November 2002 Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composite 2705 Table Il. Minicomposite Processing and Heat-Treatment Conditions Fiber tow perature (C Time(h) Atmosphere Strength(GPa) Weibull modulus Coefficient of variation ABCDE 200/1200 2/100 Argon/air 0.28 0.41 1200/1100 Argon/air 1200/1200 2/100 Argon/air 636468 0.21 1200 100 0.28 fFrom Table 1. *Control composites with no fiber coating. procedure has been described in detail previously.23, 34,35 Tows and vol% carbon and those with 50 vol%. Based on the absence of any inicomposites were mounted with epoxy on card stock that was gross segregation of phases, we assumed that the volume fraction used as a fixture. Uncoated tows were first pulled through ethanol of porosity in the YAG fiber coatings after oxidation of the in an attempt to align individual filaments before mounting. The fugitive carbon equaled the initial carbon volume fraction, as gauge length was 25.4 mm. At least 20 sections of fiber tow and determined by TGA. at least 10 minicomposites were tested for each sample where Heat-treating the coated tows in air resulted in a loss of carbon strengths were reported. Tow and minicomposite tensile strengths and an increase in grain size. As the temperature was increased te were determined from maximum loads and total fiber areas( based 1200C, the coatings sintered, and the grains reached a size on an average fiber diameter of 12 um and 420 filaments/tow), comparable to the thickness of some of the coatings, as shown in there was usually no porosity left. Thicker coatings still had (5) Phase and Microstructure Characterization residual porosity, but it was reduced from -30 and 50 vol%to Powders of the YAG pre were prepared by heating the 10 vol%. These results are similar to those found for fiber solutions on a hot plate to "200C to rate the solvent and coatings that were heat-treated within a porous matrix, as dis decompose the majority of the organics before heating in argon to cussed later temperatures of =1000C for I h Powder XRD was used to verify Table I contains the strengths of coated and uncoated (control) phase development in the precursor. Final carbon content was tows after undergoing various heat treatments to determine the effects of coating composition on fiber strength, As-received and determined by thermogravimetric analysis (tga) in air on pow- desized tows of Nextel 610(without a coating)have a strength of ders previously heated to 1000C Surfaces of coated and heat-treated fibers and urfaces 1.6 GPa. Heating the same tow at 1200C for 2 and 100 h of minicomposites were characterized by SEM 360FE, reduces its strength to 1. I and 1.0 GPa, respectively.The reduction LEO, Cambridge, U. K ). Coated fiber tows er/matrⅸx interfaces in the composites were examined by TEM(Model CM200FEG, Philips, Eindhoven, The Netherlands). Energy- dispersive X-ray spectroscopy (EDS) was used for elemental analysis. TEM specimens were prepared by impregnating coated tows with a high-temperature epoxy and thinning to electron transparency with diamond lapping films, followed by low-angle ion-beam milling, as described in detail elsewhere. 3, II. Results and discussion (1 Coated Fiber Tows Characterization of the coated tows by SEM and TEM indicated at minimal fiber bridging remained, and the filaments appeared well-coated(Fig. 2). Evidence of prior bridging during the coating process was present, but the wheels were effective at breaking them. Multiple passes of the tow through the coating liquid were used to increase coverage of fiber surfaces and to increase coating thickness. The coated tows were not stiffened by the coating and felt the same as the uncoated tows on handling. Coating thickness anged from <10 to 100 nm. A few fibers appeared to not contain any coatings at all (via TEM); however, yttrium was always detectable at fiber surfaces by EDs. Whether this was due to the limited amount of precursor that wet the fiber surface durin coating or whether it was due to residual yttrium present after the had spalled off was difficult to determin .e. It was not ssible to differentiate a lack of fiber coating from loss of fiber coating during handling. It was also difficult to correlate coating thickness with precursor concentration of such a narrow range because the number of coatings measured by tEM was smal e The coatings were amorphous and homogeneous as-coated(Fig D). After heating for I h at 1000.C in a a two-phase coatin of intimately mixed YAG and carbon was obtained with equiaxed 10 nm in size(Figs. 3(b)and(c). Over this dual-phase coating, a dense shell of YAG-10 nm thick was usually observed Similar features have also been observed in other oxide/carbon Fig. 2. SEM images of tow 1, indicating good covithin the tow due to the f the filaments ber coatings using the same coating procedure. 1, 22 39 There was by Y AG/C coating and minimal bridging of fibers little noticeable difference between the coatings that contained 30 use of lower-concentrated solution and wheels to break bridgesprocedure has been described in detail previously.23,34,35 Tows and minicomposites were mounted with epoxy on card stock that was used as a fixture. Uncoated tows were first pulled through ethanol in an attempt to align individual filaments before mounting. The gauge length was 25.4 mm. At least 20 sections of fiber tow and at least 10 minicomposites were tested for each sample where strengths were reported. Tow and minicomposite tensile strengths were determined from maximum loads and total fiber areas (based on an average fiber diameter of 12 m and 420 filaments/tow), neglecting any contributions from either the coating or matrix.36,37 (5) Phase and Microstructure Characterization Powders of the YAG precursors were prepared by heating the solutions on a hot plate to
200°C to evaporate the solvent and decompose the majority of the organics before heating in argon to temperatures of
1000°C for 1 h. Powder XRD was used to verify phase development in the precursor. Final carbon content was determined by thermogravimetric analysis (TGA) in air on pow￾ders previously heated to 1000°C in argon. Surfaces of coated and heat-treated fibers and fracture surfaces of minicomposites were characterized by SEM (Model 360FE, LEO, Cambridge, U.K.). Coated fiber tows and fiber/matrix interfaces in the composites were examined by TEM (Model CM200FEG, Philips, Eindhoven, The Netherlands). Energy￾dispersive X-ray spectroscopy (EDS) was used for elemental analysis. TEM specimens were prepared by impregnating coated tows with a high-temperature epoxy and thinning to electron transparency with diamond lapping films, followed by low-angle ion-beam milling, as described in detail elsewhere.38,39 III. Results and Discussion (1) Coated Fiber Tows Characterization of the coated tows by SEM and TEM indicated that minimal fiber bridging remained, and the filaments appeared well-coated (Fig. 2). Evidence of prior bridging during the coating process was present, but the wheels were effective at breaking them. Multiple passes of the tow through the coating liquid were used to increase coverage of fiber surfaces and to increase coating thickness. The coated tows were not stiffened by the coating and felt the same as the uncoated tows on handling. Coating thickness ranged from 10 to 100 nm. A few fibers appeared to not contain any coatings at all (via TEM); however, yttrium was always detectable at fiber surfaces by EDS. Whether this was due to the limited amount of precursor that wet the fiber surface during coating or whether it was due to residual yttrium present after the coating had spalled off was difficult to determine, i.e., it was not possible to differentiate a lack of fiber coating from loss of fiber coating during handling. It was also difficult to correlate coating thickness with precursor concentration of such a narrow range because the number of coatings measured by TEM was small. The coatings were amorphous and homogeneous as-coated (Fig. 3(a)). After heating for 1 h at 1000°C in argon, a two-phase coating of intimately mixed YAG and carbon was obtained with equiaxed particles
10 nm in size (Figs. 3(b) and (c)). Over this dual-phase coating, a dense shell of YAG
10 nm thick was usually observed. Similar features have also been observed in other oxide/carbon fiber coatings using the same coating procedure.21,22,39 There was little noticeable difference between the coatings that contained 30 vol% carbon and those with 50 vol%. Based on the absence of any gross segregation of phases, we assumed that the volume fraction of porosity in the YAG fiber coatings after oxidation of the fugitive carbon equaled the initial carbon volume fraction, as determined by TGA. Heat-treating the coated tows in air resulted in a loss of carbon and an increase in grain size. As the temperature was increased to 1200°C, the coatings sintered, and the grains reached a size comparable to the thickness of some of the coatings, as shown in Fig. 4. When YAG grains spanned the thickness of the coating, there was usually no porosity left. Thicker coatings still had residual porosity, but it was reduced from
30 and
50 vol% to
10 vol%. These results are similar to those found for fiber coatings that were heat-treated within a porous matrix, as dis￾cussed later. Table I contains the strengths of coated and uncoated (control) tows after undergoing various heat treatments to determine the effects of coating composition on fiber strength. As-received and desized tows of Nextel 610 (without a coating) have a strength of 1.6 GPa. Heating the same tow at 1200°C for 2 and 100 h in air reduces its strength to 1.1 and 1.0 GPa, respectively. The reduction Fig. 2. SEM images of tow 1, indicating good coverage of the filaments by YAG/C coating and minimal bridging of fibers within the tow due to the use of lower-concentrated solution and wheels to break bridges. Table II. Minicomposite Processing and Heat-Treatment Conditions Minicomposite Fiber tows† Temperature (°C) Time (h) Atmosphere Strength (GPa) Weibull modulus Coefficient of variation A 2 1200 2 Air 1.1 6 0.22 B 6 1200/1200 2/100 Argon/air 0.28 3 0.41 C 8 1200/1100 2/2 Argon/air 0.80 6 0.18 D 8 1200/1200 2/100 Argon/air 0.28 4 0.28 E‡ 12 1200 2 Air 0.66 6 0.21 F‡ 12 1200 100 Air 0.28 8 0.16 † From Table I. ‡ Control composites with no fiber coating. November 2002 Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites 2705