./. Appl. Ceram. Technol, 7/3/276-290(2010) DO:10IJI174-7402.200902422x International Journal o pplied Ceramic TECHNOLOGY ceramic Product D Effects of Fiber Architecture on Matrix Cracking for Melt-Infiltrated SiC/SiC Composites Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio 44142 James A DiCarlo and James D. Kiser NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135 Hee Mann Yun Matech GSM, 31304 Via Colinas, Suite 102, Westlake Village, California 91362 The matrix cracking behavior of slurry cast melt-infiltrated SiC matrix composites consisting of Sylramic-iBN fibers with wide variety of fiber architectures were compared. The fiber architectures included 2D woven, braided, 3D orthogonal, and angle interlock architectures. Acoustic emission was used to monitor in-plane matrix cracking during unload-reload tensile tests. Two key parameters were found to control matrix-cracking behavior: the fiber volume fraction in the loading direction and the area of the weakest portion of the structure, that is, the largest tow in the architecture perpendicular to the loading direction. Empirical models that support these results are presented and discussed. Introduction originally started as a NASA Glenn IRD Project and was continued Silicon carbide fiber-reinforced silicon carbide ce- under NASA's ARMD Supersonics program. ramic matrix composites (SiC/SiC CMC)are actively be ing pursued for high-temperature structural applications No daim to U.S. Government works. gine combustor liners, turbine components
Effects of Fiber Architecture on Matrix Cracking for Melt-Infiltrated SiC/SiC Composites Gregory N. Morscher* Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio 44142 James A. DiCarlo and James D. Kiser NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135 Hee Mann Yun Matech GSM, 31304 Via Colinas, Suite 102, Westlake Village, California 91362 The matrix cracking behavior of slurry cast melt-infiltrated SiC matrix composites consisting of Sylramic-iBN fibers with a wide variety of fiber architectures were compared. The fiber architectures included 2D woven, braided, 3D orthogonal, and angle interlock architectures. Acoustic emission was used to monitor in-plane matrix cracking during unload–reload tensile tests. Two key parameters were found to control matrix-cracking behavior: the fiber volume fraction in the loading direction and the area of the weakest portion of the structure, that is, the largest tow in the architecture perpendicular to the loading direction. Empirical models that support these results are presented and discussed. Introduction Silicon carbide fiber-reinforced silicon carbide ceramic matrix composites (SiC/SiC CMC) are actively being pursued for high-temperature structural applications such as engine combustor liners, turbine components, Int. J. Appl. Ceram. Technol., 7 [3] 276–290 (2010) DOI:10.1111/j.1744-7402.2009.02422.x Ceramic Product Development and Commercialization Funding for this work originally started as a NASA Glenn IRD project and was continued under NASA’s ARMD Supersonics program. *gregory.n.morscher@nasa.gov Journal compilation r 2009 The American Ceramic Society No claim to U.S. Government works
Efects of Fiber Architecture on SiC/SiC Composites 277 and exhaust nozzles. 2 These applications will require by the high-performance Sylramic-iBN SiC fiber cur- fiber architectures that can not only provide the com- rently represents the state-of-the-art in high-temperature ponent shape forming capability properties, but also the SiC/SiC composites because of its high-thermal and optimum in tensile strength, creep-rupture properties, structural performance at use temperatures beyond those and thermal conductivity in multiple directions. of current metallic alloys. The key in-plane mechanical Although conventional 2D woven lay-up architectures properties of interest are the elastic modulus, the onse offer a good degree of shape capability, they currently stress at which through-thickness matrix cracks forn ult in poorer CMC performance due to such issues as and the ultimate strength of as-fabricated panels at room reduced in-plane strength related to stress risers at ends temperature. Particular focus for this study was the onset low through-the-thickness tens matrIX cracking stress because it is above this stress that strength, shear strength, and thermal conductivity re- life-limiting environmental degradation of SiC compos- lated to the CMC need for weak fiber-matrix interfaces ites occurs at elevated(>600oC)temperature for matrix crack defection For this reason. more com Other studies on matrix cracking in 3D plex 3D weaves and braids with through-the-thickness SiC/SiC composites have focused on the chemically va- fiber reinforcement are currently being pursued. por infiltrated(CVI)matrix system" or the polymer-in At the current stage of development for SiC/Sic filtrated pyrolysis(PIP)-derived matrix system, both of CMC, it is thus important to develop an understanding those studies with low-modulus SiC-based fiber rein- of the fber-architecture effects on key properties in or- forcement. This study is distinguished in that high-mod der to better design the architectures both for shape and ulus polycrystalline SiC fibers were the primary fiber ent performance. Ty ners and reinforcement, a wider of fiber architectures were modelers will initially need fber-architecture compared including unbalanced fiber proportions in dif nability and associated composite processing models ferent directions, the composites were all of the MI va- in order to decide on what architectures can provide all riety, and the emphasis is on use and design based on the special shape features of the component. To matrix cracking stress. Unbalanced architectures were mize machining issues, such as added cost and fiber chosen in order to maximize the fiber content in one di- damage, it is generally desirable that SiC/SiC compo- rection, which is expected to be potentially desirable for nent be fabricated with near-net shape. With possible applications which require higher load-carrying ability in fiber architecture types in mind, designers would then one direction. In a future paper, it will be demonstrated need constituent properties and composite propert how improvements in the fiber architecture increased models in order to down-select which architecture stress capability at high temperatures in oxidizing envi- type would allow the component to best meet its struc ronments(G.N. morsche tural performance requirements. For high-temperature applications and thin-walled near net shaped compo- nents,a key CMC material design goal is to typically Experimental Procedure seek tensile strength and thermal conductivity through he component walls that are as high as possible in order The four SiC/SiC composite panels fabricated to withstand high thermal gradients. 3D woven and for this study were all processed with the slurry cast braided architectures offer enhanced capability in this Power Systems Composites, New regard by enabling directional tailoring of properties, ark, DE)0. CVI BN fiber coating, followed by CVI both in-plane and out-of-plane, using fibers of hi Sic infiltration (a few micrometers in thickness), SiC strength and high conductivity. However, adding fibers particulate slurry infiltration, and molten Si infiltra out-of-plane can sometimes degrade in-plane properties tion. These panels were tailored to be unbalanced where high structural performance is most needed with a higher fber volume fraction in one in-plane With these modeling needs in view, this study has direction than in the other orthogonal in-plane direc- sought to vary the fiber architecture and volume fraction tion. As-fabricated panel dimensions of the four pan- in five SiC/SiC slurry-cast melt-infiltrated(MI) panels in els were approximately 150 mm x 80 mm x 2mm order to measure the effects of multidirectional architec- Detailed descriptions of the as-produced architectures tures and fiber content on key in-plane mechanical prop- for these panels are shown in the top portion of erties. This ceramic composite system when reinforced Table I. In all cases, the in-plane X and/or Y fibers
and exhaust nozzles.1,2 These applications will require fiber architectures that can not only provide the component shape forming capability properties, but also the optimum in tensile strength, creep–rupture properties, and thermal conductivity in multiple directions. Although conventional 2D woven lay-up architectures offer a good degree of shape capability, they currently result in poorer CMC performance due to such issues as reduced in-plane strength related to stress risers at ends of plies,3 and to low through-the-thickness tensile strength, shear strength, and thermal conductivity related to the CMC need for weak fiber–matrix interfaces for matrix crack deflection. For this reason, more complex 3D weaves and braids with through-the-thickness fiber reinforcement are currently being pursued.4–6 At the current stage of development for SiC/SiC CMC, it is thus important to develop an understanding of the fiber-architecture effects on key properties in order to better design the architectures both for shape and final component performance. Typically, designers and process modelers will initially need fiber-architecture formability and associated composite processing models in order to decide on what architectures can provide all the special shape features of the component. To minimize machining issues, such as added cost and fiber damage, it is generally desirable that SiC/SiC component be fabricated with near-net shape. With possible fiber architecture types in mind, designers would then need constituent properties and composite property models in order to down-select which architecture type would allow the component to best meet its structural performance requirements. For high-temperature applications and thin-walled near net shaped components, a key CMC material design goal is to typically seek tensile strength and thermal conductivity through the component walls that are as high as possible in order to withstand high thermal gradients. 3D woven and braided architectures offer enhanced capability in this regard by enabling directional tailoring of properties, both in-plane and out-of-plane, using fibers of high strength and high conductivity. However, adding fibers out-of-plane can sometimes degrade in-plane properties where high structural performance is most needed. With these modeling needs in view, this study has sought to vary the fiber architecture and volume fraction in five SiC/SiC slurry-cast melt-infiltrated (MI) panels in order to measure the effects of multidirectional architectures and fiber content on key in-plane mechanical properties. This ceramic composite system when reinforced by the high-performance Sylramic-iBN SiC fiber currently represents the state-of-the-art in high-temperature SiC/SiC composites because of its high-thermal and structural performance at use temperatures beyond those of current metallic alloys. The key in-plane mechanical properties of interest are the elastic modulus, the onset stress at which through-thickness matrix cracks form, and the ultimate strength of as-fabricated panels at room temperature. Particular focus for this study was the onset matrix cracking stress because it is above this stress that life-limiting environmental degradation of SiC composites occurs at elevated (46001C) temperatures.7 Other studies on matrix cracking in 3D architecture SiC/SiC composites have focused on the chemically vapor infiltrated (CVI) matrix system8 or the polymer-in- filtrated pyrolysis (PIP)-derived matrix system,9 both of those studies with low-modulus SiC-based fiber reinforcement. This study is distinguished in that high-modulus polycrystalline SiC fibers were the primary fiber reinforcement, a wider variety of fiber architectures were compared including unbalanced fiber proportions in different directions, the composites were all of the MI variety, and the emphasis is on use and design based on matrix cracking stress. Unbalanced architectures were chosen in order to maximize the fiber content in one direction, which is expected to be potentially desirable for applications which require higher load-carrying ability in one direction. In a future paper, it will be demonstrated how improvements in the fiber architecture increased stress capability at high temperatures in oxidizing environments (G. N. Morscher, unpublished data). Experimental Procedure The four SiC/SiC composite panels fabricated for this study were all processed with the slurry cast MI technique (GE Power Systems Composites, Newark, DE)10,11: CVI BN fiber coating, followed by CVI SiC infiltration (a few micrometers in thickness), SiC particulate slurry infiltration, and molten Si infiltration. These panels were tailored to be unbalanced with a higher fiber volume fraction in one in-plane direction than in the other orthogonal in-plane direction. As-fabricated panel dimensions of the four panels were approximately 150 mm 80 mm 2 mm. Detailed descriptions of the as-produced architectures for these panels are shown in the top portion of Table I. In all cases, the in-plane X and/or Y fibers www.ceramics.org/ACT Effects of Fiber Architecture on SiC/SiC Composites 277
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Table I. Description of Different Architectures for MI SiC/SiC Panels Architecture description No. plies, test direction Tow epcm, test direction Panel thickness (mm) Fiber type (no. of combined tows), volume fraction and predominate angle to test direction X (warp) Y (fill) Z (stuffer) SiC/SiC panel type (reference) AI-UNI 1D angle interlock through-the-thickness, Y-aligned, unbalanced 6.5 11 2.0 ZMI and rayon o0.05, 901 Sylramic-iBN (1) fo 5 0.23, 01 3DO-Un-R 3D orthogonal, unbalanced Y 5 test direction 7 9.8 1.53 Sylramic-iBN (1) B0.10, 901 Sylramic-iBN (1) fo 5 0.28, 01 Rayon o0.05, 901 3DO-Un-Z 3D orthogonal, unbalanced Y 5 test direction 7 9.8 1.58 Sylramic-iBN (1) B0.10, 901 Sylramic-iBN (1) fo 5 0.27, 01 ZMI o0.05, 901 LTLAI 3D layer-to-layer angle interlock 3 5.5 0.96 Sylramic-iBN (1) B0.10, 901 Sylramic-iBN (1) fo 5 0.10, 01 MI SiC/SiC panels from previous studies 2D 5HS10,14 2D five-harness satin balanced 0/90 lay-up 4–8 4.9–8.7 1.5–2.2 Sylramic-iBN (1) fo 5 0.12–0.2, 01 Sylramic-iBN (1) 0.12–0.2, 901 2D 5HS DT10 2D five-harness satin (double tow), balanced 0/90 lay-up 8 3.9 2.1 Sylramic-iBN (2) fo 5 0.19, 01 Sylramic-iBN (2) 0.19, 901 Braid15 2D triaxial braid (double tow); cut/tooled into panel, X 5 axial, Y 5 hoop 5 test direction 4 4.7 1.8 Sylramic-iBN (2) B0.1, 901 Sylramic-iBN (2) fo 5 0.26, 7231 3DO-Bal-R-Y6 3D orthogonal, nearly balanced; Y 5 test direction 7 7.9 1.95 Sylramic (2) B0.18, 901 Sylramic (1) fo 5 0.20, 01 rayon o0.05, 901 3DO-Bal-Z-Y6 3D orthogonal, nearly balanced; Y 5 test direction 7 7.1 2.05 Sylramic (2) B0.18, 901 Sylramic (1) fo 5 0.17, 01 ZMI o0.05, 901 3DO-Bal-Z-X6 3D orthogonal, nearly balanced; X 5 test direction 8 3.9 2 Sylramic (2) fo 5 0.18, 01 Sylramic (1) 0.17, 901 ZMI o0.05, B901 ‘‘Double Tow’’ refers to two tows woven or braided together which means Nfibers/tow 5 1600; emboldened fo values refer to test direction. MI, melt-infiltrated. 278 International Journal of Applied Ceramic Technology—Morscher, et al. Vol. 7, No. 3, 2010
wwceramics. org/ACT Efects of Fiber Architecture on SiC/SiC Composites were the high-performance Sylramic-iBN SiC fibers, (0.5 um thick) and MI SiC matrix. For the prey which display high strength, high creep-rupture ous 3D orthogonal panels where rayon fibers were resistance,and high thermal conductivity. These fi- used, the preferred NASA conversion process for bers were originally produced as Sylramic SiC fibers in Sylramic-iBN preforms was not used because of initial 800 filament tow form by Dow Corning, Midland, MI. concern related to degradation of the architectures The tows were then heat treated at NASA either as in- Thus the in-plane fibers for these panels were only dividual tows or preferably within a textile-formed pre- Sylramic fibers which, other than a significant differ- form in a nitrogen-containing environment to convert ence in creep and rupture behavior, show essentially the Sylramic fibers into Sylramic-ibN fibers the same elastic and tensile strength behavior as the uch as the Ube Sylramic-iBN fibers when incorporated into MI SiC/ (Yamaguchi, Japan)"Tyranno ZMI"SiC and/or rayon SiC composites fiber tows were incorporated into the initial The densities of all of the par Imens architectures because of their low-modulus and better evaluated in this study were >2.85 g/cm. This implies ormability, for example, as the warp-weaver fibers. For that most of the porosity is resident within the tow those architectures that used the polymer-based rayon minicomposites(5% total porosity) typical of good fiber, significant degradation was observed in these MI infiltration as observed in Fig. I fibers due to its fugitive nature at the high fiber It should also be noted in Table i that in contrast to coating and matrix processing temperatures with only the other panels, the in-plane Sylramic-iBN fibers of the a remnant char remaining that was subsequently coated Braid panel were not oriented in the testing direction. with BN and SiC during matrix processing. The in situ For this panel, a 2D tri-axial braided architecture was architectures for the four SiC/SiC panels fabricated in first formed on a 50 mm diameter mandrel with the this study can be seen in Fig. 1 which shows tensile hoop fibers +67 from the axial direction. The ar- specimen cross-sections transverse to the test direction chitecture was then cut axially and laid up in tooling to Table I lists the key properties related to each of these form the final panel with the hoop fibers oriented +23 architectures from the Y loading direction As described in the top half of Table I, one panel Tensile specimens, 12.6 mm wide and 150 mm for this study, AI-UNI, was fabricated in order to ap- long(see Table I for average thickness values),were roach a unidirectional ID composite. The Al-UNI, an machined into a dogbone shape where the gage section angle interlock architecture was woven with a high frac- width and length were 10 and 40 mm, respectively tion of Sylramic-iBN fibers(0. 23)aligned in the fill The ends of the tensile bars were encased in a wire or test direction and both rayon and ZMI warp weaver mesh to alleviate grip stresses and bending moments hrough-the-thickness fibers(a small fraction of each). at and near the pneumatic pressure grips. Room tem- Two other panels, 3DO-Un-R and 3DO-Un-Z, were perature tensile tests were performed according to fabricated with 3D-orthogonal fber architectures where ASTM C1275 using a universal testing machine Sylramic fibers were woven in different proportions in (Model 8562, Instron, Canton, MA). Specimens were he warp and fill directions, and either rayon fibers or loaded at a constant rate( kN/min). Two clip-on strain ZMI fibers, respectively, were used as the through-thick- gages(2.5% max strain) were attached (one on each ness or Z direction fber tows. For these 3D orthogonal face)and the average of the two strain gages was used for panels, the final volume fractions of the Sylramic-ibn the tests. Unload-reload interruptions were also per fibers were - 0. 1 in the X direction, and -0 27 in the Y formed, usually at least two per test, in order to deter direction (test direction). The fifth panel, LTLAL, was a mine the residual compressive stress in the composite Sylramic-iBN layer-to-layer angle interlock consisting of three layers and a low fiber volume fraction in the yor Modal acoustic emission was also monitored dur- est direction ing the room temperature tensile tests. A Digital Wave Also shown in the bottom half of Table I are Fracture Wave Detector with two wide-band pass(50- MI SiC/SiC panels from earlier studies that will be 2000 kHz) frequency sensors also from Digital Wave used for property comparison and model development Corporation (Model B1025, Englewood, CO)was in this study. All of these panels were processed in the used. The two AE sensors were placed on the face of same manner regarding the BN fiber coating the specimen, one on each side of the gage section,ap-
were the high-performance Sylramic-iBN SiC fibers, which display high strength, high creep–rupture resistance, and high thermal conductivity.12 These fi- bers were originally produced as Sylramict SiC fibers in 800 filament tow form by Dow Corning, Midland, MI. The tows were then heat treated at NASA either as individual tows or preferably within a textile-formed preform in a nitrogen-containing environment to convert the Sylramic fibers into Sylramic-iBN fibers.13 In some cases, other fiber types such as the Ube (Yamaguchi, Japan) ‘‘Tyranno ZMI’’ SiC and/or rayon fiber tows were incorporated into the initial architectures because of their low-modulus and better formability, for example, as the warp-weaver fibers. For those architectures that used the polymer-based rayon fiber, significant degradation was observed in these fibers due to its fugitive nature at the high fibercoating and matrix processing temperatures with only a remnant char remaining that was subsequently coated with BN and SiC during matrix processing. The in situ architectures for the four SiC/SiC panels fabricated in this study can be seen in Fig. 1 which shows tensile specimen cross-sections transverse to the test direction. Table I lists the key properties related to each of these architectures. As described in the top half of Table I, one panel for this study, AI-UNI, was fabricated in order to approach a unidirectional 1D composite. The AI-UNI, an angle interlock architecture was woven with a high fraction of Sylramic-iBN fibers (B0.23) aligned in the fill or test direction and both rayon and ZMI warp weaver through-the-thickness fibers (a small fraction of each). Two other panels, 3DO-Un-R and 3DO-Un-Z, were fabricated with 3D-orthogonal fiber architectures where Sylramic fibers were woven in different proportions in the warp and fill directions, and either rayon fibers or ZMI fibers, respectively, were used as the through-thickness or Z direction fiber tows. For these 3D orthogonal panels, the final volume fractions of the Sylramic-iBN fibers were B0.1 in the X direction, and B0.27 in the Y direction (test direction). The fifth panel, LTLAI, was a Sylramic-iBN layer-to-layer angle interlock consisting of three layers and a low fiber volume fraction in the Y or test direction. Also shown in the bottom half of Table I are MI SiC/SiC panels from earlier studies that will be used for property comparison and model development in this study. All of these panels were processed in the same manner regarding the BN fiber coating (B0.5 mm thick) and MI SiC matrix. For the previous 3D orthogonal panels where rayon fibers were used, the preferred NASA conversion process for Sylramic-iBN preforms was not used because of initial concern related to degradation of the architectures. Thus the in-plane fibers for these panels were only Sylramic fibers which, other than a significant difference in creep and rupture behavior, show essentially the same elastic and tensile strength behavior as the Sylramic-iBN fibers when incorporated into MI SiC/ SiC composites. The densities of all of the panels and specimens evaluated in this study were 42.85 g/cm3 . This implies that most of the porosity is resident within the tow minicomposites (B5% total porosity) typical of good MI infiltration as observed in Fig. 1. It should also be noted in Table I that in contrast to the other panels, the in-plane Sylramic-iBN fibers of the Braid panel were not oriented in the testing direction. For this panel, a 2D tri-axial braided architecture was first formed on a 50 mm diameter mandrel with the hoop fibers 7671 from the axial direction.15 The architecture was then cut axially and laid up in tooling to form the final panel with the hoop fibers oriented 7231 from the Y loading direction. Tensile specimens, 12.6 mm wide and 150 mm long (see Table I for average thickness values), were machined into a dogbone shape where the gage section width and length were B10 and B40 mm, respectively. The ends of the tensile bars were encased in a wire mesh to alleviate grip stresses and bending moments at and near the pneumatic pressure grips. Room temperature tensile tests were performed according to ASTM C127516 using a universal testing machine (Model 8562, Instron, Canton, MA). Specimens were loaded at a constant rate (4 kN/min). Two clip-on strain gages (2.5% max strain) were attached (one on each face) and the average of the two strain gages was used for the tests. Unload–reload interruptions were also performed, usually at least two per test, in order to determine the residual compressive stress in the composite matrix. Modal acoustic emission was also monitored during the room temperature tensile tests. A Digital Wave Fracture Wave Detector with two wide-band pass (50– 2000 kHz) frequency sensors also from Digital Wave Corporation (Model B1025, Englewood, CO) was used. The two AE sensors were placed on the face of the specimen, one on each side of the gage section, apwww.ceramics.org/ACT Effects of Fiber Architecture on SiC/SiC Composites 279
280 International yournal of Applied Ceramic Techmolog Vol.7,No.3,2010 Y⊕ (d) Fig. 1. Cross-sections of MI SiC/SiC specimens:( a)A/,(b)3DO-Un-R(c) 3DO-Un-Z and(d)LTLAl. The widths of the specimens are 10mm(= warp direction and y= fill direction). A high magnification image of3Do-Un-Z(e)shows some of the details of the MI microstructure proximately 50-60 mm from one another. The two aE curred in the gage section were sorted out using a sensors were synchronized, that is, both sensors would threshold voltage crossing technique"and used for record the waveform from the same event at the same analysis according to the ocala each event based time if either sensor was triggered. Events which oc- on the speed of sound of the extensional wave, which
proximately 50–60 mm from one another. The two AE sensors were synchronized, that is, both sensors would record the waveform from the same event at the same time if either sensor was triggered. Events which occurred in the gage section were sorted out using a threshold voltage crossing technique17 and used for analysis according to the location of each event based on the speed of sound of the extensional wave, which Fig. 1. Cross-sections of MI SiC/SiC specimens: (a) AI-UNI, (b) 3DO-Un-R, (c) 3DO-Un-Z and (d) LTLAI. The widths of the specimens are 10 mm (X 5 warp direction and Y 5 fill direction). A high magnification image of 3DO-Un-Z (e) shows some of the details of the MI microstructure. 280 International Journal of Applied Ceramic Technology—Morscher, et al. Vol. 7, No. 3, 2010