Part A: applied scienc and manufacturing ELSEVIER Composites: Part A 30(1999)483-488 Ceramic composites for thermal protection systems J B. Davis, D B. Marshall, K.S. Oka",R. M. Housley, P.E. D Morgan Abstract d coating systems based on monazite, a weak interphase for oxide composites, are being in as a means to increase the service temperatures of thermal protection blankets for re-entry space craft. Preliminary evaluations, including chemical compatibility tensile strengths of coated, heat-treated fibers and fabrics, and durability in a modulated wind tunnel facility have been conducted. c 1999 Elsevier Science Ltd. All rights reserved Keywords: Thermal protection systems; Coatings; Oxide composites 1. Introduction alumino-silicate fiber (Nextel 440), while the coating (matrix) is a tw Thermal protection blankets, consisting of refractory and Al2O,. Recent studies have shown that LaPO4 forms fiber batting sandwiched between two sheets of woven cera- weak interfaces in oxide composites and is compatible at mic fabric, are of interest as a lower cost alternative to rigid high temperatures(> 1400C)with several oxides that have tiles for protection of re-entry vehicles. These blankets potential as high temperature reinforcements(e.g. Al2O3, require a coating on the outer woven sheet that infiltrates ZrO2, mullite and YAG)[2-4] and stiffens the fabric to provide an aerodynamic surface The coating must act as a high temperature starch, without causing embrittlement of the fabric. Since the coated fabric 2 Experimental procedure layer is essentially a thin ceramic matrix composite(the infiltrated coating being the matrix) the requirements for 2/Coating development blanket durability are the same as those for damage toler ance in structural CMCs a weak bond is needed between the To achieve optimum properties, LaPOa-based coatings matrix and the fibers to prevent embrittlement must be formed with a 1: I ratio of La: P. The presence of Blankets consisting of silica fiber fabrics and insula excess La or P causes detrimental reactions. In this study, are used to protect the upper surface of the Space Shuttle the monazite was formed from an aqueous precursor solu- Orbiter. These blankets which are coated with a silica-based tion of La ions and P-containing complexes. The precursor coating commonly referred to as C-9 are rated for multiple was used both to coat fabrics directly and to produce mona use at service temperatures to 650C [1]. At higher temp tures, the silica-based coatings become strongly bonded to coatings. The La P ratio in the precursor was adjusted using the fibers, embrittling the outer fabric and limiting their an iterative method. A sample of the precursor was used to useful life. Development of more refractory blanket materi form a powder by calcining at low temperatures in a closed als and compatible coatings, with temperature capabilities crucible; the powder was mixed with small sapphire crystals in the range 1000-1100C. would allow use of blankets on and cold pressed into a pellet; the pellet was packed in additional surfaces of the Orbiter, as well as on new spa additional loose monazite buffer powder in a closed crucible vehicles and fired to 1200-1400C, the fired pellet was then broken This paper presents a preliminary assessment of a new apart and the sapphire crystals which were exposed on the fabric/coating system suitable for use in this temperature fracture surface were examined by scanning electron micro- range. The fabric consists of a commercially available scopy and energy-dispersive X-ray spectroscopy to identify any reaction products. When the solutions were rich in La either LaAlO3 or LaAl1O1s phases formed on the sapphire orresponding author depending on the firing temperature: when the solutions (/99/.see front matter e 1999 Elsevier Science Ltd. All rights reserved 59-835X(98)00138-9
Ceramic composites for thermal protection systems J.B. Davisa , D.B. Marshalla , K.S. Okab,*, R.M. Housley, P.E.D. Morgan a Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360, USA b The Boeing Company, Boeing Defense and Space Group, 12214 Lakewood Boulevard, Downey, CA 90242, USA Abstract Advanced coating systems based on monazite, a weak interphase for oxide composites, are being investigated as a means to increase the service temperatures of thermal protection blankets for re-entry space craft. Preliminary evaluations, including chemical compatibility, tensile strengths of coated, heat-treated fibers and fabrics, and durability in a modulated wind tunnel facility have been conducted. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Thermal protection systems; Coatings; Oxide composites 1. Introduction Thermal protection blankets, consisting of refractory fiber batting sandwiched between two sheets of woven ceramic fabric, are of interest as a lower cost alternative to rigid tiles for protection of re-entry vehicles. These blankets require a coating on the outer woven sheet that infiltrates and stiffens the fabric to provide an aerodynamic surface. The coating must act as a high temperature starch, without causing embrittlement of the fabric. Since the coated fabric layer is essentially a thin ceramic matrix composite (the infiltrated coating being the matrix) the requirements for blanket durability are the same as those for damage tolerance in structural CMCs: a weak bond is needed between the matrix and the fibers to prevent embrittlement. Blankets consisting of silica fiber fabrics and insulation are used to protect the upper surface of the Space Shuttle Orbiter. These blankets which are coated with a silica-based coating commonly referred to as C-9 are rated for multiple use at service temperatures to 6508C [1]. At higher temperatures, the silica-based coatings become strongly bonded to the fibers, embrittling the outer fabric and limiting their useful life. Development of more refractory blanket materials and compatible coatings, with temperature capabilities in the range 1000–11008C, would allow use of blankets on additional surfaces of the Orbiter, as well as on new space vehicles. This paper presents a preliminary assessment of a new fabric/coating system suitable for use in this temperature range. The fabric consists of a commercially available alumino-silicate fiber (Nextel 440), while the coating (matrix) is a two-phase mixture of LaPO4 (La-monazite) and Al2O3. Recent studies have shown that LaPO4 forms weak interfaces in oxide composites and is compatible at high temperatures ( . 14008C) with several oxides that have potential as high temperature reinforcements (e.g. Al2O3, ZrO2, mullite and YAG) [2–4]. 2. Experimental procedures 2.1. Coating development To achieve optimum properties, LaPO4-based coatings must be formed with a 1:1 ratio of La:P. The presence of excess La or P causes detrimental reactions. In this study, the monazite was formed from an aqueous precursor solution of La ions and P-containing complexes. The precursor was used both to coat fabrics directly and to produce monazite powders which were dispersed in water to form slurry coatings. The La:P ratio in the precursor was adjusted using an iterative method. A sample of the precursor was used to form a powder by calcining at low temperatures in a closed crucible; the powder was mixed with small sapphire crystals and cold pressed into a pellet; the pellet was packed in additional loose monazite buffer powder in a closed crucible and fired to 1200–14008C; the fired pellet was then broken apart and the sapphire crystals which were exposed on the fracture surface were examined by scanning electron microscopy and energy-dispersive X-ray spectroscopy to identify any reaction products. When the solutions were rich in La, either LaAlO3 or LaAl11O18 phases formed on the sapphire, depending on the firing temperature: when the solutions Composites: Part A 30 (1999) 483–488 1359-835X/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S1359-835X(98)00138-9 * Corresponding author
al/Composites: Part A 30(1999)483-48 were rich in P, AlPO4 was observed. The original precursor composition was then adjusted based on these observations and the process was repeated until no reaction products could be detected on the sapphire in the fired pellet. The coatings developed in this study were optimized specifically for Nextel 440 fibers(3M Corporation, St hemical composition of 70% Al2O3, 28% SiO2 and 2% B2O3 and are weavable. a dipping procedure was used to sauce coatin ngs from low viscosity compositions including the neat solution precursor, and powder-containing slurries with low solids loadings (20 vol% solids)and coatings applied to complex lated wind tunnel experiments geometries such as quilted blankets were brush coated,or painted, onto the fabric surface. The powders used were tension with a 2.54 cm gauge length using monazite, produced by spray drying the stoichiometric solu- grips tion precursors(2-5 m granual size), and high purity AlO3 (average particle size.2 m from Sumitomo Chemical monazite solution or in water and the pH of the composl- coated blankets re and modulated wind tunnel testing of Company, New York, NY). These were dispersed in the 2.3. Thermal ex tions was adjusted by adding NH4OH. The chemical compatibility of the Nextel 440 fibers and the coatings The thermal and acoustic loads experienced by thermal was evaluated using X-ray difiraction and energy-dispersive protection systems during atmosphere re-entry are severe X-ray spectroscopy techniques after heat treating the coated To simulate such conditions, small blanket test specimens fiber tows and fabrics at 1 100 or 1200.C for I h. Finally, (16X 16 cm) were fabricated and subjected to radiant heat- lished cross-sections of coated fabrics were examined by ing and wind tunnel experiments. These blankets consisted scanning electron microscopy to assess the uniformity of the of Nextel 440 face-sheets and quartz fabric backing. The coatings and to determine whether the coatings infiltrated two fabric layers were quilted together with Nextel 440 both the inter- and intra-fiber tow spaces effectively sewing thread through approximately 2 cm of fibrous insu- Heat treatments of coated blankets were performed at low 2. 2. Tensile testing of coated fiber tow and fabric pressure(<I torr)in a facility equipped with quartz lamps Temperatures were monitored at the exposed fabric face and Tensile testing of coated, heat-treated fiber tows and at the backside of the blanket using calibrated thermocou- fabrics was used to evaluate the effects of various coatings ples. The maximum face-sheet exposure temperature of on the retained strengths, as well as to allow direct observa- either 1100 or 1200C was typically reached within 1 tion of the fracture behavior. Test specimens consisted of 5 min and was maintained for 30 min, after which the spe 2000 denier fiber tows and fabric coupons. Handling mens were slowly cooled Heat-treated blankets were exam damage of the fiber tows was minimized by securing them ined for evidence of coating spallation and degradation to an alumina frame, which provided support both during A modulated wind tunnel was used to expose the coated the thermal treatment used to remove the sizing and during blankets to aerodynamic flow and a fluctuating pressure that subsequent coating and firing. Uncoated fiber tows and tows simulates the acoustic loading of re-entry. After heat treat coated with a silica-based slurry were subjected to identical ment, the specimens were mounted in a wooden frame handling and heat treatments to compare their performance which was mechanically fastened between aluminum plates directly to that of tows with monazite-based coatings After (Fig. 1). The aluminum face-plate contained a rectangular the final processing step, the fiber tows were removed from hole(-105X 14 cm) to expose the coated blanket surface the frame and attached with epoxy to slotted aluminum tab The testing apparatus consisted of a compressor to flow air used as grips for tensile testing (with gauge length (at a constant total pressure of 52 MPa)through a rectangu 2.54 cm). The tabs were attached via vacuum grease to lar wind tunnel and a pneumatically driven rotor located a glass slide to allow transport to a tensile testing machine downstream from the blanket (which formed the bottom (Micropull Sciences)without risk of damaging the fiber tow face of the tunnel). The paddle-wheel shaped rotor had by flexure. Testing was carried out at room temperature one blade which restricted airflow when vertically oriented using self aligning grips As the rotor turned throughout the test(100 Hz), an alter- Woven fabric specimens(3-ply angle interlock) were cut nating change in air fow restriction set up a back pressure into strips approximately I cm x 5 cm prior to desizing and fluctuation equivalent to 172 dB with a frequency of twice coating. After a final heat treatment, they were also tested in the rotor speed. These conditions are standards set by nasa
were rich in P, AlPO4 was observed. The original precursor composition was then adjusted based on these observations and the process was repeated until no reaction products could be detected on the sapphire in the fired pellet. The coatings developed in this study were optimized specifically for Nextel 440 fibers (3M Corporation, St. Paul, MN). These small diameter fibers (12 mm) have a chemical composition of 70% Al2O3, 28% SiO2 and 2% B2O3 and are weavable. A dipping procedure was used to produce coatings from low viscosity compositions including the neat solution precursor, and powder-containing slurries with low solids loadings (,10 vol%). More concentrated slurries (.20 vol% solids) and coatings applied to complex geometries such as quilted blankets were brush coated, or painted, onto the fabric surface. The powders used were monazite, produced by spray drying the stoichiometric solution precursors (2–5 m granual size), and high purity Al2O3 (average particle size ,0.2 m from Sumitomo Chemical Company, New York, NY). These were dispersed in the monazite solution or in water and the pH of the compositions was adjusted by adding NH4OH. The chemical compatibility of the Nextel 440 fibers and the coatings was evaluated using X-ray diffraction and energy-dispersive X-ray spectroscopy techniques after heat treating the coated fiber tows and fabrics at 1100 or 12008C for 1 h. Finally, polished cross-sections of coated fabrics were examined by scanning electron microscopy to assess the uniformity of the coatings and to determine whether the coatings infiltrated both the inter- and intra-fiber tow spaces effectively. 2.2. Tensile testing of coated fiber tow and fabric Tensile testing of coated, heat-treated fiber tows and fabrics was used to evaluate the effects of various coatings on the retained strengths, as well as to allow direct observation of the fracture behavior. Test specimens consisted of 2000 denier fiber tows and fabric coupons. Handling damage of the fiber tows was minimized by securing them to an alumina frame, which provided support both during the thermal treatment used to remove the sizing and during subsequent coating and firing. Uncoated fiber tows and tows coated with a silica-based slurry were subjected to identical handling and heat treatments to compare their performance directly to that of tows with monazite-based coatings. After the final processing step, the fiber tows were removed from the frame and attached with epoxy to slotted aluminum tabs used as grips for tensile testing (with gauge length ,2.54 cm). The tabs were attached via vacuum grease to a glass slide to allow transport to a tensile testing machine (Micropull Sciences) without risk of damaging the fiber tow by flexure. Testing was carried out at room temperature using self aligning grips. Woven fabric specimens (3-ply angle interlock) were cut into strips approximately 1 cm × 5 cm prior to desizing and coating. After a final heat treatment, they were also tested in tension with a 2.54 cm gauge length using wedge-shaped grips. 2.3. Thermal exposure and modulated wind tunnel testing of coated blankets The thermal and acoustic loads experienced by thermal protection systems during atmosphere re-entry are severe. To simulate such conditions, small blanket test specimens (16 × 16 cm) were fabricated and subjected to radiant heating and wind tunnel experiments. These blankets consisted of Nextel 440 face-sheets and quartz fabric backing. The two fabric layers were quilted together with Nextel 440 sewing thread through approximately 2 cm of fibrous insulation (ICI). Heat treatments of coated blankets were performed at low pressure (,1 torr) in a facility equipped with quartz lamps. Temperatures were monitored at the exposed fabric face and at the backside of the blanket using calibrated thermocouples. The maximum face-sheet exposure temperature of either 1100 or 12008C was typically reached within 1– 5 min and was maintained for 30 min, after which the specimens were slowly cooled. Heat-treated blankets were examined for evidence of coating spallation and degradation. A modulated wind tunnel was used to expose the coated blankets to aerodynamic flow and a fluctuating pressure that simulates the acoustic loading of re-entry. After heat treatment, the specimens were mounted in a wooden frame which was mechanically fastened between aluminum plates (Fig. 1). The aluminum face-plate contained a rectangular hole (,10.5 × 14 cm) to expose the coated blanket surface. The testing apparatus consisted of a compressor to flow air (at a constant total pressure of 52 MPa) through a rectangular wind tunnel and a pneumatically driven rotor located downstream from the blanket (which formed the bottom face of the tunnel). The paddle-wheel shaped rotor had one blade which restricted airflow when vertically oriented. As the rotor turned throughout the test (,100 Hz), an alternating change in air flow restriction set up a back pressure fluctuation equivalent to 172 dB with a frequency of twice the rotor speed. These conditions are standards set by NASA 484 J.B. Davis et al. / Composites: Part A 30 (1999) 483–488 Fig. 1. Thermal protection blanket test coupon for radiant heat and modulated wind tunnel experiments
J.B. Davis et al. /Composites: Part A 30 (1999)483-488 100 -Nextel 440 1205 C ---- Nextel 440 1100 C f+=-se--aC 2 sh eta onazite Powder Filled 100 人(从 0-N ”只。、 2-theta Fig. 2. X-ray diffraction results. (a) Nextel 440 fibers after heat treatment for I hat 1 100 and 1200.C. Mullite peaks are identified. (b)Monazite- based coating after heat treatment for I h at 1100"C (F fabric peaks, M= monazite peaks, A= alumina peaks, S= SiC peaks) Ames Research Center for TPS evaluation. The tests were peaks and high background levels characteristic of amor- conducted for 600 s phous material. After heating to 1200.C, the pattern consisted primarily of narrow crystalline mullite peaks The monazite powders obtained from stoichiometric 3. Results solutions were in some instances mixed with powders of Al2O3 and Sic (which is commonly used as an additive to 3.1. Chemical compatibility and coating microstructure TPS coatings to increase their emittance). In all cases, the only peaks found in the X-ray diffraction patterns after firing X-ray diffraction studies of uncoated, heat-treated Nextel to 1200"C were those associated with the individual powder 440 fibers revealed a change in their structure at tempera- phases. No reactions between the coating constituents were tures between 1100 and 1200.C(Fig. 2). In the as-received observed state and after heat treatment at temperatures up to 1 100C, Coated fabrics were heat treated and X-rayed the X-ray diffraction pattern consisted of a few very broad conditions identical to those used for the uncoated
Ames Research Center for TPS evaluation. The tests were conducted for 600 s. 3. Results 3.1. Chemical compatibility and coating microstructure X-ray diffraction studies of uncoated, heat-treated Nextel 440 fibers revealed a change in their structure at temperatures between 1100 and 12008C (Fig. 2). In the as-received state and after heat treatment at temperatures up to 11008C, the X-ray diffraction pattern consisted of a few very broad peaks and high background levels characteristic of amorphous material. After heating to 12008C, the pattern consisted primarily of narrow crystalline mullite peaks. The monazite powders obtained from stoichiometric solutions were in some instances mixed with powders of Al2O3 and SiC (which is commonly used as an additive to TPS coatings to increase their emittance). In all cases, the only peaks found in the X-ray diffraction patterns after firing to 12008C were those associated with the individual powder phases. No reactions between the coating constituents were observed. Coated fabrics were heat treated and X-rayed under conditions identical to those used for the uncoated fabric J.B. Davis et al. / Composites: Part A 30 (1999) 483–488 485 Fig. 2. X-ray diffraction results. (a) Nextel 440 fibers after heat treatment for 1 h at 1100 and 12008C. Mullite peaks are identified. (b) Monazite-based coatings after heat treatment for 1 h at 11008C (F fabric peaks, M monazite peaks, A alumina peaks, S SiC peaks)
J.B. Davis et al./Composites: Part A 30(1999)483-488 Fiber tow strengths Coating composition Average strength SD(MPa) 350±170 solution and Al,O, 220±30 Monazite powder slurry 50±110 monazite powder slurry 590±50 Monazite powder and Alo, 830±50 Monazite powder and AlO, 1050±60 Silica-based slurry 270±50 Al2O, constituents enhanced the powder packing efficiency around the filaments(Fig. 3(b). The monazite volume frac tions in these coatings greatly exceed that needed to satisfy the percolation criterion, and the weak monazite phase is continuous throughout the coating Fig. 3. Scanning electro ing produced with monazite precursor solution with Al2 O3 powder. (b) 3.2. Tensile testing of coated fiber tows and fabric Coating produced with m and Al2O3 powder slur The measured strengths of fiber tows after heat treatment d for coatings materials alone. Again, no new reaction at 1100C for I h are summarized in Table 1. These tow phases were detected(Fig. 2). However, differences existed strengths were obtained from the peak load normalized by between the patterns obtained from fabrics coated with slur- the total cross-sectional area of fiber within each tow ries containing monazite in powder form and patterns from(assuming 650 filaments of 12 um diameter).Average fabrics coated with monazite solutions For powder slurry strengths for each condition were determined from 15-30 coatings after heat treatment to 1100.C, the peaks were separate tow measurements Filament misalignment within primarily those of the coating, indicating that the fabric each tow, which certainly affects the peak load, was not remained amorphous. For the same heat treatment, the solu- accounted for in these data. Therefore, these values are tion-coated samples exhibited different relative peak inten- lower-bound estimates of single filament properties and sities: mullite peaks were present, sometimes overlapping are used primarily to compare the relative performance of the coating peaks. The fibers of the fabric had crystallized at the materials investigated in this study a lower temperature in this case The strengths of the uncoated tows exceeded those of Slurry coatings produced using either monazite precursor coated specimens. This indicates that some loss of tow olution or monazite powder infiltrated fabrics well. strength occurred for all the coatings investigated. The However. the infiltration of each was further magnitude of this loss varied significantly with coating the addition of the submicron-sized Al2O, filler powders. composition. The monazite solution precursors alone For the coatings produced using the monazite solution, the reduced the fiber tow strength most severely. A strength spaces between filaments in each tow were well filled and enhancement was observed for specimens coated with the each filament exhibited a continuous coverage of monazite solution precursors when AlO, powder was added, the bright phase in the backscattered scanning electron although the strengths were still unacceptably low and microscope image shown in Fig. 3(a)). This is usually difi- approximately equal to those measured for the silica-based cult to accomplish with liquid precursor coating methods composition. The retained strengths of the tows coated with since, during drying, capillary forces normally tend to redis- monazite powder-based slurries were much higher and tribute the liquid to areas of contact between adjacent fibers improved further with the addition of the Al2O3 powde producing fiber bridging and incomplete coverage. The filler A further improvement was realized by raising the slurry powders, however, provide a network of particle-fiber pH prior to coating the fibers. Neutral or basic slurries contacts around the filaments and draw the liquid to the produced the strongest specimens fiber surface. For the slurries containing monazite in powder The relative strengths of the coated fabric specimens are form, the difference in the particle sizes of the monazite and not reported but were similar to those of the tows in Table 1
and for coatings materials alone. Again, no new reaction phases were detected (Fig. 2). However, differences existed between the patterns obtained from fabrics coated with slurries containing monazite in powder form and patterns from fabrics coated with monazite solutions. For powder slurry coatings after heat treatment to 11008C, the peaks were primarily those of the coating, indicating that the fabric remained amorphous. For the same heat treatment, the solution-coated samples exhibited different relative peak intensities: mullite peaks were present, sometimes overlapping the coating peaks. The fibers of the fabric had crystallized at a lower temperature in this case. Slurry coatings produced using either monazite precursor solution or monazite powder infiltrated fabrics well. However, the infiltration of each was further improved by the addition of the submicron-sized Al2O3 filler powders. For the coatings produced using the monazite solution, the spaces between filaments in each tow were well filled and each filament exhibited a continuous coverage of monazite (the bright phase in the backscattered scanning electron microscope image shown in Fig. 3(a)). This is usually diffi- cult to accomplish with liquid precursor coating methods since, during drying, capillary forces normally tend to redistribute the liquid to areas of contact between adjacent fibers producing fiber bridging and incomplete coverage. The filler powders, however, provide a network of particle-fiber contacts around the filaments and draw the liquid to the fiber surface. For the slurries containing monazite in powder form, the difference in the particle sizes of the monazite and Al2O3 constituents enhanced the powder packing efficiency around the filaments (Fig. 3(b)). The monazite volume fractions in these coatings greatly exceed that needed to satisfy the percolation criterion, and the weak monazite phase is continuous throughout the coating. 3.2. Tensile testing of coated fiber tows and fabric The measured strengths of fiber tows after heat treatment at 11008C for 1 h are summarized in Table 1. These tow strengths were obtained from the peak load normalized by the total cross-sectional area of fiber within each tow (assuming 650 filaments of 12 mm diameter). Average strengths for each condition were determined from 15–30 separate tow measurements. Filament misalignment within each tow, which certainly affects the peak load, was not accounted for in these data. Therefore, these values are lower-bound estimates of single filament properties and are used primarily to compare the relative performance of the materials investigated in this study. The strengths of the uncoated tows exceeded those of any coated specimens. This indicates that some loss of tow strength occurred for all the coatings investigated. The magnitude of this loss varied significantly with coating composition. The monazite solution precursors alone reduced the fiber tow strength most severely. A strength enhancement was observed for specimens coated with the solution precursors when Al2O3 powder was added, although the strengths were still unacceptably low and approximately equal to those measured for the silica-based composition. The retained strengths of the tows coated with monazite powder-based slurries were much higher and improved further with the addition of the Al2O3 powder. A further improvement was realized by raising the slurry pH prior to coating the fibers. Neutral or basic slurries produced the strongest specimens. The relative strengths of the coated fabric specimens are not reported but were similar to those of the tows in Table 1. 486 J.B. Davis et al. / Composites: Part A 30 (1999) 483–488 Fig. 3. Scanning electron micrographs showing infiltrated fabrics. (a) Coating produced with monazite precursor solution with Al2O3 powder. (b) Coating produced with monazite and Al2O3 powder slurry. Table 1 Fiber tow strengths Coating composition Average strength ^ SD (MPa) Uncoated 1350 ^ 170 Monazite solution 180 ^ 60 Monazite solution and Al2O3 powder 220 ^ 30 Monazite powder slurry 350 ^ 110 pH ,2 Monazite powder slurry 590 ^ 50 pH ,8 Monazite powder and Al2O3 powder 830 ^ 50 slurry, pH ,2 Monazite powder and Al2O3 powder 1050 ^ 60 slurry, pH ,7 Silica-based slurry 270 ^ 50
J.B. Davis et al. /Composites: Part A 30 (1999)483-488 Monazite& Alo, Powder Slurry Coating Silica-Based Coating 050100150200250300350400 Displacement (um) Fig. 4. Load-displacement measurements during tensile testing of coated fiber tows after heat treatment at 1100.C for I h However, fewer woven specimens were tested and the varia- peak load is non-linear, with significant load-bearing bility in the strengths was greater because of the unavoid- capability beyond the peak(Fig. 4), while the fractured able non-uniform loading of the tows within the fabric specimen has a brushy appearance due to fiber pullout The higher strengths of fabrics and tows coated with ( Fig. 5(a). In contrast, the lower strength silica-coated monazite slurry are accompanied by a distinctive non-brittle specimens exhibited brittle behavior characterized by a fracture mode, characteristic of tough ceramic matrix catastrophic load drop at failure(Fig 4)and nearly planar composites. The load-displacement response near the fracture surface(Fig. 5(b)) 3.3. Thermal exposure and modulated wind tunnel testing of coated blanket The blanket specimens were coated with the slurry composition that produced the highest tow strengths powders of monazite and alumina in an aqueous slurry dj usted to pH 7. Exposure of the coated surfaces to temperatures up to 1200.C for I h did no ly discern ible spallation of the coating, or large-scale cracking. The heat treatment did. however. increase the stiffness of the face-sheet fabric as desired. This stiffness enhancement is associated with partial sintering of the coating(matrix) The stiffened outer blanket surface performed well under acoustic loading during the wind tunnel testing. Four sepa rate blankets were evaluated. In no case was there any evidence of coating degradation detected by visual inspec tion during or after the tests. Out-of-plane displacements (pillowing) of the fabric during the tests was minimal and the face-sheet fabrics were not embrittled 4. Discussion The processing route for monazite-based coatings sign antly affected the retained strengths of heat-treated Nextel 440 fabric. Monazite precursor solutions caused significant Coated Nextel 440 fabric specimens after tensile testing: (a)silica- strength losses. This was observed despite rigorous verifica coating heat treated at 1 100C for I h;(b)monazite -alumina slurry tion of the precursor stoichiometry and the absence of reac g heat treated at 1 100C for 1 h tion phases in X-ray diffraction patterns. The fibers
However, fewer woven specimens were tested and the variability in the strengths was greater because of the unavoidable non-uniform loading of the tows within the fabric. The higher strengths of fabrics and tows coated with monazite slurry are accompanied by a distinctive non-brittle fracture mode, characteristic of tough ceramic matrix composites. The load–displacement response near the peak load is non-linear, with significant load-bearing capability beyond the peak (Fig. 4), while the fractured specimen has a brushy appearance due to fiber pullout (Fig. 5(a)). In contrast, the lower strength silica-coated specimens exhibited brittle behavior characterized by a catastrophic load drop at failure (Fig. 4) and nearly planar fracture surface (Fig. 5(b)). 3.3. Thermal exposure and modulated wind tunnel testing of coated blankets The blanket specimens were coated with the slurry composition that produced the highest tow strengths: powders of monazite and alumina in an aqueous slurry adjusted to pH 7. Exposure of the coated surfaces to temperatures up to 12008C for 1 h did not cause any discernible spallation of the coating, or large-scale cracking. The heat treatment did, however, increase the stiffness of the face-sheet fabric as desired. This stiffness enhancement is associated with partial sintering of the coating (matrix). The stiffened outer blanket surface performed well under acoustic loading during the wind tunnel testing. Four separate blankets were evaluated. In no case was there any evidence of coating degradation detected by visual inspection during or after the tests. Out-of-plane displacements (pillowing) of the fabric during the tests was minimal and the face-sheet fabrics were not embrittled. 4. Discussion The processing route for monazite-based coatings significantly affected the retained strengths of heat-treated Nextel 440 fabric. Monazite precursor solutions caused significant strength losses. This was observed despite rigorous verification of the precursor stoichiometry and the absence of reaction phases in X-ray diffraction patterns. The fibers J.B. Davis et al. / Composites: Part A 30 (1999) 483–488 487 Fig. 4. Load–displacement measurements during tensile testing of coated fiber tows after heat treatment at 11008C for 1 h. Fig. 5. Coated Nextel 440 fabric specimens after tensile testing: (a) silicabased coating heat treated at 11008C for 1 h; (b) monazite–alumina slurry coating heat treated at 11008C for 1 h
opposites: Part A 30(1999)483- crystallized at slightly lower temperatures when this type of avoid degrading the fiber strength during the coating process oating was used. However, the crystallization process itself and subsequent thermal exposure of the blanket did degrade uncoated fibers in related work. The largest increase in strength occurred when the ph of the solution and slurries was increased, suggesting that the acidic nature Acknowledgements of the precursor solutions(pH 1) plays a role in the degradation. A further improvement in retained strength The authors thank May-Lin DeHaan for preparing the was achieved through the addition of Al2O3 filler powders precursor solutions, Don Dietrich and Ed Hsieh for their to the monazite coatings. The role of these powders remains assistance with the X-ray diffraction work and Jim McGee to be verified but they may serve as an internal buffer; in for performing the wind tunnel tests. This work was essence decreasing the likelihood of reaction between the supported by Boeing and Rockwell independent research fiber and coating by making the system less sensitive to and development funds. Development of monazite solution slight monazite stoichiometry changes precursors and powders was funded by the US Office of Naval Research under contract No. 0014-95-C-0057 5. Conclusion References Monazite-based coatings for thermal protection blankets are stable and compatible with Nextel 440 fibers at tempera [Korb LJ, Morant CA, Calland RM, Thatcher CS. The shuttle orbiter tures at least as high as 1200C, much higher than currently thermal protection system. Am Ceramic Soc Bull 1981: 60(11): 1188 used silica-based systems. The monazite-coated outer fabric [2] Morgan PED, Marshall DB. Ceramic composites of monazite and behaves as a tough ceramic composite, allowing adequate alumina. J Am Ceramic Soc 1995, 78(6): 1553-1563 performance in wind tunnel tests that simulate aerodynamic 3 Morgan PED, Marshall DB, Housley RM. High temperature stability of loading. Control of the chemistry, processing and morphol monazite-alumina composites J Mat Sci Eng 1995; A195: 215-222 [4] Morgan PED, Marshall DB. Functional interfaces in oxide-oxide ogy of monazite-based coatings is, however, needed to composites. J Mat Sci Eng 1993: A162(1-2): 15-2
crystallized at slightly lower temperatures when this type of coating was used. However, the crystallization process itself did degrade uncoated fibers in related work. The largest increase in strength occurred when the pH of the solution and slurries was increased, suggesting that the acidic nature of the precursor solutions (pH , 1) plays a role in the degradation. A further improvement in retained strength was achieved through the addition of Al2O3 filler powders to the monazite coatings. The role of these powders remains to be verified but they may serve as an internal buffer; in essence decreasing the likelihood of reaction between the fiber and coating by making the system less sensitive to slight monazite stoichiometry changes. 5. Conclusion Monazite-based coatings for thermal protection blankets are stable and compatible with Nextel 440 fibers at temperatures at least as high as 12008C, much higher than currently used silica-based systems. The monazite-coated outer fabric behaves as a tough ceramic composite, allowing adequate performance in wind tunnel tests that simulate aerodynamic loading. Control of the chemistry, processing and morphology of monazite-based coatings is, however, needed to avoid degrading the fiber strength during the coating process and subsequent thermal exposure of the blankets. Acknowledgements The authors thank May-Lin DeHaan for preparing the precursor solutions, Don Dietrich and Ed Hsieh for their assistance with the X-ray diffraction work and Jim McGee for performing the wind tunnel tests. This work was supported by Boeing and Rockwell independent research and development funds. Development of monazite solution precursors and powders was funded by the US Office of Naval Research under contract No. 0014-95-C-0057. References [1] Korb LJ, Morant CA, Calland RM, Thatcher CS. The shuttle orbiter thermal protection system. Am Ceramic Soc Bull 1981;60(11):1188. [2] Morgan PED, Marshall DB. Ceramic composites of monazite and alumina. J Am Ceramic Soc 1995;78(6):1553–1563. [3] Morgan PED, Marshall DB, Housley RM. High temperature stability of monazite—alumina composites. J Mat Sci Eng 1995;A195:215–222. [4] Morgan PED, Marshall DB. Functional interfaces in oxide–oxide composites. J Mat Sci Eng 1993;A162(1–2):15–25. 488 J.B. Davis et al. / Composites: Part A 30 (1999) 483–488
