Am, Cern.So.852703-1002002) ournal Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites Michael K. Cinibulk, *Triplicane A. Parthasarathy,,f Kristin A. Keller, f and Tai-lI Mah*f Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Ohio 45433-7817 A porous oxide fiber coating was investigated for Nextel 610 mullite, zircon, and rare-earth aluminates(garnets and magneto- fibers in an alumina matrix. Polymeric-solution-derived yt lumbites, are good candidate porous fiber coatings. Porous trium aluminum garnet (YAG, Y3Al5O12) with a fugitive coatings based on alumina, mixed zirconia and silica, mullite carbon phase was used to develop the porous fiber coating lanthanum hexaluminate have been applied to small-d Ultimate tensile strengths of tows and minicomposites follow alumina and alumina-mullite fibers in tows. Porous ing heat treatments in argon and/or air were used to evaluate and lanthanum hexaluminate coatings have been applied the effect of the porous fiber coating. The porous YAG fiber phire monofilament coatings did not reduce the strength of the tows when heated in If an energy-based crack-deflection criterion is considered argon,and they degraded tow strength by only -20% after within a porous coating,2 a minimum pore volume fraction of air at 1200C for 100 h. minico posites containing +0.3 is needed to reduce the fracture energy of polycrystalline porous YAG-coated fibers were nearly twice as strong as those alumina to 25% of that of the dense material. 5, 6 For crack containing uncoated fibers. However after heating at 1200oC deflection at a coating/fiber interface, the required pore fraction for 100 h, the porous YAG coatings densified to >90%, at may be lower because of the higher elastic modulus of the fiber which point they were ineffective at protecting the fibers, compared with that of the porous coating. This paper focuses on a resulting in identical strengths for minicomposites with and porous yttrium aluminum garnet (YAG, Y3 Al, O12) fiber coating without a fiber coating that does not degrade fiber strength in air. Fiber degradation has been a major concern with most oxide coatings. Aspects of precursor synthesis, fiber coating, coating-microstructure develop- L. Introduction T HAS been well-documented that increasing the porosity of a ceramic decreases its mechanical properties. -This concept has been used to weaken matrixes of oxide composites to the extent Il. Experimental Procedure that vs 10-14 coating is not needed to protect the fibers from matrix ()) Precursor and Coating Synthesis YAG is the most creep-resistant oxide known. 26-28YAG trength of the composite; its function is mainly to hold the fibers synthesis by conventional solid-state reaction of the element in place and prevent matrix cracks from developing enough energy oxides requires temperatures >1600oC for extended period to be able to penetrate the fibers. However, in many applications Commercially available alumina fibers, such as Nextel 610(3M where hermeticity, compressive and/or transverse strength, or wear Corp, Minneapolis, MN), cannot be exposed to temperatures 1200C for more than very short periods without degrading not adequate. For such applications, a dense matrix is preferred, strength via grain growth. This limits the processin ng window fo to provide crack deflection at the fiber/matrix interfacial region. the pplication of a coating and subsequent matrix processing of and, therefore, some type of fiber coating is likely to be required the ≤1200°C. or a dense matrix composite, the concept of porosity has been Recently, the synthesis of phase-pure YAG at temperatures applied sparingly to weaken the fiber/matrix interface. Porous 2800%C within I h has been reported. The polymeric precursor coatings have been applied to large-diameter monofilaments where crystallizes directly into the garnet structure from an amorphous discrete carbon or polymer particles are used as a fugitive phase powder starting at 600C when heated in an oxidizing atmosphere that is later burned out to create porosity. 5-8 The polymer In argon, peaks of primarily hexagonal YAlO, are present along particles are usually >100 nm in size and are of the appropriate with traces of YAG and amorphous alumina at temperatures of scale to provide a fugitive phase for fiber coatings that are >l um 700-900 C At 1000oC within I h, YAlO )3 reacts with the residual in thickness alumina to form YAG, which is then the only crystalline phase The requirement for a much thinner coating on the 10-um- present. The precursor is well-suited for this work because an diameter filaments in commercially available tows and cloths has intimate mixture of nanosized YAG and carbon is obtained when led to the development of porous coatings derived from intimate it is heated in an inert atmosphere at temperatures below which mixtures of oxide and carbon particles on the order of 10 nm in strength degradation of Nextel 610 fiber occurs. Details of the diameter. 9-2 With such a fine-scale microstructure, the ability of mixed-metal citric acid/ethylene glycol/ethanol solution precursor the porous coating to resist sintering and/or coarsening when synthesis have been discussed elsewhere. 3 In the present study onstrained between fiber and matrix during exposure to elevated two solutions are prepared to obtain coatings with carbon contents temperatures is paramount. Oxides with low self-diffusion, such as (derived from thermal decomposition of the organic components) of 30 and 50 vol% on a solids basis. The carbon acts as a fugitive phase to establish and maintain porosity during subsequent matrix infiltration and densification; only after matrix processing is the F W. Zok--contributing editor carbon removed by heating in air (2) Fiber Coating ly30,20 xtel 610 fiber tow was first pass 33615-96C-5258. furnace of a fiber coating apparatus, shown in Fig. 1, at 900oC in filiated with UES. In OH45432 air to remove the sizing. The hot zone of the in-line furnace was 2703
Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites Michael K. Cinibulk,* Triplicane A. Parthasarathy,* ,† Kristin A. Keller,* ,† and Tai-Il Mah* ,† Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright–Patterson Air Force Base, Ohio 45433-7817 A porous oxide fiber coating was investigated for Nextel™ 610 fibers in an alumina matrix. Polymeric-solution-derived yttrium aluminum garnet (YAG, Y3Al5O12) with a fugitive carbon phase was used to develop the porous fiber coating. Ultimate tensile strengths of tows and minicomposites following heat treatments in argon and/or air were used to evaluate the effect of the porous fiber coating. The porous YAG fiber coatings did not reduce the strength of the tows when heated in argon, and they degraded tow strength by only 20% after heating in air at 1200°C for 100 h. Minicomposites containing porous YAG-coated fibers were nearly twice as strong as those containing uncoated fibers. However, after heating at 1200°C for 100 h, the porous YAG coatings densified to >90%, at which point they were ineffective at protecting the fibers, resulting in identical strengths for minicomposites with and without a fiber coating. I. Introduction I T HAS been well-documented that increasing the porosity of a ceramic decreases its mechanical properties.1–9 This concept has been used to weaken matrixes of oxide composites to the extent that a fiber coating is not needed to protect the fibers from matrix cracks.10–14 In such composites, the matrix contributes little to the strength of the composite; its function is mainly to hold the fibers in place and prevent matrix cracks from developing enough energy to be able to penetrate the fibers. However, in many applications where hermeticity, compressive and/or transverse strength, or wear resistance is required, for example, porous materials are usually not adequate. For such applications, a dense matrix is preferred, and, therefore, some type of fiber coating is likely to be required to provide crack deflection at the fiber/matrix interfacial region. For a dense matrix composite, the concept of porosity has been applied sparingly to weaken the fiber/matrix interface. Porous coatings have been applied to large-diameter monofilaments where discrete carbon or polymer particles are used as a fugitive phase that is later burned out to create porosity.15–18 The polymer particles are usually 100 nm in size and are of the appropriate scale to provide a fugitive phase for fiber coatings that are 1 m in thickness. The requirement for a much thinner coating on the 10-mdiameter filaments in commercially available tows and cloths has led to the development of porous coatings derived from intimate mixtures of oxide and carbon particles on the order of 10 nm in diameter.19–22 With such a fine-scale microstructure, the ability of the porous coating to resist sintering and/or coarsening when constrained between fiber and matrix during exposure to elevated temperatures is paramount. Oxides with low self-diffusion, such as mullite, zircon, and rare-earth aluminates (garnets and magnetoplumbites), are good candidate porous fiber coatings. Porous coatings based on alumina, mixed zirconia and silica, mullite, and lanthanum hexaluminate have been applied to small-diameter alumina and alumina–mullite fibers in tows.19–23 Porous zirconia and lanthanum hexaluminate coatings have been applied to sapphire monofilaments.15,18,24 If an energy-based crack-deflection criterion is considered within a porous coating,25 a minimum pore volume fraction of 0.3 is needed to reduce the fracture energy of polycrystalline alumina to 25% of that of the dense material.5,6 For crack deflection at a coating/fiber interface, the required pore fraction may be lower because of the higher elastic modulus of the fiber compared with that of the porous coating. This paper focuses on a porous yttrium aluminum garnet (YAG, Y3Al5O12) fiber coating that does not degrade fiber strength in air. Fiber degradation has been a major concern with most oxide coatings. Aspects of precursor synthesis, fiber coating, coating-microstructure development, and composite processing and evaluation are discussed. II. Experimental Procedure (1) Precursor and Coating Synthesis YAG is the most creep-resistant oxide known.26–28 YAG synthesis by conventional solid-state reaction of the elemental oxides requires temperatures 1600°C for extended periods.29 Commercially available alumina fibers, such as Nextel™ 610 (3M Corp., Minneapolis, MN), cannot be exposed to temperatures 1200°C for more than very short periods without degrading strength via grain growth.30 This limits the processing window for the application of a coating and subsequent matrix processing of the composite to temperatures 1200°C. Recently, the synthesis of phase-pure YAG at temperatures 800°C within 1 h has been reported.31 The polymeric precursor crystallizes directly into the garnet structure from an amorphous powder starting at 600°C when heated in an oxidizing atmosphere. In argon, peaks of primarily hexagonal YAlO3 are present along with traces of YAG and amorphous alumina at temperatures of 700°–900°C. At 1000°C within 1 h, YAlO3 reacts with the residual alumina to form YAG, which is then the only crystalline phase present.31 The precursor is well-suited for this work because an intimate mixture of nanosized YAG and carbon is obtained when it is heated in an inert atmosphere at temperatures below which strength degradation of Nextel 610 fiber occurs. Details of the mixed-metal citric acid/ethylene glycol/ethanol solution precursor synthesis have been discussed elsewhere.31 In the present study, two solutions are prepared to obtain coatings with carbon contents (derived from thermal decomposition of the organic components) of 30 and 50 vol% on a solids basis. The carbon acts as a fugitive phase to establish and maintain porosity during subsequent matrix infiltration and densification; only after matrix processing is the carbon removed by heating in air. (2) Fiber Coating Nextel 610 fiber tow was first passed continuously through the furnace of a fiber coating apparatus,32 shown in Fig. 1, at 900°C in air to remove the sizing. The hot zone of the in-line furnace was F. W. Zok—contributing editor Manuscript No. 187773. Received April 16, 2001; approved July 30, 2002. Supported by AFRL, under Contract No. F33615-96-C-5258. *Member, American Ceramic Society. † Also affiliated with UES, Inc., Dayton, OH 45432. J. Am. Ceram. Soc., 85 [11] 2703–10 (2002) 2703 journal
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 aluminainfiltrated 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 minicomposites 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
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 bridges
procedure 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 powders 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). Energydispersive 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 discussed 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
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 healing. 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 unconstrained 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
Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composite 2707 N610-Desized 1.6, 10 日N610-1200C2h11,6 2.1,19 1.6,9 b-1200c2h(A)1.3.12 1200c2h(Air)0.8,5 12000,100h(Air) 与4 44.555.56657758 Ln stress, MPa I 5. Weibull plot of tows 1-5 that were coated with 50 vol% YAG/C precursor and tows 12 and 13(desized, without coating) heat-treated under various here residual porosity is <10% in much thicker coatings; any flaws in the coating(or matrix) now penetrate the fiber readily in the absence of any crack-deflecting mech Incomp Table ll summarizes the st ns of minicomposites Minicom- osite A had an average strength that was nearly twice as high as the control minicomposite F. Minicomposites B and D, heat- treated for 100 h in air at 1200oC, had strengths that were equal to that of the control minicomposite F heated under the same conditions but without a fiber coating. Figure 6 contains sem images of fracture surfaces of minicomposites A and B, showing the difference in fracture behavior of the two composites. Long ber pullout lengths and holes left behind by fibers were clearly visible in the fracture surfaces of minicomposite A, as opposed to nly fibers exposed by matrix disintegration, as is often seen in (a) orous matrix composites. Minicomposite B displayed a brittle fracture surface with little, if any, fiber pullout A TEM image of the fiber/matrix interfacial region from minicomposite A is shown in Fig. 7. T and pore sizes in the YAG fiber coating increase to 40-50 nm, and particles begi to sinter following matrix processing at 1200%C for 2 h(F b), (c)). Because the matrix is porous as well and could pected to deflect cracks, the presence of the porous YAG coati er tensile strengths. Either rosity distribution in the matrix alone is not adequate to deflect acks, whereas that in the fiber coating is, or the coating protects the fiber from degradation during matrix processing. Sintering of matrix alumina particles to the alumina fiber is observed in the absence of a coating, which could lead to fiber degradation in the control composites by enhanced fiber/matrix bonding and stress concentration Figures 8 and 9 show TEM images of the fiber/ in minicomposites C and D, which contain an I matrix ite aes of Yag that
where residual porosity is 10% in much thicker coatings; any flaws in the coating (or matrix) now penetrate the fiber readily in the absence of any crack-deflecting mechanism.35 (2) Minicomposites Table II summarizes the strengths of minicomposites. Minicomposite A had an average strength that was nearly twice as high as the control minicomposite F. Minicomposites B and D, heattreated for 100 h in air at 1200°C, had strengths that were equal to that of the control minicomposite F heated under the same conditions but without a fiber coating. Figure 6 contains SEM images of fracture surfaces of minicomposites A and B, showing the difference in fracture behavior of the two composites. Long fiber pullout lengths and holes left behind by fibers were clearly visible in the fracture surfaces of minicomposite A, as opposed to only fibers exposed by matrix disintegration, as is often seen in porous matrix composites.41 Minicomposite B displayed a brittle fracture surface with little, if any, fiber pullout. A TEM image of the fiber/matrix interfacial region from minicomposite A is shown in Fig. 7. The grain and pore sizes in the YAG fiber coating increase to 40–50 nm, and particles begin to sinter following matrix processing at 1200°C for 2 h (Fig. 3(b),(c)). Because the matrix is porous as well and could be expected to deflect cracks, the presence of the porous YAG coating results in composites with higher tensile strengths. Either the porosity distribution in the matrix alone is not adequate to deflect cracks, whereas that in the fiber coating is, or the coating protects the fiber from degradation during matrix processing. Sintering of matrix alumina particles to the alumina fiber is observed in the absence of a coating, which could lead to fiber degradation in the control composites by enhanced fiber/matrix bonding and stress concentration. Figures 8 and 9 show TEM images of the fiber/matrix interfaces in minicomposites C and D, which contain an initial 30 vol% porosity coating and are heat-treated at 1100°C for 2 h in air and 1200°C for 100 h in air, respectively. Clearly evident is the grain growth, pore coarsening, and sintering of YAG that is occurring. In Fig. 5. Weibull plot of tows 1–5 that were coated with 50 vol% YAG/C precursor and tows 12 and 13 (desized, without coating) heat-treated under various conditions. Fig. 6. SEM images of (a) minicomposite A and (b) minicomposite B following heating at 1200°C for 2 and 100 h in air, respectively. Note fiber pullout and troughs left by fiber pulled out in Fig. 6(a). November 2002 Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites 2707