FIGURE 3.18 Resistance to commonly pumped chemicals. This CFCC was made by polymer infiltration and pyrolysis (PIP) process.

temperature of 1204°C (2200°F), and 15% steam atmosphere. Test specimen were evaluated at 500-h intervals, and the results compared to specimen exposed in actual engines operated on typical duty cycles.

Tensile and flexural CFCC specimens were exposed to the pressurized atmosphere mentioned above. Test specimens were evaluated every 500 h.

The CFCC evaluated was a CVI silicon carbide matrix reinforced with silicon carbide fiber, with a pyrocarbon interface coating and a silicon carbide seal coat. When specimen thickness was measured at 500 and 100 h, it was determined that surface recession was occurring at a steady rate of 45 ^m/500 h, identical to the actual engine experience operating on a typical duty cycle (Fig. 3.19). See the gas turbine engine description in Section 3.2.2 of this chapter for additional actual engine gas turbine performance data.

Additional simulated gas turbine engine data had been generated earlier. Instead of recession rate, this data focused on strength retention of CFCC combustion liners. In these tests, the CFCC consisted of a silicon-carbide-reinforced silicon carbide/silicon matrix composite formed by melt infiltration. The fiber-matrix interface coating was boron nitride based applied by CVD.

Rectangular tensile bars measuring 150 x 12 x 2.5 mm with a gage length of 2.5 mm were exposed in a induction-heated tube furnace to a temperature of 1200°C (2192°F), 90% water and 10% oxygen atmosphere (water vapor pressure of 0.9 atms) with a gas velocity of 0.04 m/s. The test was conducted for 500 h, with the temperature cycled to room temperature every 2 h. The test specimen

FIGURE 3.19 Seal coat of silicon carbide protects CFCC and prolongs its life. Note low erosion rate under these harsh conditions.

bars were pulled at a strain rate of 0.0002 m/s. The results are given in Fig. 3.20. Note the excellent retention of strength.

In a second test, both apparatus and conditions were changed to simulate other duty cycles. In this second test, tensile bars were exposed for 100 h in the exhaust section of a high-pressure combustion rig. The exposure conditions replicated an actual engine combustion atmosphere using natural-gas fuel at an equivalency ratio of 0.32, a gas temperature of 980°C (1800°F), a gas velocity of 85 m/s, and a pressure of 10 bar. During the 100 h exposure, the temperature was cycled to room temperature six times.

MI CFCC Various Conditions

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