D. Liet al. Ceramics International 30(2004)213-217 SiC+additive 2. 2. SB test To evaluate damage resistance and r-curve behavi Milling(24h)+ persant the hot-pressed billets of both LSB and Ms were cut and ground into rectangular specimens; the prospectiv tensile surfaces of the specimens were perpendicular to Plasticizer Second milling(24h)+ Binder the hot-pressing direction, normal to the layer plane The nominal dimensions of the test specimens were 3x4x30 mm. The test specimens were indented at the Degassing Tamer center of the polished prospective tensile surface(4-mm- wide side) using a Vickers diamond pyramid indenter under loads ranging from 0. 1 to 300 N. Care was taken Tape-casting to orient one set of the indentation cracks to be parallel to the longitudinal axis of the rectangular specimens After indentation, the specimens were tested in three point bending with a span of 24 mm and a crosshead speed of 0.5 mm/min(Testing machine: instron 1195) Four tests were performed for each indentation load Conversion of vickers ISB data o(P), to generate a toughness curve, T(c) [T(c)=KgI is as follows [16] Punching and stacking Conversion is accomplished using an objective indenta tion-strength K-field analysis. Under the action of applied stress oA, radial cracks of size c produced at an Binder removal indentation load P extend according to the equilibrium ↓ condition ot-pressing sintering KA(c)=roAc/2+xPc-3/2=T(c) Fig. 1. Flow chart of fabrication process of LSB where KA(c) is a global applied stress intensity factor corresponding to an applied stress oA, T(c) the tough ness curve, and y the crack shape factor which depend slurry. After coating, the green laminates were dried, on crack and specimen geometry. x the residual contact stacked, and the organic additives were removed. coefficient which is often expressed in terms of Young Finally, they were sintered under a 30 MPa pressure at modulus E and hardness H[7]x=5(E/H 5 being 1850C for I h in a nitrogen atmosphere. The obtained factor of proportionality equals to 0.016[18]. The value layered structure is shown in Fig. 2. As can be seen, the of E/H can be measured by the Knoop indentation Sic layers(dark regions) and bn(gray regions) are method proposed by Marshall et al. 19 reasonably uniform and the interfaces are straight and For a given indentation load P, failure occurs at tha cated by stacking SiC green tapes without BN coating condill tress o well-distinguishable. For comparison, MS were fabri f which satisfies the " tangency using the same fabrication process as for LSB dKa(c)/dc= dT(c)/dc (3) Accordingly, given an appropriate calibration of the coefficients y and x, families of K'a(c) curves can be generated from the o p)data. T(c) then can be deter- mined objectively as envelopes of tangency points to these families of curves [16 3. Results and discussio 3. 1. Load-displacement curves Fig. 3 shows the load-displacement curves of the indented specimens for LSB and MS. Evidently, the fracture behavior of LSB was quite different from that Fig. 2. SEM micrograph of the cross section of LSB of MS. Ms fractured catastrophically, while LSBslurry. After coating, the green laminates were dried, stacked, and the organic additives were removed. Finally, they were sintered under a 30 MPa pressure at 1850 C for 1 h in a nitrogen atmosphere. The obtained layered structure is shown in Fig. 2. As can be seen, the SiC layers (dark regions) and BN (gray regions) are reasonably uniform and the interfaces are straight and well-distinguishable. For comparison, MS were fabri￾cated by stacking SiC green tapes without BN coating using the same fabrication process as for LSB. 2.2. ISB test To evaluate damage resistance and R-curve behavior, the hot-pressed billets of both LSB and MS were cut and ground into rectangular specimens; the prospective tensile surfaces of the specimens were perpendicular to the hot-pressing direction, normal to the layer plane. The nominal dimensions of the test specimens were 3430 mm. The test specimens were indented at the center of the polished prospective tensile surface (4-mm￾wide side) using a Vickers diamond pyramid indenter under loads ranging from 0.1 to 300 N. Care was taken to orient one set of the indentation cracks to be parallel to the longitudinal axis of the rectangular specimens. After indentation, the specimens were tested in three￾point bending with a span of 24 mm and a crosshead speed of 0.5 mm/min (Testing machine: instron 1195). Four tests were performed for each indentation load. Conversion of Vickers ISB data
f (P), to generate a toughness curve, T(c) [T(c)=KR] is as follows [16]. Conversion is accomplished using an objective indenta￾tion-strength K-field analysis. Under the action of applied stress
A; radial cracks of size c produced at an indentation load P extend according to the equilibrium condition, K0 AðcÞ ¼
Ac1=2 þ Pc 3=2 ¼ TðcÞ ð1Þ where K0 A(c) is a global applied stress intensity factor corresponding to an applied stress
A, T(c) the tough￾ness curve, and the crack shape factor which depends on crack and specimen geometry.  the residual contact coefficient which is often expressed in terms of Young’s modulus E and hardness H [17], ¼ ðE=HÞ 1=2  being a factor of proportionality equals to 0.016 [18]. The value of E/H can be measured by the Knoop indentation method proposed by Marshall et al. [19]. For a given indentation load P, failure occurs at that applied stress
A=
f which satisfies the ‘‘tangency condition’’, dK0 AðcÞ=dc ¼ dTðcÞ=dc ð3Þ Accordingly, given an appropriate calibration of the coefficients and , families of K0 A(c) curves can be generated from the
f(P) data. T(c) then can be deter￾mined objectively as envelopes of tangency points to these families of curves [16]. 3. Results and discussion 3.1. Load–displacement curves Fig. 3 shows the load–displacement curves of the indented specimens for LSB and MS. Evidently, the fracture behavior of LSB was quite different from that of MS. MS fractured catastrophically, while LSB Fig. 1. Flow chart of fabrication process of LSB. Fig. 2. SEM micrograph of the cross section of LSB. 214 D. Li et al. / Ceramics International 30 (2004) 213–217