SiC/SiC composites are promising high-temperature structural materials for advanced nuclear and aero-space applications. The advantage of SiC/SiC composites comes from their low specific mass, superior thermomechanical properties and low activation [1-3]. As fabrication processes of SiC/SiC composites, there are three common processes, such as chemical vapor infiltration (CVI) [4], polymer infiltration and pyrolysis (PIP) [5] and reaction sintering/melting infiltration (RS/MI) processes [6]. However, total performances of these composites are still not satisfied for going of industrial stage. Nano-infiltration and transient eutectic-phase (NITE) method is one of the most attractive processes for SiC/ SiC composites fabrication to provide high performance on thermo-mechanical properties, size and shape flexibility and acceptable cost [7-9]. In order to produce the complex shape components of SiC/SiC composites by NITE method, the near-net shaping technique is necessary. In general, large volumetric shrinkage (−50 vol%) occurs during ceramic matrix composites fabrication by hot-pressing like NITE method. This volumetric shrinkage is caused due to infiltration and densification process of powder for matrix formation, resulting in unfortunately significant fiber-architecture and strength damage.

Therefore, the method development for suppression of large volumetric shrinkage during hot-pressing is essential to fabricate the production of complex shape by the damage-less near-net shaping, and one of method for that is preform densification before hot-pressing. In fact, the preform densification demonstrated the maintainability of fiber-architecture in composites due to suppression of large volumetric shrinkage and the improvement of composites’ density and mechanical properties [10]. However, optimization of conditions (temperature, holding time and applied pressure) during preform densification is insufficient.

The objective of this study is to clarify the effects of conditions of preform densification on SiC/SiC composites. In particular, the effects of pressure during preform densification were investigated on microstructure and mechanical property of preforms and SiC/SiC composites.

Pyrocarbon (PyC) coated-Tyranno^TM SA fibers (Ube Industrials Ltd., Japan) were used as reinforcement for SiC/SiC composites fabrications. The PyC coating was appropriately chosen at the thickness of 0.5 μm by chemical vapour deposition (CVD) process. β-SiC nanopowder (IEST, Japan, mean grain size of 32 nm) and sintering additives with Al₂O₃ (Kojundo Chemical Laboratory Co. Ltd., Japan, mean grain size of 0.3 µm, 99.99%) and Y₂O₃ (Kojundo Chemical Laboratory Co. Ltd., Japan, mean grain size of 0.4 µm, 99.99%) were used for matrix formation. For the fabrication of prepreg sheets, PyC-coated Tyranno-SA fibers were impregnated in “nano”-slurry, which consisted of the mixture of SiC nano-powders and sintering additives. Prepreg sheets were stacked for preparation of UD preforms, which is followed by preform densification. The preform densification is performed during heating under isostatic pressures of 1 - 17 MPa. The preforms prepared were hotpressed at 1870˚C for 1.5 h in Ar under a pressure of 20 MPa. The bulk density and open porosity of the preforms and the composites fabricated were measured by the Archimedes’ principle. Mechanical property evaluation was performed by three point bending test with the crosshead speed of 0.5 mm/min and a support span of 16 mm at room temperature. The specimens were straight bar type, which measured 26^L ´ 3^W ´ 1.2^T mm³. Microstructural evaluation was inspected by a JEOL JSM- 6700 F field emission scanning electron microscope (FESEM).