Journal of the American Ceramic Sociery-Cinibulk et al. Vol. 85. No. 1I Fibe Fiber 200mn n 100nm Epoxy YAG YAG+C 100 nm Fig. 4. TEM images of tow 4 showing grain growth and sintering of YAG coating after heating at 1200C for 2 h in air. Where the uncon strained porous coating is thin, grains grow to film thickness and eliminate 100mm porosity. Epoxy with heat treatment, either of which may be attributable to enviro All tows initially increased in strength after coating with YAG/C and heat-treating in argon. Figure 5 contains a Weibul plot to illustrate the strength increase with 5 g/L YAG containing 50 vol% carbon then heat-treated under various conditions. After an initial strength of 2. 1 GPa for the as-coated fiber. the strength increased further to 2.2 GPa after heating at 1000C for I h in argon. Heating at 1200oC in argon for 2 h decreased the fiber strength to 1.6 GPa while. at 1200 C heating in air for 2 h reduced the strength to 1.3 GPa. a 100 h heat treatment at 1200% C reduced the strength of the coated fiber to 0.8 Pa, which was 80% of the strength of the uncoated fiber heated 20 nm under the same conditions. Similar results were obtained for tows coated with different solution concentrations and 30 vol% carbon The initial increase in strength that is often observed for fibers Fig 3. TEM images of (a)tow I, showing amorphous coating on fiber as with carbon-containing coatings could be attributed to flaw heal and 10 nm crystalline YAG after heating at 1000C for I h in argon, and straightening of filaments within the coated tow, which then allows (c)tow 8, showing intimate mixture of 30 vol% amorphous carbon and 10 nm crystalline YAG after heating at 1000 C for I h in argon. Note a greater fraction of the filaments to bear load during a tension test formation of a surface layer of dense Y AG following heat treatment. Inset however, the desized tows(without the coating) are pulled throu n Fig. 3(b)is a selected-area diffraction pattern of the coating that ethanol to help straighten the filaments before mounting for orresponds with randomly orient stalline YAG tension testing. As the carbon is removed by oxidation, strength decreases and approaches that of the uncoated fibers with a simila thermal history. This subsequent reduction in strength is most likely due to weakening of the fiber by either flaw-size increase or been attributed arily to grain growth; however, environmental effects. as is observed for uncoated fibers. rather the ure to than any detrimental effect due to the presence of the YAG coa correlation between size and fiber strength following heat tself. The presence of subsequent porosity also enhances the treatment. For example, in the present study, for a 1.6 GPa material to be reduced in strength to 1. 1 GPa, as measured for tows heated at 1200.C for 2 h, the critical flaw size would have to be increased lowever. a coated fiber heat-treated at 1200.C for 100 h does by over a factor of 2, given o /o,=a7al, where o and a are show strength reductions of 19%0-26% compared with a similarl strength and flaw size. res ely. Figure 3(a) shows a coate thermally processed uncoated tow. In this case, the coating ber after no heat treatment other than the brief time spent in the densify to the point of full density for thin coatings and to levels coating furmace, and Fig. 4 shows a fiber after heating for 2 h at 1200C. Clearly, there is not a twofold increase in grain size following heat treatment. Either critical flaws grow at different Weibull analysis does not necessarily imply that fiber-bundle prop rates than the grains or the toughness at the crack tip decreases follow weakest-link statistics.in strength has been attributed primarily to grain growth; however, there is little evidence in the literature to support a direct correlation between grain size and fiber strength following heat treatment. For example, in the present study, for a 1.6 GPa material to be reduced in strength to 1.1 GPa, as measured for tows heated at 1200°C for 2 h, the critical flaw size would have to be increased by over a factor of 2, given 1/2  a2 1/2/a1 1/2, where  and a are strength and flaw size, respectively. Figure 3(a) shows a coated fiber after no heat treatment other than the brief time spent in the coating furnace, and Fig. 4 shows a fiber after heating for 2 h at 1200°C. Clearly, there is not a twofold increase in grain size following heat treatment. Either critical flaws grow at different rates than the grains or the toughness at the crack tip decreases with heat treatment, either of which may be attributable to environmental effects. All tows initially increased in strength after coating with YAG/C and heat-treating in argon. Figure 5 contains a Weibull plot to illustrate the strength increase of tows coated three times with 5 g/L YAG containing 50 vol% carbon then heat-treated under various conditions.‡ After an initial strength of 2.1 GPa for the as-coated fiber, the strength increased further to 2.2 GPa after heating at 1000°C for 1 h in argon. Heating at 1200°C in argon for 2 h decreased the fiber strength to 1.6 GPa while, at 1200°C, heating in air for 2 h reduced the strength to 1.3 GPa. A 100 h heat treatment at 1200°C reduced the strength of the coated fiber to 0.8 GPa, which was 80% of the strength of the uncoated fiber heated under the same conditions. Similar results were obtained for tows coated with different solution concentrations and 30 vol% carbon. The initial increase in strength that is often observed for fibers with carbon-containing coatings could be attributed to flaw heal￾ing. The strength increase could also be partially attributed to the straightening of filaments within the coated tow, which then allows a greater fraction of the filaments to bear load during a tension test; however, the desized tows (without the coating) are pulled through ethanol to help straighten the filaments before mounting for tension testing. As the carbon is removed by oxidation, strength decreases and approaches that of the uncoated fibers with a similar thermal history. This subsequent reduction in strength is most likely due to weakening of the fiber by either flaw-size increase or environmental effects, as is observed for uncoated fibers, rather than any detrimental effect due to the presence of the YAG coating itself. The presence of subsequent porosity also enhances the permeability of any trapped gases out of the coating that may otherwise cause stress corrosion at fiber grain boundaries.40 However, a coated fiber heat-treated at 1200°C for 100 h does show strength reductions of 19%–26% compared with a similarly thermally processed uncoated tow. In this case, the coatings densify to the point of full density for thin coatings and to levels ‡ Use of Weibull analysis does not necessarily imply that fiber-bundle properties follow weakest-link statistics. Fig. 3. TEM images of (a) tow 1, showing amorphous coating on fiber as coated; (b) tow 2, showing intimate mixture of 50 vol% amorphous carbon and 10 nm crystalline YAG after heating at 1000°C for 1 h in argon; and (c) tow 8, showing intimate mixture of 30 vol% amorphous carbon and 10 nm crystalline YAG after heating at 1000°C for 1 h in argon. Note formation of a surface layer of dense YAG following heat treatment. Inset in Fig. 3(b) is a selected-area diffraction pattern of the coating that corresponds with randomly oriented polycrystalline YAG. Fig. 4. TEM images of tow 4 showing grain growth and sintering of YAG coating after heating at 1200°C for 2 h in air. Where the uncon￾strained porous coating is thin, grains grow to film thickness and eliminate porosity. 2706 Journal of the American Ceramic Society—Cinibulk et al. Vol. 85, No. 11