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 fibersHowever, fewer woven specimens were tested and the varia￾bility in the strengths was greater because of the unavoid￾able 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 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 signif￾icantly affected the retained strengths of heat-treated Nextel 440 fabric. Monazite precursor solutions caused significant strength losses. This was observed despite rigorous verifica￾tion of the precursor stoichiometry and the absence of reac￾tion 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) silica￾based coating heat treated at 11008C for 1 h; (b) monazite–alumina slurry coating heat treated at 11008C for 1 h