386 Zan Qingfeng et al. /Ceramics International 33(2007)385-388 2. Experimental procedure 3. Results and discussion 2. 1. Materials and preparation 3. 1. Mechanical properties The Al2O,/Ti3SiC2 multilayer ceramics were prepared The mechanical properties of the Al2O3/Ti3SiC2 multilayer by in situ hot-pressing method according to a procedure ceramics, including bending strength and work of fracture, are described elsewhere [14]. The raw materials of Ti3 SiC2 layers shown in Table I and Fig. 2. With increasing the content of Sic were Ti, SiC and activated carbon powders. Otherwise, the whisker, the bending strength of the materials slightly increased Al2O3 layers were sintered from Al2O3 powders with MgO as firstly, and then decreased when the content of Sic whisker sintering aid and Sic whiskers as mechanical property exceeded 20 vol%. A similar trend was found for the work of is shown in Fig. 1. Firstly, powders of Al2O3(99.5% pure mechanical properties of 688 MPa for bending strength and Ceramic Co., China)with 0.3 wt% MgO(99.9% pure) and 2583 J/m- for work of fracture were obtained. Compared to the given certain quantities of SiC whiskers for the Al2O3 layer and multilayer without whisker (wO), the gain in of strength and powders of Ti(99.9% pure), SiC(99.9% pure) and activated work of fracture for materials w20 were about 4.4 and 19.6%0, carbon with the atomic ratio of 3: 1: 1 for the Ti3SiC2 layer were respectivel milled for 24 h in alcohol medium, respectively. Secondly, the Otherwise. it was found that when whisker content was green tapes of Al2O3 and Ti-SiC-C with thickness of about larger than 30 vol%, the specimens were broken after sintering 200 um both were manufactured by rolling after adding PVA, and cooling, which looked likely because of the residual glycerol and liquid affix into the raw powders. Then these stresses(as described in the following section in detail) ree vere dried in air and punched into rounds (0 50 mm). They were packed into a graphite die by turns of 3.2. Residual stresses analysis Al2O3 tapes and the Ti3SiC2 raw owders tape The green body underwent a conventional binder burnout through heating in For multilayer ceramics, especially with'strong'-interface, ing air. Finally, the multilayer ceramic was hot pressed the mechanical properties were reported to be influence 1600C for 4 h in Ar atmosphere with a pressure of about effectively by residual stress states [13], which was produced O MPa due to the thermal expansion mismatch between the interphase- layer and matrix-layer during cooling. The residual stresses 2.2. Tests and characterization would lead to cracking and three primary crack morphologies would be observed: transverse. longitudinal and debonding The specimens were sliced into test bars with the cracks if the residual stresses are large enough or the material dimensions of 4 mm x 3 mm x 36 mm for bending strength loaded. For multilayer materials, the transverse and debonding 30 mm for fracture toughness and the cracks are favorable to improve the toughness fracture work. Three-point bending test was carried out at room The residual stresses states in Al2O3/Ti3 SiC2 multilayer temperature with a span of 30 mm and a crosshead rate of ceramics without SiCw were already analyzed in our previous 0.5 mm/min. The work of fracture and fracture toughness were paper [16]. When SiC whiskers were added into Al2O3 layers, determined by the single-edge-notch-beam method (sENB)at the residual stresses states also changed. The residual room temperature with a span of 24 mm and a crosshead rate of stresses were calculated in the present work by the same 0.05 mm/min. All the tests were performed on A-2000 method, the parameters adopted in the calculation method are Shimadzu universal materials testing machine. The loads listed in Table 2. The sintering temperature was 1600C, and were applied perpendicular to the plane of laminates. The the thermal expansion coefficients of the Al2O3, Ti3 SiC2, TiC microstructure of the materials was observed by sEM and Sic whisker were assumed to be constant from room (CSM900) temperature to sintering temperature. The thickness of Al2O3 layer and Ti3 SiC2 layer were both about 100 um. For the two layers both containing two components, these mechanical parameters of each layer all need to be calculated by the rule of Ao3→Bmng Table 1 The mechanical properties of the Al2O,Ti]SiC2 multilayer ceramics with SiCw PVA+Glycerol+Liquid Paffi SiCw% Work of fracture (vol%) (MPa) Ti+SiC+C 0 659.53±47.3 2159±153 W10 674.38±25.6 0505 2341±146 Hot pressing Burning out 68359士34.1 2507±14 W20 Fig. 1. Flow chart for the preparation of Al,O,/Ti3 SiC, multilayer materials 68844±35.2 2583±142 W2 2326±135 with Sic whiskers 534.87±68.22. Experimental procedure 2.1. Materials and preparation The Al2O3/Ti3SiC2 multilayer ceramics were prepared by in situ hot-pressing method according to a procedure described elsewhere [14]. The raw materials of Ti3SiC2 layers were Ti, SiC and activated carbon powders. Otherwise, the Al2O3 layers were sintered from Al2O3 powders with MgO as sintering aid and SiC whiskers as mechanical property adjusting agent. The flow chart for the preparation of the multilayer materials is shown in Fig. 1. Firstly, powders of Al2O3 (99.5% pure, Jixin Ceramic Co., China) with 0.3 wt% MgO (99.9% pure) and given certain quantities of SiC whiskers for the Al2O3 layer and powders of Ti (99.9% pure), SiC (99.9% pure) and activated carbon with the atomic ratio of 3:1:1 for the Ti3SiC2 layer were milled for 24 h in alcohol medium, respectively. Secondly, the green tapes of Al2O3 and Ti-SiC-C with thickness of about 200 mm both were manufactured by rolling after adding PVA, glycerol and liquid paffix into the raw powders. Then these green tapes were dried in air and punched into rounds (Ø 50 mm). They were packed into a graphite die by turns of Al2O3 tapes and the Ti3SiC2 raw powders tapes. The green body underwent a conventional binder burnout through heating in flowing air. Finally, the multilayer ceramic was hot pressed at 1600 8C for 4 h in Ar atmosphere with a pressure of about 20 MPa. 2.2. Tests and characterization The specimens were sliced into test bars with the dimensions of 4 mm 3 mm 36 mm for bending strength and 4 mm 6 mm 30 mm for fracture toughness and the fracture work. Three-point bending test was carried out at room temperature with a span of 30 mm and a crosshead rate of 0.5 mm/min. The work of fracture and fracture toughness were determined by the single-edge-notch-beam method (SENB) at room temperature with a span of 24 mm and a crosshead rate of 0.05 mm/min. All the tests were performed on A-2000 Shimadzu universal materials testing machine. The loads were applied perpendicular to the plane of laminates. The microstructure of the materials was observed by SEM (CSM900). 3. Results and discussion 3.1. Mechanical properties The mechanical properties of the Al2O3/Ti3SiC2 multilayer ceramics, including bending strength and work of fracture, are shown in Table 1 and Fig. 2. With increasing the content of SiC whisker, the bending strength of the materials slightly increased firstly, and then decreased when the content of SiC whisker exceeded 20 vol%. A similar trend was found for the work of fracture expected for the improved extent. Therefore, when whisker content was about 20 vol% (W20), the optimal mechanical properties of 688 MPa for bending strength and 2583 J/m2 for work of fracture were obtained. Compared to the multilayer without whisker (W0), the gain in of strength and work of fracture for materials W20 were about 4.4 and 19.6%, respectively. Otherwise, it was found that when whisker content was larger than 30 vol%, the specimens were broken after sintering and cooling, which looked likely because of the residual stresses (as described in the following section in detail). 3.2. Residual stresses analysis For multilayer ceramics, especially with ‘strong’-interface, the mechanical properties were reported to be influenced effectively by residual stress states [13], which was produced due to the thermal expansion mismatch between the interphase￾layer and matrix-layer during cooling. The residual stresses would lead to cracking and three primary crack morphologies would be observed: transverse, longitudinal and debonding cracks if the residual stresses are large enough or the material loaded. For multilayer materials, the transverse and debonding cracks are favorable to improve the toughness. The residual stresses states in Al2O3/Ti3SiC2 multilayer ceramics without SiCW were already analyzed in our previous paper [16]. When SiC whiskers were added into Al2O3 layers, the residual stresses states were also changed. The residual stresses were calculated in the present work by the same method, the parameters adopted in the calculation method are listed in Table 2. The sintering temperature was 1600 8C, and the thermal expansion coefficients of the Al2O3, Ti3SiC2, TiC and SiC whisker were assumed to be constant from room temperature to sintering temperature. The thickness of Al2O3 layer and Ti3SiC2 layer were both about 100 mm. For the two layers both containing two components, these mechanical parameters of each layer all need to be calculated by the rule of 386 Zan Qingfeng et al. / Ceramics International 33 (2007) 385–388 Fig. 1. Flow chart for the preparation of Al2O3/Ti3SiC2 multilayer materials with SiC whiskers. Table 1 The mechanical properties of the Al2O3/Ti3SiC2 multilayer ceramics with SiCW Samples SiCW% (vol%) Bending strength (MPa) Work of fracture (J/m2 ) W0 0 659.53 47.3 2159 153 W10 10 674.38 25.6 2341 146 W15 15 683.59 34.1 2507 143 W20 20 688.44 35.2 2583 142 W25 25 534.87 68.2 2326 135