths in laminated SiiNj/BN coiiiposites showing (a) crack detlectioii, (b) crack biturcation, and (c) matrix layer pull-out. 20-I000"C) and Si.N^ (3,2 X lO'^/K. 20-1000°C). the interfa￾cial layer is under ten.sile stress after cooling from the sintering state, which is unfavorable for strength and toughness of the coniposite, Theretore. there exists an optimal moderate amount of AliO, doped in the BN intertacial layer, at which the strength and Sample Dopant in BN sepiirrttjng iiiyer Dopani conieni in BN Mjparaiing luver (wt%) Apparcni fraciure toughness (MPa-m"'') Bending strength (MPa) AI2O3 0 25 50 75 100 18,7 24.9 28,9 23.4 7,7 ± 2,0 ± 1.6 ±4.1 ±2.0 ± 1,8 465.9 ± 43. 623.1 ± 27, 709.5 ± 89, 651.5 ± 74. 666.8 ± 37, 3 1 6 9 9 toughness of the composites are the highest. The above experi￾ments show that this optimal interface composition is 50 wt% BN + 50 wt% AUO3. (B) Adding Si^N^ as a Separating Layer Modifier: Table III and Fig. 6 show the effects of Si,N4 as another separating layer modifier on the bulk mechanical properties of laminated Si ,N^/BN composites. The results show that Si,N4 as a separating layer modifier appears a little bit different from AUO^. With an increase of Si-,N4 content in the BN layers, rhe bending strength of the composites increases and reaches a maximum at 75 wtVr Si,N^. while the apparent fracture toughness of the composites reaches a maximum at 10 wt'Jf SiiN4 and then decreases. The effect of Si,N4 doping in the BN interfacial layer is different from that of AUO^ doping. During sintering of the laminated Si,N4/BN composites, the sintering aids in the SiiN^ matrix layer diffuse into the BN interfacial layer and assist with sintering the Si^Nj doped in ihe BN interfaciat layer. With an increase of Si^Nj content in the BN interfacial layer, the sintering improves and thereby the bonding strength of the interfacial layer increases gradually. When the content of SiiNj in the BN interfacial layer is close to 100 wf^/r. the interfacial layer is almost the same as the matrix layer, and the laminated composite behaves as a monolithic brittle .Si^Nj ceramic. Figure 7 shows the effect of SiiNj content in the BN interfacial layer on the shear strength of the interfacial layer,"" It can be found that the shear strength of the interfaeial layer linearly increa.ses with respect to the content of Si3N4 in the BN interfacial layer. In the case of this interfacial state, the stronger the interfacial layer, the higher the strength of the composites, but at the expense of the apparent fracture toughness of the composite. As a compromise, about 10 wt% OJD 2 •5 c 900 800 700 600 500 CO 400 - Hu ^^ es b y 0 20 40 60 80 100 Al^Oj content in BN separating layer (wt%) Fig. 5. Mechanical properties of the laminated Si,N4/BN composites with AUO, as a separaUng layer mtKlifier