333 0.5 mml Fig. 6. Optical micrograph showing a typical microstructure of Nextel 720 porous matrix composites, There is distributed porosity in the matrix, along with large shrinkage cracks d150 100 Fig. 7. Tensile test results for Nextel 720/mullite/alumina porous matrix mposites. Samples were tested in the as-processed condition and after heat treatments at 1150 C for 100 and 500 h. Overall strengths of Ten posites with uncoated fabric were superior to those with"C"fabric ile testing of fiber tows revealed an y loss in strength in the "C fabric. (See Fig. 8 for normalized composite strength data. roving were coated by indep sources, it appeared the strength loss was associated ertain deposition conditions. The degradation mechanism is the scope of this paper and is not discussed further 1 mm IV. Discussion Fig. 5. SEM of Nextel 720/CAS off-axis(+45)composite fracture surfaces: (a)uncoated, (b)0.04 um"C",(c) fugitive(0.04 um "C" initial) In the case of dense CAs matrix composites, the measured modulus, ultimate fracture strength, ugitive"C" composites, when compared with those of the control significant difference can be seen between the uncoated fibers, o composites(uncoated and""composites), showed that the ugitive coating concept is a mechanically via able conce pt fibers, and fugitive composites Nextel 720-reinforced(>90% dense)CAs matrix composites In all cases, the composites exhibited nonl inearity prior to failure These composites were also found to be quite stable at 1000.C for The long-term heat treatment of the porous matrix composites did not <500 h. For the porous matrix(85/15 Al2O SiO2, -30% overal appear to affect the composite strength. In fact, the average strength porosity, -43% matrix porosity) composites, there was no sign was slightly higher than in the as-processed case. The porous matrix icant difference between the uncoated, "C, and fugitive compos- composites also exhibited bundle pullout in all cases. A typical ites. The following is a more detailed discussion of the results to fracture surface is shown in Fig 9. The presence or absence of"C" did ain further insights from the data. not affect the appearance of the composite fracture surface, and little difference was seen after the long-term heat treatment (I Dense Matrix Composites (3) Nextel 720 Tow Testing (A Modulus: The modulus of a unidirectional composite is Tow test results are incorporated in Table Il. These results well-known to obey the simple rule of mixtures, viz indicate that the Nextel 720 as-received roving and fabric dis- pl strengths to the roving coated with 0.02 um carbon (~750 lowever, both the 0.04 um"C roving and"C Using the values listed in Table l, for a fiber volume percentage of fabric strength losses of >A. Because the fabric and 32.5+2.5, we obtain a composite modulus of 151.3 4 GPa. Thecomparison. The normalized data are shown in Fig. 8. No significant difference can be seen between the uncoated fibers, “C” fibers, and fugitive composites. In all cases, the composites exhibited nonlinearity prior to failure. The long-term heat treatment of the porous matrix composites did not appear to affect the composite strength. In fact, the average strength was slightly higher than in the as-processed case. The porous matrix composites also exhibited bundle pullout in all cases. A typical fracture surface is shown in Fig. 9. The presence or absence of “C” did not affect the appearance of the composite fracture surface, and little difference was seen after the long-term heat treatment. (3) Nextel 720 Tow Testing Tow test results are incorporated in Table II. These results indicate that the Nextel 720 as-received roving and fabric dis￾played similar strengths to the roving coated with 0.02 mm carbon (;750 MPa). However, both the 0.04 mm “C” roving and “C” fabric exhibited strength losses of .1⁄3. Because the fabric and roving were coated by independent sources, it appeared the strength loss was associated with certain deposition conditions. The degradation mechanism is beyond the scope of this paper and is not discussed further. IV. Discussion In the case of dense CAS matrix composites, the measured modulus, ultimate fracture strength, and fractography of the fugitive “C” composites, when compared with those of the control composites (uncoated and “C” composites), showed that the fugitive coating concept is a mechanically viable concept for the Nextel 720-reinforced (.90% dense) CAS matrix composites. These composites were also found to be quite stable at 1000°C for #500 h. For the porous matrix (85/15 Al2O3/SiO2, ;30% overall porosity, ;43% matrix porosity) composites, there was no signif￾icant difference between the uncoated, “C,” and fugitive compos￾ites. The following is a more detailed discussion of the results to gain further insights from the data. (1) Dense Matrix Composites (A) Modulus: The modulus of a unidirectional composite is well-known to obey the simple rule of mixtures,34 viz., Ec 5 EfVf 1 EmVm (1) Using the values listed in Table I, for a fiber volume percentage of 32.5 6 2.5, we obtain a composite modulus of 151.3 6 4 GPa. The Fig. 5. SEM of Nextel 720/CAS off-axis (645°) composite fracture surfaces: (a) uncoated, (b) 0.04 mm “C”, (c) fugitive (0.04 mm “C” initial). Fig. 6. Optical micrograph showing a typical microstructure of Nextel 720 porous matrix composites. There is distributed porosity in the matrix, along with large shrinkage cracks. Fig. 7. Tensile test results for Nextel 720/mullite/alumina porous matrix composites. Samples were tested in the as-processed condition and after heat treatments at 1150°C for 100 and 500 h. Overall strengths of composites with uncoated fabric were superior to those with “C” fabric. Tensile testing of fiber tows revealed an ; 1⁄3 loss in strength in the “C” fabric. (See Fig. 8 for normalized composite strength data.) February 2000 Fugitive Interfacial Carbon Coatings for Oxide/Oxide Composites 333