Cera-NITE^®:
Advanced SiC/SiC Composites by NITE Process

Cera-NITE is the first generation high performance advanced SiC/SiC composite material products based on the NITE (Nano-Infiltration and Transient Eutectic Phase) Process, invented and patented by Professor A. Kohyama and his research group at Kyoto University.
As the well known SiC/SiC Composite material fabrication processes, PIP (Polymer Impregnation and Pyrolysis) RS/MI Reaction Sintering/Melt Infiltration) and CVI (Chemical Vapor Infiltration) can be applied. However, as shown in Fig.1, inter-fiber bundle pores, matrix micro-cracks and/or micro-segregation of impurities causes degradation under severe environments, such as ultra-high temperature, radiation environments.
Only Cera-NITE, with highly dense SiC matrix and very limited porosity, can provide excellent gas tightness and stability of many properties under severe environments.

Fig. 1: Difference in microstructure on different fabrication processes;Excellence of Cera-NITE

Fig. 2: TEM microstructure of Cera-NITE

Cera-NITE uses highly crystalized near stoichiometry SiC fiber, Tyranno-SA. The production of Cef-NITE (Tyranno-SA grade SiC fibers produced under the licensing of their patents) is now under preparation and Cef-NITE will be available from early 2007 and will be used as reinforcement of Cera-NITE. SiC matrix is introduced using a few tens of nm level nano-SiC powders. As shown in Fig. 2, highly dense, very low porosity SiC matrix (grain size is very similar with that of fibers) is formed inter- and intra-fiber bundles. Carbon coating is used as fiber-matrix interfacial layer. Other options are available including C/SiC multi-layers. Continuous coating line is under development and this process may improve total properties including uniformity and reduction of intra-bundle porosity.

Density, Tensile Properties;

Cera-NITE has a highly dense SiC matrix which makes the density of Cera-NITE higher than 3Mg/m³. This density can be obtained with nearly the same process time, almost regardless of material size and shape, which is the advantage of NITE process. Cera-NITE has an excellent mechanical properties, where tensile test results, based on the ASTM C1275 standard, provided tensile strength, elastic modulus and proportional limit strength (PLS), as 400 MPa, 310 GPa and 125 MPa, respectively. (Cera-NITE UD Lot#0408case)

Thermal Loading/Thermal Shock Resistance and Oxidation Resistance;

Owing to the well controlled microstructure with the similar crystal grain size in both matrix and fibers, Cera-NITE presents excellent thermal loading/thermal shock resistance. The very severe thermal cycling test done at Nagasaki Works, Mitsubishi Heavy Industries, is an good example. 1350 C heating for 125 second by burning gas flame followed by the quick quenching by cool air caused cracking before 50cycles for the case of advanced PIP-SiC/SiC developed by Japanese national program from 1991 to 2000, whereas the first product from Pilot Grade Cera-NITE at Ube Industries had no detectable cracks after 100 cycles and showed no mechanical property degradation (Shown in Fig. 3). Thermal stress figure of merit for Cera-NITE is outstanding from room temperature to 1600C and higher, is beating almost all high temperature structural materials in this temperature range. The high dense matrix of Cera-NITE prevents the oxidation of carbon interface which gives higher resistance to oxidation of SiC/SiC composites.

PIP SiC/SiC Liner: Formation of SiO2 due to the oxide impurities inside the SiC fiber and matrix. * Brittle fracture before 50 cycles

NITE SiC/SiC Liner: Excellent thermal conductiveity and excellent total properties,excellent M,made It very resistant to thermal shock * No crack at least in 100 cycles

Fig. 3: Very Severe Thernmal Cycling Test Results

Thermal Conductivity;

	Fig.4: Room temperature thermal conductivity of Cera-NITE

Thermal conductivity of Cera-NITE is tailorable by changing the carbon interface structure and thickness and fiber architecture. By using high termal conductivity carbon fibers and/or changing the interface, the control of thermal conductivity can be more flexible. Figure 4 is an example of the property for the case of Cera-NITE with 300nm carbon interface for the uni-directionally reinforced?UD?Cera-NITE and ±90 Cross-ply (CP)Cera-NIT, measured by laser flash method. The average value for UD and CP is 20.3 W/mK and 20.38W/mK, respectably.

Gas Tightness (He Permeability)

	Fig. 5: He gas permeability of Ceramics and their composites

The highly dense, well crystallized SiC structure of Cera-NITE with low porosity and few cracks provides not only excellent mechanical properties and high thermal properties but presents outstanding gas tightness. The He gas permeability test done at Hino laboratory, Hokkaido University provides excellent in He gas permeability as high as 1.0-10m2/s, which was even maintained even after the loading larger than the proportional limit strength. In this figure, PG#1-#3 indicate the progress of processing during the pilot grade production at Ube Industries. showing the improvement in He gas tightness more than 3 orders of magnitude from #1 to #3.

This He tightness was quite stable without significant degradation by minor plastic deformation caused by 10% higher loading than the proportional limit stress. This is quite encouraging results showing the slow crack growth at the beginning of the elastic deformation and the insensitibity of gas tightness by this level of crack development.