《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_j.1151-2916.1999.tb02086.x[1]

ournal . An. Cera.soc,8292321-3l(1999 Continuous Coating of Oxide Fiber Tows Using Liquid Precursors Monazite Coatings on Nextel 720TM Emmanuel Boakye, ,t Randall S. Hay, , and M. Dennis Petryt nC, Dayton, Ohio 45432; Air Force Research Laboratory, Materials Directorate, Wright-Patterson Air Force Base, Ohio 45433 Seven different aqueous or ethanolic precursors were used fiber-matrix interface. 28, 29 Ideally, fiber coatings should be to continuously coat monazite (LapO4) on Nextel 720TM smooth, the same thickness, and the correct composition. The fiber tows. Immiscible liquid displacement was used to coatings also should not degrade filament tensile strength. Un- minimize bridging of coating between filaments. Precursor fortunately, there is little information about the degree to which viscosities, densities, and concentrations were measured these ideal coating qualities must be approached to get accept and solids were characterized by DTA/TGA and X-ray able CMC mechanical performance Coatings were cured in-line at 1200%-14000C and charac. Current cmc fiber-matrix interfaces are either carbon 30,3 terized for thickness, microstructure and composition by or BN. 32,33 However, oxidation is a major limitation to car- optical microscopy, SEM, and TEM. Tensile strengths of bon 4 and BN3,35 interfaces, particularly in the presence o he coated fibers varied with the precursor used and were water, 35, 36 and"pest "oxidation is a severe problem at inter- ing thicknesses were typically -50-100 nm for precur- high tempera Po), that.as 25% to >50% lower than those of the as-received fiber The mediate temperatures. 7, 38 The e problems motivate research oating stoichiometry and coating thickness of a particular on oxide Cmcs with carbon and Bn replacements, such as Monazite is stable with many common oxides, and some nesses predicted by theory for 12 mm diameter monofila evidence suggests monazite-alumina interphase boundaries de- ments. There were significant thickness variations from flect cracks. 9- Similar results were found for YPO,(Xeno- filament to filament, but usually little variation in ce time)yttrium aluminum garnet interphase boundaries. Mona- sition or microstructure. Amorphous AlPOa layers forme zite has a coefficient of thermal expansion(CTE)of 9.6x from phosphate- rich precursors. Factors that could affect 106C, modulus of 130 GPa, 0 and density of 5.08 g/cm coating characteristics and tensile strength reduction were Hydrated LapO4 converts to monazite at -500oC and is called discusse rhabdophane. Nonstoichiometric monazite precursors can react with the fiber or matrix, and carbothermal reduction of phos . Introduction phate in monazite is also possible. 43 Precise control of mona- zite stoichiometry during fiber coating is, therefore, desirable nd some work has been done on cloths. ,/on thick. 3,46Ss 0 eled s g of plates has been extensively investigated and mod- 10 Dip coating of fiber monofilaments is also a com racking can be caused by shrinkage from rapid Coating fiber tows is also of interest. Most ceramic fiber sht loss, capillary forces, and sintering at high heating process I .8,48 or by residual stress from thermal expansion mis tows have 200-500 filaments that are 10-15 um in diameter match- Continuous fiber coating generally uses high heating each. 8-20 It is difficult to coat individual filaments uniformly rates, therefore, cracking from weight loss and shrinkage is of in a fiber tow, because coating cements the filaments to- particular concern. Micrometer-thick coatings require multiple gether, 21 and a thick crust often forms around the tow perim layers and can still crack if thermal expansion mismatch is eter. 22,23 Displacement of some coating liquid by an immis- large. Currently, there is no consensus for optimal coating cible liquid reduces, but does not eliminate, cementation and thickness; it can vary widely from one fiber-coating-matrix crusting 22-24 Immiscible liquid displacement is a common system to another. rocess in oil recovery, chemical engineering, and hydrol- Fiber coatings from seven different monazite liquid-phase precursors on Nextel 720TM(3M, St. Paul, MN) fibers are ogy. 8, 25 Simple geometries have been studied, but the process described. Tows of Nextel 720 consist of 420 alumina-mullite is still poorly understood, and there is little data on fiber tows filaments 12 um in diameter with a CtE of 6x 10 bAnd One application for fiber tow coating is ceramic fiber-matrix an elastic modulus of 260 GPa. 20 Overall goals are to(1)coat composites(CMCs). 26, 27 CMCs are flaw-tolerant, if the fiber coating promotes crack deflection and fiber pull out at the fibers to evaluate the monazite interface concept,(2)develop a rotocol to screen precursors for fiber coating and the criteria for evaluating coating quality, and (3)collect data to under- stand continuous tow coating by liquids. This paper is con- cerned with the last two points. The work is part of a general K. T. Faber--contributing editor effort to identify fiber coatings and coating methods for CMcs The methods and apparatus used for coating are described else- where, 122-24 1 including a demonstration with ethanolic pre lo. 190312 Received April 22, 1998; approved January 18, 1999 cursors e build off previous work on precipitation of rials Directorate, Wright monazite powder from aqueous and ethanolic solutions, Base under Contract No general methods for homogenous erican Ceramic Society rare-earth orthophosphate sols 53,55 The relationships between erson Air Force Base. OH. recursor and coating characteristics are reported and discussed 2321

Continuous Coating of Oxide Fiber Tows Using Liquid Precursors: Monazite Coatings on Nextel 720™ Emmanuel Boakye,*,† Randall S. Hay,*,‡ and M. Dennis Petry† UES, Inc., Dayton, Ohio 45432; and Air Force Research Laboratory, Materials Directorate, Wright-Patterson Air Force Base, Ohio 45433 Seven different aqueous or ethanolic precursors were used to continuously coat monazite (LaPO4) on Nextel 720™ fiber tows. Immiscible liquid displacement was used to minimize bridging of coating between filaments. Precursor viscosities, densities, and concentrations were measured, and solids were characterized by DTA/TGA and X-ray. Coatings were cured in-line at 1200°–1400°C and charac￾terized for thickness, microstructure, and composition by optical microscopy, SEM, and TEM. Tensile strengths of the coated fibers varied with the precursor used and were 25% to >50% lower than those of the as-received fiber. The coating stoichiometry and coating thickness of a particular precursor did not correlate with tensile strength. Median coating thicknesses were typically ∼50–100 nm for precur￾sors with 40–80 g/L monazite, much larger than thick￾nesses predicted by theory for 12 mm diameter monofila￾ments. There were significant thickness variations from filament to filament, but usually little variation in compo￾sition or microstructure. Amorphous AlPO4 layers formed from phosphate-rich precursors. Factors that could affect coating characteristics and tensile strength reduction were discussed. I. Introduction SMOOTH, uniform ceramic coatings can be made from liquid￾phase precursors by dip1,2 and spin coating3,4 on plates. Dip coating of plates has been extensively investigated and mod￾eled.5–10 Dip coating of fiber monofilaments is also a common process,11–15 and some work has been done on cloths.16,17 Coating fiber tows is also of interest. Most ceramic fiber tows have 200–500 filaments that are 10–15 mm in diameter each.18–20 It is difficult to coat individual filaments uniformly in a fiber tow, because coating cements the filaments to￾gether,21 and a thick crust often forms around the tow perim￾eter.22,23 Displacement of some coating liquid by an immis￾cible liquid reduces, but does not eliminate, cementation and crusting.22–24 Immiscible liquid displacement is a common process in oil recovery, chemical engineering, and hydrol￾ogy.8,25 Simple geometries have been studied, but the process is still poorly understood, and there is little data on fiber tows. One application for fiber tow coating is ceramic fiber–matrix composites (CMCs).26,27 CMCs are flaw-tolerant, if the fiber coating promotes crack deflection and fiber pull out at the fiber–matrix interface.28,29 Ideally, fiber coatings should be smooth, the same thickness, and the correct composition. The coatings also should not degrade filament tensile strength. Un￾fortunately, there is little information about the degree to which these ideal coating qualities must be approached to get accept￾able CMC mechanical performance. Current CMC fiber–matrix interfaces are either carbon30,31 or BN.32,33 However, oxidation is a major limitation to car￾bon34 and BN33,35 interfaces, particularly in the presence of water,35,36 and “pest” oxidation is a severe problem at inter￾mediate temperatures.37,38 These problems motivate research on oxide CMCs with carbon and BN replacements, such as monazite (LaPO4), that are stable with the matrix and fiber at high temperatures. Monazite is stable with many common oxides, and some evidence suggests monazite–alumina interphase boundaries de￾flect cracks.39–41 Similar results were found for YPO4 (Xeno￾time)/yttrium aluminum garnet interphase boundaries.42 Mona￾zite has a coefficient of thermal expansion (CTE) of 9.6 × 10−6 °C−1 , modulus of 130 GPa,40 and density of 5.08 g/cm3 . Hydrated LaPO4 converts to monazite at ∼500°C and is called rhabdophane. Nonstoichiometric monazite precursors can react with the fiber or matrix, and carbothermal reduction of phos￾phate in monazite is also possible.43 Precise control of mona￾zite stoichiometry during fiber coating is, therefore, desirable. The stoichiometry of oxides, such as LaPO4, can be easily controlled with sols8,44 or solutions,4,45 but coatings from such precursors usually crack if they are over ∼0.1–0.4 mm thick.2,46–48 Cracking can be caused by shrinkage from rapid weight loss, capillary forces, and sintering at high heating rates,2,8,48 or by residual stress from thermal expansion mis￾match.49 Continuous fiber coating generally uses high heating rates; therefore, cracking from weight loss and shrinkage is of particular concern. Micrometer-thick coatings require multiple layers and can still crack if thermal expansion mismatch is large. Currently, there is no consensus for optimal coating thickness; it can vary widely from one fiber–coating–matrix system to another.26,50 Fiber coatings from seven different monazite liquid-phase precursors on Nextel 720™ (3M, St. Paul, MN) fibers are described. Tows of Nextel 720 consist of 420 alumina–mullite filaments 12 mm in diameter with a CTE of 6 × 10−6 °C−1 and an elastic modulus of 260 GPa.20 Overall goals are to (1) coat fibers to evaluate the monazite interface concept, (2) develop a protocol to screen precursors for fiber coating and the criteria for evaluating coating quality, and (3) collect data to under￾stand continuous tow coating by liquids. This paper is con￾cerned with the last two points. The work is part of a general effort to identify fiber coatings and coating methods for CMCs. The methods and apparatus used for coating are described else￾where,11,22–24,51 including a demonstration with ethanolic pre￾cursors.52 We build off previous work on precipitation of monazite powder from aqueous and ethanolic solutions,53–58 general methods for homogenous precipitation,45 and work on rare-earth orthophosphate sols.53,55 The relationships between precursor and coating characteristics are reported and discussed K. T. Faber—contributing editor Manuscript No. 190312. Received April 22, 1998; approved January 18, 1999. Supported by Air Force Research Laboratory Materials Directorate, Wright￾Patterson Air Force Base under Contract No. F33615-91-C-5663. *Member, American Ceramic Society. † UES, Inc., Dayton, OH. ‡ Wright-Patterson Air Force Base, OH. J. Am. Ceram. Soc., 82 [9] 2321–31 (1999) Journal 2321

urnal of the American Ceramic Sociery-Boakye et al. Vol. 82. No 9 with use of existing coating models for flat plates and mono- Take Up Spool filaments. The strength and Weibull moduli of coated ceramie fibers are also very important for CMCs; 9,60 therefore, tensile trengths of coated filaments are also measured and discussed More detailed work on tensile strength appears in a subsequent IL. Experimental Procedures () Materials 3 The following chemicals were used: trimethyl phosphate, drated lanthanum nitrate, and diammonium hydrogen phos- Furnace hate(Aldrich Chemical Co., Milwaukee, WI), nitric acid ( Mallinckrodt Co., Phillipsburg, NJ); 1-octanol and hexadec- ane(Matheson, Coleman, and Bell, Houston, TX); phosphoric acid(Fisher Scientific Co., Pittsburgh, PA); Darvan 821-A (R. T. Vanderbilt Co., Norwalk, CT); and Duramax B-1043 (Rohm and Haas Co., Montgomeryville, PA). Water was pu- rified by deionization of distilled water with nanopure ultra- pure system(Model D4744, Barnstead/Thermolyne Corp Surge Tank Dubuque, IA)for all experiments 过 Gas iniet (2) Precursor Characterization Viscosities were measured in a programmable rheomete nmiscihl Coater (Model DV-Ill, Brookfield Engineering Labs, Stoughton, MA) at a shear rate of 1/300(unless noted otherwise ) Densities were measured with a 25 mL bottle. Differential thermal analy- sis(DTA)and thermogravimetric analysis(TGa)were done (Model STA-409, Netzsch, Exton, PA)at 10@C/min Powder samples for DTA and TGA analysis were formed by evapora- Fiber tow diffractometer(Mode/ rory diffraction(XRD)was done in a tion at 140C for 18h. x-r Unless stated otherwise, powder for XRD was heat-treated at 1200C for I h. Monazite(LaPO4): La3 PO, and monazite ( LaPO4): LaPs Oo ratios were estimated from the 20= 28.6 diffraction of LapO4, the 20= 29.2 diffraction of La PO, Recireulation and the 20 23.9 diffraction of LaP3 Oo. 6 Qualitative esti mates of monazite crystallite size were calculated using the Scherer formula. 62 The effect of temperature on crystallinity Fig. 1. Schematic diagram of fiber coater. was studied for precursors No. I and No. 2 after heat treatments at La: PO, ratios from 1: 1 to 1: 1. 8 were measured for precursor spooled; therefore, temperatures s1200.C were used for these precursors. Multiple coatings were applied with precursor No Precursor concentrations referred to monazite yield per unit 2. The different coating liquids and coating conditions are volume liquid and were determined by weighing the residue listed in Table I after evaporation of a fixed precursor volume in a tared alu- (4) Coating Characterization mina crucible followed by heat treatment at 1400oC for I h in Fiber coatings were characterized for uniformity, thickness, treatment at 1400 C was omitted to preserve the carbon with- with energy-dispersive spectroscopy(EDS)(Model 360FE, rhabdophane Tokyo, Japan). TEM specimens of coated fiber cross sections ( Fiber Coating vere prepared by a method described elsewhere. b3 Phase pres- A vertical coater, described elsewhere, was used(Fig ence was evaluated from EDS in scanning TEM(STEM)mode 1). 11 22-24, 51 To minimize filament bridging, an immiscible hy (40 nm spot size)and tron diffraction. Typical lly coating drocarbon was floated on the precursor to help displace excess thickness, grain size, and percentage of coverage were mea- precursor 22-24 The hydrocarbon layer was typically 2 cm sured from 20 to 50 filaments. The number of coating bridges thick. I-octanol was used for aqueous precursors, and hexa- per filament and prevalence of crust were qualitatively esti- decane was used for ethanolic precursors. 2 A nonionic surfac mated from at least 100 filaments. Tem was used for thick- tant(Triton X-100, Lab Chem, Inc, Pittsburgh, PA)was added nesses less than-05 um; SEM was used for larger thicknesses to some aqueous precursors to improve filament wetting and Filament tensile strengths were measured with 75 tests using 7)and pure monazite(No. 1-No. 5)coatings were heat-treated average l, auge length. Failure stress was calculated from nt diameter. Details of the method are discussed in-line in argon and air, respectively, at-11000-1400oC at 0.7-1.4 cm/s. The furnace hot zone was -8 cm in length, and total furnace length was 30 cm. The fibers and coatings were, lIL. Results and Discussion therefore, heated very rapidly and held at near-maximum tem perature for 10 s at most. The furnace core was alumina with a ()General platinum rhodium 60: 40 wire wound on the outside. for som TEM and SEM micrographs of coatings from different liquid aqueous precursors(No. 1, No. 5), temperatures >1200C de precursors are shown in Figs. 2-8. Bridging of the coating graded fiber strength so severely that the fibers could not be between filaments was observed for all precursors(Figs. 2-8)

with use of existing coating models for flat plates and mono￾filaments. The strength and Weibull moduli of coated ceramic fibers are also very important for CMCs;59,60 therefore, tensile strengths of coated filaments are also measured and discussed. More detailed work on tensile strength appears in a subsequent paper. II. Experimental Procedures (1) Materials The following chemicals were used: trimethyl phosphate, hydrated lanthanum nitrate, and diammonium hydrogen phos￾phate (Aldrich Chemical Co., Milwaukee, WI); nitric acid (Mallinckrodt Co., Phillipsburg, NJ); 1-octanol and hexadec￾ane (Matheson, Coleman, and Bell, Houston, TX); phosphoric acid (Fisher Scientific Co., Pittsburgh, PA); Darvan 821-A (R. T. Vanderbilt Co., Norwalk, CT); and Duramax B-1043 (Rohm and Haas Co., Montgomeryville, PA). Water was pu￾rified by deionization of distilled water with nanopure ultra￾pure system (Model D4744, Barnstead/Thermolyne Corp., Dubuque, IA) for all experiments. (2) Precursor Characterization Viscosities were measured in a programmable rheometer (Model DV-III, Brookfield Engineering Labs, Stoughton, MA) at a shear rate of 1/300 (unless noted otherwise). Densities were measured with a 25 mL bottle. Differential thermal analy￾sis (DTA) and thermogravimetric analysis (TGA) were done (Model STA-409, Netzsch, Exton, PA) at 10°C/min. Powder samples for DTA and TGA analysis were formed by evapora￾tion at 140°C for 18 h. X-ray diffraction (XRD) was done in a diffractometer (Model Rotaflex, Rigaku Co., Tokyo, Japan). Unless stated otherwise, powder for XRD was heat-treated at 1200°C for 1 h. Monazite (LaPO4):La3PO7 and monazite (LaPO4):LaP3O9 ratios were estimated from the 2u 4 28.6° diffraction of LaPO4, the 2u 4 29.2° diffraction of La3PO7, and the 2u 4 23.9° diffraction of LaP3O9. 61 Qualitative esti￾mates of monazite crystallite size were calculated using the Scherer formula.62 The effect of temperature on crystallinity was studied for precursors No. 1 and No. 2 after heat treatments of 140°, 800°, 900°, 1000°, and 1200°C for 1 h. Phase presence at La:PO4 ratios from 1:1 to 1:1.8 were measured for precursor No. 1. Precursor concentrations referred to monazite yield per unit volume liquid and were determined by weighing the residue after evaporation of a fixed precursor volume in a tared alu￾mina crucible followed by heat treatment at 1400°C for 1 h in air. For precursors with organic binders (No. 6 and No. 7), heat treatment at 1400°C was omitted to preserve the carbon with￾out reducing the phosphate; therefore, LaPO4 was present as rhabdophane. (3) Fiber Coating A vertical coater, described elsewhere, was used (Fig. 1).11,22–24,51 To minimize filament bridging, an immiscible hy￾drocarbon was floated on the precursor to help displace excess precursor.22–24 The hydrocarbon layer was typically 2 cm thick. 1-octanol was used for aqueous precursors, and hexa￾decane was used for ethanolic precursors.52 A nonionic surfac￾tant (Triton X-100, Lab Chem, Inc., Pittsburgh, PA) was added to some aqueous precursors to improve filament wetting and enhance thin-film formation. The monazite–carbon (No. 6, No. 7) and pure monazite (No. 1–No. 5) coatings were heat-treated in-line in argon and air, respectively, at ∼1100°–1400°C at 0.7–1.4 cm/s. The furnace hot zone was ∼8 cm in length, and total furnace length was 30 cm. The fibers and coatings were, therefore, heated very rapidly and held at near-maximum tem￾perature for 10 s at most. The furnace core was alumina with a platinum:rhodium 60:40 wire wound on the outside. For some aqueous precursors (No. 1, No. 5), temperatures >1200°C de￾graded fiber strength so severely that the fibers could not be spooled; therefore, temperatures #1200°C were used for these precursors. Multiple coatings were applied with precursor No. 2. The different coating liquids and coating conditions are listed in Table I. (4) Coating Characterization Fiber coatings were characterized for uniformity, thickness, microstructure, and composition by optical microscopy, SEM with energy-dispersive spectroscopy (EDS) (Model 360FE, Leica, Inc., Thorwood, NY) and TEM (Model 2000 FX, JEOL, Tokyo, Japan). TEM specimens of coated fiber cross sections were prepared by a method described elsewhere.63 Phase pres￾ence was evaluated from EDS in scanning TEM (STEM) mode (40 nm spot size) and electron diffraction. Typically coating thickness, grain size, and percentage of coverage were mea￾sured from 20 to 50 filaments. The number of coating bridges per filament and prevalence of crust were qualitatively esti￾mated from at least 100 filaments. TEM was used for thick￾nesses less than ∼0.5 mm; SEM was used for larger thicknesses. Filament tensile strengths were measured with 75 tests using a 2.54 cm gauge length. Failure stress was calculated from average filament diameter. Details of the method are discussed elsewhere.64 III. Results and Discussion (1) General TEM and SEM micrographs of coatings from different liquid precursors are shown in Figs. 2–8. Bridging of the coating between filaments was observed for all precursors (Figs. 2–8). Fig. 1. Schematic diagram of fiber coater. 2322 Journal of the American Ceramic Society—Boakye et al. Vol. 82, No. 9

September 1999 Continuous Coating of Oxide Fiber Tows Using Liquid Precursors 2323 Table I. Coating Liquids and Conditions Precursor tmosphere coatings ILa-IP 1.020.011286 ILa -IP Air 0.0303.81 ILa-l 1.060.0613.91 1.060.0644.100.4428 1300 0.075 50 147 404.971.040.11414400.6 ILa-IP 404971.04 ILa -IP 4.9 0.84 8338911m12 La-rich 0.0875.500 ILa -IP 8044.101.0 1300 P-rich 3.1 0.147 P-rich 1.070.378 Crust and normal coating not distinguishable. fLow tensile strength-fiber could not be spooled During fiber spooling and handling, the bridges often broke ortional to the solution concentration(C) for C between 20 halfway between the filaments and became axial fins. Coating and 80 g/L by a constant(K)of -0.83 x 10-6(m/kg).As thickness tapered off away from fins. Attempts to quantify the discussed later, this is the expected result if precursor viscosity number of bridges per filament were unsuccessful, because m) does not change significantly with C. There was some compaction of filaments for SEM and TEM observation often bridging of coating between filaments(Fig. 2(b). Some coat crushed fins. However, some qualitative observations of bridge gs had large bubbles, possibly from gas evolved during dry bundance were made for different precursors. Despite the high ng(Figs. 2(d), (e), and(g). Relatively little coating crust was Coating crust was often present around the tow perimeter 40 nm (Table I). Monazite grain size averaged -25 nm for Crust was distinguishable from normal coating by a thickness coatings done at 1200@C Grain size was slightly smaller(17 10 times higher. Thickness histograms tended to best fit a nm)in thinner coatings and slightly larger(33 nm)for coating nal distribution(Fig 9)and, with crusting, tended to be made at 1300 C. Lower concentration solutions made bimodal. Crusts were broken during sample preparation but smoother coatin were still visible as thick areas. Thickness and abundance of In coatings deliberately made phosphorus-rich, there were crust could be estimated from the histograms, if the larger amorphous AlPOA layers of variable thickness between the thickness distribution was assigned to crust and the smaller to monazite and the fiber(Fig. 2(f)). Othe rwIse monazite was normal coating. The bimodal distributions were separated into only phase observed. Occasionally ly a thin AlPOA layer was best fit sum of two unimodal distributions and median values found in coatings made from 1: 1 solutions. Lanthanum-rich of the separate distributions were used for thicknesses of hases were not observed by tEM crusted and normal coating Coated fiber tensile strength (o)was comparatively low (2) Precursor No. I (Table I). Fibers coated at 1300 C could not be spooled when stoichiometric solutions were used. However, phosphate-rich (A) Sol/Solution Characterization: Lanthanum nitrate coatings could be spooled; tensile strength was 0.99 GPa for a and trimethyl phosphate were dissolved in 20% nitric acid 4s Solutions with La: pO ratios of 1: 1. 1: 1.12. 1: 1.20.1- 1.60 La: PO4 =1: 1.59 solution and 1. 12 GPa for a La PO4 =1: 1.82 solution. At 1200oC, all coatings could be spooled, and tensile 1:1.80,and 1:3 were made. The viscosity and density of the l: I strengths ranged from 0.94 for a 20 g/L solution to 0.97 GPa g/cmy, respectively, as concentration changed from 20 to 80 there was no dependence of tensile strength on 8 from 1. 1 to 1.22 mPas and 1.06 to 1.07 g/cm, respectively, (3) Precursor No. 2 the La: POa ratio changed from 1: 1 to 1: 3 (A) Sol/Solution Characterization: Colloidal rhabdo A 5%TGA weight loss between 100 to 180C was consis- phane(LaPOaxH2O) particles were formed in water from lan- tent with loss of -I mol of water/ (mol of monazite)(Fig. 10). thanum nitrate and diammonium hydrogen phosphate If this behavior was similar to that for other mixed nitrate phosphates, the product could have been lanthanum methyl La(NO3)3+(NH,)2HPO4 - hosphate, (Lao(H(CH3)( PO4))lH20). 45,65 A DTA exotherm LaPO4 xH,O+ 2NH NO3+ HNO (1) gested that this intermediate converted to monazite above The sols were peptized with 70% nitric acid to OM-0.48M. Sol 540°C(Fg.10) m had a strong dependence on acid concentration, with a maxi- After heat treatment at 140C, the powder was X-ray amor- mum of 6. 74 mPas at 0. 14/, 5.00 mPas at 0.07M, 3.31 mPas phous. After heat treatment at 800C, XRD was consistent at 0.0M, and 1.21 mPas at 0. 48M for 40 g/L sol concentrations ith pure monazite for the 1: I and 1: 1. 12 solutions. Mixtures Sol density(p)was 1.04 g/cm of monazite and lap,Oo formed from the 1: 1. 2 to 1- 1. 8 solu- A 3% TGA weight loss between 140%.C corresponded tions(Fig. 11). Crystallite size for the 1: I solutions was -32 to a loss of -0.55 mol of water/(mol of monazite )and was nm after 800%C heat treatment and -38 nm after 1200.C heat consistent with dehydration of rhabdophane(ApOA O), treatment where x=0.5(Fig. 10).54 58 A further 25% TGA weight loss (B) Fiber Coatings: Nearly f the filament sur- and sharp endotherm between -2600-380oC was attributed to faces were coated when 40 and L concentrations were decomposition of ammonium nitrate and nitric acid sed (Fig. 2). Coatings done g/L solution covered The powder was a mixture of monazite and nitrammite 95% of the surface. Med thickness(8)was pro- after heat treatment at 140.C and pure monazite after

During fiber spooling and handling, the bridges often broke halfway between the filaments and became axial fins. Coating thickness tapered off away from fins. Attempts to quantify the number of bridges per filament were unsuccessful, because compaction of filaments for SEM and TEM observation often crushed fins. However, some qualitative observations of bridge abundance were made for different precursors. Despite the high heating rates, thin coatings were not cracked. However, when the coating was crusted and >1 mm thick, it often cracked off. Coating crust was often present around the tow perimeter. Crust was distinguishable from normal coating by a thickness ∼10 times higher. Thickness histograms tended to best fit a lognormal distribution (Fig. 9) and, with crusting, tended to be bimodal. Crusts were broken during sample preparation but were still visible as thick areas. Thickness and abundance of crust could be estimated from the histograms, if the larger thickness distribution was assigned to crust and the smaller to normal coating. The bimodal distributions were separated into a best fit sum of two unimodal distributions, and median values of the separate distributions were used for thicknesses of crusted and normal coating. (2) Precursor No. 1 (A) Sol/Solution Characterization: Lanthanum nitrate and trimethyl phosphate were dissolved in 20% nitric acid.45,65 Solutions with La:PO4 ratios of 1:1, 1:1.12, 1:1.20, 1:1.60, 1:1.80, and 1:3 were made. The viscosity and density of the 1:1 mixture changed from 0.90 to 1.07 mPazs and 1.02 to 1.06 g/cm3 , respectively, as concentration changed from 20 to 80 g/L. The viscosity and density of an 80 g/L solution changed from 1.1 to 1.22 mPazs and 1.06 to 1.07 g/cm3 , respectively, as the La:PO4 ratio changed from 1:1 to 1:3. A 5% TGA weight loss between 100° to 180°C was consis￾tent with loss of ∼1 mol of water/(mol of monazite) (Fig. 10). If this behavior was similar to that for other mixed nitrate phosphates, the product could have been lanthanum methyl phosphate, (LaO(H(CH3)(PO4))z1H2O).45,65 A DTA exotherm suggested that this intermediate converted to monazite above 540°C (Fig. 10).55–57 After heat treatment at 140°C, the powder was X-ray amor￾phous. After heat treatment at 800°C, XRD was consistent with pure monazite for the 1:1 and 1:1.12 solutions. Mixtures of monazite and LaP3O9 formed from the 1:1.2 to 1:1.8 solu￾tions (Fig. 11).61 Crystallite size for the 1:1 solutions was ∼32 nm after 800°C heat treatment and ∼38 nm after 1200°C heat treatment. (B) Fiber Coatings: Nearly 100% of the filament sur￾faces were coated when 40 and 80 g/L concentrations were used (Fig. 2). Coatings done with 20 g/L solution covered ∼95% of the surface. Median coating thickness (d) was pro￾portional to the solution concentration (C) for C between 20 and 80 g/L by a constant (K) of ∼0.83 × 10−6 (m4 /kg). As discussed later, this is the expected result if precursor viscosity (h) does not change significantly with C. There was some bridging of coating between filaments (Fig. 2(b)). Some coat￾ings had large bubbles, possibly from gas evolved during dry￾ing (Figs. 2(d), (e), and (g)). Relatively little coating crust was present around the tow perimeter, and thickness uniformity was better than average (Fig. 9). Median crust thickness (dcrust) was 440 nm (Table I). Monazite grain size averaged ∼25 nm for coatings done at 1200°C. Grain size was slightly smaller (17 nm) in thinner coatings and slightly larger (33 nm) for coatings made at 1300°C. Lower concentration solutions made smoother coatings. In coatings deliberately made phosphorus-rich, there were amorphous AlPO4 layers of variable thickness between the monazite and the fiber (Fig. 2(f)). Otherwise monazite was the only phase observed. Occasionally a thin AlPO4 layer was found in coatings made from 1:1 solutions. Lanthanum-rich phases were not observed by TEM. Coated fiber tensile strength (s) was comparatively low (Table I). Fibers coated at 1300°C could not be spooled when stoichiometric solutions were used. However, phosphate-rich coatings could be spooled; tensile strength was 0.99 GPa for a La:PO4 4 1:1.59 solution and 1.12 GPa for a La:PO4 4 1:1.82 solution. At 1200°C, all coatings could be spooled, and tensile strengths ranged from 0.94 for a 20 g/L solution to 0.97 GPa for an 80 g/L solution. The proportionality of C with d implied there was no dependence of tensile strength on d. (3) Precursor No. 2 (A) Sol/Solution Characterization: Colloidal rhabdo￾phane (LaPO4zxH2O) particles were formed in water from lan￾thanum nitrate and diammonium hydrogen phosphate: La(NO3)3 + (NH4)2HPO4 → LaPO4zxH2O + 2NH4NO3 + HNO3 (1) The sols were peptized with 70% nitric acid to 0M–0.48M. Sol h had a strong dependence on acid concentration, with a maxi￾mum of 6.74 mPazs at 0.14M, 5.00 mPazs at 0.07M, 3.31 mPazs at 0.0M, and 1.21 mPazs at 0.48M for 40 g/L sol concentrations. Sol density (r) was 1.04 g/cm3 . A 3% TGA weight loss between 140°–200°C corresponded to a loss of ∼0.55 mol of water/(mol of monazite) and was consistent with dehydration of rhabdophane (LaPO4zxH2O), where x ≈ 0.5 (Fig. 10).54,58 A further 25% TGA weight loss and sharp endotherm between ∼260°–380°C was attributed to decomposition of ammonium nitrate and nitric acid. The powder was a mixture of monazite and nitrammite53,61 after heat treatment at 140°C and pure monazite after heat Table I. Coating Liquids and Conditions Precursor Composition Atmosphere Number of coatings C (g/L) h (mPa?s) r (g/cm3 ) d (mm) h (mm) dcrust (mm) hcrust (mm) Temperature (°C) s (GPa) 1 1La − 1P Air 1 20 0.90 1.02 0.011 2.86 1200 0.94 1 1La − 1P Air 1 40 0.030 3.81 1200 0.96 1 1La − 1P Air 1 80 1.07 1.06 0.061 3.91 1200 0.97 1 1La − 1P Air 1 80 1.07 1.06 0.064 4.10 0.44 28 1300 ‡ 1 1La − 1.59P Air 1 80 1300 0.99 1 1La − 1.82P Air 1 80 1300 1.12 1 1La − 3P Air 1 80 1.22 1.07 1300 2 1La − 1P Air 1 40 4.97 1.04 0.075 9.50 0.77 98 1300 1.47 2 1La − 1P Air 2 40 4.97 1.04 0.114 14.40 0.62 79 1300 2 1La − 1P Air 5 40 4.97 1.04 0.170 21.50 0.81 102 1300 2 1La − 1P Air 8 40 4.97 1.04 1300 1.04 2 1La − 1P Air 10 40 4.97 1.04 0.280 35.50 0.80 101 1300 3 La-rich Air 1 50 1.39 0.84 0.055 5.60 0.39 40 1300 1.21 3 La-rich Argon 1 50 1.39 0.84 1300 1.24 4 1La − 1P Air 1 80 2.84 1.27 0.087 5.50 0.68 43 1300 1.07 5 1La − 1P Air 1 80 44.10 1.07 1300 ‡ 6 P-rich Argon 1 60 3.12 1.05 0.147 † † 1300 7 P-rich Argon 1 108 2.24 1.07 0.378 † † 1300 1.56 † Crust and normal coating not distinguishable. ‡Low tensile strength—fiber could not be spooled. September 1999 Continuous Coating of Oxide Fiber Tows Using Liquid Precursors 2323

2324 Journal of the American Ceramic Sociery-Boakye et al. Vol. 82. No 9 treatment at 600C(Fig. 11). Monazite crystallite size in- creased from -23 to -37 nm between 600-1000%C. but there was little change between I000°1200°C (B) Fiber Coatings. Coating coverage was nearly con- tinuous for 40-80 g/L sols with 0M-0 14M nitric acid, but sols ith 0.4M and 1.9M nitric acid had only 40%-50% coverage Addition of TX-100 surfactant did not improve coverage Monazite was the only crystalline phase in the coatings Granu- lar monazite formed at -1200oC. while vermicular monazite Sum formed at -1300C (Fig. 3). When coatings were extremel thin(<20 nm), they were sometimes amorphous, but lanthanum and phosphorus peaks with a 1: I height ratio suggestive of monazite were still observed by EDS. Irregularly shaped ag- glomerates were sometimes seen(Fig 3(b). The average grain size of the single-and double-pass coatings were 28 and 45 nm, respectively Coating morphology changed with nitric acid concentration. Coatings from sols with no nitric acid were granular, while sols with 0.07M and 0. 14M nitric acid made mixed vermicular and granular coatings(Fig. 3(f). Nitric acid reduced the particle size by dissolution of colloidal rhabdophane(Eq.(1)and consequently, could reduce granularity. Coatings with poor coverage tended to be clumped platelets or elongated particles This could be related to nucleation of dissolved LaPO a as rhab- dophane from colloidal particles that spotted filaments rather than continuously covered them, forming clumped platelets nstead of a film There were large variations in 8(Figs. 3, 9). For a 40 g/L sol with 0.07M nitric acid, single-pass coatings had a bimodal 8 distribution with medians at 75 and 770 nm for normal coating and crust, respectively (Table I). The 8 of normal coating in- creased with the number of coats(N)(Fig 12) (2) An explanation for the 0.55 power law(instead of linear)de- pendence was not obvious. It was possible that coating fins and 100 nm filament compaction after coating allowed less precursor to surround filaments Crust thickness was independent of N(Fig 12). This was attributed to debonding once a critical thickness of-I um was reached, Debonding of thick coatings was par- ticularly prevalent in TEM sections(Fig. 3(e)) Tensile strengths (o) were comparatively high(Table I) Fibers coated at 1300C with a 40 g/L sol had 1. 47 GPa aver- age o. Those coated eight times and those coated at 1400oC had o of 1.04 and 1.06 GPa, respectively(Table D) (4 Pecl 500nm (A Sol/Solution Characterisation: This precursor was supplied by Sankar Sambasivan of Northwestern University ( Evanston, IL). A 1: 0.5 molar mixture of lanthanum nitrate and ()100mm phosphorus pentoxide was dissolved in dry ethanol. The m and p were 1.39 mPas and 0.84 g/cm, respectively, for a 50 g/ solution As expected for an anhydrous system, no weight loss attrib- utable to rhabdophane dehydration was found. 3 A DTA en dotherm at 900C corresponded to a slight increase in weight loss. A DTA exotherm at 375C corresponded to monazite crystallization(Fig. 10) XRD after 1200%C heat treatment was consistent with mona zite with slight excess La, PO,(Fig. 11). Unmeasured adsorp- tion of water by P2Os during sol preparation might have caused the phosphorus deficiency. Crystallite size was 30 nm e.(B) Fiber Coatings. These coatings had continuous cov- rage and were relatively smooth and uniform in thickness (Fig. 4). The thickness histogram was relatively narrow, with less bimodality from crust(Fig. 9). Bridging(Fig. 4(c))was relatively less common. Median 8 of normal coating was 55 for a solution with 50 g/L concentration (Table 1). Median 8 Fig. 2. TEM graphs of coatings from precursor No. 1: (aH(d) was 390 nm, The coating had intragranular pores 5-20 nm in 200°C,80g/L 00°C,80gL, honeycombs;(f)1300°C,80gL diameter that varied sporadically in abundance from <5 to >30 La:P=1: 1.8.(g)SEM micrograph of coated fiber surface, 1200%C, 80 vol%. Average grain size was -200 nm, much larger than that gl of other coatings. Although X-ray analysis of the coating liquid

treatment at 600°C (Fig. 11). Monazite crystallite size in￾creased from ∼23 to ∼37 nm between 600°–1000°C, but there was little change between 1000°–1200°C. (B) Fiber Coatings: Coating coverage was nearly con￾tinuous for 40–80 g/L sols with 0M–0.14M nitric acid, but sols with 0.4M and 1.9M nitric acid had only 40%–50% coverage. Addition of TX-100 surfactant did not improve coverage. Monazite was the only crystalline phase in the coatings. Granu￾lar monazite formed at ∼1200°C, while vermicular monazite formed at ∼1300°C (Fig. 3). When coatings were extremely thin (<20 nm), they were sometimes amorphous, but lanthanum and phosphorus peaks with a 1:1 height ratio suggestive of monazite were still observed by EDS. Irregularly shaped ag￾glomerates were sometimes seen (Fig. 3(b)). The average grain size of the single- and double-pass coatings were 28 and 45 nm, respectively. Coating morphology changed with nitric acid concentration. Coatings from sols with no nitric acid were granular, while sols with 0.07M and 0.14M nitric acid made mixed vermicular and granular coatings (Fig. 3(f)). Nitric acid reduced the particle size by dissolution of colloidal rhabdophane (Eq. (1)) and, consequently, could reduce granularity. Coatings with poor coverage tended to be clumped platelets or elongated particles. This could be related to nucleation of dissolved LaPO4 as rhab￾dophane from colloidal particles that spotted filaments rather than continuously covered them, forming clumped platelets instead of a film. There were large variations in d (Figs. 3, 9). For a 40 g/L sol with 0.07M nitric acid, single-pass coatings had a bimodal d distribution with medians at 75 and 770 nm for normal coating and crust, respectively (Table I). The d of normal coating in￾creased with the number of coats (N) (Fig. 12): d(nm) 4 80N0.55 (2) An explanation for the 0.55 power law (instead of linear) de￾pendence was not obvious. It was possible that coating fins and filament compaction after coating allowed less precursor to surround filaments. Crust thickness was independent of N (Fig. 12). This was attributed to debonding once a critical thickness of ∼1 mm was reached. Debonding of thick coatings was par￾ticularly prevalent in TEM sections (Fig. 3(e)). Tensile strengths (s) were comparatively high (Table I). Fibers coated at 1300°C with a 40 g/L sol had 1.47 GPa aver￾age s. Those coated eight times and those coated at 1400°C had s of 1.04 and 1.06 GPa, respectively (Table I). (4) Precursor No. 3 (A) Sol/Solution Characterization: This precursor was supplied by Sankar Sambasivan of Northwestern University (Evanston, IL). A 1:0.5 molar mixture of lanthanum nitrate and phosphorus pentoxide was dissolved in dry ethanol. The h and r were 1.39 mPazs and 0.84 g/cm3 , respectively, for a 50 g/L solution. As expected for an anhydrous system, no weight loss attrib￾utable to rhabdophane dehydration was found.53 A DTA en￾dotherm at 900°C corresponded to a slight increase in weight loss. A DTA exotherm at 375°C corresponded to monazite crystallization (Fig. 10). XRD after 1200°C heat treatment was consistent with mona￾zite with slight excess La3PO7 (Fig. 11). Unmeasured adsorp￾tion of water by P2O5 during sol preparation might have caused the phosphorus deficiency. Crystallite size was 30 nm. (B) Fiber Coatings: These coatings had continuous cov￾erage and were relatively smooth and uniform in thickness (Fig. 4). The thickness histogram was relatively narrow, with less bimodality from crust (Fig. 9). Bridging (Fig. 4(c)) was relatively less common. Median d of normal coating was 55 nm for a solution with 50 g/L concentration (Table I). Median dcrust was 390 nm. The coating had intragranular pores 5–20 nm in diameter that varied sporadically in abundance from <5 to >30 vol%. Average grain size was ∼200 nm, much larger than that of other coatings. Although X-ray analysis of the coating liquid Fig. 2. TEM micrographs of coatings from precursor No. 1: (a)–(d) 1200°C, 80 g/L; (e) 1300°C, 80 g/L, honeycombs; (f) 1300°C, 80 g/L, La:P 4 1:1.8. (g) SEM micrograph of coated fiber surface, 1200°C, 80 g/L. 2324 Journal of the American Ceramic Society—Boakye et al. Vol. 82, No. 9

200 100 monazite fiber 100nm 100m 400mm (d) 100 nm 300nm uIn (d) 100 (f) 500nm y (g) 10m 围己c则cmmp(2ma4(0(0) TEM micrographs of coatings from precursor No3 (c)I pass, (d) morphology, and cracking of coating), (f)SEM micrograph of coated coatings were done at 1300C with a 50 g/L solution fiber surface( Note vermi microstructure)

Fig. 3. TEM micrographs of coatings from precursor No. 2 made at 1300°C with a 40 g/L sol at 1.4 cm/s: (a) 1 pass, (b) 1 pass, agglom￾erate, (c) 1 pass, (d) 2 passes, (e) 10 passes (Note fins, irregular morphology, and cracking of coating), (f) SEM micrograph of coated fiber surface (Note vermicular microstructure). Fig. 4. (a)–(f) TEM micrographs of coatings from precursor No. 3 (Note bridges in (c)). (g) SEM micrograph of coated fiber surface. All coatings were done at 1300°C with a 50 g/L solution