Journal of the American Ceramic Societyacobson et al. Vol. 82. No 6 3c0 23x103m2 55 53 HxByozlg 2 SiC Intact Fig. 11. Schematic of volatilization processes for exposed BN Table ll. Calculated Molecular, Knudsen, and Effective Diffusivities for Various HB, O(g)Species Knudsen diffusivity Effective diffusivity H 4.6 3.8 Time(hrs) 1200 cm/s), I H (c) 1000 H,BO3(g) 4.0 H3.(g) 2.5 0.64 Sealing K=3.7x 10"um/r Table Ill. Calculated Vapor Pressures above B,O3 with a(B2O3)=0.5 0.92 Pressure at700°cbar essure at8o0°cbar) 0% HOO, 1%H2O/O2 % HOO HBO 3.6×10-61.1×10-6 H3B03(g) 3.8×10 H3B3O6(g)2.8×10 88×10-6 This expression can be integrated to Time(hrs 2aND1BP1B2(,4_(F2+4B d 12dB ured interphase thickness shown. All BN interphase -1000oC:(a)-05 um BN interphase, 700C in 10%H,O/O2,(b)-1 The full expression for recession distance is then the sum of the BN interphase, 700C in 1% H2O/O2, and(c)-I um contributions from each of the three vapor species, analogous phase, 800oC in 1% H,O/O to Eq(9)This expression can be integrated to yHxByOz 2 = 2VBN DHxByOz PHxByOz RT St + At d − ~A2 + 4Bt! 3/2 12dB D (19) The full expression for recession distance is then the sum of the contributions from each of the three vapor species, analogous to Eq. (9): Fig. 10. Results of pore recession calculations (–––) showing para￾bolic rate constants used and comparison to experiment (d) with mea￾sured interphase thickness shown. All BN interphases deposited at ∼1000°C: (a) ∼0.5 mm BN interphase, 700°C in 10% H2O/O2, (b) ∼1 mm BN interphase, 700°C in 1% H2O/O2, and (c) ∼1 mm BN inter￾phase, 800°C in 1% H2O/O2. Fig. 11. Schematic of volatilization processes for exposed BN. Table II. Calculated Molecular, Knudsen, and Effective Diffusivities for Various HxByOz(g) Species (A) 700°C Species Molecular diffusivity in O2 (cm2 /s) Knudsen diffusivity (cm2 /s) Effective diffusivity (cm2 /s) 0.5 mm 1 mm 0.5 mm 1 mm HBO2(g) 0.90 2.3 4.6 0.64 0.75 H3BO3(g) 0.80 1.9 3.8 0.56 0.66 H3B3O6(g) 0.72 1.3 2.6 0.46 0.56 (B) 800°C Species Molecular diffusivity in O2 (cm2 /s) Knudsen diffusivity (cm2 /s), 1 mm Effective diffusivity (cm2 /s), 1 mm HBO2(g) 1.1 4.8 0.90 H3BO3(g) 0.95 4.0 0.77 H3B3O6(g) 0.86 2.5 0.64 Table III. Calculated Vapor Pressures above B2O3 with a(B2O3) = 0.5 Species Pressure at 700°C (bar)† Pressure at 800°C (bar)† 10% H2O/O2 1% H2O/O2 1% H2O/O2 HBO2(g) 3.6 × 10−6 1.1 × 10−6 8.6 × 10−6 H3BO3(g) 1.2 × 10−4 3.8 × 10−6 3.0 × 10−6 H3B3O6(g) 2.8 × 10−4 8.8 × 10−6 5.7 × 10−6 † 1 bar 4 105 Pa. 1480 Journal of the American Ceramic Society—Jacobson et al. Vol. 82, No. 6