1974 Joumal of the American Ceramic Society-Boakye et al. Vol 87. No. 10 1000C,1h epoxy m-sircouia 50 nm s.Nextel 1000C,100h epoxy 。 Nexte720 200nm Nextel 720 Fig. 11. TEM images of ZNS-C coatings on Nextel 720 that and transformation to a twinned sImms show coarsening of ZrO, particles monoclinic phase. Coatings were at 1000@C in argon and heat- reated for 1 and 100 h at 1200.C where it may promote grain growth and degrade fiber strength Finely dispersed multiphase fiber coatings of I-ZrO2-SiOx-carbon bon phase is rapidly filled by SiO,, and dense I-4 oatings form. Consequently, such (-ZrO2-SiO2 coatings are n weak and. therefore, are not useful as fiber-matrix interfaces in eramIc-matrix composites A more promising approach to synthesis of porous ZrSiO4-fiber coatings might involve formation of a coating green body that is a uniform dispersion of fine ZrSiO and carbon powder. Matrix densification and subsequent carbon burnout would have to be done below the temperature at which ZrSiOa is carbothermal 10m reduced, and the Zrsio and carbon particle sizes would have to be significantly smaller than coating thicknesses, typically 100 nm. Another possibility is use of a flux for ZrSiO4 formation that Fig 10. TEM images of ZNS-C-derived coatings on NextelTM 720 that functions in a reducing atmosphere, without affecting fibers show densification of the coating and oxidation of carbon in air that yields a dispersion of t-ZrO2 particles in SiOz. Coatings were applied at 1000C in argon and heat-treated for I and 100 h at 1000.C in ai In earlier studies, it was noted that surface-active decomposition products were more easily trapped in coatings that were less porous and more likely to be locally hermetic. The SiOz that formed from the ZNS-C precursor at high temperatures in air formed a dense, t Zirconia hermetic coating(Fig. 10)that may have trapped even very small amounts of surface-active species at very high local partial pressures. This may have accounted for the unusually low strengths of Nextel 720 and Hi-Nicalon-s with ZNS-C-derived coatings after high- temperature exposure to air. However, very small amounts of Sio2 might have been expected to seal fiber surface flaws and increase fiber strength, if behavior was analogous to that observed for trace AlPO lass in monazite-coated fibers .5 V. Summary and Conclusions Methods using fluxes that readily form ZrSiO Nextel720 fiber 1200-1400C do not do so in the presence of carbon. The 20 nm ommonly used V,Os liquid-phase flux is reduced in the presenceIn earlier studies, it was noted that surface-active decomposition products were more easily trapped in coatings that were less porous and more likely to be locally hermetic.51 The SiO2 that formed from the ZNS-C precursor at high temperatures in air formed a dense, hermetic coating (Fig. 10) that may have trapped even very small amounts of surface-active species at very high local partial pressures. This may have accounted for the unusually low strengths of Nextel 720 and Hi-Nicalon-S with ZNS-C-derived coatings after high￾temperature exposure to air. However, very small amounts of SiO2 might have been expected to seal fiber surface flaws and increase fiber strength, if behavior was analogous to that observed for trace AlPO4 glass in monazite-coated fibers.51 IV. Summary and Conclusions Methods using fluxes that readily form ZrSiO4 in air at 1200°–1400°C do not do so in the presence of carbon. The commonly used V2O5 liquid-phase flux is reduced in the presence of carbon and, therefore, is ineffective. In an open system, ZrSiO4 can be carbothermally reduced. Flux methods also are inappropri￾ate for fiber coatings because flux can be easily lost to the fibers, where it may promote grain growth and degrade fiber strength. Finely dispersed multiphase fiber coatings of t-ZrO2–SiO2– carbon can be deposited in argon from appropriate precursors. However, on heat treatment in air at
1000°C, porosity from the fugitive carbon phase is rapidly filled by SiO2, and dense t-ZrO2–SiO2 coatings form. Consequently, such t-ZrO2–SiO2 coatings are not weak and, therefore, are not useful as fiber–matrix interfaces in ceramic-matrix composites. A more promising approach to synthesis of porous ZrSiO4-fiber coatings might involve formation of a coating green body that is a uniform dispersion of fine ZrSiO4 and carbon powder. Matrix densification and subsequent carbon burnout would have to be done below the temperature at which ZrSiO4 is carbothermally reduced, and the ZrSiO4 and carbon particle sizes would have to be significantly smaller than coating thicknesses, typically
100 nm. Another possibility is use of a flux for ZrSiO4 formation that Fig. 10. TEM images of ZNS-C-derived coatings on NextelTM 720 that functions in a reducing atmosphere, without affecting fibers. show densification of the coating and oxidation of carbon in air that yields a dispersion of t-ZrO2 particles in SiO2. Coatings were applied at 1000°C in argon and heat-treated for 1 and 100 h at 1000°C in air. Fig. 11. TEM images of ZNS-C-derived coatings on NextelTM 720 that show coarsening of ZrO2 particles in SiO2 and transformation to a twinned monoclinic phase. Coatings were applied at 1000°C in argon and heat￾treated for 1 and 100 h at 1200°C in air. Fig. 12. TEM images of ZP-C-derived coatings on NextelTM 720 that show t-ZrO2 particles without SiO2. Coatings were applied at 1000°C in argon and heat-treated for 1 h at 1000°C in air. 1974 Journal of the American Ceramic Society—Boakye et al. Vol. 87, No. 10