October 2005 laminated silicon nitride stacks s-I 1000 Cer 60 Cer 20.4 0.1 Fig, 4 of the screen-printing process and measured viscosi- Fig. 6. Microstructure of sintered tapes laminated by compression ties as a to the set change of shear rates for laminating pastes coarse B-Si3N4 grains surrounded by fine grains(light gray) and ith different contents of the pre-ceramic polymer condary phase(white)(scanning electron microscopy, SEM, plasma ganic material by separation of organic molecules of low mass The pyrolysis of a laminated stack may lead to stresses and such as hydrocarbons and ammonia. This is accompanied by cracks in the interlayer because of the shrinkage of the highly mass loss of about 22 mass% and a linear shrinkage of 25% precursor-containing interlayer and(almost)no shrinkage of the A normal organic binder, in comparison, burns out completely, green tapes. Such cracks were indeed observed for thick inter- leaving behind porosity. Thus, by using the ceramic precursor, a layers (interlayer thickness a tape thickness)and high amour dense interlayer can be expected. The product after pyrolysis is an amorphous SiC,N, material. During a further heat Thus, cracks during pyrolysis can be avoided by thin interlayers treatment-in this work, the subsequent sintering step--it crys- and medium precursor contents allizes to Si3N4, SiC, and C. These phases will undergo com- In conclusion, the pastes containing 30-50 vol% of the ce- plex reactions with the atmosphere and/or the glassy phase ramic precursor(Cer 30-50) turned out to be best suited for during sintering. lamination of the green tapes With lower amounts of precursor, the effect of the pre-ceramic polymer as a binder and therefore the green strength become too low. For higher fractions of pre- cursors(>50 vol%), problems arise because the sintering be- havior differs very much from that of the tape material a15 (2) Microstructural Development After lamination by compression, the single layers cannot be distinguished in the fracture surface of green parts(Fig. 2). The analysis of polished samples by SEM at high magnification re- veals a homogeneous material as well( Fig. 6) In terlayers of a paste-laminated stack 005 be easily detected after pyrolysis because their density is higher 14% than that of the pyrolyzed green tapes(Fig. 7). The green tapes contain about 25 vol% binder and 30 vol% porosity, -or Temperature/C about 55 vol% porosity, respectively, after binder burno (assuming no shrinkage). In comparison, the measured tot 015 porosity of a sample Cer60(prepared from a mixture of 60 vol% precursor and 40 vol% silicon nitride)is only 33 vol%because of the high ceramic yield of the ceramic precursor. The different densities after pyrolysis affect the sintering be -6vol% precur vior. Generally, the lower the starting density, the higher the 005o shrinkage to achieve full density. Thus, tape material with lower density needs a higher shrinkage for complete densi- fication than the interlayers. During sintering of a stack, the 扌2 difference in shrinkage can lead to internal stresses, incomplet densification of the tape material, or even cracks in the tapes perpendicular to the layers, which in fact had been observed under non-optimized processing conditions(Fig. 8). The pre- cautions mentioned above(thin interlayers, medium precursor contents) helped to prevent cracks. 0.10 The overall densities of sintered stacks decrease slightly with ncreasing precursor content of the laminating pastes: stack T/C Fig. 5. Mass loss of (a)the green tape and(b) the cured ceramic pre- hereas the densities of stacks made with the pastes decrease to cursor in mixtures from 100 vol%(Cer100) to 50 vol% precursor and 3.247(Cer30)and 3. 241( Cer50)g/cm. This proves that the powder(Cer50)(flowing nitrogen atmosphere, 5 K/min heating rate). ering behavior is negatively influenced by the presence of theganic material by separation of organic molecules of low mass such as hydrocarbons and ammonia.16 This is accompanied by a mass loss of about 22 mass% and a linear shrinkage of 25%.17 A normal organic binder, in comparison, burns out completely, leaving behind porosity. Thus, by using the ceramic precursor, a dense interlayer can be expected. The product after pyrolysis is an amorphous SiCxNy material. During a further heat treatment—in this work, the subsequent sintering step—it crys￾tallizes to Si3N4, SiC, and C.16 These phases will undergo com￾plex reactions with the atmosphere and/or the glassy phase during sintering. The pyrolysis of a laminated stack may lead to stresses and cracks in the interlayer because of the shrinkage of the highly precursor-containing interlayer and (almost) no shrinkage of the green tapes. Such cracks were indeed observed for thick inter￾layers (interlayer thickness  tape thickness) and high amounts of pre-ceramic precursors in the laminating paste (  70%). Thus, cracks during pyrolysis can be avoided by thin interlayers and medium precursor contents. In conclusion, the pastes containing 30–50 vol% of the ce￾ramic precursor (Cer30–50) turned out to be best suited for lamination of the green tapes. With lower amounts of precursor, the effect of the pre-ceramic polymer as a binder and therefore the green strength become too low. For higher fractions of pre￾cursors (450 vol%), problems arise because the sintering be￾havior differs very much from that of the tape material. (2) Microstructural Development After lamination by compression, the single layers cannot be distinguished in the fracture surface of green parts (Fig. 2). The analysis of polished samples by SEM at high magnification re￾veals a homogeneous material as well (Fig. 6). In comparison, the interlayers of a paste-laminated stack can be easily detected after pyrolysis because their density is higher than that of the pyrolyzed green tapes (Fig. 7). The green tapes contain about 25 vol% binder and 30 vol% porosity,10—or about 55 vol% porosity, respectively, after binder burnout (assuming no shrinkage). In comparison, the measured total porosity of a sample Cer60 (prepared from a mixture of 60 vol% precursor and 40 vol% silicon nitride) is only 33 vol%17 because of the high ceramic yield of the ceramic precursor. The different densities after pyrolysis affect the sintering be￾havior. Generally, the lower the starting density, the higher the shrinkage to achieve full density. Thus, tape material with a lower density needs a higher shrinkage for complete densi- fication than the interlayers. During sintering of a stack, the difference in shrinkage can lead to internal stresses, incomplete densification of the tape material, or even cracks in the tapes perpendicular to the layers,18 which in fact had been observed under non-optimized processing conditions (Fig. 8). The pre￾cautions mentioned above (thin interlayers, medium precursor contents) helped to prevent cracks. The overall densities of sintered stacks decrease slightly with increasing precursor content of the laminating pastes: stacks made by compression have a sintering density of 3.252 g/cm3 , whereas the densities of stacks made with the pastes decrease to 3.247 (Cer30) and 3.241 (Cer50) g/cm3 . This proves that the sintering behavior is negatively influenced by the presence of the Fig. 4. Simulation of the screen-printing process and measured viscosi￾ties as a response to the set change of shear rates for laminating pastes with different contents of the pre-ceramic polymer. Fig. 5. Mass loss of (a) the green tape and (b) the cured ceramic pre￾cursor in mixtures from 100 vol% (Cer100) to 50 vol% precursor and powder (Cer50) (flowing nitrogen atmosphere, 5 K/min heating rate). Fig. 6. Microstructure of sintered tapes laminated by compression: coarse b-Si3N4 grains surrounded by fine grains (light gray) and secondary phase (white) (scanning electron microscopy, SEM, plasma￾etched). October 2005 Laminated Silicon Nitride Stacks 2717