/. Am Cern. So, 87 ITI In4-12C0KH1 urna 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 441 15 James A. DiCarlo NASA Glenn Research Center. Cleveland Ohio 44135 Linus Thomas-Ogbuji* QSS Group, Inc, Cleveland, Ohio 44135 ally, the debonding and sliding interface enabling fibe interfaces as well as oxidation of the fiber surface (Fig. I(a). The it for Sic-tiber-reinforeed Sic-matrix composites with liquid boria reaction product reacts with the SiC fiber to form phase. Recently, composites have been fabricated where inter- oxidation of the SiC. Also. B,O, reacts with water vapor in the occur between the BN interp atmosphere to form volatile B-containing hydrated species result and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure phenomena result in a solid oxidation product (glass) that strongly 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 l(an higher stresses were observed in air at 815 C. In addition, no One proposal to curtail this type of rapid oxidative process that oss 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 the 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" 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. me 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 occurrence of outside debonding was believed to occur during demonstrated for stress-rupture of minicomposites with multilayer C/SiC coating infiltration For SiC/SiC composites with BN interphases. if the debonding and sliding layer was between the BN and the matrix, a simil benefit proposed for the multilayer approach could be achieved I. Introduction Oxidation of the Bn would occur from the "outside"of the BN F OR woven SiC/SiC composites with BN interphases, the typical inwards toward the fiber. The resulting boria oxidation product interface where debonding and sliding occur is between the would react with the SiC matrix to eventually form a borosilicate fiber and the BN interphase, We refer to this phenomenon as glass that would act as a"sealant"slowing diffusion of oxidizing "inside debonding. "Unfortunately, the inside debonding of the species to the BN. In order for the fibers to be fused together or to interphase exacerbates the environmental durability problem of the matrix, oxidation of the entire thickness of the bN would have SiC/SiC composites with BN interphases at intermediate temper- to occur(Fig. 1(b). This may take a considerable amount of time considering the effects of sealing and the reduced surface area of spheres. "2 When matrix cracks are formed, the environment has "inside"debonding case(Figs. I(a)and (b). Therefore, the major BN interphase preferentially at both the fiber/BN and BN/CVI SiC benefit expected from an outside-debonded interphase in SiC/SiC composites would be improved intermediate-temperature mechan cal properties, e.g., stress-rupture. in oxidizing environments. Such behavior has been demonstrated and will be described and R. Naslain--contributing editor discussed in this work I. Experimental Procedure npt No. 186784 Received August 2: approved April 23. 203. the NASA UEET program. Sic-tiber-reinforced melt-infiltrated SiC-matrix composite pan els that exhibited outside debonding were fabricated from 2D. ntist al NASA Glenn Rescarch Center. Cleveland, OH woven, balanced, 5 harness satin, 0/90 fabric, by General Electric
January 200-4 Effect of a BN Interphase Thar Debonds between the Interphase and the Matrix in SiC/SiC Composites a: Inside Debonding Oxidation SiO+B, O, b: Outside Debonding Fig. I. Schematic representation of oxidation of the interphase for (a) debonding and sliding between the fiber and the BN interphase, i c,inside debonding, and (b) between the BN interphase and the matrix, i.e. "outside debonding he composite fabri machine (Instron Model 8562 Instron, Ltd. Canton, MA). Modal cation process involves the following steps: chemical vapor ic emission (AE) was monitored during the room- infiltration (CVI) of a stacked (152 mm x 229 mm) 2D-woven fabric with BN, CVI SiC infiltration, SiC particle slurry infiltra laced rature tests with two wide- band (50 kHz to 2.0 MHz) sensors outside the tapered region of the tensile bar. The AE tion, 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 Corning. Midland, MI hat occurred outside the gauge section fibers, Hi-Nicalon type S(Nippon Carbon, Tokyo, Japan, referred Intermediate-temperature stress-rupture tests were performed to as HNS in the following), and Sylramic-iBN ( treated Sylramic on dogbone specimens using a different universal-testing machine fibers that possess an in sitm BN coating"). Most of the panels (Instron Model 4502 Instron, Ltd.. Canton MA) in air at 815 as were fabricated with Sylramic-iBN or Sylramic fibers and ranged in Ref. 3. Specimens were tabbed with graphite-epoxy composite in fiber volume fraction in the loading direction from 0. 13 to 0.2 tabs. The test specimens were gripped with water-cooled hydraulic (i.e, total fiber volume fraction of 0.26 to 0.40). Table I lists some grips, and a very low load (100 N) was applied to account for f the variations in the physical characteristics of composite thermal expansion of the material during heating. The specimens panels were exposed to elevated temperature using a resistance-heated furnace( MoSi, elements ). Although the fumace 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, 815 C, the load was raised to the each panel. Dogbone specimens, 152 mm long, were cut so that the rupture stress where it was held until failure auge section was 10 mm wide and the grip section was 12.5 mm Specimens from some panels were also subjected to an atr wide Both monotonic and load/unload/reload hysteresis tensile spheric pressure burner-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 T(MPa ocation of Estimated trum EIGPat a,n (MPar 7.1/8 0.17 HNS-inside 7.1/8 0.38 SYL--outsid 7.1/6 0.13 224 5.0/8 SYL-mixe 0.19 246 0.33 SYL-insid 8.7/8 0.2 64 SYL-inside 0.17 270 0.31 70 SYL-iBN outside 0.2 SYL-iBN outsid 0.17 220 39 SYL-iBN mixed 0.19 >476 SYLiBn inside 8.7/8 73 SYL-iBN insid 7.9/8 0.2 248 502 0.42 SYI-ibn inside 5.0/8 0.12 279 284 0.2 63 Tow ends per centimeter
106 Journal of the American Ceramic SocietyMorscher et al. Vol, 87. No. I determine the retained strength. The low-pressure burner rig(1.0 measure of the residual stress can be approximated from the atm)uses a high-velocity (Mach 0.3) flame and is designed to intersection of the average slopes of the hysteresis loops for simulate the combustion environments of turbine engines stresses 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-debondi a field emission scanning electron microscope(FESEM), Hitachi composite and -35 MPa for the outside-debonding composite Model S-4700. A fiber push-in technique.4 was performed on Figure 3 shows typical stress-strain curves(hysteresis loops 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 some specimens was examined using Auger electron spectroscopy composites were evident for room-temperature stress-strain be- (AES) and transmission electron microscopy (TEM), For AES. havior:"outside-debondingcomposites had (1) lower elastic mall slivers of composite material were fractured in bending in situ under vacuum to prevent the fracture surface from contami moduli (Table I)and (2)a higher strain at a given applied stress including higher strains to failure (Table I and Fig. 3). However. nation. Depth profiles were then performed at regions where the 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. Figure 4 shows examples of composite fracture surfaces after IL. 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 lout 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 nd AE activity are shown in Fig. 2 for MI SYL-iBN/SiC c)and (d)). Note the adherence of the BN layer to the fibers for omposite 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 t 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 he close packing of fibers in woven bundles, debonding between SYL-IBN 14 35087emBf=02 E=280 GPa 岳 0 04日 Strain, 500 SYL-BN 1.4 8. epcm: 8 ply, f0.2 E=216 GP 350 300 250 150 50 0.1 0.2 0.3 0.4 0.5 0.6 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
January 2004 Effect of a BN Interphase That Debonds between the Interphase and the Matrit in SiC/SiC Composites 600 was estimated from the measured final crack density of failed composites multiplied by the normalized cumulative AE energy 500 Outside Debond ng ( Fig. 2), assuming the latter represented the stress-dependent Inside Debonding distribution of matrix cracks, which has been demonstrated for Inside Debonding similar systems. Therefore. the only variable not known was T. E f=0.18 which was adjusted to best fit the predicted stress-strain curve to d30018py the experimental stress-strain curve. For the case where the sliding B SYL-iBN lengths overlap, Ahn and Curtin showed that if the cracks are utside Debonding f=0.17 ll equally spaced, the composite strain could then be modeled by 100 18 epi e=o/(,+au/Er-a(o+oM4E S(c)p I (for p<28) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Therefore, for higher applied stress conditions, if pe I<28 was Strain, predicted, Eq. (4) was used. Examples of best-fit stress-strain curves for inside-debonding (T 73 MPa) and outside-debonding Fig. 3. Room-temperature tensile stress-strain curves for 8.7 epem (T-18 MPa) composite specimens are shown in Figs. 2(a) and (b). respectively (hysteresis 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 for 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 in Fig. 3(a)) posites often displayed a secondary modulus before significar 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 different volume fractions. The initial elastic moduli were very Stress rupture at 815C was performed on SYL and SYL-iBN onsistent(218 GPa) and all of the curves showed an inflection composites with inside and outside debonding(Fig. 6). The MPa). This inflection was not associated with any AE activity: i.e., debonding have been reported in Refs. 20 and 21. Since the panels it appears that this inflection was not due to matrix crack varied in fiber volume fraction, the rupture stress data are plotted formation as the stress on the fibers, ie. the load in a matrix crack that wa Finally, the interfacial shear strength of several different inside carried by the fibers. For comparison, the rupture stress come. 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 fiber com- 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 are natu E=U/ES +a8(o)p/Eo +on) rally spread apart from one another with the formation of the100 (forp2>26) nm BN layer on the fiber surface. The rupture life depends on the time it takes to bond nearest-neighbor fibers together, which takes where o is the applied stress. o,, is the residual (thermal)stress in more time with increasing separation distance. In addition, the 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. In other respectively, and pe is the matrix crack density. The first 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 ber surface, which contained SiC. extra strain (displacement) of the fibers at and away from a For both fiber composite systems possessing an outside- through-thickness matrix crack dictated by the sliding length debonding interface. further improvements in intermediate- temperature stress-rupture life were observed (Fig. 6). For SYL 2 debonding composites, stress-rupture improved by over 250 MPa where in fiber stress (-50 MPa for an /=0.2 composite). For a=(1-f)E=∥E outside-debonding SYL-iBN composites in comparison to inside debonding SYL- BN composites, there was over an order of and ris the fiber radius, f is the fiber volume fraction in the loading magnitude in time improvement at high stresses and --200 MP direction, and T is the interfacial shear strength, Ec and o,were improvement in fiber stress (40 MPa for an/=0,2 composite determined from the stress-strain curves. Er is 380 GPa and E at lower stresses near the run-out condition. It should be noted that was determined from the rule-of-mixtures. The stress-dependent p hese high-stress conditions for stress-rupture are significantly
Journal of the American Ceramie Society-Morscher er al Vol 87. No. 1 2014 Fib BN Fig. 4. FESEM images of fracture surfaces of SYL- iBN composites showing outside debonding (a, b) and inside debonding (e, d). higher than the stresses for matrix cracks to penetrate the load- contact. the thinner areas of Bn were oxidized and fiber-to-fiber bearing fibers (determined from the onset of hysteresis loop fusion occurred for rupture times greater than 80 h. There were activity,-175 MPa for/= 0.2 composites used in this study ) In several regions of significant fiber pullout throughout the cross 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 did not fail after long periods of time. L A few specimens(SYL and SYL-iBN) were precracked at room mperature and compared with the rupture behavior of pristine Examination of the rupture specimen fracture surfaces con- specimens from the same panel (Fig. 8). It was evident that firmed the survival of most of the BN around the fibers in the inside-debonding SYL composites with nominally good matrix crack even though significant oxidation had occurred in the properties were significantly poorer in rupture behavior with matrix crack (Fig. 7). However, at regions of near fiber-to-fiber precracking as has been observed in another study. On the other 6001 77 GPa 50050 f=0.18 f 400 Strain. f=0.17 = Change in slope at 70 MPa not associated with occurrence of ae 0.1 030 Strain, Fig. 5. Room-temperature tensile stress-strain curves for a number of outside-debonding composites with difterent fiber volume tractions