J.x. Zhang et al. /Ceramics International 30(2004)697-703 699 als and Y203(2.85 wt %)and Al203(2.15%)as sintering (SENB) method on test bars of 4 mm high, 3 mm wide and additives. Details concerning the aqueous tape casting pro- 36 mm long by a three-point bend using a span of 30mm cess could be obtained from previous published papers and a cross head speed of 0.05 mm-min. A straight notch [36, 37]. The thickness of the Sic green tapes was adjusted with fine diamond saw was introduced with a depth at about to -150 um To introduce weak interface between SiC lay- 2 mm. Work of fracture was obtained by dividing the area un ers, thin C-SiC sheets were also prepared by tape casting der the load-displacement curve by twice the cross-section process using doctor blade equipment. The C powders(Op- area of the sample [381 tical pure, Shanghai Carbon Element Factory) were firstly dispersed in deionized water followed by the addition of Sic powders with controlled composition. PVAl788 was 23. Microstructure characterization selected as binder and glycerol as plasticizer. The thickness of C sheets was adjusted to 30 um. Much thinner layer Microstructure characterization was performed by op- were also prepared by the so-called"screen printing"pro- cal microscopy and SEM. Energy dispersive X-ray cess. The surface of each SiC sheets was coated by passing (EDX)spectra were obtained across the interfacial layer the aforementioned C slurries through a 200-mesh screen. to determine the diffusion of elements from adjacent 46-20 um range through the adjustment of solid content of characterize Crack pattern of fractured surface was also The thickness of the interfacial layers was varied in the said slurry. After coating, the sheets were dried and stacked in the repeating sequence of SiC and C-sic layers Subsequently, organic additives were removed by heating to 800C in a flowing argon atmosphere. a slow heating rate was selected to minimize bloating and cracking during py- rolysis, which might result in distortion of the layers. After pyrolysis, the billets were placed in a BN-coated graphite die. Consolidation was performed by hot pressing at 35 MPa under an argon atmosphere at temperature of 1850C for 0.5h 2. 2. Mechanical and microstructure evaluation of Denection Specimens for flexural test were cut and ground into rect- angular bars. A schematic illustration of bending test process for laminated SiC composites is shown in Fig. 1. Three-point bending strength was determined at room temperature on five to six 3 mm x 4 mm x 36 mm bars with a span of 30 mm and a cross head speed of 0.5- min-.Apparent toughness was measured by the single-edge-notched-beam 0.10.2030.40.5 Denection(mm) p→ Fracture toughness 5 三品 s Content of C(wt%)/12 6 20 0.30.4 Content of C in interfacial layers(wt%) Deflection (mm) Fig. 4. Influence of interfacial composition on the mechanical properties Fig. 5. Load-deflection curves of SiC laminates(a)SiC30-C70,(b) of Sic laminates SiC40-C60,(c)SiC50-C50.J.X. Zhang et al. / Ceramics International 30 (2004) 697–703 699 als and Y2O3 (2.85 wt.%) and Al2O3 (2.15%) as sintering additives. Details concerning the aqueous tape casting pro￾cess could be obtained from previous published papers [36,37]. The thickness of the SiC green tapes was adjusted to ∼150
m. To introduce weak interface between SiC lay￾ers, thin C–SiC sheets were also prepared by tape casting process using doctor blade equipment. The C powders (Op￾tical pure, Shanghai Carbon Element Factory) were firstly dispersed in deionized water followed by the addition of SiC powders with controlled composition. PVA1788 was selected as binder and glycerol as plasticizer. The thickness of C sheets was adjusted to ∼30
m. Much thinner layers were also prepared by the so-called “screen printing” pro￾cess. The surface of each SiC sheets was coated by passing the aforementioned C slurries through a 200-mesh screen. The thickness of the interfacial layers was varied in the 5–20
m range through the adjustment of solid content of the said slurry. After coating, the sheets were dried and stacked in the repeating sequence of SiC and C–SiC layers. Subsequently, organic additives were removed by heating to 800 ◦C in a flowing argon atmosphere. A slow heating rate was selected to minimize bloating and cracking during py￾rolysis, which might result in distortion of the layers. After pyrolysis, the billets were placed in a BN-coated graphite die. Consolidation was performed by hot pressing at 35 MPa under an argon atmosphere at temperature of 1850 ◦C for 0.5 h. 2.2. Mechanical and microstructure evaluation of laminated composites Specimens for flexural test were cut and ground into rect￾angular bars. A schematic illustration of bending test process for laminated SiC composites is shown in Fig. 1. Three-point bending strength was determined at room temperature on five to six 3 mm × 4 mm × 36 mm bars with a span of 30 mm and a cross head speed of 0.5 mm·min−1. Apparent toughness was measured by the single-edge-notched-beam 20 40 60 80 0 250 500 750 1000 20 40 60 0 1500 3000 4500 Work of fracture ( J.m-2) Content of C (wt%) Strength Fracture toughness Content of C in interfacial layers (wt%) Strength (MPa) 6 8 10 12 14 16 18 Fracture toughness (MPa.s-1 ) Fig. 4. Influence of interfacial composition on the mechanical properties of SiC laminates. (SENB) method on test bars of 4 mm high, 3 mm wide and 36 mm long by a three-point bend using a span of 30 mm and a cross head speed of 0.05 mm·min−1. A straight notch with fine diamond saw was introduced with a depth at about 2 mm. Work of fracture was obtained by dividing the area un￾der the load–displacement curve by twice the cross-section area of the sample [38]. 2.3. Microstructure characterization Microstructure characterization was performed by op￾tical microscopy and SEM. Energy dispersive X-ray (EDX) spectra were obtained across the interfacial layer to determine the diffusion of elements from adjacent layers. The crack pattern of fractured surface was also characterized. Fig. 5. Load–deflection curves of SiC laminates (a) SiC30-C70, (b) SiC40-C60, (c) SiC50-C50