J. An. Ceram Soc..87[101967-197602004) ournal Zirconia-Silica-Carbon Coatings on Ceramic Fibers Emmanuel E Boakye, *T Randall S. Hay, *f M. Dennis Petry, and Triplicane A Parthasarathy* f UES, Inc, Dayton, Ohio 45432 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 Precursors for zircon-carbon mixtures were made to coat ZrSiO4 forms from ZrO, and Sio, at -1400-1600C.,For fibers for ceramic-matrix composites. Precursors were char- NextelM 720 fiber coatings, fib er strength degra acterized using XRD, TGA, and DTA. Zircon formed from with grain growth in the fiber requires ZrSiO4 to form at vanadium-or lithium-doped precursors after heat treatments <1300.3. However. near-stoichiometric SiC fibers surviv at≥900° C in air, but it did not form at1200°-1400° C in argon much higher temperatures in argon without such degradation. 4-37 when large amounts of carbon were added. Some precursors Reacting ZrO, with various Sio, allotropes has little effect on were used to coat Nextel 720 and Hi-Nicalon fibers. The ZrSiOa formation. Sol-gel precursors decreas se the formation coatings were characterized using SEM and TEM, and coated- temperature to 1200C,3 but temperatures >1350%C are neces- fiber tensile strengths were measured. Although zircon formed sary for high purity. 3 Seeding and fluxes can significantly in powders, only tetragonal-zirconia-silica mixed phases formed in fiber coatings at 1200.C in air. Loss of vanadium air has been studied extensively, 239. 0.45-47and there is one study oxide flux to the fibers may have caused the lack of conversion for a nitrogen atmosphere, but there is little information for other to zircon. The strengths of the coated fibers were severely atmospheres. It has been claimed that ZrSiOa should be stable with degraded after heat treatment at 21000C in air, but not in carbon at <1527 C, based on thermodynamic calculations, but carbon-coated fibers made using similar methods. Mecha- reduction of ZrSiO4 at 1450C Ran be produced by carbothermal argon. The coated fibers were compared with zirconia- other work shows that Zro isms for fiber strength degradation are discussed. In this work, attempts to synthesize precursors for porous ZrSiO4 fiber coatings are reported. The precursors use carbon as a fugitive phase to hold porosity open during coating deposition. Conditions that may promote ZrSiO4 formation in the presence of carbon are examined. Fibers were coated with these precursors. here has been extensive research on development of conversion to ZrSio, was attempted, and strengths of the coated oxidation-resistant fiber-matrix interfaces for ceramic-matrix composites(CMCs). Recent results demonstrate the utility of tensile strengths of the coated fibers are discussed composites are another widely used approach. Here, a discrete coatings were previously discussed in a minicomposite study of intrinsic weakness of the porous matrix does not allow buildup of a matrix crack-tip stress that is sufficient to break fibers. A weakness of this approach is that matrix-dominated mechanical properties of porous-matrix CMCs tend to be poor Fretting and Il. Experimental Procedures wear at contact and attachment points can severely degrade porous matrices.A modified approach eserve matn Fiber coatings are typically 100 nm thick on 12 um fila- dominated properties is use of a porous fiber-matrix interface in a ments. 0-33 For a homogenous coating, the oxide--carbon disper dense-matrix CMC. -8 Candidate materials for a porous fiber sion should be homogeneous at a "20 nm scale. Separate matrix interface must be themochemically stable with the fiber and precipitation of oxides and carbon from precursors introduces the matrix, and they must be sufficiently refractory so that porosity extensive filament-to-filament variation in coatings. One approach designed into the interface material does not coarsen or densify is to bond the carbon and oxide precursors through electrostatic omposite processing and use and steric interactions. After the matrix is processed, carbon SiO4) decomposes to SiO and ZrO, at-16000- can be oxidized to leave a porous-oxide fiber-matrix interface. 1700C,,and it has a coefficient of thermal expansion(CTE)of The following chemicals were used: zirconyl nitrate hydrate 4.3X 10/C, which is a good match with SiC.,4 Conse- ammonium vanadate. lithium nitrate,(Aldrich Chemical Co uently, ZrSiOa has been used as a matrix in CMCs with SiC milwaukee, Wn), tetraethoxysilane(Alfa Chemical Co., Ward Hil fibers.- The low diffusion and creep rates of ZrSiO4 suggest it MA), and poly(acrylic acid)(PA; Fischer Scientific Co.,Pitts should resist densification and pore coarsening.-- Except for burgh, PA). Water was purified by deionization of distilled water HF, it is chemically resistant to acids. These properties make with a nanopure ultrapure system (Model D4744, Barnstead/ ZrSiO4 an attractive candidate for a porous fiber-matrix interface. Thermolyne Corp, Dubuque, IA) (2)ZNS Precursor F. w. Zok--contributing editor Zirconyl nitrate hydrate(ZN, 25 g) was dissolved in 500 mL of absolute ethanol and refluxed at 45C for 30 min. Tetraethoxy lane (TEoS, 22.5 added and the mixture was refluxed at anuscript No 10557. Received September 25. 2003; approved May 3, 2004. 55°Cfor24h.A g)was a m vanadate or lithium nitrate was added as a dopant to mber, American Ceramic Society ZrSiOa formation temperat concentration was 1.2 g/L, except where stated otherwise. In some ir Force Research Laboratory. formulations, TEOS was prehydrolyzed for 2 h at pH 2, with anZirconia–Silica–Carbon Coatings on Ceramic Fibers Emmanuel E. Boakye,* ,† Randall S. Hay,* ,‡ M. Dennis Petry,† and Triplicane A. Parthasarathy* ,† UES, Inc., Dayton, Ohio 45432 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright–Patterson Air Force Base, Ohio 45433 Precursors for zircon– carbon mixtures were made to coat fibers for ceramic-matrix composites. Precursors were char￾acterized using XRD, TGA, and DTA. Zircon formed from vanadium- or lithium-doped precursors after heat treatments at >900°C in air, but it did not form at 1200°–1400°C in argon when large amounts of carbon were added. Some precursors were used to coat NextelTM 720 and Hi-NicalonTM fibers. The coatings were characterized using SEM and TEM, and coated￾fiber tensile strengths were measured. Although zircon formed in powders, only tetragonal-zirconia–silica mixed phases formed in fiber coatings at 1200°C in air. Loss of vanadium oxide flux to the fibers may have caused the lack of conversion to zircon. The strengths of the coated fibers were severely degraded after heat treatment at >1000°C in air, but not in argon. The coated fibers were compared with zirconia– carbon-coated fibers made using similar methods. Mecha￾nisms for fiber strength degradation are discussed. I. Introduction THERE has been extensive research on development of oxidation-resistant fiber–matrix interfaces for ceramic-matrix composites (CMCs).1 Recent results demonstrate the utility of monazite (LaPO4, NdPO4) for this interface.2–5 Porous-matrix composites are another widely used approach.6 –10 Here, a discrete phase does not exist at the fiber–matrix interface; instead, the intrinsic weakness of the porous matrix does not allow buildup of a matrix crack-tip stress that is sufficient to break fibers. A weakness of this approach is that matrix-dominated mechanical properties of porous-matrix CMCs tend to be poor. Fretting and wear at contact and attachment points can severely degrade porous matrices.8 A modified approach that can preserve matrix￾dominated properties is use of a porous fiber–matrix interface in a dense-matrix CMC.11–18 Candidate materials for a porous fiber– matrix interface must be themochemically stable with the fiber and the matrix, and they must be sufficiently refractory so that porosity designed into the interface material does not coarsen or densify during composite processing and use. Zircon (ZrSiO4) decomposes to SiO2 and ZrO2 at
1600°– 1700°C,19,20 and it has a coefficient of thermal expansion (CTE) of 4.3 106 /°C, which is a good match with SiC.21,22 Conse￾quently, ZrSiO4 has been used as a matrix in CMCs with SiC fibers.22–24 The low diffusion and creep rates of ZrSiO4 suggest it should resist densification and pore coarsening.25–29 Except for HF, it is chemically resistant to acids.30 These properties make ZrSiO4 an attractive candidate for a porous fiber–matrix interface. ZrSiO4 forms from ZrO2 and SiO2 at
1400°–1600°C.31,32 For NextelTM 720 fiber coatings, fiber strength degradation associated with grain growth in the fiber requires ZrSiO4 to form at 1300°C.33 However, near-stoichiometric SiC fibers survive much higher temperatures in argon without such degradation.34 –37 Reacting ZrO2 with various SiO2 allotropes has little effect on ZrSiO4 formation.32 Sol– gel precursors decrease the formation temperature to
1200°C,32 but temperatures 1350°C are neces￾sary for high purity.38 Seeding and fluxes can significantly decrease ZrSiO4 formation temperature.39 – 42 ZrSiO4 formation in air has been studied extensively,32,39,40,43– 47 and there is one study for a nitrogen atmosphere,42 but there is little information for other atmospheres. It has been claimed that ZrSiO4 should be stable with carbon at 1527°C, based on thermodynamic calculations,22 but other work shows that ZrO2 can be produced by carbothermal reduction of ZrSiO4 at 1450°C.48 In this work, attempts to synthesize precursors for porous ZrSiO4 fiber coatings are reported. The precursors use carbon as a fugitive phase to hold porosity open during coating deposition.13,49 Conditions that may promote ZrSiO4 formation in the presence of carbon are examined. Fibers were coated with these precursors, conversion to ZrSiO4 was attempted, and strengths of the coated fibers were measured. Characteristics of the fiber coatings and tensile strengths of the coated fibers are discussed and compared with those of ZrO2– carbon-coated fibers. Some features of these coatings were previously discussed in a minicomposite study of porous ZrO2–SiO2 fiber coatings.15 II. Experimental Procedures (1) General Fiber coatings are typically
100 nm thick on 12 m fila￾ments.50 –53 For a homogenous coating, the oxide– carbon disper￾sion should be homogeneous at a
20 nm scale. Separate precipitation of oxides and carbon from precursors introduces extensive filament-to-filament variation in coatings. One approach is to bond the carbon and oxide precursors through electrostatic and steric interactions.13,49 After the matrix is processed, carbon can be oxidized to leave a porous-oxide fiber–matrix interface. The following chemicals were used: zirconyl nitrate hydrate, ammonium vanadate, lithium nitrate, (Aldrich Chemical Co., Milwaukee, WI), tetraethoxysilane (Alfa Chemical Co., Ward Hill, MA), and poly(acrylic acid) (PA; Fischer Scientific Co., Pitts￾burgh, PA). Water was purified by deionization of distilled water with a nanopure ultrapure system (Model D4744, Barnstead/ Thermolyne Corp., Dubuque, IA). (2) ZNS Precursor Zirconyl nitrate hydrate (ZN, 25 g) was dissolved in 500 mL of absolute ethanol and refluxed at 45°C for 30 min. Tetraethoxysi￾lane (TEOS, 22.5 g) was added, and the mixture was refluxed at 55°C for 24 h. Ammonium vanadate or lithium nitrate was added as a dopant to decrease ZrSiO4 formation temperature. Their concentration was 1.2 g/L, except where stated otherwise. In some formulations, TEOS was prehydrolyzed for 2 h at pH 2, with an F. W. Zok—contributing editor Manuscript No. 10557. Received September 25, 2003; approved May 3, 2004. Supported by the Air Force Office of Scientific Research. *Member, American Ceramic Society. † UES, Inc. ‡ Air Force Research Laboratory. J. Am. Ceram. Soc., 87 [10] 1967–1976 (2004) 1967 journal