N24-A CMC system

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.%). 

FIGURE 3. SEM micrographs showing (a) outside debonding for the N24-B CMC system and (b) inside debonding for the N24-A system. Note that the BN adheres to the Sylramic-iBN fibers in the outside debonding composites. 

To address the temperature issues related to excess silicon, all the same constituents in the N24-C system are used for potential N26 CMC generations, but remaining open pores in the CVI SiC matrix are filled by silicon-free ceramics, rather than by melt infiltration of silicon. In particular, for the N26-A CMC system, a SiC-yielding polymer from Starfire Inc. [13] is infiltrated into the matrix porosity at room temperature and then pyrolyzed at temperatures up to 2912°F (1600°C). This polymer infiltration and pyrolysis (PIP) process was repeated a few times until composite porosity was reduced to 14 vol,%, At this point, the total CMC system is then thermally treated at NASA to improve its thermal conductivity and creep-resistance. Thus although more porous than the other CMC systems, the N26-A system has no free silicon in the matrix, thereby allowing long-time structural use at 2600°F... 

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. 

FIGURE 13. Typical total creep strain versus time behavior at 2642°F (1450°C) in air at an applied stress of 69 MPa for the N24-A and N26-A CMC systems. 

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 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. 

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