J. Zhang et al. Scripta Materialia 52(2005) Table l Sic Mechanical properties of monolithic and multilayered SiC samples Bulk sic Pressureless Hot Y2O3: Al2O3 3:7 Relative density (%)99.5 98 99 Strength(MPa) 754±47650±54724±31662±4I Toughne 4.5±1.06.8±1.29.5±1.17.58±1.1 (MPa·m-) 10203040如670 tion are also listed for comparison. Our samples with Fig.3. XRD patterns of as sintered SiC samples with initial Al,O3 and Y,O3 molar ratio as 3: 5 really show a high Y2O3: Al2O3 ratio as 3: 7. fracture toughness of 9.5 MPa m 2. There are two explanations for the improved mechanical properties 3. 2. Thermal residual stress Perovskite-like YAlO3 has an orthorhombic crystal tructure, its thermal expansion coefficients are .5×10-°/C,43×10-6°Cand10.8×10-°/Cale the a, b or c axis, respectively [16]. Since this value is al ways larger than that of SiC(4.5 x 10/C), a residual tensile stress is expected between a YAlO3 particle and surrounding matrix on cooling The radial matrix stress (omr) and the tangential matrix stress(ome=-omr2)are based on the hydrostatic stress ah developed surround the YAlO3 grain. It can be estimated from the followi equation [171 (x-xm)△T where ap, am, Vp, Vm, Ep and Em are the thermal expan sion coefficient. Poisson s ratio and youngs modulus of YAlO3 and SiC, respectively. Due to the lack of both ”1:6 elastic modulus and Poissons ratio for YAlO3, we calcu late instead the stress generated by YAG particle in Sic Fig 4. Micrographs of the fracture surface. matrix as a reference. Using 8x 10-6/C and 4.5 x 10-6/ C[18] for ap (YAG)and am(SiC), 333 GPa [19] and 440 GPa [17 for Ep and Em, and 0.30 and 0.25 [20]for SEM micrographs of the fracture surface of SiC sam- Vp and Vm, respectively, we find that the substantial ma ples are shown in Fig 4. From the microstructure of the trix stress can be developed as high as me =-668 MPa laminates, it was difficult to determine the interface be- in compression and omr =1335 MPa in tension for ATof tween the adjacent SiC layers. It is worth noting in 1000C. The existence of such a large stress may weaken Fig. 4 that the Al2O3/Y203 doped Sic ceramics exhibit the inter-phase boundaries, leading to a fracture along a predominantly intergranular fracture mode, Most of such boundaries [19]. Fig. 5 shows that the cracks prop- the grains are uniform and equiaxed. The existence of agated along the grain boundary and the interface be- thermal residue stress may weaken the interphase tween second phase and Sic matrix. It indicates that a boundaries, leading to intergranular fracture behavior. relatively weak interface comes from the amorphous grain boundary films and the thermal mismatch stress 3.2. Mechanical properties at hetero phase boundaries, which facilitated the inter granular fracture The effect of additive composition on various Based on the calculation above it is postulated that mechanical properties of Sic laminates is demonstrated the thermal expansion mismatch may result in micro- in Table 1. The properties of hot pressed and pressure- cracking at the SiC-YAlO3 boundary ahead of a pri less sintered monolithic samples with the same composi mary propagating crack, similar to that in SiC-TiSEM micrographs of the fracture surface of SiC sam￾ples are shown in Fig. 4. From the microstructure of the laminates, it was difficult to determine the interface be￾tween the adjacent SiC layers. It is worth noting in Fig. 4 that the Al2O3/Y2O3 doped SiC ceramics exhibit a predominantly intergranular fracture mode, Most of the grains are uniform and equiaxed. The existence of thermal residue stress may weaken the interphase boundaries, leading to intergranular fracture behavior. 3.2. Mechanical properties The effect of additive composition on various mechanical properties of SiC laminates is demonstrated in Table 1. The properties of hot pressed and pressure￾less sintered monolithic samples with the same composi￾tion are also listed for comparison. Our samples with Al2O3 and Y2O3 molar ratio as 3:5 really show a high fracture toughness of 9.5 MPa Æ m1/2. There are two explanations for the improved mechanical properties. 3.2.1. Thermal residual stress Perovskite-like YAlO3 has an orthorhombic crystal structure, its thermal expansion coefficients are 9.5 · 10
6 /C, 4.3 · 10
6 /C and 10.8 · 10
6 /C along the a, b or c axis, respectively [16]. Since this value is al￾ways larger than that of SiC (4.5 · 10
6 /C), a residual tensile stress is expected between a YAlO3 particle and surrounding matrix on cooling. The radial matrix stress (rmr) and the tangential matrix stress (rmh =
rmr/2) are based on the hydrostatic stress rh developed surround the YAlO3 grain. It can be estimated from the following equation [17]: rmr ¼ rh ¼ ap
am
DT ½ð Þ 1 þ mm =2Emþ½ 1
2mp
=Ep ð1Þ where ap, am, mp, mm, Ep and Em are the thermal expan￾sion coefficient, Poisson
s ratio and Young
s modulus of YAlO3 and SiC, respectively. Due to the lack of both elastic modulus and Poisson
s ratio for YAlO3, we calcu￾late instead the stress generated by YAG particle in SiC matrix as a reference. Using 8 · 10
6 /C and 4.5 · 10
6 / C [18] for ap (YAG) and am (SiC), 333 GPa [19] and 440 GPa [17] for Ep and Em, and 0.30 and 0.25 [20] for mp and mm, respectively, we find that the substantial ma￾trix stress can be developed as high as rmh =
668 MPa in compression and rmr = 1335 MPa in tension for DT of 1000 C. The existence of such a large stress may weaken the inter-phase boundaries, leading to a fracture along such boundaries [19]. Fig. 5 shows that the cracks prop￾agated along the grain boundary and the interface be￾tween second phase and SiC matrix. It indicates that a relatively weak interface comes from the amorphous grain boundary films and the thermal mismatch stress at hetero phase boundaries, which facilitated the inter￾granular fracture. Based on the calculation above, it is postulated that the thermal expansion mismatch may result in micro￾cracking at the SiC–YAlO3 boundary ahead of a pri￾mary propagating crack, similar to that in SiC–TiB2 Fig. 4. Micrographs of the fracture surface. Table 1 Mechanical properties of monolithic and multilayered SiC samples Sintering process Bulk SiC Laminated SiC Hot pressed Pressureless sintering Hot pressed Y2O3:Al2O3 3:7 3:7 3:5 3:7 Relative density (%) 99.5 98 99 99.1 Strength (MPa) 754 ± 47 650 ± 54 724 ± 31 662 ± 41 Toughness (MPa Æ m1/2) 4.5 ± 1.0 6.8 ± 1.2 9.5 ± 1.1 7.58 ± 1.1 Fig. 3. XRD patterns of as sintered SiC samples with initial Y2O3:Al2O3 ratio as 3:7. J. Zhang et al. / Scripta Materialia 52 (2005) 381–385 383