ournal J.Am.Ceum.Soc.8104-1212004 Effect of a boron Nitride Interphase that Debonds between the Interphase and the matrix in SiC/SiC Composites Gregory N Morscher Ohio Aerospace Institue, Cleveland, Ohio 44142 Hee Mann yun*, Cleveland State University, Cleveland, Ohio 44115 James A. Dicarlo NASA Glenn Research Center. Cleveland, Ohio 44135 Linus Thomas-Ogbuji·t QSs Group, Inc, Cleveland, Ohio 44135 Typically, the debonding and sliding interface enabling fiber interfaces as well as oxidation of the fiber surface(Fig. 1(a). The pullout for SiC-fiber-reinforced SiC-matrix composites with liquid boria reaction product reacts with the Sic fiber to form a BN-based interphases occurs between the fiber and the inter borosilicate liquid that increases in SiO2 content with phase. Recently, composites have been fabricated where inter oxidation of the SiC. Also, B,, reacts with water vapor face debonding and sliding occur between the BN interphase atmosphere to form volatile B-containing hydrated species os result ind the matrix. This results in two major improvements in ing in an even higher Sio content in the oxidation product. These mechanical properties. First, significantly higher failure phenomena result in a solid oxidation product(glass) that strongly strains were attained due to the lower interfacial shear bonds fibers bridging the matrix crack to one another or to the strength with no loss in ultimate strength properties of the matrix itself and causes subsequent composite embrittlement( Fig composites. Second, significantly longer stress-rupture times at 1(a) higher stresses were observed in air at s15C. In addition, no One proposal to curtail this type of rapid oxidative process that loss in mechanical properties was observed for composites that leads to composite embrittlement would be for the debonding and did not possess a thin carbon layer between the fiber and th sliding interface to be some distance away from the reinforcing interphase when subjected to burner- rig exposure Two pri fibers. For SiC/SiC composites this has been attempted with mary factors were hypothesized for the occurrence of debond C/SiC multilayers as the"interphase"3-5 and more recently with ng and sliding between the bn interphase and the siC matrix: BN/SiC multilayers. In theory, debonding and sliding would a weaker interface at the BN/matrix interface than the fi- occur in some of the outer layers, prohibiting or complicating the ber/BN interface and a residual tensile/shear stress-state at the diffusion of oxidizing species to the inner fiber/interphase region BN/matrix interface of melt-infiltrated composites. Also, the that leads to composite embrittlement. Some benefit has been occurrence of outside debonding was believed to occur during demonstrated for stress-rupture of minicomposites with multilayer cooldown after molten silicon infiltration For SiC/SiC composites with Bn interphases, if the debonding and sliding layer was between the bn and the matrix, a similar benefit proposed for the multilayer approach could be I. Introduction Oxidation of the bn would occur from the"outside" f the bn OR woven SiC/Sic composites with BN interphases, the typical would react with the Sic matrix to eventually form a borosilicate interface where debonding and sliding occur is between the fI er and the bn interphase. We refer to this phenomenon glass that would act as a"sealant "slowing diffusion of oxidizing species to the Bn. In order for the fibers to be fused together or to interphase exacerbates the environmental durability problem of the mcat i:ioi dabo h s m en tr thiconsid er ble am wn of ae terphases at intermediate temper- considering the effects of sealing and the reduced surface area of atures(600 to 1000C)in the presence of oxidizing atmo- BN exposed to oxidizing species when compared with the typical direct access to the fibers themselves. This causes oxidation of the benefit expected from an outside debonded interphase in SiC/Sic Bn interphase preferentially at both the fiber/BN and BN/CVI SiC composites would be improved intermediate-temperature mechan ical properties, e. g, stress-rupture, in oxidizing environments Such behavior has been demonstrated and will be described and discussed in this work I. Experimental Procedure se NASA UF: 2002: approved April 23, 2003 0784 Received A SiC-fiber-reinforced melt-infiltrated SiC-matrix Ceram els that exhibited outside debonding were fabricated from 2D- cientist at NASA Glenn Research Center, Cleveland, OH woven, balanced, 5 harness satin, 0/90 fabric, by General Electric
J. Am. Cerum. Soc., 87 [I] 104-12 (2004) journal Effect of a Boron Nitride lnterphase That Debonds between the lnterphase and the Matrix in SiC/SiC Composites Gregory N. Morscher*.+ Ohio Aerospace Institue, Cleveland, Ohio 44142 Hee Mann Yun*.t Cleveland State University, Cleveland, Ohio 44 I 15 James A. DiCarlo* NASA Glenn Research Center, Cleveland, Ohio 44135 Linus Thomas-Ogbuji*7t QSS Group, Inc., Cleveland, Ohio 44135 Typically, the debonding and I ding interface enabling fiber pullout for Sic-fiber-reinforced Sic-matrix composites with BN-based interphases occurs between the fiber and the interphase. Recently, composites have been fabricated where interface debonding and sliding occur between the BN interphase and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure strains were attained due to the lower interfacial shear strength with no loss in ultimate strength properties of the composites. Second, significantly longer stress-rupture times at higher stresses were observed in air at 815°C. In addition, no loss in mechanical properties was observed for composites that did not possess a thin carbon layer between the fiber and the interphase when subjected to burner-rig exposure. Two primary factors were hypothesized for the Occurrence of debonding and sliding between the BN interphase and the Sic matrix: a weaker interface at the BNhatrix interface than the fiber/BN interface and a residual tensilekhear stress-state at the BN/matrix interface of melt-infiltrated composites. Also, the occurrence of outside debonding was believed to occur during composite fabrication, i.e., on cooldown after molten silicon infiltration. I. Introduction OR woven SiClSiC composites with BN interphases, the typical F interface where debonding and sliding occur is between the fiber and the BN interphase. We refer to this phenomenon as “inside debonding.” Unfortunately, the inside debonding of the interphase exacerbates the environmental durability problem of SiC/SiC composites with BN interphases at intermediate temperatures (600” to lO00”C) in the presence of oxidizing atmospheres.’.’ When matrix cracks are formed, the environment has direct access to the fibers themselves. This causes oxidation of the BN interphase preferentially at both the fiberlBN and BN/CVI Sic R. Naslain-ontributing editor Manuscript No. 186784. Received August 7, 2002: approved April 23,2003. This work was supported by the NASA UEET program. ‘Member, American Ceramic Society. ‘Senior Research Scientist at NASA Glenn Research Center, Cleveland, OH. interfaces as well as oxidation of the fiber surface (Fig. l(a)). The liquid boria reaction product reacts with the Sic fiber to form a borosilicate liquid that increases in SiO, content with further oxidation of the Sic. Also, B,O, reacts with water vapor in the atmosphere to form volatile B-containing hydrated species resulting in an even higher SiO, content in the oxidation product. These phenomena result in a solid oxidation product (glass) that strongly bonds fibers bridging the matrix crack to one another or to the matrix itself and causes subsequent composite embrittlement (Fig. One proposal to curtail this type of rapid oxidative process that leads to composite embrittlement would be for the debonding and sliding interface to be some distance away from the reinforcing fiber^.^ For SiC/SiC composites this has been attempted with C/SiC multilayers as the “interpha~e”~-~ and more recently with BN/SiC multilayers.6 In theory, debonding and sliding would occur in some of the outer layers, prohibiting or complicating the diffusion of oxidizing species to the inner fibedinterphase region that leads to composite embrittlement. Some benefit has been demonstrated for stress-rupture of minicomposites with multilayer C/SiC coating^.'.^ For SiC/SiC composites with BN interphases, if the debonding and sliding layer was between the BN and the matrix. a similar benefit proposed for the multilayer approach could be achieved. Oxidation of the BN would occur from the “outside” of the BN inwards toward the fiber. The resulting boria oxidation product would react with the Sic matrix to eventually form a borosilicate glass that would act as a “sealant” slowing diffusion of oxidizing species to the BN. In order for the fibers to be fused together or to the matrix, oxidation of the entire thickness of the BN would have to occur (Fig. l(b)). This may take a considerable amount of time considering the effects of sealing and the reduced surface area of BN exposed to oxidizing species when compared with the typical “inside” debonding case (Figs. I(a) and (b)). Therefore, the major benefit expected from an outside-debonded interphase in SiC/SiC composites would be improved intermediate-temperature mechanical properties, e.g., stress-rupture, in oxidizing environments. Such behavior has been demonstrated and will be described and discussed in this work. 1 (a)). II. Experimental Procedure Sic-fiber-reinforced melt-infiltrated SiC-matrix composite panels that exhibited outside debonding were fabricated from 2Dwoven, balanced, 5 harness satin, 0/90 fabric, by General Electric 104
January 2004 Effect of a BN interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composite a: Inside Debonding Fiber Oxidation Matrix Crack SiO +B O, Oxidation SiO2+BO, b: Outside Debonding Fig. 1. Schematic representation of oxidation of the interphase for(a)debonding and sliding between the fiber and the Bn interphase, i. e, "inside debonding. "and(b)between the bn interphase and the matrix, i.e, " outside debonding Power Systems Composites(Newark, DE). The composite fabri- machine(Instron Model 8562, Instron, Ltd, Canton, MA). Modal cation process involves the following steps: chemical vapo acoustic emission (AE) was monitored during the room infiltration(CVi)of a stacked(152 mm X 229 mm)2D-woven temperature tests with two wide-band (50 kHz to 2.0 MHz)sensors fabric with bN, cvI SiC infiltration Sic ele slurry infiltra- laced outside the tapered region of the tensile bar The ae ion, and final liquid Si infiltration. The occurrence of outside waveforms were recorded and digitized using a fracture wave debonding was initially a processing aberration, but has since been detector(FWD, Digital Wave Corp, Englewood, CO). The AE under study to optimize and control its occurrence. Outside data were filtered using the location software provided by the debonding was observed for over 20 different SiC/SiC composite FWD manufacturer, after the tensile test, to separate out the aE panels fabricated with Sylramic( Dow Corming, Midland, MI) S(NI Intermediate-temperature stress-rupture tests were performed to as HNS in the following), and Sylramic-iBN (treated Sylrami on dogbone specimens using a different universal-testing machine fibers that possess an in situ Bn coating). Most of the pane (Instron Model 4502, Instron, Ltd, Canton, MA)in air at 815C as were fabricated with Sylramic in Ref. 3. Specimens were tabbed with graphite-epoxy composite in fiber volume fraction in the (i. e, total fiber volume fraction to 0.40) Table I lists some grips, and a very low load(100 N) was applied to account for of the variations in the physical characteristics of composite thermal expansion of the material during heating The specimens were exposed to elevated temperature using a resistance-heated Mechanical property evaluation included room- and furnace(MoSi2 elements). Although the furnace was 75 mm long intermediate-temperature tensile testing. Room-temperature ten- the hot zone region was only about 15 mm. When the furnace sile testing was performed on at least two dogbone specimens from reached the desired temperature, 815oC, the load was raised to the each panel. Dogbone specimens, 152 mm long, were cut so that the pture stress where it was held until failure Specimens from some panels were also subjected to an atmo wide. Both monotonic and load/unload/reload hysteresis tensile spheric pressure burmer-rig under zero-stress exposure at 815C tests were performed at room temperature using a universal-testing i.e., uncracked, and then tensile tested at room temperature to Table I. Physical and Mechanical Properties of Some of the SiC/SiC Composites Tested Estimated from epcm/No plies E(GPa) 0.13 224 SYL-outside 0.15 YL-inside SYL-inside 8.7/8 389 0. SYL-iBN outside 8.7/8 SYL-ibN outside 0.17 0.49 R 248 N inside 50/8 0.12 79 Tow ends per centimeter
January 2004 Effect of a BN Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites 105 Fig. 1. Schematic representation of oxidation of the interphase for (a) debonding and sliding between the fiber and the BN interphase, i.e., “inside debonding,” and (b) between the BN interphase and the matrix, i.e., “outside debonding.” Power Systems Composites (Newark, DE). The composite fabrication process involves the following steps: chemical vapor infiltration (CVI) cf a stacked (-152 mm X 229 mm) 2D-woven fabric with BN, CVI Sic infiltration, Sic particle slurry infiltration, and final liquid Si infiltration.” The occurrence of outside debonding was initially a processing aberration, but has since been under study to optimize and control its occurrence. Outside debonding was observed for over 20 different SiCISiC composite panels fabricated with SylramicO (Dow Coming, Midland, MI) fibers, Hi-Nicalon type S (Nippon Carbon, Tokyo, Japan, referred to as HNS in the following), and Sylramic-iBN (treated SylramicO fibers that possess an in situ BN coating’’). Most of the panels were fabricated with Sylramic-iBN or SylramicO fibers and ranged in fiber volume fraction in the loading direction from 0.13 to 0.2 (i.e., total fiber volume fraction of 0.26 to 0.40). Table I lists some of the variations in the physical characteristics of composite panels. Mechanical property evaluation included room- and intermediate-temperature tensile testing. Room-temperature tensile testing was performed on at least two dogbone specimens from each panel. Dogbone specimens, 152 mm long, were cut so that the gauge section was 10 mm wide and the grip section was 12.5 mm wide. Both monotonic and loadlunloadlreload hysteresis tensile tests were performed at room temperature using a universal-testing machine (Instron Model 8562, Instron, Ltd, Canton, MA). Modal acoustic emission (AE) was monitored during the roomtemperature tests with two wide-band (50 kHz to 2.0 MHz) sensors placed outside the tapered region of the tensile bar.” The AE waveforms were recorded and digitized using a fracture wave detector (FWD, Digital Wave Corp., Englewood, CO). The AE data were filtered using the location software provided by the FWD manufacturer, after the tensile test, to separate out the AE that occurred outside the gauge section. Intermediate-temperature stress-rupture tests were performed on dogbone specimens using a different universal-testing machine (Instron Model 4502, Instron, Ltd., Canton, MA) in air at 815°C as in Ref. 3. Specimens were tabbed with graphite-epoxy composite tabs. The test specimens were gripped with water-cooled hydraulic grips, and a very low load (100 N) was applied to account for thermal expansion of the material during heating. The specimens were exposed to elevated temperature using a resistance-heated furnace (MoSi, elements). Although the furnace was 75 mm long, the hot zone region was only about 15 mm. When the furnace reached the desired temperature, 815°C. the load was raised to the rupture stress where it was held until failure. Specimens from some panels were also subjected to an atmospheric pressure burner-rig under zero-stress exposure at 8 15”C, i.e., uncracked, and then tensile tested at room temperature to Table I. Physical and Mechanical Properties of Some of the SiC/SiC Composites Tested T (MPa) Specimen-location of Estimated from Measured from debonding epcm+/No. plies f E (GPa) u,,~, (MPa) e,,, (%) ak curve push-in test HNS-outside 7.118 0.17 200 352 0.46 - HNS-inside 7.118 0.18 240 311 0.38 - SYL-outside 7.116 0.13 224 224 0.27 37 - SYL-outside 5.018 0.15 219 297 0.43 25 - SYL-mixed 7.918 0.19 246 353 0.33 45 26 SYL-inside 6.318 0.19 246 397 0.36 - - SYL-inside 8.718 0.2 265 389 0.3 65 64 SYL-inside 7.118 0.17 270 310 0.3 1 63 70 SYL-iBN outside 8.718 0.2 216 456 0.5 18 7 SYL-iBN outside 7.118 0.17 220 395 0.49 11 6 SYL-iBN mixed 7.918 0.19 228 >476 0.5 I 43 31 SYL-iBN inside 8.718 0.2 277 404 0.31 73 83 SYL-iBN inside 7.918 0.2 248 502 0.42 - - SYL-iBN inside 5.018 0.12 279 284 0.21 63 - ‘Tow ends per centimeter
Journal of the American Ceramic Society--Morscher et al etermine the retained strength. 2 The low-pressure burner rig(1.0 measure of the residual stress can be mated from th tm)uses a high-velocity(Mach 0.)flame and is designed to intersection of the average slopes of the hysteresis loops for simulate the combustion environments of turbine engines tresses higher than approximately half the peak stress of the fracture surfaces of the failed composites were examined with hysteresis loop(Fig. 2), 5-60 MPa for the inside-debonding a field emission scanning electron microscope (FESEMD), Hitachi composite and -35 MPa for the outside-debonding composite polished sections of untested panels to determine the interfacial removed) for the same architecture MI composites with"outside shear stress of the sliding interface. At least 20 different fibers and"inside"debonding. In general, although similar in ultimate were tested for each specimen. Finally, the interphase region of strength, two differences between outside- and inside-debonding small slivers of composite material were fractured in bending in moduli (Table I)and (2)a higher strain at a given applied stress nation. Depth profiles were then performed at regions wher mi situ under vacuum to prevent the fracture surface from conta including higher strains to failure(Table I and Fig. 3). However, one panel, which exhibited a mixture of inside and outside Bn layer adhered to the matrix and at other regions where the Bn debonding, was an exception and had a high elastic modulus (246 layer adhered to the fiber GPa) Figure 4 shows examples of composite fracture surfaces after II. Results room-temperature tensile failure. Some bundle pullout was ob- served for both types of composites; however, individual fiber (1) Room-Temperature Tensile Stress-Strain Behavior pullout was significantly longer for outside-debonding composites Typical unload-reload tensile hysteresis stress-strain curves Figs. 4(a) and(b) than for inside-debonding composites(Figs and AE activity are shown in Fig. 2 for MI SYL-iBN/SiC 4(c)and(d ). Note the adherence of the BN layer to the fibers for composite that displays inside and outside debonding. It was he outside -debonding composites(Fig. 4(b))compared with the observed that the first detectable aE that occurs in the gauge outside-debonding composites(Fig. 4(d). It would be ideal if section occurs at 110 20 MPa for both inside- and outside- debonding outside the bn interphase occurred for each fiber debonding composites. Also note that on unloading the material independently from one another(e. g, Fig. 1). However, because of tiffens, indicating that the matrix is in residual compression. a the close packing of fibers in woven bundles, debonding between 8.epcm 8 ply: f=0.2 E=280 GPa 0.2 Strain. a) 500 8.epcm; 8 ply: f0.2 E=216 GPa 350 00 0.1 02 0.5 Strain, % Fig. 2. Tensile load-unload-reload hysteresis curves for(a) inside-debonding and (b)outside-debonding SYL-iBN SiC/SiC composites. Also plotted is the normalized cumulative AE energy. Squares are stress-strain model for best-fit interfacial shear stress
106 Journal of the American Ceramic Society--Morscher et al. Vol. 87, No. 1 determine the retained strength.” The low-pressure burner rig (1.0 atm) uses a high-velocity (Mach 0.3) flame and is designed to simulate the combustion environments of turbine engines. Fracture surfaces of the failed composites were examined with a field emission scanning electron microscope (FESEM), Hitachi Model S-4700. A fiber push-in was performed on polished sections of untested panels to determine the interfacial shear stress of the sliding interface. At least 20 different fibers were tested for each specimen. Finally, the interphase region of some specimens was examined using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). For AES, small slivers of composite material were fractured in bending in situ under vacuum to prevent the fracture surface from contamination. Depth profiles were then performed at regions where the BN layer adhered to the matrix and at other regions where the BN layer adhered to the fiber. III. Results (I) Room-Temperature Tensile Stress-Strain Behavior Typical unload-reload tensile hysteresis stress-strain curves and AE activity are shown in Fig. 2 for MI SYL-iBN/SiC composite that displays inside and outside debonding. It was observed that the fist detectable AE that occurs in the gauge section occurs at 110 2 20 MPa for both inside- and outsidedebonding composites. Also note that on unloading the material stiffens, indicating that the matrix is in residual compression. A measure of the residual stress can be approximated from the intersection of the average slopes of the hysteresis loops for stresses higher than approximately half the peak stress of the hysteresis loop (Fig. 2),15 -60 MPa for the inside-debonding composite and -35 MPa for the outside-debonding composite. Figure 3 shows typical stress-strain curves (hysteresis loops removed) for the same architecture MI composites with “outside” and “inside” debonding. In general, although similar in ultimate strength, two differences between outside- and inside-debonding composites were evident for room-temperature stress-strain behavior: “outside-debonding” composites had (1) lower elastic moduli (Table I) and (2) a higher strain at a given applied stress including higher strains to failure (Table I and Fig. 3). However, one panel, which exhibited a mixture of inside and outside debonding, was an exception and had a high elastic modulus (246 GPa). Figure 4 shows examples of composite fracture surfaces after room-temperature tensile failure. Some bundle pullout was observed for both types of composites; however, individual fiber pullout was significantly longer for outside-debonding composites (Figs. 4(a) and (b)) than for inside-debonding composites (Figs. 4(c) and (d)). Note the adherence of the BN layer to the fibers for the outside-debonding composites (Fig. 4(b)) compared with the outside-debonding composites (Fig. 4(d)). It would be ideal if debonding outside the BN interphase occurred for each fiber independently from one another (e.g., Fig. 1). However, because of the close packing of fibers in woven bundles, debonding between 450 i I SY 400 4 8.7e~cm: L-iBN 8 ply: f = 0.2 Ir #Y 1.4 * 1.2 i1 15 0.4 0 0.4 ’/’ Strain, % (a) 1 ______ 500 I M0 O*I 0.2 0.3 0.4 0.5 Straln, % (b) 1.4 I 0.6 * C 1w W 0.8 a 1.2 f E 0.4 10.2 s -40 0.6 Fig. 2. Tensile load-unload-reload hysteresis curves for (a) inside-debonding and (b) outside-debonding SYL-iBN SiC/SiC composites. Also plotted is the normalized cumulative AE energy. Squares are stress-strain model for best-fit interfacial shear stress
January 2004 Effect of a BN Interphase That Debonds between the interphase and the Matrix in SiC/Sic Composites was estimated from the measured final crack density of failed composites multiplied by the normalized cumulative AE energy (Fig. 2), assuming the latter represented the stress-dependent Inside Debonair distribution of matrix cracks, which has been demonstrated for inside Debonding" similar systems'.8 Therefore, the only variable not known was T which was adjusted to best fit the predicted stress-strai 3008吵 the experimental stress-strain curve. For the case where SYL-IBN lengths overlap Ahn and Curtin"showed that if the ° Outside Debonding still equally spaced, the composite strain could then be modeled by o/(fE+ao/ -(o+o4ES(o)p] 00.10.20.304050607 E E品 <28 Strain stress-strain curves for ide-debonding Fig. 3. Room-temperature tensile stress-strain curves for 8.7 epcm ecimens are shown in Figs. 2(a) and SYL- iBN SiC/SiC composites and 7, I epcm HNS SiC/SiC composites (b), respectively. steresis loops removed). Note the HNS composites are displaced by The interfacial shear stress was also measured directly from the 0. 2% in strain for clarity. fiber push-in technique. Results of the two techniques are listed fo individual specimens in Table I for systems that displayed global outside debonding, mixed outside/inside debonding, and global the bn interphase and the matrix was often observed to occur inside debonding. Both techniques confirmed that the interfacial around groups of fibers that were linked to one another by the thin shear strength of global outside-debonding composites(-10 MPa Bn that was deposited on two closely spaced fibers. Usually, these was significantly less than that of inside-debonding composites fiber groups were made up of a few fibers that formed a row of (-70 MPa). Mixed outside/inside debonding had intermediate fibers as shown in Fig 4(b ). Debonding at the BN interphase/Sic values of interfacial shear strength. It is important to note that even matrix was observed for individual fibers that were well separated though the interfacial shear strength of outside debonding is lower from other fibers. For some composites, regions of outside than that of inside-debonding composites, there was no loss in debonding and inside debonding were observed in different re- ultimate strengths for outside-debonding composites and often u gions or bundles of the fracture surface, i.e., mixed debonding ultimate strength increased(e.g, compare the f= 0.2 composites In addition to a low elastic modulus, outside-debonding com- in Fig 3(a). posites often displayed a secondary modulus before significant matrix cracking. Figure 5 shows a family of stress-strain curves for a number of different outside-debonding composites with (2) Intermediate-Temperature Mechanical Behavior ifferent volume fractions, The initial elastic moduli were very Stress rupture at 815C was performed on SYL and SYL- iBN consistent(218 GPa)and all of the curves showed an inflection composites with inside and outside debonding(fig. 6).The x70 MPa that resulted in stress-rupture data for SYL SiC/SiC composites displaying inside MPa). This inflection was not associated with any AE activity; i.e debonding have been reported in refs 20 and 21. since the par it appears that this inflection was not due to matrix crack varied in fiber volume fraction, the rupture stress data are plotted ormation as the stress on the fibers i e. the load in a matrix crack that was Finally, the interfacial shear strength of several different inside- carried by the fibers. For comparison, the rupture stress corre- debonding and outside-debonding composites was determined sponding to a composite with f=0.2 in the loading direction is using two techniques. 4 First, the interfacial shear strength was shown on the right axis. Each set of data for the different types of estimated by modeling the stress-strain curve based on the composites had at least one panel with f =0.2. stress-dependent crack density(from AE) Composite strain was First, note that there is a difference in rupture behavior between determined in the same fashion as Pryce and Smith. Using the inside-debonding SYL-ibN fiber composites and SYL nomenclature of Curtin et al, composite strain can be modeled posites. Inside-debonding SYL-iBn composites outperform (i.e assuming equally spaced cracks fail after a longer time at a given stress) inside-debonding SYL composites because the fibers in SYL-iBn composites E=a/Ee+a6()pE0+σt) rally spread apart from one another with the formation of the 100 the fiber surface. 2I The where o is the applied stress. the residual (thermal)stress in more time with increasing separation distance. In addition, the the matrix(compression is negative), E is the elastic modulus, debonding interface for inside-debonding SYL-ibN actually oc- subscripts m, f, and c refer to matrix, fiber, and composite, curs between the in situ BN and the CVI-deposited BN. 0In other respectively, and p. is the matrix crack density. The fir rst part of the words, for inside-debonding SYL-iBN composites, the debonding equation corresponds to the elastic strain response of an uncracked and sliding interface was some distance(-100 nm)away from the composite and the second part of the equation corresponds to the fiber surface, which contained Sic in(displacement) of the fibers at and away from a or both fiber composite systems possessing an outside- through-thickness matrix crack dictated by the sliding lengt debonding interface, further improvements in intermediate mperature stress-rupture life were observed(Fig. 6). For SYL (2) composites with outside debonding compared with SYL inside bonding co ed by over 25 a=(1-∫)Em/E outside-debonding SYL-ibn composites in comparison to inside- debonding SYL-BN composites, the and r is the fiber radius, f is the fiber volume fraction in the magnitude in time improvement at resses and -200 MPa direction, and T is the interfacial shear strength. E and o improvement in fiber stress(40 determined from the stress-strain curves. Er is 380 GPa and at lower stresses near the run-out c It should be noted that was determined from the rule-of-mixtures. The stress-dependent p. these high-stress conditions for stress-rupture are significantly
January 2004 Effect of a BN Interphase That Debonds between the Interphase and the Matrix in SiC/SiC Composites 107 6oo 1 500 - 'Outside Debonding' "Inside Debonding" 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Strain, % Fig. 3. Room-temperature tensile stress-strain curves for 8.7 epcm SYL-iBN SiC/SiC composites and 7. I epcm HNS SiC/SiC composites (hysteresis loops removed). Note the HNS composites are displaced by 0.2% in strain for clarity. the BN interphase and the matrix was often observed to occur around groups of fibers that were linked to one another by the thin BN that was deposited on two closely spaced fibers. Usually, these fiber groups were made up of a few fibers that formed a row of fibers as shown in Fig. 4(b). Debonding at the BN interphaselSiC matrix was observed for individual fibers that were well separated from other fibers. For some composites, regions of outside debonding and inside debonding were observed in different regions or bundles of the fracture surface, i.e., mixed debonding. In addition to a low elastic modulus, outside-debonding composites often displayed a secondary modulus before significant matrix cracking. Figure 5 shows a family of stress-strain curves for a number of different outside-debonding composites with different volume fractions. The initial elastic moduli were very consistent (-218 GPa) and all of the curves showed an inflection at -70 MPa that resulted in a lower secondary modulus (-177 MPa). This inflection was not associated with any AE activity; i.e., it appears that this inflection was not due to matrix crack formation. Finally, the interfacial shear strength of several different insidedebonding and outside-debonding composites was determined using two technique^.'^ First, the interfacial shear strength was estimated by modeling the stress-strain curve based on the stress-dependent crack density (from AE). Composite strain was determined in the same fashion as Pryce and Smith.I6 Using the nomenclature of Curtin et aZ.,I7 composite strain can be modeled assuming equally spaced cracks: E zz U/E, + CX~(U)~,JE~(U + uth) (1) (for pi' > 26) where u is the applied stress, uIh is the residual (thermal) stress in the matrix (compression is negative), E is the elastic modulus, subscripts m, f, and c refer to matrix, fiber, and composite, respectively, and p, is the matrix crack density. The first part of the equation corresponds to the elastic strain response of an uncracked composite and the second part of the equation corresponds to the extra strain (displacement) of the fibers at and away from a through-thickness matrix crack dictated by the sliding length: 6 = cir(u + u,h)/2T (2) where a = (1 -f)E,,/fE, (3) and r is the fiber radius, f is the fiber volume fraction in the loading direction, and T is the interfacial shear strength. Ec and ulh were determined from the stress-strain curves. Ef is 380 GPa and Em was determined from the rule-of-mixtures. The stress-dependent pc was estimated from the measured final crack density of failed composites multiplied by the normalized cumulative AE energy (Fig. 2), assuming the latter represented the stress-dependent distribution of matrix cracks, which has been demonstrated for similar systems.''*'8 Therefore, the only variable not known was T, which was adjusted to best fit the predicted stress-strain curve to the experimental stress-strain curve. For the case where the sliding lengths overlap, Ahn and Curtin'' showed that if the cracks are still equally spaced, the composite strain could then be modeled by E = a/(fEJ + CKU~/E~ - CK(U + U~~)/[~E&(U)P,] (4) (for p;' < 26) Therefore, for higher applied stress conditions, if pC-' < 26 was predicted, Eq. (4) was used. Examples of best-fit stress-strain curves for inside-debonding (T - 73 MPa) and outside-debonding (T - 18 MPa) composite specimens are shown in Figs. 2(a) and (b), respectively. The interfacial shear stress was also measured directly from the fiber push-in technique. Results of the two techniques are listed for individual specimens in Table I for systems that displayed global outside debonding, mixed outsidelinside debonding, and global inside debonding. Both techniques confirmed that the interfacial shear strength of global outside-debonding composites (- 10 MPa) was significantly less than that of inside-debonding composites (-70 MPa). Mixed outsidehide debonding had intermediate values of interfacial shear strength. It is important to note that even though the interfacial shear strength of outside debonding is lower than that of inside-debonding composites, there was no loss in ultimate strengths for outside-debonding composites and often the ultimate strength increased (e.g., compare thef = 0.2 composites in Fig. 3(a)). (2) Intermediate-Temperature Mechanical Behavior Stress rupture at 815°C was performed on SYL and SYL-iBN composites with inside and outside debonding (Fig. 6). The stress-rupture data for SYL SiC/SiC composites displaying inside debonding have been reported in Refs. 20 and 21. Since the panels varied in fiber volume fraction, the rupture stress data are plotted as the stress on the fibers, i.e., the load in a matrix crack that was carried by the fibers. For comparison, the rupture stress corresponding to a composite withf = 0.2 in the loading direction is shown on the right axis. Each set of data for the different types of composites had at least one panel with f = 0.2. First, note that there is a difference in rupture behavior between inside-debonding SYL-iBN fiber composites and SYL fiber composites. Inside-debonding SYL-iBN composites outperform (i.e., fail after a longer time at a given stress) inside-debonding SYL composites because the fibers in SYL-iBN composites are naturally spread apart from one another with the formation of the - 100 nm BN layer on the fiber surface.*' The rupture life depends on the time it takes to bond nearest-neighbor fibers together, which takes more time with increasing separation distance. In addition, the debonding interface for inside-debonding SYL-iBN actually occurs between the in situ BN and the CVI-deposited BN." In other words, for inside-debonding SYL-iBN composites, the debonding and sliding interface was some distance (-100 nm) away from the fiber surface, which contained Sic. For both fiber composite systems possessing an outsidedebonding interface, further improvements in intermediatetemperature stress-rupture life were observed (Fig. 6). For SYL composites with outside debonding compared with SYL insidedebonding composites, stress-rupture improved by over 250 MPa in fiber stress (-50 MPa for an f = 0.2 composite). For outside-debonding SYL-iBN composites in comparison to insidedebonding SYL-BN composites, there was over an order of magnitude in time improvement at high stresses and -200 MPa improvement in fiber stress (-40 MPa for an f = 0.2 composite) at lower stresses near the run-out condition. It should be noted that these high-stress conditions for stress-rupture are significantly
Journal of the American Ceramic Society-Morscher et aL. 3w× 80SE(M 11/e2 11bm号2aw1 nox SE1an Fiber BN Fiber BN sav 113mmx20c4 0401110620v14 Fig. 4. FESEM images of fracture surfaces of SYL- iBn composites showing outside debonding(a, b)and inside debonding(c, d) higher than the stresses for matrix cracks to penetrate the load- contact, the thinner areas of BN earing fibers (determined from the onset of hyste op fusion occurred for rupture times activity, 175 MPa for f=0. 2 composites used in this study ) In several regions of significant fib other words, the SYL-iBN composites are significantly cracked at section of the fracture surface the stress-rupture conditions of this study, even for specimens that A few specimens(SYL and SYL-iBN)were precracked at room did not fail after long periods of time. temperature and compared with the rupture behavior of pristine Examination of the rupture specimen fracture surfaces con- from the same panel(Fig. 8). It was evident that firmed the survival of most of the bn around the fibers in the onding SYL composites with nominally good rupture matrix crack even though significant oxidation had occurred in the were significantly poorer in rupture behavior with matrix crack( Fig. 7). However, at regions of near fiber-to-fiber ng as has been observed in another study. On the other 600 f=0.18 f。=0.2 f。=017 200 Change in slope at-70 MPa not associated with occurrence of ae 0203040.50.6 Strain. Fig. 5. Room-temperature tensile stress-strain curves for a number of outside-debonding composites with different fiber volume fractions
108 Journal of the American Ceramic Society-Morscher et al. Vol. 87, No. 1 Fig. 4. FESEM images of fracture surfaces of SYL-iBN composites showing outside debonding (a,b) and inside debonding (c,d). higher than the stresses for matrix cracks to penetrate the loadbearing fibers (determined from the onset of hysteresis loop activity, - 175 MPa for f = 0.2 composites used in this study). In other words, the SYL-iBN composites are significantly cracked at the stress-rupture conditions of this study, even for specimens that did not fail after long periods of time. Examination of the rupture specimen fracture surfaces confirmed the survival of most of the BN around the fibers in the matrix crack even though significant oxidation had occurred in the matrix crack (Fig. 7). However, at regions of near fiber-to-fiber 600 500 6 300 b a 200 12 100 200 -I contact, the thinner areas of BN were oxidized and fiber-to-fiber fusion occurred for rupture times greater than 80 h. There were several regions of significant fiber pullout throughout the cross section of the fracture surface. A few specimens (SYL and SYL-iBN) were precracked at room temperature and compared with the rupture behavior of pristine specimens from the same panel (Fig. 8). It was evident that inside-debonding SYL composites with nominally good rupture properties were significantly poorer in rupture behavior with precracking as has been observed in another study." On the other 1& Change in slope at - 70 MPa not associated with occurrence of AE 0 0 0.1 0.2 0.3 0.4 0.5 0.6 Strain, % , = 0.18 Fig. 5. Room-temperature tensile stress-strain curves for a number of outside-debonding composites with different fiber volume fractions