Journal of the American Ceramic Society-Zhu et al. Vol 81. No. 3 Huger et al. reported results of a thermogravimetric analysis TGA)of Nicalon TM fibers in air in the range of 700-1200oC They also converted the mass gain that was measured using TGa to an oxide thickness as a function of oxidation time The experimen ata are con pared with the present model (Eq. (22)and the Deal and Grove(D&G) model in Fig. 4 Both models agree well with experimental data for oxidation temperatures in the range of 7000-1000.C. As will be dis- cussed later(see Fig. 6), the present model and the Deal and Grove model show a pronounced difference only at the later stage of oxidation. Therefore. we cannot determine which model agrees better with the experimental data shown in Fig. 4 because these data are only from the of oxidation For oxidation temperatures of 1 100 and 1200C and times >30 h. the actual oxide thickness decreases below that which is predicted by both models; this is because these models are not valid anymore for temperatures >1100oC, because of the rea- son discussed below 2·kmm Several factors may affect the accuracy of Eq.(22), when compared with the experimental data of the NicalonTM fibers an oxide layer; the fiber has been treated at 1200oC for 8 h in an sion of co, which is a reaction product at the oxidaton. Fig. 2. Typical SEM micrograph of the cross section of a fiber with First, the present theory does not consider the outward diffu- oxygen partial pressure of 0. 14 atm (-0.014 MPa) face when SiC reacts with O2. In addition, further pyrolytic reaction may occur in the core of Nicalon TM fibers SiC121O04(3)0.9058(s)+0.095s0(g)+0.305C0g) The present theory(Eq(22)) is compared with the experi- lental data in Fig. 3. Because initial oxidation is inevitabl The outward diffusion of the pyrolysis products-gaseous Sic an initial oxide thickness of 0.08 um(which would have re- and Co-will exaggerate the situation. As the oxide layer be- uired -30 min to form if the fibers would have been intro comes thicker, the outward diffusion of CO and Sio may be- duced directly into a furnace preheated to 1200C)is assumed come a limiting factor and slow the oxidation-reaction kinetics to be present and is used in the calculation of Eq(22). Figure If the oxide thickness is large enough, gas bubbles may form in shows that the measured oxide thicknesses are in statistical he oxide layer, as shown in Fig. 5, which renders the present agreement with the values calculated using Eq(22) for oxida ion durations of 4 and 8 h but is smaller than the thickness that The scatter of the fiber diameter may also affect the accuracy is calculated for an oxidation duration of 16 h. The large scatter of Eq (22). An average fiber diameter of 16 um has been used in the data can be attributed to the fact that Nicalon M fiber in the calculations shown in Figs. 3 and 4, whereas the actual have a large diameter variation(from 8 um to 22 um), and ter scatters over a range of 8-22 um. 8 The initial compositions may vary from fiber to fiber although they are during the heating of the fibers to the designated from the same tow. Note that the error bars in Fig. 3 give the ror when using Eq actual range of data scatter zzi and Naslain' and Costello et aL. 22 also attributed Average thicknes Time Fig 3. Compa of the present theory(Eq(22)) with experimental data Parameters for the calculation using Eq (22)are E= 225, F=0.025. and G= 4.5 verage fiber radius R of 8 um has been the calculationThe present theory (Eq. (22)) is compared with the experi￾mental data in Fig. 3. Because initial oxidation is inevitable during the heating of fibers from room temperature to 1200°C, an initial oxide thickness of 0.08 mm (which would have re￾quired ∼30 min to form if the fibers would have been intro￾duced directly into a furnace preheated to 1200°C) is assumed to be present and is used in the calculation of Eq. (22). Figure 3 shows that the measured oxide thicknesses are in statistical agreement with the values calculated using Eq. (22) for oxida￾tion durations of 4 and 8 h but is smaller than the thickness that is calculated for an oxidation duration of 16 h. The large scatter in the data can be attributed to the fact that Nicalon™ fibers have a large diameter variation (from 8 mm to 22 mm),18 and compositions may vary from fiber to fiber although they are from the same tow. Note that the error bars in Fig. 3 give the actual range of data scatter. Huger et al.8 reported results of a thermogravimetric analysis (TGA) of Nicalon™ fibers in air in the range of 700°–1200°C. They also converted the mass gain that was measured using TGA to an oxide thickness as a function of oxidation time. Their experimental data are compared with the present model (Eq. (22)) and the Deal and Grove (D&G) model in Fig. 4. Both models agree well with experimental data for oxidation temperatures in the range of 700°–1000°C. As will be dis￾cussed later (see Fig. 6), the present model and the Deal and Grove model show a pronounced difference only at the later stage of oxidation. Therefore, we cannot determine which model agrees better with the experimental data shown in Fig. 4, because these data are only from the early stage of oxidation. For oxidation temperatures of 1100° and 1200°C and times >30 h, the actual oxide thickness decreases below that which is predicted by both models; this is because these models are not valid anymore for temperatures >1100°C, because of the rea￾son discussed below. Several factors may affect the accuracy of Eq. (22), when compared with the experimental data of the Nicalon™ fibers. First, the present theory does not consider the outward diffu￾sion of CO, which is a reaction product at the oxidation inter￾face when SiC reacts with O2. In addition, further pyrolytic reaction may occur in the core of Nicalon™ fibers:9 SiC1.21O0.40(s) → ← 0.905SiC(s) + 0.095SiO(g) + 0.305CO(g) (37) The outward diffusion of the pyrolysis products—gaseous SiO and CO—will exaggerate the situation. As the oxide layer be￾comes thicker, the outward diffusion of CO and SiO may be￾come a limiting factor and slow the oxidation-reaction kinetics. If the oxide thickness is large enough, gas bubbles may form in the oxide layer, as shown in Fig. 5, which renders the present theory totally inapplicable. The scatter of the fiber diameter may also affect the accuracy of Eq. (22). An average fiber diameter of 16 mm has been used in the calculations shown in Figs. 3 and 4, whereas the actual fiber diameter scatters over a range of 8–22 mm.18 The initial oxidation during the heating of the fibers to the designated temperature can be another source of error when using Eq. (22). Filipuzzi and Naslain1 and Costello et al.22 also attributed Fig. 2. Typical SEM micrograph of the cross section of a fiber with an oxide layer; the fiber has been treated at 1200°C for 8 h in an oxygen partial pressure of 0.14 atm (∼0.014 MPa). Fig. 3. Comparison of the present theory (Eq. (22)) with experimental data. Parameters for the calculation using Eq. (22) are E 4 225, F 4 0.025, and G 4 4.5; an average fiber radius R of 8 mm has been used in the calculation. 658 Journal of the American Ceramic Society—Zhu et al. Vol. 81, No. 3