P. Mogileusky, 4. Zanguil/ Materials Science and Engineering 4262(1999)16-24 8104 Calculated values of the permeabilities of silica(Ps)and matrix(Pm) MZY30P. n=I MZY30P MZY35W 6104 P,Mol·s-1.8.4×10-178.1×10-1797×10-181.1×10-17 18×10-151.2×10-157.5×10-165.2×10-16 m/P, P/P=21. 4(best fit A rather surprising finding is the lower value of matrix permeability obtained for the MzY35w mate- rial, which has higher ZrO, content and was oxidized at P /P=5 higher temperature, than for the MZY30P material 210 This may be partially due to the difference in the mullite stoichiometry and microstructure resulted from differ- ent preparation and processing procedures employed in the studies [8, 10]. However, the most plausible reason for the decrease of the apparent matrix permeability is 0+LLLiLiLrcL-er 0050.10.150.20.250.3 -TC 150014001300120011001000 Fig. 8. The effect of matrix oxygen permeability on the oxidation profile according to the present model. 10157a ormation of zircon(ZrSio4) in the reaction be- ZrO, and SiO,, which becomes important at [28]. In this reaction, high permeability Zro phase in the matrix is replaced with zircon which has very low oxygen permeability [29-31]. This may result in a drastic decrease of the matrix oxygen permeability Fig. 8 shows the effect of matrix oxygen permeability 91- OI 9 MZY30P on the oxidation profile according to the present model using the example of the previously discussed experi mental data for the mzy30P material. The value of parameter n= l and the corresponding value of silica oxygen permeability from Table 3, P,=8.4 x 10-17 molcm-Is-l bar-I were used for the calculations of the oxidation curves for different values of matrix oxygen permeability, Pm. This figure shows that the oxidation profiles are sensitive to the value of matrix oxygen permeability and can be used for experimenta determination of P. On the other hand, this means 5.56.06.5707,58.08.59.0 that calculated permeability values, or even experimen- tal data measured for similar materials, should be used Fig. 7. Comparison of the calculated values of Ps from the present with caution for predictions of oxidation behavior, analysis with the literature data on oxidation of Si and Sic materia since relatively small variation of the matrix permeabil- in dry oxygen and oxygen permeation through silica: . MZY35W; ity can result in significant change of the oxidation MZY30P: 1, oxygen permeation through vitreous silica [11]: 2, si profile. single crystal [17]: 3, Si single crystal [18]: 4, Si single crystal (19:5, Si single crystal [20]: 6, SiC single crystal [21, 22]: 7a, SiC single crystal Based on the above model, a simple criterion which (0001)[17] 7b, SiC single crystal (0001)[17]: 8a, SiC single crystal determines whether the oxidation follows mode i or (0001)[23: 8b, SiC single crystal (0001)(23]: 9, hot pressed Sic [17 mode ii can be evaluated. It should be noted first 10, CVD SIC [24]: Il, CVD SiC [25]. however, that in the framework of this simple model22 P. Mogile6sky, A. Zang6il / Materials Science and Engineering A262 (1999) 16–24 Table 3 Calculated values of the permeabilities of silica (Ps) and matrix (Pm). MZY30P MZY35W n=1 n=4 n=1 n=4 8.4×10−17 1.1×10−17 8.1×10−17 Ps, Mol · s−1 · 9.7×10−18 cm−1 · bar−1 1.8×10−15 1.2×10−15 7.5×10−16 Pm, Mol · 5.2×10−16 s−1 · cm−1 · bar1/n ) Fig. 8. The effect of matrix oxygen permeability on the oxidation profile according to the present model. A rather surprising finding is the lower value of matrix permeability obtained for the MZY35W mate￾rial, which has higher ZrO2 content and was oxidized at higher temperature, than for the MZY30P material. This may be partially due to the difference in the mullite stoichiometry and microstructure resulted from differ￾ent preparation and processing procedures employed in the studies [8,10]. However, the most plausible reason for the decrease of the apparent matrix permeability is Fig. 7. Comparison of the calculated values of Ps from the present analysis with the literature data on oxidation of Si and SiC materials in dry oxygen and oxygen permeation through silica: , MZY35W; ", MZY30P; 1, oxygen permeation through vitreous silica [11]; 2, Si single crystal [17]; 3, Si single crystal [18]; 4, Si single crystal [19]; 5, Si single crystal [20]; 6, SiC single crystal [21,22]; 7a, SiC single crystal (0001() [17]; 7b, SiC single crystal (0001) [17]; 8a, SiC single crystal (0001() [23]; 8b, SiC single crystal (0001) [23]; 9, hot pressed SiC [17]; 10, CVD SiC [24]; 11, CVD SiC [25]. the formation of zircon (ZrSiO4) in the reaction be￾tween ZrO2 and SiO2, which becomes important at 1200°C [28]. In this reaction, high permeability ZrO2 phase in the matrix is replaced with zircon which has very low oxygen permeability [29–31]. This may result in a drastic decrease of the matrix oxygen permeability. Fig. 8 shows the effect of matrix oxygen permeability on the oxidation profile according to the present model, using the example of the previously discussed experi￾mental data for the MZY30P material. The value of parameter n=1 and the corresponding value of silica oxygen permeability from Table 3, Ps=8.4×10−17 mol · cm−1 · s−1 · bar−1 , were used for the calculations of the oxidation curves for different values of matrix oxygen permeability, Pm. This figure shows that the oxidation profiles are sensitive to the value of matrix oxygen permeability and can be used for experimental determination of Pm. On the other hand, this means that calculated permeability values, or even experimen￾tal data measured for similar materials, should be used with caution for predictions of oxidation behavior, since relatively small variation of the matrix permeabil￾ity can result in significant change of the oxidation profile. Based on the above model, a simple criterion which determines whether the oxidation follows mode I or mode II can be evaluated. It should be noted first, however, that in the framework of this simple model