February 2000 ugitive Interfacial Carbon Coatings for Oxide/Oxide Composites were used in evaluating the strengths of both dense and porous 200+ matrIx composites. 醞As- processed 1000°C/500h/ 150 IlL. Results (I Dense Matrix Composites (A) Microstructure: The CVD"C 100 smooth and continuous There was fiber contact during the coating ocess, which was unavoidable because of the static position of the fibers during deposition. This contact led to significant areas of exposed fiber surface because of spalling of the bridged coating 50 Such exposure could be detrimental if the fiber and matrix wer reactive and/or if exposure was excessive, leading to substantial Iber/matrix sintering. The "C thicknesses were not directly measured. There were some variations in the coating thickness Calculated 0.02 micron 0.04 micron within the tow: however this was not considered in the current work. Because of possible coating variations, any demonstrated property dependence on carbon thickness should be considered qualitative CAs glass-ceramic (anorthite) melted at >1400C, but the 250 hot-pressing temperature was kept below this to minimize strength degradation of the fibers, which was significant at >1300C. Some As-Processed pertinent physical properties of the Nextel 720 fiber and CAS △200 N Oxidized 650 C/24 h/Oxygen matrix are given in Table L 端1000°c/500h/air Microstructural evaluation of the hot-pressed samples revealed that the fibers were relatively well dispersed, with 30-35 vol% 150 fibers. The bulk porosity, arising primarily from incomplete binder burnout, was determined to be -5%-10% using the archimedes technique. The composites were black in the as-processed condi- tion, and a small amount of glassy phase appeared optically to be left in the matrix. The appearance of the Nextel 720/0.04 um C"/CAS sample changed to white after oxidative heat treatment at 650C, indicating that the carbon had burned away. The oxidation onditions were chosen based on the model of Cawley et al. After both the 650 c and the long-term heat treatment, the Uncoated 0.02 micron 0.04 micron microstructures of the composites remained essentially unchanged, aside from the removal of the carbon. No large pores were seen in (b) the composites, indicating that CO, evolution and removal did not Fig. 2.(a)Measured elastic modulus ause significant damage, as might be expected from the pressure CAS composites(>90% dense)ar No significant difference can be seen m the0° Nextel720~ ith the calculated value various conditions.(b) The gap formed by the oxidation of"C was too thin to be Ultimate strengths of composites Uncoated specimens exhibit extremely imaged in the SEM; further imaging was impaired by the rounding poor strengths of the fiber edges caused by the different polishing rates of th constituents. a dark ring was seen around the fiber but it was not ear whether it was a polishing artifact or an actual gap different conditions studied. The first column shows the modulu Transmission electron microscopy redicted through rule-of-mixtures calculations (see Section usable for the oxidized samples because of difficulties associated iV(1A). Comparing the control composites, viz., 0.02 and 0.04 with sample preparation. Indications of gap stability in this work were therefore evaluated through mechanical testing of the com- m"C "specimens, with the composites of interest, viz., the same composites tested after a 1000C, 500 h heat treatment in air, posites and the resultant fiber pullout observed on the fracture shows that there is not a significant difference in the modulus surfaces values. This indicates that the presence of"C its thickness, or its (B) Mechanical Properties: The test results of the 0%(uni- removal has little effect on the measured modulus directional) composites are shown in Fig. 2. In Fig. 2(a), the elastic In Fig. 2(b), the ultimate failure strengths of the composites moduli calculated from the stress-strain plots are shown for the under different conditions are compared. The uncoated composites clearly have very poor strength; in fact, many specimens failed before testing. The "C" composites exhibit much better strengths Table I. Select Physical Properties of CAs and Nextel 720 the 0.04 um C composites are seen to have slightly better composite strengths than the 0.02 um" composites, particularly operty CAS glass-ceramic Nextel 720 if the tow strengths are taken into consideration. The strengths of Cao-Al,Ox2Si02 85 wt% Al the composites of interest, where the "C was burned away, are also shown in Fig. 2(b). These composites were tested after the Density C -removal heat treatment(650C, 24 h, O2) and after a long Elastic modulus(GP term(1000 C, 500 h, air) heat treatment. It can be seen that oefficient of composite strength decreases slightly with carbon removal; how- ever, the strengths after"C" removal of the 0.04 um composites Flexure strengt 124 Tensile strength(GPa) are comparable to those of the 0.02 um"C" composites Fabric strength(MPa) In Fig. 3, the results of tests on the t45composites are show (fill), 3 in. gauge(Ib/in) Fig 3(a), thewere used in evaluating the strengths of both dense and porous matrix composites. III. Results (1) Dense Matrix Composites (A) Microstructure: The CVD “C” appeared generally smooth and continuous. There was fiber contact during the coating process, which was unavoidable because of the static position of the fibers during deposition. This contact led to significant areas of exposed fiber surface because of spalling of the bridged coating. Such exposure could be detrimental if the fiber and matrix were reactive and/or if exposure was excessive, leading to substantial fiber/matrix sintering. The “C” thicknesses were not directly measured. There were some variations in the coating thickness within the tow; however, this was not considered in the current work. Because of possible coating variations, any demonstrated property dependence on carbon thickness should be considered qualitative. CAS glass-ceramic (anorthite) melted at .1400°C, but the hot-pressing temperature was kept below this to minimize strength degradation of the fibers, which was significant at .1300°C. Some pertinent physical properties of the Nextel 720 fiber and CAS matrix are given in Table I. Microstructural evaluation of the hot-pressed samples revealed that the fibers were relatively well dispersed, with 30–35 vol% fibers. The bulk porosity, arising primarily from incomplete binder burnout, was determined to be ;5%–10% using the Archimedes technique. The composites were black in the as-processed condi￾tion, and a small amount of glassy phase appeared optically to be left in the matrix. The appearance of the Nextel 720/0.04 mm “C”/CAS sample changed to white after oxidative heat treatment at 650°C, indicating that the carbon had burned away. The oxidation conditions were chosen based on the model of Cawley et al.33 After both the 650°C and the long-term heat treatment, the microstructures of the composites remained essentially unchanged, aside from the removal of the carbon. No large pores were seen in the composites, indicating that COx evolution and removal did not cause significant damage, as might be expected from the pressure of an entrapped gas. The gap formed by the oxidation of “C” was too thin to be imaged in the SEM; further imaging was impaired by the rounding of the fiber edges caused by the different polishing rates of the constituents. A dark ring was seen around the fiber, but it was not clear whether it was a polishing artifact or an actual gap. Transmission electron microscopy (TEM) was also not readily usable for the oxidized samples because of difficulties associated with sample preparation. Indications of gap stability in this work were therefore evaluated through mechanical testing of the com￾posites and the resultant fiber pullout observed on the fracture surfaces. (B) Mechanical Properties: The test results of the 0° (uni￾directional) composites are shown in Fig. 2. In Fig. 2(a), the elastic moduli calculated from the stress–strain plots are shown for the different conditions studied. The first column shows the modulus predicted through rule-of-mixtures calculations (see Section IV(1A)). Comparing the control composites, viz., 0.02 and 0.04 mm “C” specimens, with the composites of interest, viz., the same composites tested after a 1000°C, 500 h heat treatment in air, shows that there is not a significant difference in the modulus values. This indicates that the presence of “C”, its thickness, or its removal has little effect on the measured modulus. In Fig. 2(b), the ultimate failure strengths of the composites under different conditions are compared. The uncoated composites clearly have very poor strength; in fact, many specimens failed before testing. The “C” composites exhibit much better strengths; the 0.04 mm “C” composites are seen to have slightly better composite strengths than the 0.02 mm “C” composites, particularly if the tow strengths are taken into consideration. The strengths of the composites of interest, where the “C” was burned away, are also shown in Fig. 2(b). These composites were tested after the “C”-removal heat treatment (650°C, 24 h, O2) and after a long￾term (1000°C, 500 h, air) heat treatment. It can be seen that composite strength decreases slightly with carbon removal; how￾ever, the strengths after “C” removal of the 0.04 mm composites are comparable to those of the 0.02 mm “C” composites. In Fig. 3, the results of tests on the 645° composites are shown. In Fig. 3(a), the measured moduli are compared for the control and the fugitive “C” composites. A decrease in modulus occurs with Table I. Select Physical Properties of CAS and Nextel 720 Property CAS glass-ceramic Nextel 720 Composition CaO–Al2O3–2SiO2 85 wt% Al2O3, 15 wt% SiO2 Density 2.8 3.4 Elastic modulus (GPa) 98 262 Coefficient of thermal expansion (31026 /°C) 5 6 Flexure strength (MPa) 124 Tensile strength (GPa) 2.1 Fabric strength (MPa) (fill), 3 in. gauge (lb/in.) 230 Fig. 2. (a) Measured elastic modulus values for the 0° Nextel 720/“C”/ CAS composites (.90% dense) are compared with the calculated value. No significant difference can be seen between various conditions. (b) Ultimate strengths of composites. Uncoated specimens exhibit extremely poor strengths. February 2000 Fugitive Interfacial Carbon Coatings for Oxide/Oxide Composites 331