N22 CMC system

For the N22 CMC system, remaining open porosity in the CVI SiC matrix was filled by room-temperature infiltration of SiC particulate or slurry casting, followed by the melt-infiltration (MI) of silicon metal near 1400°C. This yielded a final composite with 2% closed porosity within the fiber tows and 0% porosity between the tows. The final composite system (often referred to as a slurry-cast MI composite) typically displayed a thermal conductivity about double that of a full CVI SiC composite system in which the CVI matrix process was carried to completion. Also the composite did not require an oxidation-protective over-coating to seal open porosity. Decreasing the porosity of the hybrid matrix also increased the N22 CMC elastic modulus, which in turn contributed to a high proportional limit stress. However, since the filler contained some low-modulus silicon, the modulus increase was not as great as if the filler were completely dense SiC. 

FIGURE 10. Typical room-temperature stiess-strain curves for the N22 CMC system with Sylramic and Hi-Nicalon Type-S fibers, and for the N24-B CMC system with Sylramic-iBN fibers before and after combustion gas exposure ofthe systems in a low-pressure burner rig at -800°C for -100 hours. The fibers in the N22 systems each had carbon on their surfaces before BN interphase deposition. 

Table 2 hsts some key constituent material and process data for five SiC fiber-reinforced CMC systems recently developed at NASA. For convenience, these systems have been labeled by the prefix N for NASA, followed by their approximate upper temperature capability in degrees Fahrenheit divided by 100 that is, N22, N24, and N26, with suffix letters A, B, and C to indicate their generation. Also shown in Table 2 are the primary organizations where the different process steps were performed to fabricate each CMC system into a test panel. However, it should be noted that these steps have also been performed at other organizations, resulting in test panels with equivalent properties. 

FIGURE 6. Typical linear thermal expansion curves in the axial and transverse directions for the N22 and N24 CMC systems panels fabricated with the silicon melt-infiltration step. Also shown for comparison is the best fit curve for monolithic SiC with the 5-phase [16]. 

FIGURE 7. Typical transverse thermal conductivity curves for thin panels with CMC systems N22, N24-A, and N24-C. Effect of fiber conductivity is shown by curve for the N22 system with the lower conductivity Hi-Nicalon Type-S fiber type. 

FIGURE 8. Typical room-temperature tensile stress-strain curves for the inside-debonding N22 and N24-A CMC systems, and the outside-debonding N24-B CMC system (total fiber content -40 vol.%). 

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