at.y. Appl Ceram. Technol, 7/ 3)304-315(2010) DOk:10.II1117447402200902469x International Journal o pplied Ceramic TECHNOLOGY ceramic Product D Cracking Resistance of Silicon Carbide Composites by Single- and Double-Notched Specimen Techniques Takashi Nozawaand Hiroyasu Tanigawa Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 66. This paper aims to evaluate fracture resistance of nanoinfiltration transient eutectic phase-sintered(NIT -reinforced silicon carbide matrix(SiC/SiC)composites, using the single-edge notched bend(SENB)test notch tensile(DNT)test. Both SENB and DNT test results identified notch insensitivity of NITE-SiC/SiC es. With the fact of notch insensitivity, unique proportional limit stress, and fracture strength were identified regardless of the presence of artificial notches. By applying the nonlinear fracture mechanics, micro- and macrocracking energies were separately stimated. Specifically, lower microcrack formation energy was identified compared with conventional low-stiffness composite system. Finally, superior crack resistance of NITE-SiC/SiC composites was clearly demonstrated Introduction line and near-stoichiometric SiC itself has inherently good chemical stability at high temperatures, strength Highly crystalline and near-stoichiometric silicon retention, specific strength, and low activation/low af- carbide fiber-reinforced silicon carbide matrix(SiC/SiC) ter-heat properties Of many composite types, new class composites are attractive materials for nuclear fusion "nuclear-grade"SiC/SiC composites namely (1)robust and advanced fission energy systems, as well as nonnu- and dense SiC/SiC composites produced by the nano- clear high-temperature industries, because high cryst infiltration transient eutectic(NITE) phase-sintered process and (2) high-purity SiC/SiC composites by This research was supported by a collaboration between the Japan Atomic Energy Ag the chemical vapor infiltration(CVD) and Institute of Energy Science and Technology Co. Ltd. This work was partly sponsor ising with perceived merits like excellent baseline me- the Japan Society for the Promotion of Science (SPS)under the aor 9860089, Grant-in-Aid for Young Scientists (Start-up). of KAKENHI chanical properties and proven radiation stability of microstructure and strength under certain irradiation 2009 The American Ceramic Society nditions. Specifically, the good gas tightness of the
Cracking Resistance of Silicon Carbide Composites by Single- and Double-Notched Specimen Techniques Takashi Nozawa* and Hiroyasu Tanigawa Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan This paper aims to evaluate fracture resistance of nanoinfiltration transient eutectic phase-sintered (NITE)–silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites, using the single-edge notched bend (SENB) test and the doublenotch tensile (DNT) test. Both SENB and DNT test results identified notch insensitivity of NITE–SiC/SiC composites. With the fact of notch insensitivity, unique proportional limit stress, and fracture strength were identified regardless of the presence of artificial notches. By applying the nonlinear fracture mechanics, micro- and macrocracking energies were separately estimated. Specifically, lower microcrack formation energy was identified compared with conventional low-stiffness composite system. Finally, superior crack resistance of NITE–SiC/SiC composites was clearly demonstrated. Introduction Highly crystalline and near-stoichiometric silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composites are attractive materials for nuclear fusion and advanced fission energy systems, as well as nonnuclear high-temperature industries, because high crystalline and near-stoichiometric SiC itself has inherently good chemical stability at high temperatures, strength retention, specific strength, and low activation/low after-heat properties. Of many composite types, new class ‘‘nuclear-grade’’ SiC/SiC composites namely (1) robust and dense SiC/SiC composites produced by the nanoinfiltration transient eutectic (NITE) phase-sintered process and (2) high-purity SiC/SiC composites by the chemical vapor infiltration (CVI) process are promising with perceived merits like excellent baseline mechanical properties1–3 and proven radiation stability of microstructure and strength under certain irradiation conditions.4 Specifically, the good gas tightness of the Int. J. Appl. Ceram. Technol., 7 [3] 304–315 (2010) DOI:10.1111/j.1744-7402.2009.02469.x Ceramic Product Development and Commercialization This research was supported by a collaboration between the Japan Atomic Energy Agency and Institute of Energy Science and Technology Co. Ltd. This work was partly sponsored by the Japan Society for the Promotion of Science (JSPS) under the contract of KAKENHI 19860089, Grant-in-Aid for Young Scientists (Start-up). *nozawa.takashi67@jaea.go.jp r 2009 The American Ceramic Society
wwceramics. org/ACT Cracking Resistance of Silicon Carbide Composites dense NITE-SiC/SiC composites would provide an ad- would impact the failure behavior. Of particular impor ditional advantage to apply this to gas-cooled systems. tance is then to separately discuss micro-and macrocrack- Accordingly, several fusion blanket designs propose to ing behaviors. A simple guess indicates that, as the failure utilize this class of SiC/SiC composites. With the behavior generally depends on the type and distribution of completion of the "proof-of-principle"phase, the R&d the internal Aaws as a crack origin, there would be an ap- on SiC/SiC composites is now shifting to the more parent difference between dense NITE-SiC/SiC compos pragmatic phase like material data generation and en- ites and conventional porous composites such as CVI- gineering design. SiC/SiC composites. The failure initiation behavior would One of key features of nuclear-grade SiC/SiC com- also vary from the choice of fabric architecture and applied osites is high stiffness due to the utilization of high-crys- loading directions due to the inherent anisotropy of the talline SiC for both reinforcing fibers and matrix. When composites. Therefore, careful discussion is necessary applying this class of composite materials, crack propaga This study aims to identify crack resistance and tion behavior, that is, matrix crack pop-in and crack ex- damage tolerance of a new class of SiC/SiC composites, tension, therefore, needs to be clarified first and the failure that is, NITE-SiC/SiC composites. For this purpose, the criterion needs to be properly determined to deliver in- existing testing methodology for fracture toughness eval formation for the engineering design activities, as it has uation was first applied. Second, the developmental frac- been addressed that the inherent "brittle-like "fracture ture resistance method based on nonlinear fracture (so-called"quasi-ductile"fracture) gives significant diffi- mechanics was applied and the damage accumulation be lty in engineering component design. Note that this havior of composites was evaluated using various notched metals as this quasi-ductility occurs as a result of cumu- lative accumulation of irreversible permanent damages Historically, many efforts have been devoted for the crack propagation analysis by fracture toughness testing Experimental Procedure methodology using notched specimens. -14 Rice devel- Materials oped a path-independent integral method, so-called J integral, and demonstrated the validity of this method Two types of pilot-grade NITE-SiC/SiC for approximate estimates of strain concentrations at ites were produced by the Institute of Energy Science smooth-ended notch tips in elastic and elastic-plastic and Technology(Ube, Japan)(Table I). For the first Hashida composite, a -250-nm-thick pyrolytic carbon(PyC et al. further developed the fracture testing technic interphase as a form of fiber/matrix(F/M) interface was for ceramic matrix composites, which have matrix chemically vapor deposited(CVD)on the fiber surface toughness comparable with bridging toughness induced prior matrix densification (hereafter"NITE-Thick by fibers in the fracture process zone. One drawback of Coat"). In contrast, for the other type, a thin PyC in uate the postpeak part of the stress-displacement relation- case, the F/M interfacial coating was not successful.r the Hashida's approach is the limited applicability to eval- terphase(<50 nm)was designed. However, in the latter ship. Another attempt for the cracking energy evaluation formed, as clearly shown in micrographs in Table I. The has been carried out using unloading/reloading hysteresis major parts of fibers were uncoated, although thin PyC curves of the load-displacement relationship. 12-14 This was formed for limited number of fibers. In this study, technique separately distinguishes irreversible energies we designate this composite"NITE-Thin-Coat " For from elastic energy and crack formation energy. Similarl both types, highly crystalline and near-stoichiometric for fast fracture property evaluation, such hysteresis ap- Tyranno-SA third-grade SiC fibers were uni-direc- nportant composite tionally reinforced ith a fiber volume fraction of parameters can be obtained successfully. However, due 0.4-0.45. Typical micrographs show well-densified ma- to the complex failure behaviors of ites, it is some- trix indicating that the porosity of this material was very how questionable whether these techniques are fully ap- low(<5%). A secondary phase(white contrast in Table plicable to composites. Indeed, there is a general D), which was reportedly an oxide phase composed of understanding that damage accumulation by microcrack sintering additives such as Al2O3, SiO2, and Y20 nitiates prior the complete fracture of composites and this was localized in the matrix, specifically within intrabun-
dense NITE–SiC/SiC composites would provide an additional advantage to apply this to gas-cooled systems.5 Accordingly, several fusion blanket designs propose to utilize this class of SiC/SiC composites.6–8 With the completion of the ‘‘proof-of-principle’’ phase, the R&D on SiC/SiC composites is now shifting to the more pragmatic phase like material data generation and engineering design. One of key features of nuclear-grade SiC/SiC composites is high stiffness due to the utilization of high-crystalline SiC for both reinforcing fibers and matrix. When applying this class of composite materials, crack propagation behavior, that is, matrix crack pop-in and crack extension, therefore, needs to be clarified first and the failure criterion needs to be properly determined to deliver information for the engineering design activities, as it has been addressed that the inherent ‘‘brittle-like’’ fracture (so-called ‘‘quasi-ductile’’ fracture) gives significant diffi- culty in engineering component design. Note that this quasi-ductility is totally different from the ductility of metals as this quasi-ductility occurs as a result of cumulative accumulation of irreversible permanent damages. Historically, many efforts have been devoted for the crack propagation analysis by fracture toughness testing methodology using notched specimens.9–14 Rice developed a path-independent integral method, so-called J integral, and demonstrated the validity of this method for approximate estimates of strain concentrations at smooth-ended notch tips in elastic and elastic–plastic materials.9 Based on this J integral approach, Hashida et al. 11 further developed the fracture testing technique for ceramic matrix composites, which have matrix toughness comparable with bridging toughness induced by fibers in the fracture process zone. One drawback of the Hashida’s approach is the limited applicability to evaluate the postpeak part of the stress–displacement relationship. Another attempt for the cracking energy evaluation has been carried out using unloading/reloading hysteresis curves of the load–displacement relationship.12–14 This technique separately distinguishes irreversible energies from elastic energy and crack formation energy. Similarly, for fast fracture property evaluation, such hysteresis approach has been widely adopted and important composite parameters can be obtained successfully.15 However, due to the complex failure behaviors of composites, it is somehow questionable whether these techniques are fully applicable to composites. Indeed, there is a general understanding that damage accumulation by microcracks initiates prior the complete fracture of composites and this would impact the failure behavior. Of particular importance is then to separately discuss micro- and macrocracking behaviors. A simple guess indicates that, as the failure behavior generally depends on the type and distribution of the internal flaws as a crack origin, there would be an apparent difference between dense NITE–SiC/SiC composites and conventional porous composites such as CVI– SiC/SiC composites. The failure initiation behavior would also vary from the choice of fabric architecture and applied loading directions due to the inherent anisotropy of the composites. Therefore, careful discussion is necessary. This study aims to identify crack resistance and damage tolerance of a new class of SiC/SiC composites, that is, NITE–SiC/SiC composites. For this purpose, the existing testing methodology for fracture toughness evaluation was first applied. Second, the developmental fracture resistance method based on nonlinear fracture mechanics was applied and the damage accumulation behavior of composites was evaluated using various notched specimens. Experimental Procedure Materials Two types of pilot-grade NITE–SiC/SiC composites were produced by the Institute of Energy Science and Technology (Ube, Japan) (Table I). For the first composite, a B250-nm-thick pyrolytic carbon (PyC) interphase as a form of fiber/matrix (F/M) interface was chemically vapor deposited (CVD) on the fiber surface prior matrix densification (hereafter ‘‘NITE-ThickCoat’’). In contrast, for the other type, a thin PyC interphase (o50 nm) was designed. However, in the latter case, the F/M interfacial coating was not successfully formed, as clearly shown in micrographs in Table I. The major parts of fibers were uncoated, although thin PyC was formed for limited number of fibers. In this study, we designate this composite ‘‘NITE-Thin-Coat.’’ For both types, highly crystalline and near-stoichiometric Tyrannot-SA third-grade SiC fibers were uni-directionally reinforced with a fiber volume fraction of 0.4–0.45. Typical micrographs show well-densified matrix indicating that the porosity of this material was very low (o5%). A secondary phase (white contrast in Table I), which was reportedly an oxide phase composed of sintering additives such as Al2O3, SiO2, and Y2O3, 16 was localized in the matrix, specifically within intrabunwww.ceramics.org/ACT Cracking Resistance of Silicon Carbide Composites 305
International ournal of Applied Ceramic Technolog-Nozawa and Tanigawa Vol.7,No.3,2010 Table I. List of SiC/SiC Composites Tested NITE-Thick-Coat NITE-Thin-Coat PIP-Coat Typic microstructure Fiber Tyranno-SA3 Tyranno-SA3 Tyranno-SA3 Architecture UD Fiber volume fraction ~045 ~0.4 0.3 F/M interface 250nm Pyc None 150nm Pyc NITE-SIC NITE-SiC PIP-SiC Density 296g/cm3 319gm3212gcm3 Porosity 0.05 0.02 ~0.20 Proportional Limit 136 MPa(Tensile)204 MPa(Tensile)62 MPa(Tensile) Stress(PLS) Fracture Strength 146 MPa(Tensile) 674 MPa(Bend) 204 MPa(Tensile) 249 MPa(Tensile Elastic Modulus 336 GPa(Tensile) 27 GPa(Bend) 410 GPa(Tensile)45 GPa(Tensile) dles. This oxide phase, however, would not significantly Single-Edge Notched Bend Test(SENB) mpact test results at room temperature. For comparison, a two-dimensional (eight-harness) accumulation ehavior was evaluated by satin-woven SiC/SiC composite was fabricated by the the SENB technique. Various size st polymer impregnation and pyrolysis(PIP)method (Ube different initial notch depth(ao) were applied ("SENB- Industries, Ube, Japan)(hereafter"PIP-Coat"). Tyr- 1-4"in Fig. 1). Note that the width(W) to length(Lo) annoSA third-grade SiC fibers were applied. A ratio was fixed for all specimen types. For comparison, 100-nm-thick CVD-SiC and a subsequent x 150- unnotched bend specimens with a size of nm-thick PyC coating were formed on the fiber surface. 20 mm x 4 mm x 1.5 mm were tested. The number of This PIP process produces the SiC matrix of stoichio- test specimens is also listed in the table in Fig. 1. The metric composition(C/Si a 1)using a blend polymer fiber longitudinal direction for both specimen types of polycarbosilane and polymethylsilane. However, set perpendicular to the loading direction. Test coupons mall pores were distributed in the PIP-SiC matrix as including an artificial straight notch were machined shown in Table I, resulting in a comparably lower den- from the composite plate by a diamond saw with blade sity of x2. 1 g/cm. Details of the piP process are de thickness of <0.5 mm. The specimen surfaces were scribed in nozawa et al. I7 then polished using a standard metallographic tech
dles. This oxide phase, however, would not significantly impact test results at room temperature. For comparison, a two-dimensional (eight-harness) satin-woven SiC/SiC composite was fabricated by the polymer impregnation and pyrolysis (PIP) method (Ube Industries, Ube, Japan) (hereafter ‘‘PIP-Coat’’). Tyrannot-SA third-grade SiC fibers were applied. A B100-nm-thick CVD-SiC and a subsequent B150- nm-thick PyC coating were formed on the fiber surface. This PIP process produces the SiC matrix of stoichiometric composition (C/Si 1) using a blend polymer of polycarbosilane and polymethylsilane. However, small pores were distributed in the PIP–SiC matrix as shown in Table I, resulting in a comparably lower density of B2.1 g/cm3 . Details of the PIP process are described in Nozawa et al.17 Single-Edge Notched Bend Test (SENB) Damage accumulation behavior was evaluated by the SENB technique. Various size specimens with a different initial notch depth (a0) were applied (‘‘SENB- 1–4’’ in Fig. 1). Note that the width (W) to length (L0) ratio was fixed for all specimen types. For comparison, unnotched bend specimens with a size of 20 mm 4 mm 1.5 mm were tested. The number of test specimens is also listed in the table in Fig. 1. The fiber longitudinal direction for both specimen types was set perpendicular to the loading direction. Test coupons including an artificial straight notch were machined from the composite plate by a diamond saw with blade thickness of o0.5 mm. The specimen surfaces were then polished using a standard metallographic technique Table I. List of SiC/SiC Composites Tested 306 International Journal of Applied Ceramic Technology—Nozawa and Tanigawa Vol. 7, No. 3, 2010
wwceramics. org/ACT Cracking Resistance of Silicon Carbide Composites ○ Materials spec. o m, ITit me. L. Depth, a ofs Fig. I. Drawing of the single-edge notched bend (SENB)test NITE-Thick-Coat with a surface finish of l um for the crack extension observation. The radius of the notch root was approx- mately 150 um. The SENB tests were conducted at Fig. 2. Drawing of the double-notch tensile(DNT) test specimen room temperature in ambient air using an electrome- chanical testing machine ng a threepoint bend fixture with a support span(L) of 16 mm for SENB-1 3 and 32 mm for SENB-4. diamond saw. The radius of the notch root was The crack opening displacement(COD) was measured 150 um. The DNT tests were conducted at room by the clip-on-type gauge. The constant crosshead dis- temperature using an electromechanical testing machine placement rate was 0.1 mm/min. Unloading/reloading with a wedge-type gripping device. The COd was mea sequences were applied to evaluate the damage accumu- sured by a pair of Cod gauges. An average of two lation behavior during the test. In this paper, test results readings was applied in the calculation. A constant of NITE-Thick-Coat are reported. No data are pres- crosshead displacement rate was 0.5 mm/min. Unload ently available for NITE-Thin-Coat. Furthermore, en- ing/ reloading sequences were applied to evaluate the tire tests for PIP-Coat were invalid because of the damage accumulation behavior during the tests. The compressive failure at the loading point. Details are de- DNT tests were applied to all SiC/SiC composite types scribed in nozawa et al listed in table i Double-Notch Tensile(DNT) Test Figure 2 shows a photo image of the test setup and Microstructural Observation a drawing of the dNT test specimen. Notched tensile A crack extension length from the initial artificial specimens with a different initial notch depth(ao)were tch was measured using optical microscopy. For this used. For comparison, unnotched rectangular tensile purpose, replica films of the surface microstructure of the specimens (40 mm x 4mm x 1.5 mm) with a gauge specimen were taken at each unloading point for SeNB length of 15 mm were tested. The number of test spec- tests. For DNT tests, the crack extension length could not mens are also listed in Fig. 2. The fiber longitudinal be measured primarily due to unstable crack extension just direction was set parallel to the loading direction. Test prior fracture. Fracture surfaces were observed using field- hined from the composite plate by mission scanning electron microscopy
with a surface finish of B1 mm for the crack extension observation. The radius of the notch root was approximately 150 mm. The SENB tests were conducted at room temperature in ambient air using an electromechanical testing machine. Test specimens were loaded using a threepoint bend fixture with a support span (L) of 16 mm for SENB-1 B3 and 32 mm for SENB-4. The crack opening displacement (COD) was measured by the clip-on-type gauge. The constant crosshead displacement rate was 0.1 mm/min. Unloading/reloading sequences were applied to evaluate the damage accumulation behavior during the test. In this paper, test results of NITE-Thick-Coat are reported. No data are presently available for NITE-Thin-Coat. Furthermore, entire tests for PIP-Coat were invalid because of the compressive failure at the loading point. Details are described in Nozawa et al.18 Double-Notch Tensile (DNT) Test Figure 2 shows a photo image of the test setup and a drawing of the DNT test specimen. Notched tensile specimens with a different initial notch depth (a0) were used. For comparison, unnotched rectangular tensile specimens (40 mm 4 mm 1.5 mm) with a gauge length of 15 mm were tested. The number of test specimens are also listed in Fig. 2. The fiber longitudinal direction was set parallel to the loading direction. Test coupons were machined from the composite plate by a diamond saw. The radius of the notch root was B150 mm. The DNT tests were conducted at room temperature using an electromechanical testing machine with a wedge-type gripping device. The COD was measured by a pair of COD gauges. An average of two readings was applied in the calculation. A constant crosshead displacement rate was 0.5 mm/min. Unloading/reloading sequences were applied to evaluate the damage accumulation behavior during the tests. The DNT tests were applied to all SiC/SiC composite types listed in Table I. Microstructural Observation A crack extension length from the initial artificial notch was measured using optical microscopy. For this purpose, replica films of the surface microstructure of the specimen were taken at each unloading point for SENB tests. For DNT tests, the crack extension length could not be measured primarily due to unstable crack extension just prior fracture. Fracture surfaces were observed using fieldemission scanning electron microscopy. Fig. 2. Drawing of the double-notch tensile (DNT) test specimen. Fig. 1. Drawing of the single-edge notched bend (SENB) test specimen. www.ceramics.org/ACT Cracking Resistance of Silicon Carbide Composites 307
International ournal of Applied Ceramic Technolog-Nozawa and Tanigawa Vol.7,No.3,2010 NITE-Thin-Coat NITE-Thick-Coat 0001002003004005006 Strain [%] Strain [%J tensile fracture behaviors of a) nanoinfltration transient eutectic(NITEr and(b) polymer-impregnation Results ductility. The big difference was in the high fracture strain (0.9%)compared with those of NITE-SiC/SiC com- Tensile Properties posites(<0.1%). Additionally, the proportional limit Figure 3 shows typical tensile stress versus strain stress(PLS), that is, an equivalent stress when matrix cracking initiates, was low ( 30 MPa)due to the brittle curves of SiC/SiC composites by the standard tensile PIp-SiC matrix For both types of NITE-SiC/SiC com- rest and reduced data are listed in Table l. the nite- Thin-Coat composite exhibited brittle fracture. The scan- posites, the PLS was quite high(<150 MPa)due to im- brittle surface probably due to the strong bonding at the by the NITe prolif Ensity of the SiC matrix produced proved stiffness and der ning electron microscope image(Fig 4a)clearly shows the F/M interface. No fiber pullout was observed in the frac- SENB Test Results ture surface. In contrast, NITE-Thick-Coat showed quasi- ductility coupled with fiber pullouts due to the cumulative Figure 5 shows a typical load versus COD curve debonding until fracture(Fig. 4b). However, the total during SENB tests for NITE-Thick-Coat and Fig.6ex- elongation was not so significant even in this case. In contrast, the PIP-Coat composite showed better quasi- ing stage of the SENB-3 specimen. From Fig NITE-Thin ick-Coa 50 Fig 4. Typical fracture surface images of nanoinfilration transient-eutectic(NITESiC/SiC composites
Results Tensile Properties Figure 3 shows typical tensile stress versus strain curves of SiC/SiC composites by the standard tensile test and reduced data are listed in Table I. The NITEThin-Coat composite exhibited brittle fracture. The scanning electron microscope image (Fig. 4a) clearly shows the brittle surface probably due to the strong bonding at the F/M interface. No fiber pullout was observed in the fracture surface. In contrast, NITE-Thick-Coat showed quasiductility coupled with fiber pullouts due to the cumulative debonding until fracture (Fig. 4b). However, the total elongation was not so significant even in this case. In contrast, the PIP-Coat composite showed better quasiductility. The big difference was in the high fracture strain (B0.9%) compared with those of NITE–SiC/SiC composites (o0.1%). Additionally, the proportional limit stress (PLS), that is, an equivalent stress when matrixcracking initiates, was low (B30 MPa) due to the brittle PIP–SiC matrix. For both types of NITE–SiC/SiC composites, the PLS was quite high (B150 MPa) due to improved stiffness and density of the SiC matrix produced by the NITE process. SENB Test Results Figure 5 shows a typical load versus COD curve during SENB tests for NITE-Thick-Coat and Fig. 6 exhibits typical specimen surface micrographs in each loading stage of the SENB-3 specimen. From Fig. 5, Fig. 4. Typical fracture surface images of nanoinfiltration transient-eutectic (NITE)–SiC/SiC composites. Fig. 3. Typical tensile fracture behaviors of (a) nanoinfiltration transient-eutectic (NITE)– and (b) polymer-impregnation and pyrolysis (PIP)–SiC/SiC composites. 308 International Journal of Applied Ceramic Technology—Nozawa and Tanigawa Vol. 7, No. 3, 2010