N26-A CMC system

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 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. 

C) and higher, Research is on-going at NASA to develop further generations ofthe N26-A system where porosity is significantly reduced, so that CMC thermal conductivity... 

FIGURE 9. Tjrpical room-temperature tensile stress-strain curves for the annealed Sylramic-iBN CMC systems N24-C and N26-A (total fiber content -34 vol.%). 

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