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 (<10 vol%). More concentrated Fig. 1. Thermal protection blanket test coupon for radiant heat and modu- slurries(>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 nasawere 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 solu￾tion 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 composi￾tions 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 observa￾tion 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 heat￾ing 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 insu￾lation (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 thermocou￾ples. 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 speci￾mens were slowly cooled. Heat-treated blankets were exam￾ined 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 treat￾ment, 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 rectangu￾lar 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 alter￾nating 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 modu￾lated wind tunnel experiments