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 2703Porous 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 yt￾trium aluminum garnet (YAG, Y3Al5O12) with a fugitive carbon phase was used to develop the porous fiber coating. Ultimate tensile strengths of tows and minicomposites follow￾ing 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-m￾diameter 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 magneto￾plumbites), 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 sap￾phire 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 develop￾ment, 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