Si3N4 matrix composites reinforced by Sic whiskers, Sic particles, or both were fabricated using the hot-pressing technique. The mechanical properties of the composites containing various amounts of these Sic reinforcing materials and different sizes of Sic particles were investigated. Fracture toughness of the composites was significantly improved by introducing Sic whiskers and particles together, compared with that obtained by adding Sic whiskers or SIC particles alone. On increasing the size of the added Sic particles, the fracture toughness of the composites reinforced by both whiskers and particles was increased. Their fracture toughness also showed a strong dependence on the amount of Sic particles (average size 40 pm) and was a maximum at the particle content of 10 ~01%. The maximum fracture toughness of these composites was 10.5 MPa-m1'2 and the flexural strength was 550 MPa after addition of 20 vol% of Sic whiskers and 10 vol% of Sic particles having an average particle size of 40 pm. These mechanical properties were almost constant from room temperature to temperatures around 1000°C. Fracture surface observations revealed that the reinforcing mechanisms acting in these composites were crack deflection and crack branching by Sic particles and pullout of Sic whiskers. [
Dense, fine-grained silicon carbide (Sic) ceramics were fabricated by a hot-pressing technique using pyrolyzed polycarbosilane powders. Hot-isostatic pressing treatments were also applied to some of these hot-pressed samples. The grainsize range of the obtained sintered bodies was from 0.2 to 1.4 pm, which was much finer than that of ordinary sintered Sic ceramics. Relationships among sintering conditions, microstructures, and fracture toughness of the obtained ceramics were investigated. A clear grain-size dependence of fracture toughness was observed in this very fine-grain region (0.2 to 1.4 pm). Fracture toughness showed its maximum (5.1 MPa-m'") at the average grain size of -0.7 pm. Also, the fracture toughness of the samples having similar grain sizes increased with increasing relative density. [
Sic-monofilament-reinforced SIC or Si3N4 matrix composites were fabricated by hot-pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filamentlmatrix interface and fracture toughness of S i c monofiament/Si,N, matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with fdament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si& monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the Sic monofilament/Si3N4 matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the Sic monofilament/Si,N, matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications. [
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