ournal In. Ceran. Soc, 82[1]1415-55(1999) Fiber Effects on minicomposite mechanical Properties for Several Silicon Carbide Fiber-Chemically Vapor-Infiltrated Silicon Carbide Matrix Systems Gregory N Morscher, t Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio Julian Martinez-Fernandez Departamento de Fisica de la Materia Condensada, University of Seville, Seville, Spain Several different types of SiC fiber tows were coated with fiber tests while mimicking, to some degree, the larger-scale BN and composited using chemically vapor- infiltrated Sic macrocomposite tensile behavior. Single-fiber tests have been to form single-tow minicomposites. The types of SiC fib used to determine the interfacial properties for a Nicalon fiber/ included NicalonM, Hi-Nicalon'M, and the new SyiramieT CVI-SiC matrix system, a CVD-fiber/glass-matrix system, 0 were determined from unload-reload tensile hysteresis- studies, the model of Marshall 2 for fiber pullout was used to loop tests. The ultimate stress and strain properties also determine the interfacial properties of the fiber/matrix system. were determined for the minicomposites. The ultimate Also, the effect of fiber roughness on fiber sliding was deter- strengths of the newer Hi-Nicalon and Sylramic fibers were mined for the CVD-SiC/glass matrix system superior to that of Nicalon minicomposites with similar Because macrocomposite are composed of several tows in iber volume fractions. The Sylramic minicomposites had the loading direction, the minicomposite represents a subele- strength, respectively, because of the high modulus of the of minicomposites is very similar to that of macrocomposite, fiber and the rough surface of this fiber type. The apparent because the interfacial and elastic properties of the constituents interfacial shear strength increased as the stress increased are the same. The matrix and fibers also have different flaw for the Sylramic minicomposites, which also was attributed distributions, as in a macrocomposite, which account for, in to the surface roughness of this fiber accordance with the interfacial properties, the stress-dependent cracking behavior and fiber failure prior to ultimate failure L. Introduction The aspect of a macrocomposite that is foreign to minicom- posites are 90 plies, which are often low-stress crack-initiation composites to be used at temperatures >1200%C, fibers with(tunnel cracking)intersect load-bearing tows. Therefore,the better creep and rupture properties than ceramic-grade(CG absolute stress-strain behavior of minicomposites is expected to differ from macrocomposite, because of the differences in able' Fiber development has been underway, and several ven- crack morphology that result from differences in matrix-flaw dors are or will be offering higher-use-temperature SiC-based distributions and fiber architecture fibers. 2-4 Currently, SylramicTM and Hi-Nicalon TM fibers are substantially more expensive than CG NicalonTM. It is es of fiber and similar interphases and matrices and then that. as these fibers find greater use ca In this study, minicomposites were fabricated with different their cost will also de- tensile tested at room temperature, to determine the effect of crease. There is a current need to assess the effect of different fiber properties on the SIiC /BN,SiCm system. Even though the fibers for a given composite system in a cost-effective manner. absolute stress-strain behaviors of minicomposites are ex- One way to accomplish this is to fabricate and test single-tow pected to differ from macrocomposite that have been made minicomposites. Minicomposite fabrication and testing are es- with the same constituents the relative difference in stress. pecially practical for ceramic composite systems that use strain behavior of the different fiber-type minicomposites is chemically vapor infiltrated(CVI) interphases and matrices expected to translate to macrocomposite stress-strain behavior such as in many SiC fiber/BN interphase/SiC matrix systems This test approach has already been used to determine roor Il. Experimental Procedure temperature interfacial and ultimate tensile properties,6 and high-temperature stress rupture, and cyclic stress properties Single-fiber-tow composites were processed in the same way in ambient air for several fiber/interphase/matrix composite as that described by Morscher. A single-fiber tow was coate with a BN interphase, and then the coated tow was infiltrated The minicomposite test incorporates the analysis of single with SiC. The three different minicomposites are listed in Table I, with pertinent physical rties of the composite components. Note that the Bn interphase for Nicalon-fiber ites was processed by a different vendor than the K.T. Faber--contributing editor Hi-Nicalon-fiber and Sylramic-fiber minicomposites. There are two reasons for this. The nicalon fibers were coated for an earlier study and at the time when the Hi-Nicalon and syl- ramic fibers were coated. the vendor who coated the nicalon lo. 190649. Received October 6, 1997, approved April 24, 1998. fibers was not in the fiber-coating business anymore. also A-Lewis Research Center. Cleveland OH Nicalon could not be coated with the BN coating applied to the 44135-3127 Hi-Nicalon or Sylramic fibers because the BN
Fiber Effects on Minicomposite Mechanical Properties for Several Silicon Carbide Fiber–Chemically Vapor-Infiltrated Silicon Carbide Matrix Systems Gregory N. Morscher*,† Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio Julian Martinez-Fernandez Departamento de Fisica de la Materia Condensada, University of Seville, Seville, Spain Several different types of SiC fiber tows were coated with BN and composited using chemically vapor-infiltrated SiC to form single-tow minicomposites. The types of SiC fiber included Nicalon™, Hi-Nicalon™, and the new Sylramic™ polycrystalline SiC fiber. The interfacial shear stresses were determined from unload–reload tensile hysteresisloop tests. The ultimate stress and strain properties also were determined for the minicomposites. The ultimate strengths of the newer Hi-Nicalon and Sylramic fibers were superior to that of Nicalon minicomposites with similar fiber volume fractions. The Sylramic minicomposites had the lowest strain to failure and highest interfacial shear strength, respectively, because of the high modulus of the fiber and the rough surface of this fiber type. The apparent interfacial shear strength increased as the stress increased for the Sylramic minicomposites, which also was attributed to the surface roughness of this fiber. I. Introduction I T IS evident that, in order for SiC-fiber-reinforced ceramic composites to be used at temperatures >1200°C, fibers with better creep and rupture properties than ceramic-grade (CG) Nicalon™ (Nippon Carbon, Tokyo, Japan) need to be available.1 Fiber development has been underway, and several vendors are or will be offering higher-use-temperature SiC-based fibers.2–4 Currently, Sylramic™ and Hi-Nicalon™ fibers are substantially more expensive than CG Nicalon™. It is expected that, as these fibers find greater use, their cost will also decrease. There is a current need to assess the effect of different fibers for a given composite system in a cost-effective manner. One way to accomplish this is to fabricate and test single-tow minicomposites.5 Minicomposite fabrication and testing are especially practical for ceramic composite systems that use chemically vapor infiltrated (CVI) interphases and matrices such as in many SiC fiber/BN interphase/SiC matrix systems. This test approach has already been used to determine roomtemperature interfacial and ultimate tensile properties5,6 and high-temperature stress rupture7,8 and cyclic stress8 properties in ambient air for several fiber/interphase/matrix composite systems. The minicomposite test incorporates the analysis of singlefiber tests while mimicking, to some degree, the larger-scale macrocomposite tensile behavior. Single-fiber tests have been used to determine the interfacial properties for a Nicalon fiber/ CVI-SiC matrix system,9 a CVD-fiber/glass-matrix system,10 and a CVD-SiC fiber/CVI-SiC matrix system.11 For all these studies, the model of Marshall12 for fiber pullout was used to determine the interfacial properties of the fiber/matrix system. Also, the effect of fiber roughness on fiber sliding was determined for the CVD-SiC/glass matrix system.10 Because macrocomposites are composed of several tows in the loading direction, the minicomposite represents a subelement of the macrocomposite. The tensile stress–strain behavior of minicomposites is very similar to that of macrocomposites,5 because the interfacial and elastic properties of the constituents are the same. The matrix and fibers also have different flaw distributions, as in a macrocomposite, which account for, in accordance with the interfacial properties, the stress-dependent cracking behavior and fiber failure prior to ultimate failure. The aspect of a macrocomposite that is foreign to minicomposites are 90° plies, which are often low-stress crack-initiation sites13 and non-through-thickness cracking that may or may not (tunnel cracking14) intersect load-bearing tows. Therefore, the absolute stress–strain behavior of minicomposites is expected to differ from macrocomposites, because of the differences in crack morphology that result from differences in matrix-flaw distributions and fiber architecture. In this study, minicomposites were fabricated with different types of fiber and similar interphases and matrices and then tensile tested at room temperature, to determine the effect of fiber properties on the SiCf /BNi /SiCm system. Even though the absolute stress–strain behaviors of minicomposites are expected to differ from macrocomposites that have been made with the same constituents, the relative difference in stress– strain behavior of the different fiber-type minicomposites is expected to translate to macrocomposite stress–strain behavior. II. Experimental Procedure Single-fiber-tow composites were processed in the same way as that described by Morscher.7 A single-fiber tow was coated with a BN interphase, and then the coated tow was infiltrated with SiC. The three different minicomposites are listed in Table I, with pertinent physical properties of the composite components. Note that the BN interphase for Nicalon-fiber minicomposites was processed by a different vendor than the Hi-Nicalon-fiber and Sylramic-fiber minicomposites. There are two reasons for this. The Nicalon fibers were coated for an earlier study, and at the time when the Hi-Nicalon and Sylramic fibers were coated, the vendor who coated the Nicalon fibers was not in the fiber-coating business anymore. Also, Nicalon could not be coated with the BN coating applied to the Hi-Nicalon or Sylramic fibers because the BN processing temK. T. Faber—contributing editor Manuscript No. 190649. Received October 6, 1997; approved April 24, 1998. *Member, American Ceramic Society. † Resident Research Associate at NASA–Lewis Research Center, Cleveland, OH 44135–3127. J. Am. Ceram. Soc., 82 [1] 145–55 (1999) Journal 145
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(near
perature 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), HiNicalon-fiber (HN-PBN), and Sylramic-fiber (Syl-PBN) minicomposites, respectively. The fiber volume fraction for the minicomposites was determined 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 greatest 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 alligator clips. The length of the minicomposites between the cardboard edges ranged from 60 to 150 mm. Tensile testing was performed on a universal testing machine (Model 4502, Instron, 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 determined from the difference between the upper-tab and lower-tab edge displacements divided by the length of the minicomposites 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. Because 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 several ∼30 mm lengths, mounted in epoxy, and polished longitudinally to a 1 mm finish to determine the matrix-crack spacing. More than half of the gauge section was used to determine the matrix-crack spacing. For some of the Syl-PBN minicomposites, 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 minicomposites. The stress–strain behavior of the HN-PBN minicomposites 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 comparison 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 minicomposites 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 bearing 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 polycrystalline 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
January 1999 Fiber Effects on Minicomposite Mechanical Properties for Several SiC/cvl-SiC Matrix System 班Nic-PBN Syl-PBN 50u Se 100 Nic-3MBN 00203040.50.6070B08 Fig. 2. Hysteresis loops for three minicomposites with different types of Sic fiber just prior to failure Evidently, the Nicalon fibers were damaged during the Cvi SiC step to a greater extent than the other fibers, as has been observed by other researchers. 8 It will be shown below that the 4015 crack spacing and interfacial shear strength are similar for the Nicalon and Hi-Nicalon minicomposites, therefore, the Fig 4. SEM micrograph of the fracture surface of the Nicalon/ strength comparison between the two minicomposite systems is 3MBN/SiC minicomposite reasonable, because the actual fiber length at peak stress-the gauge length'-is approximately the same. The Hi-Nicalon and Sylramic fibers did not incur very much, if any, strength would account for this wide scatter in the measured elastic loss after minicomposite fabrication modulu The scatter in the measured elastic modulus was approxi- The hysteresis-loop modulus, ultimate strain, and hysteresis- mately the same for all three minicomposite types(at least five loop widths were very different for the three different mini- amples for each minicomposite type were tested) and ranged composites; this is most easily observed in Fig. 2. Just prior to from 330 to 380 GPa, even though the different minicompos- failure, the hysteresis-loop modulus was 175 GPa for Syl-PBN ites consisted of fibers with different moduli. The expected 74 GPa for HN-PBN, and 47 GPa for Nic-3MBN minicom- elastic moduli(rule of mixtures)and scatter due to the range of posites. The strain to failure was.27% for Syl-PBN,0.76% fiber volume fractions for each minicomposite system would for HN-PBN, and 0.9% for Nic-3MBN minicomposites. The hysteresis-loop widths were significantly larger for the HN- PBN, and Syl-PBN minicomposites, respectively. The mea- PBN and Nic-3MBN minicomposites, compared to the Syl sured values are lower and the scatter in measured values is PBN minicomposite. All these results were expected, consid- significantly larger than what would be expected. The experi ering the fiber moduli and roughnesses, as discussed below mental setup that is used in this study does introduce some The permanent deformation on unloading(Fig. 1)for the error in measuring the displacement at low strains, because Nic-3MBN minicomposite was very large(-0.2% for a peak some self-alignment occurs for the samples and grips, which stress of-320 MPa). Some of the permanent deformation could 350000 350000 军3000Nic-3MBN Syl NiC-3MBN PBN 250000 250000 Hi-NiC- 150Dn PBN 150000 世 100000 100000 Hi-Nic- PBN 1 50000 Syl-PBN 0 100150200250300350400450500001020.3040508070809 Composite Stress, MPa Composite Strain, Fig 3. Acoustic emission(AE) for different minicomposites
Evidently, the Nicalon fibers were damaged during the CVISiC step to a greater extent than the other fibers, as has been observed by other researchers.18 It will be shown below that the crack spacing and interfacial shear strength are similar for the Nicalon and Hi-Nicalon minicomposites; therefore, the strength comparison between the two minicomposite systems is reasonable, because the actual fiber length at peak stress—the ‘‘gauge length’’—is approximately the same. The Hi-Nicalon and Sylramic fibers did not incur very much, if any, strength loss after minicomposite fabrication. The scatter in the measured elastic modulus was approximately the same for all three minicomposite types (at least five samples for each minicomposite type were tested) and ranged from 330 to 380 GPa, even though the different minicomposites consisted of fibers with different moduli. The expected elastic moduli (rule of mixtures) and scatter due to the range of fiber volume fractions for each minicomposite system would be 381 ± 5, 379 ± 5, and 397 ± 1 GPa for Nic-3MBN, HNPBN, and Syl-PBN minicomposites, respectively. The measured values are lower and the scatter in measured values is significantly larger than what would be expected. The experimental setup that is used in this study does introduce some error in measuring the displacement at low strains, because some self-alignment occurs for the samples and grips, which would account for this wide scatter in the measured elastic modulus. The hysteresis-loop modulus, ultimate strain, and hysteresisloop widths were very different for the three different minicomposites; this is most easily observed in Fig. 2. Just prior to failure, the hysteresis-loop modulus was 175 GPa for Syl-PBN, 74 GPa for HN-PBN, and 47 GPa for Nic-3MBN minicomposites. The strain to failure was ∼0.27% for Syl-PBN, 0.76% for HN-PBN, and 0.9% for Nic-3MBN minicomposites. The hysteresis-loop widths were significantly larger for the HNPBN and Nic-3MBN minicomposites, compared to the SylPBN minicomposite. All these results were expected, considering the fiber moduli and roughnesses, as discussed below. The permanent deformation on unloading (Fig. 1) for the Nic-3MBN minicomposite was very large (∼0.2% for a peak stress of ∼320 MPa). Some of the permanent deformation could Fig. 2. Hysteresis loops for three minicomposites with different types of SiC fiber just prior to failure. Fig. 3. Acoustic emission (AE) for different minicomposites. Fig. 4. SEM micrograph of the fracture surface of the Nicalon/ 3MBN/SiC minicomposite. January 1999 Fiber Effects on Minicomposite Mechanical Properties for Several SiC/CVI-SiC Matrix Systems 147
Journal of the American Ceramic Sociery-Morscher and Martines-Fernandes Vol. 82. No (b) 0.5mm Matrix Cracks Fig. 5. SEM micrographs of the fracture surfaces of the Hi-Nicalon/PBN/CVI-SiC m incOmposite( (a)matrix crack 300 um away from the fracture urface and(b) matrix crack 1200 um away from the fracture surface) 868428K Fig. 6. SEM micrographs of the fracture surfaces of the Sylramic/PBN/SiC minicomposite((a) matrix crack is <100 um from the fracture surface Figs. 6(b)and (c)show that fiber pullout occurs throughout the minicomposite)
Fig. 5. SEM micrographs of the fracture surfaces of the Hi-Nicalon/PBN/CVI-SiC minicomposite ((a) matrix crack 300 mm away from the fracture surface and (b) matrix crack 1200 mm away from the fracture surface). Fig. 6. SEM micrographs of the fracture surfaces of the Sylramic/PBN/SiC minicomposite ((a) matrix crack is <100 mm from the fracture surface; Figs. 6(b) and (c) show that fiber pullout occurs throughout the minicomposite). 148 Journal of the American Ceramic Society—Morscher and Martinez-Fernandez Vol. 82, No. 1
January 1999 Fiber Efects on Minicomposite Mechanical Properties for Several SiC/Cvl-SiC Matrix System associated with the relief of residual stres fiber did seem to show some pullout(Figs. 6(b)and(c)). The mposite with matrix cracking. The fiber had a thermal ex- Sylramic fibers that did pull out a longer distance(Fig. 6(b)) pansion coefficient of-3. 1 x 10-b/C, according to the product were always the outer-tow fibers, which had a thicker BN literature from the manufacturer for CG on. Assumin ayer. that the matrix has a thermal expansion coefficient of -4 Another observation is the presence of matrix cracks near the 10/C and the minicomposite was processed at-1025C, the fracture surface. For a HN-PBN minicomposite, the nearest anent deformation due to full decoupling of the fiber from matrix cracks to the fracture surface were -300 and 1200 um matrix would result in a permanent strain of-0.1% away from the fracture surface(Figs. 5(a)and(b). For a Syl value, of course, is an overestimate, because complete PBN minicomposite, the matrix-crack spacing is very smal rever. it is evident that more th (<100 um) near the fracture surface( Fig. 6(a)) of the permanent deformation measured for the Nic minicomposite is not from the relief of residual stress in the (3) Determination of the Matrix-Crack Spacing gauge section of the minicomposite. Instead, at least half of the Polished minicomposite longitudinal sections were used anent deformation is associated with excessive damage determine the average crack spacing after failure. Figure 7 and alignment that occurs at the tabs because of the gripping shows examples, at different of the hN-pbn rrangement. This result is also probably the cause of the lar and Syl-PBN minicomposites. The crack spacings near the d decreases that occur in the Nic-3MBN stress-strain curve fracture surface are indicative of those measured at higher stresses(strains)in Fig. 1. Less permanent deforma- length of the minicomposites. The average crack spac tion was observed for the other two systems(Fig. 2) determined from the number of cracks and the length of mini Figure 3 shows typical aE data for the three minicomposite omposite examined, as listed in Table II types, as a function of minicomposite stress and strain. The AE The Nic-3MBN and HN-PBN minicomposites had fairly energy can be attributed to matrix cracking. The first matrix ell-distributed cracks; i.e., the spacing between two indi- cracking stress and strain for the different minicomposites was vidual cracks ranged from 0. 1 mm to -3 mm along the entire determined from the onset of AE activity(Fig. 3)and is tabu- minicomposite length observed. The Syl-PBN minicomposite was entirely different. Most individual crack spacings were are approximately the same for the three minicomposite sys- very small (Fig. 7(b))or very large along the length of the tems. At -0.04% strain, there is a significant deviation fro con osite. There were several regions of the minicomposite linearity on the stress-strain curve for the Nic-3MBN(Fig. 1) where dozens of cracks were spaced <0. 1 mm apart. There and HN-PBN(not shown) minicomposites; however, for the were other regions where cracks were separated by large dis Syl-PBN minicomposites, there is no significant deviation tances, the largest separation being 9 mm. In fact, for the Syl- from linearity until a strain of-0 1% is reached. The aE data PBN minicomposite, -92% of the cracks occurred in regions so implies that the rate of cracking with stress is much greater for Nic-3MBN minicomposites, at least at the onset of crack observed. In other words, there was very little cracking in ing. The HN-PBN and Syl-PBN minicomposites have similar >70%of the sample, which indicates that this system is far AE behavior(cracking trends) with stress, however, the Syl- from matrix-crack saturation the different crack. ng dis- PBN minicomposite has a rapid crack-growth regime for tributions are shown in Fig. 8 for the HN-PBN and Syl-PBN pared to that of the HN-PBN minicompos- minicomposit e. The rate of AE energy for the Nic-3MBN minicomposite decreases at -0 32% strain(-200 MPa stress). The HN-PBN IV. Analysis and Syl-PBN minicomposites do not show a decreasing rate o AE activity. All three minicomposit aturate in matrix cracks prior to the failure load a (1) Determination of the Interfacial Shear Stress fro Hysteresis Loops Scanning electron microscopy(SEM) micrographs of typical Lamon hysteresis loops were analyzed using the approach of (2) Examination of the Fracture Surface aly and v PBN, respectively ) Significant pullout lengths were observed for the Nic-3MBN and HN-3PBN minicomposites, whereas the da Syl-PBN minicomposites only had very short pullout lengths The Nic-3MBN fracture surfaces were always more frag- mented and jagged, in comparison to the other two minicom where de is the hysteresis loop width, o the peak stress of the posite types. It seems that the Sic did not infiltrate as well into hysteresis loop, o the stress where de is measured, and o min the the tow, compared to the other two minicomposite types, which minimum stress of the hysteresis loop. The inelastic strain in- resulted in the appearance of a thicker SiC"sheath"that sur dex, is determined from the relationship rounded the infiltrated tow. This phenomenon made it difficult to determine the pullout length for the Nic-3MBN minicom -a0(8) posite. The pullout-length distributions were determined for the HN-PBN and Syl-PBN minicomposite fracture surfaces. The d=b 4T-TE mean pullout lengths were 240 and 3 um for the HN-PBN and Syl-PBN minicomposites, respectively. Even though the Syl- where R is the average fiber radius, d the average matrix-crack PBN minicomposite fiber pullout is very small, almost ever spacing(number of cracks divided by minicomposite length), f Table Il. Mechanical Properties of the minicom crack Estimated interfacial Ultimate stress shear stress(MPa) Nic/3MBN Hi-Nic/PBN 15±10 -14 0.035 440±20 65-175 0.035 450±20
be associated with the relief of residual stresses in the minicomposite with matrix cracking. The fiber had a thermal expansion coefficient of ∼3.1 × 10−6/°C, according to the product literature from the manufacturer for CG Nicalon. Assuming that the matrix has a thermal expansion coefficient of ∼4 × 10−6/°C and the minicomposite was processed at ∼1025°C, the permanent deformation due to full decoupling of the fiber from the matrix would result in a permanent strain of ∼0.1%. This value, of course, is an overestimate, because complete decoupling does not occur; however, it is evident that more than half of the permanent deformation measured for the Nic-3MBN minicomposite is not from the relief of residual stress in the gauge section of the minicomposite. Instead, at least half of the permanent deformation is associated with excessive damage and alignment that occurs at the tabs because of the gripping arrangement. This result is also probably the cause of the large load decreases that occur in the Nic-3MBN stress–strain curve at higher stresses (strains) in Fig. 1. Less permanent deformation was observed for the other two systems (Fig. 2). Figure 3 shows typical AE data for the three minicomposite types, as a function of minicomposite stress and strain. The AE energy can be attributed to matrix cracking.11 The first matrixcracking stress and strain for the different minicomposites was determined from the onset of AE activity (Fig. 3) and is tabulated in Table II. The first matrix-cracking stresses and strains are approximately the same for the three minicomposite systems. At ∼0.04% strain, there is a significant deviation from linearity on the stress–strain curve for the Nic-3MBN (Fig. 1) and HN-PBN (not shown) minicomposites; however, for the Syl-PBN minicomposites, there is no significant deviation from linearity until a strain of ∼0.1% is reached. The AE data also implies that the rate of cracking with stress is much greater for Nic-3MBN minicomposites, at least at the onset of cracking. The HN-PBN and Syl-PBN minicomposites have similar AE behavior (cracking trends) with stress; however, the SylPBN minicomposite has a rapid crack-growth regime for strains >0.1%, compared to that of the HN-PBN minicomposite. The rate of AE energy for the Nic-3MBN minicomposite decreases at ∼0.32% strain (∼200 MPa stress). The HN-PBN and Syl-PBN minicomposites do not show a decreasing rate of AE activity. All three minicomposite systems probably do not saturate in matrix cracks prior to the failure load. (2) Examination of the Fracture Surface Scanning electron microscopy (SEM) micrographs of typical minicomposite fracture surfaces are shown in Figs. 4–6 for the three minicomposite types (Nic-3MBN, HN-PBN, and SylPBN, respectively). Significant pullout lengths were observed for the Nic-3MBN and HN-3PBN minicomposites, whereas the Syl-PBN minicomposites only had very short pullout lengths. The Nic-3MBN fracture surfaces were always more fragmented and jagged, in comparison to the other two minicomposite types. It seems that the SiC did not infiltrate as well into the tow, compared to the other two minicomposite types, which resulted in the appearance of a thicker SiC ‘‘sheath’’ that surrounded the infiltrated tow. This phenomenon made it difficult to determine the pullout length for the Nic-3MBN minicomposite. The pullout-length distributions were determined for the HN-PBN and Syl-PBN minicomposite fracture surfaces. The mean pullout lengths were 240 and 3 mm for the HN-PBN and Syl-PBN minicomposites, respectively. Even though the SylPBN minicomposite fiber pullout is very small, almost every fiber did seem to show some pullout (Figs. 6(b) and (c)). The Sylramic fibers that did pull out a longer distance (Fig. 6(b)) were always the outer-tow fibers, which had a thicker BN layer. Another observation is the presence of matrix cracks near the fracture surface. For a HN-PBN minicomposite, the nearest matrix cracks to the fracture surface were ∼300 and 1200 mm away from the fracture surface (Figs. 5(a) and (b)). For a SylPBN minicomposite, the matrix-crack spacing is very small (<100 mm) near the fracture surface (Fig. 6(a)). (3) Determination of the Matrix-Crack Spacing Polished minicomposite longitudinal sections were used to determine the average crack spacing after failure. Figure 7 shows examples, at different magnifications, of the HN-PBN and Syl-PBN minicomposites. The crack spacings near the fracture surface are indicative of those measured along the length of the minicomposites. The average crack spacing was determined from the number of cracks and the length of minicomposite examined, as listed in Table II. The Nic-3MBN and HN-PBN minicomposites had fairly well-distributed cracks; i.e., the spacing between two individual cracks ranged from 0.1 mm to ∼3 mm along the entire minicomposite length observed. The Syl-PBN minicomposite was entirely different. Most individual crack spacings were very small (Fig. 7(b)) or very large along the length of the composite. There were several regions of the minicomposite where dozens of cracks were spaced <0.1 mm apart. There were other regions where cracks were separated by large distances, the largest separation being 9 mm. In fact, for the SylPBN minicomposite, ∼92% of the cracks occurred in regions where the total length was ∼30% of the minicomposite length observed. In other words, there was very little cracking in >70% of the sample, which indicates that this system is far from matrix-crack saturation. The different crack-spacing distributions are shown in Fig. 8 for the HN-PBN and Syl-PBN minicomposites. IV. Analysis (1) Determination of the Interfacial Shear Stress from Hysteresis Loops The hysteresis loops were analyzed using the approach of Lamon et al.9 and Vagaggini et al.19 The hysteresis loop width was related to stress for various peak-stress hysteresis loops as follows: d« sp 2 = 2+S s sp − smin sp DS1 − s sp D (1) where d« is the hysteresis loop width, sp the peak stress of the hysteresis loop, s the stress where d« is measured, and smin the minimum stress of the hysteresis loop. The inelastic strain index, +, is determined from the relationship + = b2 ~1 − a1 f! 2 S Rf d D 4f 2 tEm (2) where Rf is the average fiber radius, d the average matrix-crack spacing (number of cracks divided by minicomposite length), f Table II. Mechanical Properties of the Minicomposites Minicomposite Average crack spacing† (mm) Estimated interfacial shear stress (MPa) First cracking Ultimate stress (MPa) Stress (MPa) Strain (%) Nic/3MBN 0.45 25 ± 10 ∼140 0.035 310 ± 20 Hi-Nic/PBN 0.58 15 ± 10 ∼140 0.035 440 ± 20 Syl/PBN 0.34 65–175 ∼140 0.035 450 ± 20 † At failure. January 1999 Fiber Effects on Minicomposite Mechanical Properties for Several SiC/CVI-SiC Matrix Systems 149