October 2004 Zirconia-Silica-Carbon Coatings on Ceramic Fibers was consistent with the results for -doped ZNS-C(Figs ostructure development of ZNS-C-derived 2(p) and (q)) and suggested that was unstable in the fiber coatings formed in argon on Nextel 720 and Hi-Nicalon -s presence of carbon above 1250c was a lower tempera- was similar in most respects to that in powders. Coatings were ture than previously suggested. 224& At 1400 C, ZrC formed(Figs deposited as small(--5 nm)t-ZrO2 particles in an amorphous Sio 20)and (k). The uniformity and small particle size of the powders and carbon matrix at 1000%-1200C(Fig 5). Coating thickness and coatings derived from the ZNs-C precursors may have ranged from 10 to >100 nm. The coatings were pliable; they influenced reduction maintained continuity over Mode ll cracks that developed at fiber surfaces during thin-section preparation(Fig. 5(a)). At 1300 (5) Fiber Coatings 1400C, the t-ZrO2 particle size was slightly larger(5-10 nm) (A)Phase and Microstructure: Coating morphology was and, at 1500%C, it was -15 nm(Fig. 6). Some coatings formed at similar to that observed for other systems.so Every filament was 1300C had an outer layer of -20 nmt-ZrOz particles and an inner oated but the thickness varied. The median coating thickness was 50 nm for Nextel 720 fibers using a 65 g/L precursor. The 1.2 present at temperatures as low as 1300.C(Fig. 7). Coatings g/L of ammonium vanadate should have yielded a V2O5 flux deposited at 1600C consisted of "50 nm ZrC mosaic crystals, concentration of 2.5 wt% relative to ZrSiO4 for this precursor with evidence of graphitic carbon, SiO2, and trace SiC from EDS Coating bridges between filaments(Fig. 5(b)and crust around the fiber-tow perimeter were common for higher-concentration and 1600%c and electron diffraction patterns(Fig. 6). At 15000 the carbon was turbostratic. Residence time at precursors, particularly for the 160 g/L precursor with viscosity of emperature was only a few seconds during coating; therefore, ZrC 5.3 cP. This was consistent with previous work on monazite formation required higher temperatures to form during coating coatings, where crust and other coating imperfections were more than it did during I h heat treatments of powders(Fig. 2). TE prevalent when precursor viscosity was >3 cP. Coating mor vanadium flux was not detected using EDS in the argon-deposited phology and thickness were not affected by the hexadecane layer coatings, except for occasional high-contrast particles with a high thickness vanadium content which were inferred to be vanadium carbide 了400 10001200 1c8 5001000& ZrC gra shite 10 nm Fig. 6. TEM images of SiO,, some turbostratic carbon, and I-ZrO2 are present after coating at 1400%and 1500C pze turbostatic graphitic carbon, and some amorphouswas consistent with the results for lithium-doped ZNS-C (Figs. 2(p) and (q)) and suggested that ZrSiO4 was unstable in the presence of carbon above
1250°C, which was a lower tempera￾ture than previously suggested.22,48 At 1400°C, ZrC formed (Figs. 2(j) and (k)). The uniformity and small particle size of the powders and coatings derived from the ZNS-C precursors may have influenced reduction.71–74 (5) Fiber Coatings (A) Phase and Microstructure: Coating morphology was similar to that observed for other systems.50 Every filament was coated, but the thickness varied. The median coating thickness was
50 nm for Nextel 720 fibers using a 65 g/L precursor. The 1.2 g/L of ammonium vanadate should have yielded a V2O5 flux concentration of 2.5 wt% relative to ZrSiO4 for this precursor. Coating bridges between filaments (Fig. 5(b)) and crust around the fiber-tow perimeter were more common for higher-concentration precursors, particularly for the 160 g/L precursor with viscosity of 5.3 cP. This was consistent with previous work on monazite coatings, where crust and other coating imperfections were more prevalent when precursor viscosity was 3 cP.50 Coating mor￾phology and thickness were not affected by the hexadecane layer thickness. The phase and microstructure development of ZNS-C-derived fiber coatings formed in argon on Nextel 720 and Hi-Nicalon-S was similar in most respects to that in powders. Coatings were deposited as small (
5 nm) t-ZrO2 particles in an amorphous SiO2 and carbon matrix at 1000°–1200°C (Fig. 5). Coating thickness ranged from 10 to 100 nm. The coatings were pliable; they maintained continuity over Mode II cracks that developed at fiber surfaces during thin-section preparation (Fig. 5(a)). At 1300°– 1400°C, the t-ZrO2 particle size was slightly larger (
5–10 nm), and, at 1500°C, it was
15 nm (Fig. 6). Some coatings formed at 1300°C had an outer layer of
20 nm t-ZrO2 particles and an inner layer of 5 nm particles (Fig. 7). Evidence of graphitic carbon was present at temperatures as low as 1300°C (Fig. 7). Coatings deposited at 1600°C consisted of
50 nm ZrC mosaic crystals, with evidence of graphitic carbon, SiO2, and trace SiC from EDS spot analysis and electron diffraction patterns (Fig. 6). At 1500° and 1600°C, the carbon was turbostratic. Residence time at temperature was only a few seconds during coating; therefore, ZrC formation required higher temperatures to form during coating than it did during 1 h heat treatments of powders (Fig. 2). The vanadium flux was not detected using EDS in the argon-deposited coatings, except for occasional high-contrast particles with a high vanadium content, which were inferred to be vanadium carbide. Fig. 6. TEM images of ZNS-C-derived coatings on Hi-NicalonTM-S SiC fiber. Coatings were applied at 1400°, 1500°, and 1600°C in argon. Amorphous SiO2, some turbostratic graphitic carbon, and t-ZrO2 are present after coating at 1400° and 1500°C. ZrC, turbostratic graphitic carbon, and some amorphous SiO2 are present after coating at 1600°C. October 2004 Zirconia–Silica–Carbon Coatings on Ceramic Fibers 1971