146 Journal of the American Ceramic Society-Morscher and Martines-Fernandes Vol. 82. Ne Table I. Data for Single-Tow Minicomposites That Have Been Tested ber diameter ic/3MBN/SIC 0.07-0.12 Hi-NIC/PBN/SIC 0.13-0.2 0.75(0.43) Syl/PBN/SIC 0.12-0.19 045(0.3-2) pyrolytic BN(Advanced Ceramics, CI land and density of the fiber tow, BN interphase, and CVI-SiC matrix. 'Values given in e for this coating would severely degrade the fiber Each individual minicomposite that failed was cut into sev- strength. The BN that was used to coat the Nicalon tows was eral -30 mm lengths, mounted in epoxy, and polished longi- rocessed at a low temperature (-1050oC)and a thin Sic tudinally to a I um finish to determine the matrix-crack spac layer(0.5 um thick) was applied on top of the bn by the More than half of the gauge section was used to determine nterphase vendor before SiC infiltration. The Hi-Nicalon and he matrix-crack spacing. For some of the Syl-PBN minicom- Sylramic tows were coated with a bn phase that was de sites. the cracks were not always observable after polishing at 1400C. 16 The abbreviations 3MBN and PbN will be used and required etching in a boiling Muriakami's solution. Matrix denote the interphases for the Nicalon -fiber(Nic-3MBN) Hi- cracks in the Nic-3MBN and HN-PBN minicomposites were icalon-fiber(HN-PBN), and Sylramic-fiber(Syl-PBN)mini- easily observed after the longitudinal polishing com The fiber volume fraction for the minicomposites was de termined from the weight of the fibers, the weight gain of the IlL. Results BN coating, and the weight gain of the CVI infiltration. The density of Bn used in the calculations for PBN was 1.76 g/cr Tensile-Test Results based on the measured value of a similar bn made in bulk Typical examples of stress-strain hysteresis loc p tests are orm. The density(atomic order) of Bn increased as the shown in Fig. I for the Nic-3MBN and Syl-PBN minicompos processing temperature increased. The density of 3MBN is ites. The stress-strain behavior of the HN-PBN minicompos unknown. A value of 1.5 g/cm was assumed for the 3MBN ites is not shown for clarity but would be located between that nterphase, because it was processed at a lower temperature. of the two minicomposite types shown in Fig. 1. The composite The Syl-PBN and HN-PBN minicomposites that were tested stress was determined by multiplying the fiber volume fractio had an average fiber volume fraction of -0.16. The average by the stress on the fibers if fully loaded(failure load divided volume fraction of the Nic-3MBN minicomposites was sligh by total fiber area). Figure 2 shows the final hysteresis loop just lower (0.1). However, there was a range of fiber volume prior to failure for the three minicomposite types that were fractions. as noted in Table I sted in this study. The fiber volume fraction for the Nic- o The interphase thickness was variable across the tow cross 3MBN, HN-PBN, and Syl-PBN minicomposites used for com- est variability. The outer layer of fibers could have a coating up respectively parison in this study( Figs. 2-6) were 0.12, 0.16, and 0.16 to a 3 um thick, whereas the interior fibers in a tow The ultimate of the HN-PBN and Syl-PBN mini much-thinner(-0 4 um)and more-uniform coatings (Table I composites we ately the same(450 MPa), wher calcular age value for the coating thickness was used in the c-3MBN minicomposites was -310 ons, based on the weight gain of the PBN MPa. Based on the number of fibers per tow, these strengths e The minicomposites were mounted with epoxy to cardboard correspond to an average fiber strength of.8 GPa for the emission(AE)sensors were attached to the epoxy withalleaesic GPa for the Nicalon fibers, assuming that the fibers were bear ing the full load just prior to failure. The as-produced fiber tor clips. The length of the minicomposites between the card- strengths were -2.8-3.0 GPa for all three of these fiber types board edges ranged from 60 to 150 mm. Tensile testing was performed on a universal testing machine( Model 4502, stron, Canton, MA). Tensile unload-reload hysteresis loops were performed with increasing loads until the minicomposite failed. The displacements of the upper and lower cardboard tabs were monitored with a laser extensometer(Model Zygo Syl-PBN ), Zygo train was mined from the difference between the upper-tab and lower-tab 5 edge displacements divided by the length of the minicompos- 9 ites between the edges. The energy of the acoustic events was monitored with an AE analyzer(Model LOCAN 320, Physical Acoustics, Princeton, NJ). The AE analyzer also included computer, which collected the load, strain, and AE data. Be cause of the gripping method and attachment of AE transducers mBN to the epoxy, significant bending moments could result near the grips. The minicomposites often failed in this region at stresses =50 lower than those achieved for gauge failures. Only samples that failed in the gauge section were used to determine the ultimate properties. However, tensile hysteresis analysis could Strain. still be performed with samples that failed prematurely(nearperature for this coating would severely degrade the fiber strength. The BN that was used to coat the Nicalon tows was processed at a low temperature (∼1050°C)15 and a thin SiC layer (∼0.5 mm thick) was applied on top of the BN by the interphase vendor before SiC infiltration. The Hi-Nicalon and Sylramic tows were coated with a BN phase that was deposited at 1400°C.16 The abbreviations 3MBN and PBN will be used to denote the interphases for the Nicalon-fiber (Nic-3MBN), Hi￾Nicalon-fiber (HN-PBN), and Sylramic-fiber (Syl-PBN) mini￾composites, respectively. The fiber volume fraction for the minicomposites was de￾termined from the weight of the fibers, the weight gain of the BN coating, and the weight gain of the CVI infiltration. The density of BN used in the calculations for PBN was 1.76 g/cm3 , based on the measured value of a similar BN made in bulk form.17 The density (atomic order) of BN increased as the processing temperature increased. The density of 3MBN is unknown. A value of 1.5 g/cm3 was assumed for the 3MBN interphase, because it was processed at a lower temperature. The Syl-PBN and HN-PBN minicomposites that were tested had an average fiber volume fraction of ∼0.16. The average volume fraction of the Nic-3MBN minicomposites was slightly lower (∼0.1). However, there was a range of fiber volume fractions, as noted in Table I. The interphase thickness was variable across the tow cross section.7 The higher-temperature PBN coatings had the great￾est variability. The outer layer of fibers could have a coating up to a 3 mm thick, whereas the interior fibers in a tow had much-thinner (∼0.4 mm) and more-uniform coatings (Table I). An average value for the coating thickness was used in the calculations, based on the weight gain of the PBN. The minicomposites were mounted with epoxy to cardboard tabs, as described in other studies.7,11 The epoxy section of the minicomposite was gripped with pneumatic grips, and acoustic emission (AE) sensors were attached to the epoxy with alliga￾tor clips. The length of the minicomposites between the card￾board edges ranged from 60 to 150 mm. Tensile testing was performed on a universal testing machine (Model 4502, In￾stron, Canton, MA). Tensile unload–reload hysteresis loops were performed with increasing loads until the minicomposite failed. The displacements of the upper and lower cardboard tabs were monitored with a laser extensometer (Model Zygo 1100, Zygo Corp., Middlefield, CT). The strain was deter￾mined from the difference between the upper-tab and lower-tab edge displacements divided by the length of the minicompos￾ites between the edges. The energy of the acoustic events was monitored with an AE analyzer (Model LOCAN 320, Physical Acoustics, Princeton, NJ). The AE analyzer also included a computer, which collected the load, strain, and AE data. Be￾cause of the gripping method and attachment of AE transducers to the epoxy, significant bending moments could result near the grips. The minicomposites often failed in this region at stresses lower than those achieved for gauge failures. Only samples that failed in the gauge section were used to determine the ultimate properties. However, tensile hysteresis analysis could still be performed with samples that failed prematurely (near the epoxy). Each individual minicomposite that failed was cut into sev￾eral ∼30 mm lengths, mounted in epoxy, and polished longi￾tudinally to a 1 mm finish to determine the matrix-crack spac￾ing. More than half of the gauge section was used to determine the matrix-crack spacing. For some of the Syl-PBN minicom￾posites, the cracks were not always observable after polishing and required etching in a boiling Muriakami’s solution. Matrix cracks in the Nic-3MBN and HN-PBN minicomposites were easily observed after the longitudinal polishing. III. Results (1) Tensile-Test Results Typical examples of stress–strain hysteresis loop tests are shown in Fig. 1 for the Nic-3MBN and Syl-PBN minicompos￾ites. The stress–strain behavior of the HN-PBN minicompos￾ites is not shown for clarity but would be located between that of the two minicomposite types shown in Fig. 1. The composite stress was determined by multiplying the fiber volume fraction by the stress on the fibers if fully loaded (failure load divided by total fiber area). Figure 2 shows the final hysteresis loop just prior to failure for the three minicomposite types that were tested in this study. The fiber volume fraction for the Nic- 3MBN, HN-PBN, and Syl-PBN minicomposites used for com￾parison in this study (Figs. 2–6) were 0.12, 0.16, and 0.16, respectively. The ultimate stresses of the HN-PBN and Syl-PBN mini￾composites were approximately the same (∼450 MPa), whereas the ultimate stress of the Nic-3MBN minicomposites was ∼310 MPa. Based on the number of fibers per tow, these strengths correspond to an average fiber strength of ∼2.8 GPa for the Sylramic fibers, 2.75 GPa for the Hi-Nicalon fibers, and 2.4 GPa for the Nicalon fibers, assuming that the fibers were bear￾ing the full load just prior to failure. The as-produced fiber strengths were ∼2.8–3.0 GPa for all three of these fiber types. Table I. Data for Single-Tow Minicomposites That Have Been Tested Minicomposite† Fiber diameter (mm) Elastic modulus (GPa) Volume fraction of fibers, vf ‡ Average interphase thickness§ Fiber Matrix (mm) Nic/3MBN/SiC 14 200 400 0.07–0.12 0.5 (0.4–1) Hi-Nic/PBN/SiC 13 280 400 0.13–0.21 0.75 (0.4–3) Syl/PBN/SiC 9 380 400 0.12–0.19 0.45 (0.3–2) † The assembly of the minicomposite is given in the format of fiber/interphase/matrix. Abbreviations in this format denote the following materials: ‘‘Nic’’ 4 Nicalon fiber and ‘‘Hi-Nic’’ 4 Hi-Nicalon fiber (Nippon Carbon, Tokyo, Japan); ‘‘Syl’’ 4 Sylramic polycrys￾talline SiC fiber (Dow Corning, Midland, MI); ‘‘3MBN’’ 4 low-temperature (1050°C)-deposited BN (The 3M Corp., St. Paul, MN); PBN 4 1400°C-deposited pyrolytic BN (Advanced Ceramics, Cleveland, OH); and ‘‘SiC’’ 4 the CVI-SiC matrix (B. F. Goodrich, Brecksville, OH). ‡ Determined from the mass and density of the fiber tow, BN interphase, and CVI-SiC matrix. § Values given in parentheses are the range of interphase thickness. Fig. 1. Typical stress–strain curves for the minicomposites. 146 Journal of the American Ceramic Society—Morscher and Martinez-Fernandez Vol. 82, No. 1