VoL. 83. No. 2 200 a 1000°C/500hair 150 Uncoated 0.02 micron 0.04 micron 100um 150 N1000°C/500hair 100 0.04 micron us values for off-axis Nextel 720/C/CAS asing carbon thickness. indicate that"C and fugitive samples are superior to creasing "C thickness, and the modulus drops even further as the"C is removed This indicates that the off-axis load transfer is not as efficient as in the 0.orientation. In Fig 3(b), the ultimate strengths in the off-axis are compared. The fugitive"C"compos- tes appear to be nearly as good as the"C composites and 100m ignificantly better than the uncoated composites Figures 4 and 5 show fracture surfaces of some typical control Fig. 4. SEM of Nextel 720/CAS unidirectional (0%) composite fracture and fugitive composites(uncoated, 0.02 um"C, and 0.04 um C") tested in the o°and±45° orientations. Generally,the uncoated composites exhibit brittle fracture surfaces with virtually matrix, and 30 vol% porosity(ASTM C20-922). There is some no fiber pullout in both orientations(Figs. 4(a) and 5(a)). For the variability(<5 vol%)in fiber volume percentage from sample to "C and fugitive composites, the fracture surfaces show significant sample, which contributes to scatter in the tensile test data. fiber pullout. The extent of fiber pullout follows the same trend as the tensile strength values. The incorporation of the 0.02 and 0.04 (B) Mechanical Properties: The results of the tensile tests, own in Fig. 7, revealed that, for the porous matrix composites um"C results in a major increase in fiber pull-out lengths(Figs the as-processed strengths of the samples containing uncoated 4(b) and 5(b)). After the 1000@C heat treatment that removes the abric were superior to samples containing"C" fabric. Subsequent arbon, the pull-out lengths diminish noticeably(Figs. 4(c)and tow testing of the Nextel 720"C" fabric revealed that there was a trength loss of >3(Table D), which could be attributed to fiber uncoated sample degradation during the coating process. To truly evaluate the (2) Porous Matrix Composites omposite strengths, therefore, it was necessary to normalize these (A) Microstructure: Microstructural analysis of the porous trengths with respect to the measured tow properties prior to matrix composites reveals areas of fine, dispersed porosity in the natrix. Aside from these pores, however, there are also large shrinkage cracks in the matrix(Fig. 6), as seen in other works Standard Test Methods for Apparent Porosity completed on pressure-infiltrated composites. The density of the ater, ASTM Designation C20-92. American So 金 West Conshohocken, P.increasing “C” thickness, and the modulus drops even further as the “C” is removed. This indicates that the off-axis load transfer is not as efficient as in the 0° orientation. In Fig. 3(b), the ultimate strengths in the off-axis are compared. The fugitive “C” compos￾ites appear to be nearly as good as the “C” composites and significantly better than the uncoated composites. Figures 4 and 5 show fracture surfaces of some typical control and fugitive composites (uncoated, 0.02 mm “C”, and 0.04 mm “C”) tested in the 0° and 645° orientations. Generally, the uncoated composites exhibit brittle fracture surfaces with virtually no fiber pullout in both orientations (Figs. 4(a) and 5(a)). For the “C” and fugitive composites, the fracture surfaces show significant fiber pullout. The extent of fiber pullout follows the same trend as the tensile strength values. The incorporation of the 0.02 and 0.04 mm “C” results in a major increase in fiber pull-out lengths (Figs. 4(b) and 5(b)). After the 1000°C heat treatment that removes the carbon, the pull-out lengths diminish noticeably (Figs. 4(c) and 5(c)) but are still significant when compared with those of the uncoated sample. (2) Porous Matrix Composites (A) Microstructure: Microstructural analysis of the porous matrix composites reveals areas of fine, dispersed porosity in the matrix. Aside from these pores, however, there are also large shrinkage cracks in the matrix (Fig. 6), as seen in other works completed on pressure-infiltrated composites.29 The density of the composites is typically 2.2 g/cm3 , with ;30 vol% fibers, 40 vol% matrix, and 30 vol% porosity (ASTM C20-92‡ ). There is some variability (,5 vol%) in fiber volume percentage from sample to sample, which contributes to scatter in the tensile test data. (B) Mechanical Properties: The results of the tensile tests, shown in Fig. 7, revealed that, for the porous matrix composites, the as-processed strengths of the samples containing uncoated fabric were superior to samples containing “C” fabric. Subsequent tow testing of the Nextel 720 “C” fabric revealed that there was a strength loss of .1⁄3 (Table II), which could be attributed to fiber degradation during the coating process. To truly evaluate the composite strengths, therefore, it was necessary to normalize these strengths with respect to the measured tow properties prior to ‡ “Standard Test Methods for Apparent Porosity, Water Absorption, Apparent Specific Gravity, and Bulk Density of Burned Refractory Brick and Shapes by Boiling Water,” ASTM Designation C20-92. American Society for Testing and Materials, West Conshohocken, PA. Fig. 3. (a) Modulus values for off-axis (645°) Nextel 720/“C”/CAS samples. There is a decrease in modulus with increasing carbon thickness. (b) Off-axis strengths indicate that “C” and fugitive samples are superior to uncoated composites. Fig. 4. SEM of Nextel 720/CAS unidirectional (0°) composite fracture surfaces: (a) uncoated, (b) 0.02 mm “C”, (c) fugitive (0.02 mm “C” initial). 332 Journal of the American Ceramic Society—Keller et al. Vol. 83, No. 2