Zan Qingfeng ef aL /Ceramics international 33(2007)385-388 3000 Table 3 Residual stresses of the AlO,Ti3SiC2 multilayer ceramics with SiCw Samples EA ET V 9Tg8Ec3 (GPa)(GPa) (x10-°/°C)(×10-6C)(MP 390.03210.3000.28.800 8.47 -121.2 W10409.03210.2900.28.370 8.47 37.8 W1541853210.2850.28.155 8.47 W20 428.03210.2800.27940 8.47 203.1 25437.53210.2750.27.725 8.47 287.3 447.03210.2700.27.510 8.47 372.3 1800 The residual compressive stresses in Al2O3 layer. SiCw(vol%) Fig. 2. Mechanical properties of the multilayer ceramics with SiC whiskers The toughening mechanisms induced by whisker in the multilayer ceramics, namely 2nd-level toughening mechan- sms, are similar to the toughening mechanisms in whisker mixture. And the calculated results containing the mechanical reinforced ceramic composites and have been studied ripely parameters and residual stresses of each layer are shown in they include crack deflection mechanism, crack bendin Table 3. With the content of whisker increasing, the residual mechanism, whisker pull-out mechanism, bridging mechanism, stresses in Al2O3 layer changed from tensile to compressive. micro-crack mechanism, and so on [1-5]. Fig. 3 shows the The residual compressive stresses could strengthen the Al2O3 microstructure of Al2O3 layer in the multilayer ceramics. The layer, thereby the fracture behavior and the toughening whiskers are observed homogeneously distributed and aligned mechanisms of the multilayer ceramics were also changed, directionally in two directions. The 2D texture of whiskers as described in Section 3.3 should be formed in the rolling procedure for green tape Otherwise, it is apparent from in Table 3 that when the preparation, and this was very useful to the whisker toughenin content of Sic whisker reaches 30 vol%, the residual tensile mechanisms. Otherwise, the pull-out mechanism selected as a stress in Ti3 SiC2 layers is up to 372 MPa. According to the representative one is shown in Fig. 4 papers about Ti3 SiC2 bulk ceramic [17, 18], the bending As we know, the toughening effects of the multilayered rength was reported to be only about 260-320 MPa which was structural toughening mechanisms depend strongly on the crack well-known higher than its tensile strength. Hence, the propagation path and the mechanical properties of the matrix multilayer ceramics with 30 vol% or more SiCw would be layer and interphase-layer [10]. When the Sic whiskers are broken by the residual stresses during sintering and following added into Al2O3 layers, the microstructure and mechanical Coo properties of the Al2O3 layer is changed at first. The change of mechanical properties of Al2O3 layer can influence the location 3.3. Toughening mechanisms and synergy effect of crack deflection and crack propagated path. In other words, it influences the 1st-level mechanisms. Otherwise. with the The high toughness of multilayer ceramics should be addition of Sic whiskers the residual stresses are changed as attributed to different kinds of toughening mechanisms. In the already analyzed above. From Table 2, the layers carry the AL2O,/Ti3 Sic2 multilayer ceramics with SiC whiskers, these residual compressive stress when whiskers are added. It makes toughening mechanisms could be: multilayered structural the Al2O3 layers difficult to fracture, and the crack is enforced toughening, whisker toughening and synergy effect by here ibefore two mechanisms The toughening mechanisms induced by multilayered structure,defined as 1st-level toughening mechanisms in this paper, have been investigated in recent years [9-14 they include crack deflection mechanism, bridging mechanism of interlocking layers, frictional sliding mechanism, and echanical parameters of Al2O. Ti SiC2, TiC and SiC whisker Materials Al,O3 Ti3SiC2 E(GPa) 0.20 0.20 10 CTE(10-°Cy 8.8 7.74 4.5 Fig 3. Fracture surface of the Al2O,Ti3 SiC2 multilayer ceramics with 20 vol% Assumed to be constant from room temperature to sintering temperaturemixture. And the calculated results containing the mechanical parameters and residual stresses of each layer are shown in Table 3. With the content of whisker increasing, the residual stresses in Al2O3 layer changed from tensile to compressive. The residual compressive stresses could strengthen the Al2O3 layer, thereby the fracture behavior and the toughening mechanisms of the multilayer ceramics were also changed, as described in Section 3.3. Otherwise, it is apparent from in Table 3 that when the content of SiC whisker reaches 30 vol%, the residual tensile stress in Ti3SiC2 layers is up to 372 MPa. According to the papers about Ti3SiC2 bulk ceramic [17,18], the bending strength was reported to be only about 260–320 MPa which was well-known higher than its tensile strength. Hence, the multilayer ceramics with 30 vol% or more SiCW would be broken by the residual stresses during sintering and following cooling procedures. 3.3. Toughening mechanisms and synergy effect The high toughness of multilayer ceramics should be attributed to different kinds of toughening mechanisms. In the Al2O3/Ti3SiC2 multilayer ceramics with SiC whiskers, these toughening mechanisms could be: multilayered structural toughening, whisker toughening and synergy effect by here￾inbefore two mechanisms. The toughening mechanisms induced by multilayered structure, defined as 1st-level toughening mechanisms in this paper, have been investigated in recent years [9–14], they include crack deflection mechanism, bridging mechanism of interlocking layers, frictional sliding mechanism, and so on. The toughening mechanisms induced by whisker in the multilayer ceramics, namely 2nd-level toughening mechan￾isms, are similar to the toughening mechanisms in whisker reinforced ceramic composites and have been studied ripely, they include crack deflection mechanism, crack bending mechanism, whisker pull-out mechanism, bridging mechanism, micro-crack mechanism, and so on [1–5]. Fig. 3 shows the microstructure of Al2O3 layer in the multilayer ceramics. The whiskers are observed homogeneously distributed and aligned directionally in two directions. The 2D texture of whiskers should be formed in the rolling procedure for green tape preparation, and this was very useful to the whisker toughening mechanisms. Otherwise, the pull-out mechanism selected as a representative one is shown in Fig. 4. As we know, the toughening effects of the multilayered structural toughening mechanisms depend strongly on the crack propagation path and the mechanical properties of the matrix￾layer and interphase-layer [10]. When the SiC whiskers are added into Al2O3 layers, the microstructure and mechanical properties of the Al2O3 layer is changed at first. The change of mechanical properties of Al2O3 layer can influence the location of crack deflection and crack propagated path. In other words, it influences the 1st-level mechanisms. Otherwise, with the addition of SiC whiskers the residual stresses are changed as already analyzed above. From Table 2, the layers carry the residual compressive stress when whiskers are added. It makes the Al2O3 layers difficult to fracture, and the crack is enforced Zan Qingfeng et al. / Ceramics International 33 (2007) 385–388 387 Fig. 2. Mechanical properties of the multilayer ceramics with SiC whiskers. Table 2 The mechanical parameters of Al2O3, Ti3SiC2, TiC and SiC whisker Materials Al2O3 Ti3SiC2 TiC SiCW E (GPa) 390 320 322 580 n 0.30 0.20 0.20 0.20 CTE (106 /8C)a 8.8 9.2 7.74 4.5 a Assumed to be constant from room temperature to sintering temperature. Table 3 Residual stresses of the Al2O3/Ti3SiC2 multilayer ceramics with SiCW Samples EA (GPa) ET (GPa) nA nT aA (106 /8C) aT (106 /8C) sR a (MPa) W0 390.0 321 0.300 0.2 8.800 8.47 121.2 W10 409.0 321 0.290 0.2 8.370 8.47 37.8 W15 418.5 321 0.285 0.2 8.155 8.47 120.0 W20 428.0 321 0.280 0.2 7.940 8.47 203.1 W25 437.5 321 0.275 0.2 7.725 8.47 287.3 W30 447.0 321 0.270 0.2 7.510 8.47 372.3 a The residual compressive stresses in Al2O3 layer. Fig. 3. Fracture surface of the Al2O3/Ti3SiC2 multilayer ceramics with 20 vol% SiC whiskers.