2704 Journal of the American Ceramic Sociery-Cinibulk et al. Vol. 85. No. 11 solutions and a furnace temperature of 1000C. After problems with severe fiber bridging by excess entrained solution, subsequent coating runs were conducted at lower concentrations and lower temperatures. The 5, 8, and 10 g/L sols were applied three times with the coated tow passing through a furnace at 800C(10 s residence time)and then over a 2.5-cm-diameter guide wheel to bend the tow by "60.(Fig. 1), as summarized in Table I. The guide wheel allowed for any weak coating bridges between filaments to be broken. The lower temperature ensured that a partially pyrolyzed amorphous coating was deposited during each pass, which minimized stratification of the coating due to multiple passes through the coater, and was more likely to produce a Furnace thicker, single homogeneous layer. Postcoating heat treatment were conducted under various conditions to evaluate their effect the coatings and on fiber tow strength; these are given in Table I All postcoating heat treatments included an initial heating for I h in argon at 1000C to convert the coating to a homogeneously dispersed mixture of nanometer-sized YAG and carbon Fiber Tow ( Minicommposite Processing In the absence of being able to fabricate an oxide composite t Argor with a dense matrix that also had a modulus close to that of the fibers, an alumina matrix with 40 vol% porosity was chosen to Immiscible Displacing liquid evaluate the porous fiber coating concept. Control composites using the same fiber volume fraction and matrix were fabricated Coating Precursor Liquid and tested for comparison. Tows(tows 2, 6, and 8 in Table D) containing the homogeneously dispersed YAG and carbon fiber ol% alumina(AKP-53, Sumitomo Chemicals, Tokyo, Japan) along with a small amount of gel-casting agents. Minicomposites x75 mm in length were prepared by inserting four alumina "1.6 mm. On heating the tubing with a heat gun, the inner diameter decreased to -l mm, expelling excess matrix slurry from the composites, which increased the fiber volume fraction. The alumina matrix was allowed to gel and then was dried under 95% elative humidity. The resulting composites were sintered at Fig. 1. Schematic of continuous fiber coater Inset shows magnified view 1200.C in either argon or air to form unidirectional minicompos of wheels to break weak fiber bridges ites containing four tows, each with a fiber volume fraction of "30%.(The processing and mini Ite fabrication procedures are discussed in detail elsewhere. , )A number of conditions were used to sinter and heat-treat the minicomposites to evaluate 8 cm in length, which resulted in a residence time of <10 s. the porous coatings, as summarized in Table Il. The final heat Desized tows were the with YAG solutions having ar treatment was always in air, which oxidized the fugitive carbon i oxide concentration of 5 15.or2 and a final carbo the coatings to yield porous YAG fiber coatings ontent of either 30 or 50 ased on total solids content. All Datings were applied using a continuous coating apparatus with exadecane as an immiscible liquid to displace excess YAG/C 4 Tensile Testin solution from between the filaments to minimize bridging(Fig. Over the past five years, a tensile test procedure has been 1). Initial coating trials were conducted with 15 and 25 g/L developed in our laboratory to evaluate novel fiber coatings. This Table L. Fiber-Coating Precursor Solutions, Heat-Treatment Conditions, and Strengths Precursor solution/coating Postcoating heat treatment Carbon content YAG content Atmosphere l000/1200 gon 1000/1200 Argon/air 0.10 33333333311 l000/1200 1/100 Argon/ai 1000/1200 l000/1200 Argon/ai 1000/12001/100rgon/air 74 13 Air 1.1 0.20 14 1200 1.0 10 0.12 dEsized at 900%C, no coating
8 cm in length, which resulted in a residence time of 10 s. Desized tows were then coated with YAG solutions having an oxide concentration of 5, 8, 10, 15, or 25 g/L and a final carbon content of either 30 or 50 vol%, based on total solids content. All coatings were applied using a continuous coating apparatus with hexadecane as an immiscible liquid to displace excess YAG/C solution from between the filaments to minimize bridging (Fig. 1).32 Initial coating trials were conducted with 15 and 25 g/L solutions and a furnace temperature of 1000°C. After problems with severe fiber bridging by excess entrained solution, subsequent coating runs were conducted at lower concentrations and lower temperatures. The 5, 8, and 10 g/L sols were applied three times with the coated tow passing through a furnace at 800°C (
10 s residence time) and then over a 2.5-cm-diameter guide wheel to bend the tow by
60° (Fig. 1), as summarized in Table I. The guide wheel allowed for any weak coating bridges between filaments to be broken. The lower temperature ensured that a partially pyrolyzed amorphous coating was deposited during each pass, which minimized stratification of the coating due to multiple passes through the coater, and was more likely to produce a thicker, single homogeneous layer. Postcoating heat treatments were conducted under various conditions to evaluate their effect on the coatings and on fiber tow strength; these are given in Table I. All postcoating heat treatments included an initial heating for 1 h in argon at 1000°C to convert the coating to a homogeneously dispersed mixture of nanometer-sized YAG and carbon. (3) Minicomposite Processing In the absence of being able to fabricate an oxide composite with a dense matrix that also had a modulus close to that of the fibers, an alumina matrix with
40 vol% porosity was chosen to evaluate the porous fiber coating concept. Control composites using the same fiber volume fraction and matrix were fabricated and tested for comparison. Tows (tows 2, 6, and 8 in Table I) containing the homogeneously dispersed YAG and carbon fiber coatings were infiltrated with an aqueous slurry containing 45 vol% alumina (AKP-53, Sumitomo Chemicals, Tokyo, Japan) along with a small amount of gel-casting agents.33 Minicomposites
75 mm in length were prepared by inserting four alumina￾infiltrated tows into heat shrink tubing with an inner diameter of
1.6 mm. On heating the tubing with a heat gun, the inner diameter decreased to
1 mm, expelling excess matrix slurry from the composites, which increased the fiber volume fraction. The alumina matrix was allowed to gel and then was dried under 95% relative humidity. The resulting composites were sintered at 1200°C in either argon or air to form unidirectional minicompos￾ites containing four tows, each with a fiber volume fraction of
30%. (The processing and minicomposite fabrication procedures are discussed in detail elsewhere.33,34) A number of conditions were used to sinter and heat-treat the minicomposites to evaluate the porous coatings, as summarized in Table II. The final heat treatment was always in air, which oxidized the fugitive carbon in the coatings to yield porous YAG fiber coatings. (4) Tensile Testing Over the past five years, a tensile test procedure has been developed in our laboratory to evaluate novel fiber coatings. This Fig. 1. Schematic of continuous fiber coater. Inset shows magnified view of wheels to break weak fiber bridges. Table I. Fiber-Coating Precursor Solutions, Heat-Treatment Conditions, and Strengths Tow Precursor solution/coating Postcoating heat treatment Strength Carbon content (vol%) YAG content (g/L) Number of passes Temperature (°C) Time (h) Atmosphere Tensile strength (GPa) Weibull modulus Coefficient of variation 1 50 5 3 2.1 19 0.06 2 50 5 3 1000 1 Argon 2.2 22 0.05 3 50 5 3 1000/1200 1/2 Argon 1.6 9 0.13 4 50 5 3 1000/1200 1/2 Argon/air 1.3 12 0.10 5 50 5 3 1000/1200 1/100 Argon/air 0.81 5 0.22 6 50 8 3 1000 1 Argon 7 50 8 3 1000/1200 1/2 Argon/air 1.2 14 0.09 8 30 10 3 1000 1 Argon 2.0 18 0.07 9 30 10 3 1000/1200 1/2 Argon 1.7 29 0.04 10 30 10 3 1000/1200 1/2 Argon/air 0.78 13 0.09 11 30 10 3 1000/1200 1/100 Argon/air 0.74 8 0.15 12† 1 1.6 10 0.11 13† 1 1200 2 Air 1.1 6 0.20 14† 1 1200 100 Air 1.0 10 0.12 † Desized at 900°C, no coating. 2704 Journal of the American Ceramic Society—Cinibulk et al. Vol. 85, No. 11