Composites Science and Technology 56(1996)1341-134 C 1yy/ Published by Elsevier Science Limited ELSEVIER PII:S0266·3538(96)00008·5 96/51500 INTERFACE COMPATIBILITY IN CERAMIC-MATRIX COMPOSITES L.R. Hwang, J.W. Fergus, H. P. Chen B. Z Jang Materials Engineering Program, 202 Ross Hall, Auburn University, Auburn, Alabama 36849, USA (Received 20 April 1995; revised 28 November 1995; accepted 15 December 1995 Abstract converted into ceramics by high-temperature pyroly Critical issues of interface modifications in ceramic- sis. Polymer precursors can be utilized to prepare matrix composites have been investigated. Minimal ceramic materials in a variety of useful forms such a interaction between the fiber and the matrix is essential powders, coatings, fibers, hollow spheres, foams, or to achieving good toughness in ceramic-fiber-reinforced monoliths, Examples of polymer-derived ceramics Si-C-O composites. Introduction of a thin barrier include silicon carbide, ilicon nitride. 8-20 boron minimize such interaction. The flexural strength of a been reviewed by several researchers, subject Polycarbosilane, first developed by Yajima et al., -o Nicalon fibers was found to be five times higher than is a commonly used precursor to silicon carbide fibers that of the composite containing uncoated Nicalon Topics related to the synthesis procedure, pyrolysis fibers. Chemical reactions between the barrier coating conditions, polymer and ceramic characterization, and and the matrix and those between the coating and the applications of polycarbosilane have been discussed in fiber must be avoided in such a three-phase material. A the litcraturc.- A commercially available continuous methodology for selecting candidate fiber coating silicon carbide fiber(Nicalon), which was derived use of chemical from polycarbosilane, following Yajima's work alculations is presented. C1997 Published by Elsevier exhibits high strength and high modulus. In the Science limited present investigation, polycarbosilane was used as the precursor to the ceramic matrix (Si-C-O)while Keywords: ceramic-matrix composites, interface com Nicalon fiber was used to reinforce the matrix. patibility, Nicalon, fiber coating The first objective of the present study was to learn more about the mechanical behavior of Si-C-o 1 INTRODUCTION matrix composites containing either coatedor uncoated fibers. The Si-C-O matrix was formed by Mechanical properties of continuous-fiber-rcinforccd converting polycarbosilane into a mixture of very ceramic-matrix composites(CMCs)depend to a great fine-grained B silicon carbide, silicon oxide, and excess extent on the characteristics of the fiber/matrix carbon through a pyrolysis process. Three types of interface. Weak fiber/ matrix interface bonds in a continuous fibers were used: mullite fiber(Nextel brittle matrix composite permit toughening mechan- 312), carbon fiber and silicon carbide(Nicalon)fiber isms such as interfacial debonding, fiber pull-out, and Without a proper interface modification, all of the crack bifurcation to operate. In contrast, a strong composites investigated exhibited a brittle fracture interfacial bond tends to allow a crack to propagate mode, leading to low flexural strengths. In order to straight through the structure, resulting in low fracture prevent the catastrophic failure of these ceramic toughness. -Interfacial chemical reactions can occur composites, two types of pyrolytic carbon thin films many fiber/matrix combinations, leading to the were coated on the fiber surface to prevent potential ormation of a strong interface bond. A barrier reaction between the fiber and the matrix. Thc coating on the fiber surface has often been used to flexural strength of the composite was improved prevent or minimize such fiber matrix interactions. considerably when a thin carbon coating layer Organometallic polymers, as a precursor to deposited onto Nicalon fibers prior to composite ceramics, generally contain elements such as silicon fabrication carbon, boron, nitrogen or titanium in their backbone Carbon may not be an ideal fiber-coating material side group or ladder structure. These polymers can be for composites used in oxidizing environments Therefore, alternative materials such as Si3 N4, bn, B To whom correspondence should be addressed TiC, ZrC, and Al,O3 were considered and evaluated
Composites Science and Technology 56 (1996) 1341-1348 0 1997 Published by Elsevier Science Limited ELSEVIER PII: SO266-3538(96)00008-S Printed in Northern Ireland. Ail rights reserved 0266.3538/96/$15.00 INTERFACE COMPATIBILITY IN CERAMIC-MATRIX COMPOSITES L. R. Hwang, J. W. Fergus, H. P. Chen & B. Z. Jang* Materials Engineering Program, 202 Ross Hall, Auburn University, Auburn, Alabama 36849, USA (Received 20 April 1995; revised 28 November 1995; accepted 15 December 1995) Abstract Critical issues of interface modifications in ceramicmatrix composites have been investigated. Minimal interaction between the fiber and the matrix is essential to achieving good toughness in ceramic-fiber-reinforced Si-C-O composites. Introduction of a thin barrier coating on the fiber surface has been utilized to minimize such interaction. The flexural strength of a Si-C-O composite containing CVD carbon-coated Nicalon fibers was found to be five times higher than that of the composite containing uncoated Nicalon jibers. Chemical reactions between the barrier coating and the matrix and those between the coating and the jiber must be avoided in such a three-phase material. A methodology for selecting candidate fiber coating materials by the use of chemical compatibility calculations is presented. 0 1997 Published by Elsevier Science Limited Keywords: ceramic-matrix composites, interface compatibility, Nicalon, fiber coating 1 INTRODUCTION Mechanical properties of continuous-fiber-reinforced ceramic-matrix composites (CMCs) depend to a great extent on the characteristics of the fiber/matrix interface. Weak fiber/matrix interface bonds in a brittle matrix composite permit toughening mechanisms such as interfacial debonding, fiber pull-out, and crack bifurcation to operate. In contrast, a strong interfacial bond tends to allow a crack to propagate straight through the structure, resulting in low fracture toughness.‘-5 Interfacial chemical reactions can occur in many fiber/matrix combinations, leading to the formation of a strong interface bond. A barrier coating on the fiber surface has often been used to prevent or minimize such fiber matrix interactions.5 Organometallic polymers, as a precursor to ceramics, generally contain elements such as silicon, carbon, boron, nitrogen or titanium in their backbone, side group or ladder structure. These polymers can be * To whom correspondence should be addressed. 1341 converted into ceramics by high-temperature pyrolysis. Polymer precursors can be utilized to prepare ceramic materials in a variety of useful forms such as powders, coatings, fibers, hollow spheres, foams, or monoliths. Examples of polymer-derived ceramics include silicon carbide,6-17 silicon nitride,18-” boron nitride,‘lmz4 and their solutions.25~26 This subject has been reviewed by several researchers.27-30 Polycarbosilane, first developed by Yajima et al.,“l’ is a commonly used precursor to silicon carbide fibers. Topics related to the synthesis procedure, pyrolysis conditions, polymer and ceramic characterization, and applications of polycarbosilane have been discussed in the literature.&” A commercially available continuous silicon carbide fiber (Nicalon), which was derived from polycarbosilane, following Yajima’s work, exhibits high strength and high modulus. In the present investigation, polycarbosilane was used as the precursor to the ceramic matrix (Si-C-O) while Nicalon fiber was used to reinforce the matrix. The first objective of the present study was to learn more about the mechanical behavior of Si-C-O matrix composites containing either coated or uncoated fibers. The Si-C-O matrix was formed by converting polycarbosilane into a mixture of very fine-grained p silicon carbide, silicon oxide, and excess carbon through a pyrolysis process. Three types of continuous fibers were used: mullite fiber (Nextel 312), carbon fiber and silicon carbide (Nicalon) fiber. Without a proper interface modification, all of the composites investigated exhibited a brittle fracture mode, leading to low flexural strengths. In order to prevent the catastrophic failure of these ceramic composites, two types of pyrolytic carbon thin films were coated on the fiber surface to prevent potential reaction between the fiber and the matrix. The flexural strength of the composite was improved considerably when a thin carbon coating layer was deposited onto Nicalon fibers prior to composite fabrication. Carbon may not be an ideal fiber-coating material for composites used in oxidizing environments. Therefore, alternative materials such as Si3N4, BN, B, Tic, ZrC, and Al,O, were considered and evaluated
1342 L. R. Hwang et al based on their physical and thermodynamic com Table 1. Basic properties of the fibers used patibility with both the fiber and the matrix. Other types of fibers dispersed in Si-C-O or other ceramic Fib matrices may require different fiber coatings. The Graphite Nextel Nicalon second objective of the present study was, therefore, to establish a general methodology for evaluating and Density(g/cm) 255 selecting proper fiber coating materials to prevent Tensile strength(MPa) 1720 3010 fiber/ matrix reactions in CMCs Tensile modulus(GPa) 152 Diameter (um) 0-12 2 EXPERIMENTAL Coeff. of thermal exp. A:-07, A: 3 5 A: 4.0 2.1 Matrix preparation and characterization (×10/°C) Two types of polycarbosilane synthesized through the thermal decomposition of polydimethyl- silane followed by condensatIo polymerization. Polydimethylsilane used in the matrix preparation was 2.3 Interface modifications supplied by Petrash Chemical Co. Without a reaction Two types of carbon, chemical vapor deposition accelerator, the polydimethylsilane powder was kept (CVD) carbon and polymeric carbon, were used to in the reaction tube at 470C for 18 h. With 3 wt of modify the interface of a Nicalon/Si-C-o composite poly(borodiphenyl siloxane)as a reaction accelerator, In the CVD process, carbon was deposited isother the polydimethylsilane powder was heated at 350C mally onto Nicalon fibers from a hydrogen/ethylene for 5h. Both reactions were conducted in a N2 gas mixture. After the Nicalon fabric was placed in the environment to prevent the reaction between center zone of the reaction tube the reaction tube was polydimethylsilane and oxygen. The flow rate of N urged with helium and hydrogen gases at flow rates gas was kept at 40 std-cm/min of 200 and 60 cm/ T/min, respectively. The furnace sO After the reaction was complete, the reaction temperature was then increased to 1050C and carbon oduct was dissolved in hexane and filtered. The was deposited from the gas mixture at 10..C and hexane was removed by heating the solution at 80'C 1 atm. The flow rates of hydrogen and ethylene were in a N2 gas flowing beaker. After the solvent 60 and 200 cm/min, respectively. The thickness of the removed, a glassy solid of yellowish brown color was carbon was varied by changing the deposition time obtained. The reaction products were characterized by After carbon was deposited, the Nicalon fabric was Fourier transf intrared (FTiR)spectroscopy, cooled in the furnace with helium and hydrogen gas nuclear magnetic resonance(NMR)spectroscopy, and flow ultraviolet (UV) spectroscopy. The weight loss of the In the polymeric carbon process, phenolic resin was reaction products during high temperature pyrolysis first dissolved in ethyl alcohol. Two different was monitored by a thermal gravimetric analyser concentrations of phenolic ethyl alcohol solution, 12 (TGA). After the ceramic precursor was pyrolyzed at and 36 wt%, were prepared and coated on the fiber a high temperature, a hard black ceramic formcd. The surfaces. After the phenolic lesin was converted into crystal structure of this ceramic was characterized by carbon. the carbon-coated fibers were characterized X-ray diffraction. by X-ray diffraction 2.2 Fabrication of composites Plain-weave ceramic fabrics used in the preparation of 2. 4 Simple indentation method composites included graphite(Hercules Aw 193P The interfacial frictional stress of licon carbide (ceramic grade Nicalon NL202), and measured by the method proposed by Marshall. Two mullite(3M Nextel-312) Basic data on these fibers are types of specimens, thin and thick, were used in the given in Table 1. The sizing agents on the fibers were measurement of frictional stress. Both types of removed before the fabrics were impregnated with specimens were cut from the composites with four polycarbosilane. A solution with polycarbosilane cycles of re-impregnation and re-pyrolysis. The dissolved in hexane was coated on the fabrics to form specimen was cut perpendicular to the fiber axis. The prepregs, which were cut, laid up, and compressio urface was first polished by silicon carbide papers molded to consolidate the laminate. The final step to followed by 3 um diamond paste. The thickness of the fabricate the CMC was to convert the polycarbosilane thicker specimen was 5 mm and that of the thinner into a SI-C-O matrix by pyrolysis. In order to specimen was about 0-25 mm. A Leco DM-400 improve the mechanical properties of these compos- hardness tester was used to apply a force to the fiber ites, several cycles of reimpregnation and re-pyrolysis end and to depress the fiber end below the matrix were applied to reduce porosity in the Si-C-O matrix. surface. The hardness of the Nicalon fiber used in the Details of this composite fabrication procedure are calculation of frictional stress was measured on the available elsewhere. I composite containing uncoated fibers at a loading
1342 L. R. Hwang et al. based on their physical and thermodynamic compatibility with both the fiber and the matrix. Other types of fibers dispersed in Si-C-O or other ceramic matrices may require different fiber coatings. The second objective of the present study was, therefore, to establish a general methodology for evaluating and selecting proper fiber coating materials to prevent fiber/matrix reactions in CMCs. 2 EXPERIMENTAL 2.1 Matrix preparation and characterization Two types of polycarbosilane were synthesized through the thermal decomposition of polydimethylsilane followed by condensation polymerization. l1 Polydimethylsilane used in the matrix preparation was supplied by Petrash Chemical Co. Without a reaction accelerator, the polydimethylsilane powder was kept in the reaction tube at 470°C for 18 h. With 3 wt% of poly(borodipheny1 siloxane) as a reaction accelerator, the polydimethylsilane powder was heated at 350°C for 5 h. Both reactions were conducted in a N2 gas environment to prevent the reaction between polydimethylsilane and oxygen. The flow rate of N, gas was kept at 40 std-cm”/min. After the reaction was complete, the reaction product was dissolved in hexane and filtered. The hexane was removed by heating the solution at 80°C in a N2 gas flowing beaker. After the solvent was removed, a glassy solid of yellowish brown color was obtained. The reaction products were characterized by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and ultraviolet (UV) spectroscopy. The weight loss of the reaction products during high temperature pyrolysis was monitored by a thermal gravimetric analyser (TGA). After the ceramic precursor was pyrolyzed at a high temperature, a hard black ceramic formed. The crystal structure of this ceramic was characterized by X-ray diffraction. 2.2 Fabrication of composites Plain-weave ceramic fabrics used in the preparation of composites included graphite (Hercules AW 193P), silicon carbide (ceramic grade Nicalon NL202), and mullite (3M Nextel-312). Basic data on these fibers are given in Table 1. The sizing agents on the fibers were removed before the fabrics were impregnated with polycarbosilane. A solution with polycarbosilane dissolved in hexane was coated on the fabrics to form prepregs, which were cut, laid up, and compression molded to consolidate the laminate. The final step to fabricate the CMC was to convert the polycarbosilane into a Si-C-O matrix by pyrolysis. In order to improve the mechanical properties of these composites, several cycles of reimpregnation and re-pyrolysis were applied to reduce porosity in the Si-C-O matrix. Details of this composite fabrication procedure are available elsewhere.” Table 1. Basic properties of the fibers used Property Density (g/cm’) Tensile strength (MPa) Tensile modulus (GPa) Diameter (pm) Fabric weave Coeff. of thermal exp. (x lO_“/OC) Fiber Graphite Nextel Nicalon 1.78 >2.7 2.55 4200 1720 3010 241 152 197 7-8 10-12 10-15 Plain Plain Plain A: -0.7, A: 3.5 A: 4.0 R: 10 2.3 Interface modifications Two types of carbon, chemical vapor deposition (CVD) carbon and polymeric carbon, were used to modify the interface of a Nicalon/Si-C-O composite. In the CVD process, carbon was deposited isothermally onto Nicalon fibers from a hydrogen/ethylene mixture. After the Nicalon fabric was placed in the center zone of the reaction tube, the reaction tube was purged with helium and hydrogen gases at flow rates of 200 and 60 cm3/min, respectively. The furnace temperature was then increased to 1050°C and carbon was deposited from the gas mixture at 1050°C and 1 atm. The flow rates of hydrogen and ethylene were 60 and 200 cm3/min, respectively. The thickness of the carbon was varied by changing the deposition time. After carbon was deposited, the Nicalon fabric was cooled in the furnace with helium and hydrogen gas flow. In the polymeric carbon process, phenolic resin was first dissolved in ethyl alcohol. Two different concentrations of phenolic ethyl alcohol solution, 12 and 36 wt%, were prepared and coated on the fiber surfaces. After the phenolic resin was converted into carbon, the carbon-coated fibers were characterized by X-ray diffraction. 2.4 Simple indentation method The interfacial frictional stress of the composite was measured by the method proposed by Marshall.31 Two types of specimens, thin and thick, were used in the measurement of frictional stress. Both types of specimens were cut from the composites with four cycles of re-impregnation and re-pyrolysis. The specimen was cut perpendicular to the fiber axis. The surface was first polished by silicon carbide papers, followed by 3 pm diamond paste. The thickness of the thicker specimen was 5 mm and that of the thinner specimen was about 0.25 mm. A Leco DM-400 hardness tester was used to apply a force to the fiber end and to depress the fiber end below the matrix surface. The hardness of the Nicalon fiber used in the calculation of frictional stress was measured on the composite containing uncoated fibers at a loading
Interface compatibility in ceramic-matrix composites 1343 force of 100 gmf. After the fiber ends were depressed crack initiated from the bottom tension side and then were measured by a JEOL-840 scanning electron The specimen with a 0.18 um carba the specimen by the Vickers pyramid, the indentation dimensions propagated through the thickncss a buckling feature on the compression side as well as tensile failure of fibers on the tension side. The 2.5 Three-point bending test buckling feature appeared at approximately the same a three-point bending test was used to measure the time as the load/ deflection curve reached the flexural strengths and flexural moduli of the maximum load. Soon after buckling, fiber breakage composites. Four rectangular bars were prepared from was observed to initiate from the surface of the each sample. The average dimensions of the bars were specimen. This feature indicated that the specimen 5 mm x 1-4 mm x 50 mm. An Instron-1125 test had a relatively weak interfacial bond. In the machine was used in this test. The crosshead speed composite containing fibers with 0.5 um carbon was maintained at 0.5 mm/min, and the span-to-depth coating, the compressive buckling feature became ratio was fixed at 32. Spe simens vere loaded obvious but fiber breakage did not occur right after perpendicular to the fabric plane. After the bending the flexural stress reached the maximum point test, the fracture surfaces of the specimens were Instead, fiber breakage took place at the final stage of examined by scanning electron microscopy. the bending test. This phenomenon implied that the specimen with 0.5 um carbon coating had a relatively 3 RESULTS AND DISCUSSION weak interfacial bond, and that compressive buckling was the dominant failure mechanism. Generally All of the composites that contained untreated fibers speaking, ceramic materials have better compressive showed a catastrophic tailure behavior and did not strength than tensile strength. However, poor exhibit any fiber pull-out phenomena during the interfacial bonding has been shown to degrade the three-Point bending test. The fexural strengths (lower compressive strength of a CMC and therefore lead to than 73 MPa in most cases) of these composites failure from the compression loaded side. ppeared to be too low for structural applications Representative flexural stress/deflection curves are shown in Fig. 1, which demonstrates that the flexural 3.1 Composites containing CVD carbon-coated strength and the work-of-fracture values of carbon fibers coated composites are superior to those of composites SEM examination of the surface of CVD carbon- without a carbon coating. Nevertheless, the flexural coated fibers indicated that coating layers were moduli of all these composites are about the same relatively uniform, and the average thickness was The flexural strength of composites containing fibers approximately 0l, 0. 18, and 0. 5 um for the three with a 0. 5 um carbon coating is about five times CVD durations. The flexural strengths and moduli of higher than that of composites without a carbon Nicalon/Si-C-O composites with and without carbon coating. coating on the fiber surface are given in Table 2. The Figure 2 shows the fracture surface of the strength of carbon coated fiber reinforced composites Nicalon/SiCo composite with uncoated fibers. was much higher than that posite without There is no fiber/ matrix debonding, fiber pull-out, or carbon coating. The addition of a carbon layer did not fracture mirrors of fiber ends on the fracture surface significantly affect the flexural modulus of composites. of the composites without carbon coating. On the fracture surface examination revealed a smooth contrary, all these features can be found on the fracture surface, indicative of a catastrophic failure fracture surface of composites with carbon coating mode, for specimens containing uncoated fibers. The Thin layers of carbon coating were observed on the specimen with a carbon coating of 0. l um in thickness pulled-out fiber surface(Fig. 3). This feature indicates had a relatively rough fracture surface. According to that interfacial debonding took place between the the observations made during the flexural test, the matrix and the coated carbon layer Table 2. Propertics of Nicalon fiber/Si-C-O composites Number of 4 RI 4 RI 4 RI Coating thickness (um) Flexural strength(MPa) 15.4 Flexural modulus(GPa) 12. 8 Density(g/cm) 1·43 er volume 039 040 033 Shear failure at y=0.24 MPa
Interface compatibility in ceramic-matrix composites 1343 force of 100 gmf. After the fiber ends were depressed by the Vickers pyramid, the indentation dimensions were measured by a JEOL-840 scanning electron microscope. 2.5 Three-point bending test A three-point bending test was used to measure the flexural strengths and flexural moduli of the composites. Four rectangular bars were prepared from each sample. The average dimensions of the bars were 5mmX1~4mmX50mm. An Instron-1125 test machine was used in this test. The crosshead speed was maintained at 0.5 mm/min, and the span-to-depth ratio was fixed at 32. Specimens were loaded perpendicular to the fabric plane. After the bending test, the fracture surfaces of the specimens were examined by scanning electron microscopy. 3 RESULTS AND DISCUSSION All of the composites that contained untreated fibers showed a catastrophic failure behavior and did not exhibit any fiber pull-out phenomena during the three-point bending test. The flexural strengths (lower than 73 MPa in most cases) of these composites appeared to be too low for structural applications.” 3.1 Composites containing CVD carbon-coated fibers SEM examination of the surface of CVD carboncoated fibers indicated that coating layers were relatively uniform, and the average thickness was approximately 0.1, 0.18, and 0.5 pm for the three CVD durations. The flexural strengths and moduli of Nicalon/Si-C-O composites with and without carbon coating on the fiber surface are given in Table 2. The strength of carbon-coated fiber-reinforced composites was much higher than that of the composite without carbon coating. The addition of a carbon layer did not significantly affect the flexural modulus of composites. Fracture surface examination revealed a smooth fracture surface, indicative of a catastrophic failure mode, for specimens containing uncoated fibers. The specimen with a carbon coating of 0.1 pm in thickness had a relatively rough fracture surface. According to the observations made during the flexural test, the crack initiated from the bottom tension side, and then propagated through the thickness of the specimen. The specimen with a 0.18 pm carbon coating showed a buckling feature on the compression side as well as tensile failure of fibers on the tension side. The buckling feature appeared at approximately the same time as the load/deflection curve reached the maximum load. Soon after buckling, fiber breakage was observed to initiate from the surface of the specimen. This feature indicated that the specimen had a relatively weak interfacial bond. In the composite containing fibers with 0.5 pm carbon coating, the compressive buckling feature became obvious but fiber breakage did not occur right after the flexural stress reached the maximum point. Instead, fiber breakage took place at the final stage of the bending test. This phenomenon implied that the specimen with 0.5 pm carbon coating had a relatively weak interfacial bond, and that compressive buckling was the dominant failure mechanism. Generally speaking, ceramic materials have better compressive strength than tensile strength. However, poor interfacial bonding has been shown to degrade the compressive strength of a CMC and therefore lead to failure from the compression loaded side.32 Representative flexural stress/deflection curves are shown in Fig. 1, which demonstrates that the flexural strength and the work-of-fracture values of carboncoated composites are superior to those of composites without a carbon coating. Nevertheless, the flexural moduli of all these composites are about the same. The flexural strength of composites containing fibers with a 0.5 pm carbon coating is about five times higher than that of composites without a carbon coating. Figure 2 shows the fracture surface of the Nicalon/Si-C-O composite with uncoated fibers. There is no fiber/matrix debonding, fiber pull-out, or fracture mirrors of fiber ends on the fracture surface of the composites without carbon coating. On the contrary, all these features can be found on the fracture surface of composites with carbon coating. Thin layers of carbon coating were observed on the pulled-out fiber surface (Fig. 3). This feature indicates that interfacial debonding took place between the matrix and the coated carbon layers. Table 2. Properties of Nicalon fiber/Si-C-O composites Number of re-impregations 0 RI 4 RI 4 RI 4 RI 4 RI Coating thickness (pm) 0 0 0.1 0.18 0.5 Flexural strength (MPa) 15.4 43.7 151.2 198.6 239.4 Standard deviation 3.2 3.6 7.9 4.27 23.9 Flexural modulus (GPa) 12.8 51.7 52.9 56.5 54.0 Standard deviation 2,4 2.0 1.7 1.8 3.9 Density (g/cm”) 1.43 1.99 1.99 1.98 1.96 Fiber volume fraction 0.39 0.40 0.39 0.32 0.33 “Shear failure at y = 0.24 MPa
1344 L. R. Hwang et al. NICALON/Si-C-O COMPOSITES C:0.5 um c:0.18μm C: 0.1 um Uncoated Fig 3. Thin carbon layer remaining adhered to the nical 0.002040.60.81.01.21,41,61.82.0 fibcr surfacc aftcr thc flexural tcst Deflection(mm) nicalon/s Representative flexural stress/deflection /Si-C-O composites containing uncoated where fiber sliding could take place: at the those containing CVD carbon coated (three thicknes fiber/carbon interface or the carbon/ matrix interface By testing the thick specimens, the interfacial frictional stress could be measured, but the position where fiber sliding took place was very difficult to 3.1.1 Interfacial bond measurement identify. Therefore, thin specimens containing carbon The interfacial frictional stress and the interfacial coated fibers were prepared and tested. The bonding of Nicalon/Si-C-O composites, with and thicknesses of these specimens were about 0. 25 mm without carbon coating on the fiber, were studied by a The thickness of the thin specimen has to be less than imple indentation method table 3 gives the results the sliding length of the fiber, which can be of the simple indentation test of Nicalon/Si-C-O determined by testing the corresponding thick composites. When the thickness of carbon coating on sdecimenats of the forward inde the fiber surface was increased. the interfacial frictional stress in the thick specimen decreased. SEM two thin specimens were not much different those on the thick specimens. After the indentation carbon-coated fibers had moved a few micrometers test, fibers were found to be pushed out of the bottom relative to the surrounding matrix. On the contrary surface of the specimen( Fig. 5). In most cases, carbon the fibers without carbon coating were not moved by layers could be found on the pushed-out fiber the indenter for a specimen with identical thickness surfaces. This implies that fiber sliding occurred at the instead, the fibers were cracked by the indenter. interface between the matrix and the carbon layer or Figure 4 shows the fibers in a composite after the that the carbon layer failed by shearing. After the indentation test. There are two possible positions fibers were pushed out, a back ward indentation test was applied on these fibers. As given in Table 3, the back ward frictional stresses are about 15 MPa less than those obtained in the forward indentation test This difference could be due to the existence of not only a frictional stress but also an intrinsic intertacial bond between the matrix and the fiber coating or due to the cncrgy associated with shear of the carbon high and of bonding exists between the coated carbon and the matrix, the composites containing CVD carbon-coated fibers exhibit better ite with a flexural strength of 240 MPa can be achieved with 0 5 um of carbon m冒 coated on the fiber surface, but the strength of the can be furth Fig. 2. Fracture surface of Nicalon/Si-C-O composite One method is to increase the fiber volume fraction of the d the other is to re
1344 L. R. Hwang et al. NICALONISI-C-O COMPOSITES 240 I lc: 0.1 Fm\ I I 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1,4 I,6 1.8 2.0 Deflection (mm) Fig. 1. Representative flexural stress/deflection curves of Nicalon/Si-C-O composites containing uncoated fibers, and those containing CVD carbon coated (three thickness values). 3.1.1 Interfacial bond measurement The interfacial frictional stress and the interfacial bonding of Nicalon/Si-C-O composites, with and without carbon coating on the fiber, were studied by a simple indentation method. Table 3 gives the results of the simple indentation test of Nicalon/Si-C-O composites. When the thickness of carbon coating on the fiber surface was increased, the interfacial frictional stress in the thick specimen decreased. SEM and optical microscopy clearly show that the carbon-coated fibers had moved a few micrometers relative to the surrounding matrix. On the contrary, the fibers without carbon coating were not moved by the indenter for a specimen with identical thickness; instead, the fibers were cracked by the indenter. Figure 4 shows the fibers in a composite after the indentation test. There are two possible positions Fig. 2. Fracture surface of Nicalon/Si-C-O composite containing uncoated fibers. Fig. 3. Thin carbon layer remaining adhered to the Nicalon fiber surface after the flexural test. where fiber sliding could take place: at the fiber/carbon interface or the carbon/matrix interface. By testing the thick specimens, the interfacial frictional stress could be measured, but the position where fiber sliding took place was very difficult to identify. Therefore, thin specimens containing carboncoated fibers were prepared and tested. The thicknesses of these specimens were about 0.25 mm. The thickness of the thin specimen has to be less than the sliding length of the fiber, which can be determined by testing the corresponding thick specimen. The results of the forward indentation test on these two thin specimens were not much different from those on the thick specimens. After the indentation test, fibers were found to be pushed out of the bottom surface of the specimen (Fig. 5). In most cases, carbon layers could be found on the pushed-out fiber surfaces. This implies that fiber sliding occurred at the interface between the matrix and the carbon layer or that the carbon layer failed by shearing. After the fibers were pushed out, a backward indentation test was applied on these fibers. As given in Table 3, the backward frictional stresses are about 15 MPa less than those obtained in the forward indentation test. This difference could be due to the existence of not only a frictional stress but also an intrinsic interfacial bond between the matrix and the fiber coating or due to the energy associated with shear of the carbon coating. Although the interfacial frictional stress is relatively high and a certain extent of bonding exists between the coated carbon and the matrix, the composites containing CVD carbon-coated fibers exhibit better mechanical properties than the composites with uncoated fibers. A composite with a flexural strength of 240 MPa can be achieved with 0.5 pm of carbon coated on the fiber surface, but the strength of the composite can be further increased by two methods. One method is to increase the fiber volume fraction of the composite, and the other is to reduce the volume
Interface compatibility in ceramic-matrix composites 1345 Table 3. Matrix/fiber frictional stresses in Nicalon/Si-C-O composites with and without 0 018 05 Thick Frictional stress(MPa) >200 196 Standard deviation Thin forward Shear strength(MPa) (MPa) 168 172 104 Standard deviation Bonding strength(MPa)(forward-backward 172 150 13-4 fraction of pores. Table 4 gives the strengths of the 3.2 Composites containing polymeric carbon-coated composites with different densities. The strength of the composite with a higher density is always higher After the phenolic resin was converted into carbon than that of the composite with a lower density. This the carbon-coated fibers were characterized by X-ray henomenon can be attributed to lower volume diffraction. The X-ray diffraction patterns of these fraction of pores resulting in lower residual stresses phenolic carbon-coated fibers were practically identi- and smaller microcracks in the matrix. The strength of cal to that of the fiber without coating. The C-(002) ite can be further increased by peak was not found in the X-ray diffraction patterns density of the composite, i.e. by decreasing the One can conclude that the phenolic carbon coated on volume fraction of pores the fiber surface is a glassy carbon Microscopic examinations showed that carbon oating produced from phe uniform, and the surface of coating was not smooth. A large fraction of the fiber surface was not covered by carbon. In addition filaments tended to be bonded together by the coated carbon. The composites with the rough, non-uniform carbon coating on the fiber surface demonstrated catastrophic failure modes leading to low flexural strengths. The flexural strengths of the composites containing polymeric carbon-coated fibers are 41 MPa(12% phenolic) and 46 MPa(36% phenolic), which are about the same as that of the composite containing uncoated fibers 10 um Fiber pull-out and fiber/matrix debonding phenomena were not pronounced. Only a small amount of 10 um 10 um Fig. 4. Fiber ends that were indented with a vicke Fig. 5. Fiber end in a thin Nicalon/Si-C-O specimen pyramid at a load of 0. 98 N:(a)uncoated fibers; pushed out by a Vickers pyramid from the other end of this
Interface compatibility in ceramic-matrix composites 1345 Table 3. Matrix/fiber frictional stresses in Nicalon/Si-C-O composites with and without carbon coating Thickness of carbon coating (pm) 0 Thick specimen Frictional stress (MPa) Standard deviation Thin forward Shear strength (MPa) Standard deviation Thin backward Frictional stress (MPa) Standard deviation Bonding strength (MPa) (forward - backward) >200 0.1 0.18 0.5 32.3 29.2 19.6 7.0 6.2 8.0 34.0 32.2 23.8 4.8 5.1 4.4 16.8 17.2 10.4 5.1 4.2 4.0 17.2 15.0 13.4 fraction of pores. Table 4 gives the strengths of the composites with different densities. The strength of the composite with a higher density is always higher than that of the composite with a lower density. This phenomenon can be attributed to lower volume fraction of pores resulting in lower residual stresses and smaller microcracks in the matrix. The strength of a composite can be further increased by increasing the density of the composite, i.e. by decreasing the volume fraction of pores. 3.2 Composites containing polymeric carbon-coated fibers After the phenolic resin was converted into carbon, the carbon-coated fibers were characterized by X-ray diffraction. The X-ray diffraction patterns of these phenolic carbon-coated fibers were practically identical to that of the fiber without coating.” The C-(002) peak was not found in the X-ray diffraction patterns. One can conclude that the phenolic carbon coated on the fiber surface is a glassy carbon. Microscopic examinations showed that carbon coating produced from phenolic resin was not uniform, and the surface of coating was not smooth. A large fraction of the fiber surface was not covered by carbon. In addition, filaments tended to be bonded together by the coated carbon. The composites with the rough, non-uniform carbon coating on the fiber surface demonstrated catastrophic failure modes, leading to low flexural strengths. The flexural strengths of the composites containing polymeric carbon-coated fibers are 41 MPa (12% phenolic) and 46 MPa (36% phenolic), which are about the same as that of the composite containing uncoated fibers. Fiber pull-out and fiber/matrix debonding phenomena were not pronounced. Only a small amount of fiber Fig. 4. Fiber ends that were indented with a Vickers Fig. 5. Fiber end in a thin Nicalon/Si-C-O specimen pyramid at a load of 0.98N: (a) uncoated fibers; (b) pushed out by a Vickers pyramid from the other end of this carbon-coated fibers. fiber
1346 L. R. Hwang et al Table 4. Flexural strengths of Nicalon/Si-C-O composites with different densities Thickness of carbon coating(um) 0-1 018 0-5 Number of tions 4 RI 3 RI RI Flexural strength(MPa) 151.2 1771 1986 Standard deviation 16 Density (g/cm) Fiber volume fraction 038 038 032 033 pull-out and fiber matrix debonding could be observed 4.1 Assessment of potential coating materials at the positions where the fibers were perpendicular to Potential coating materials can be divided into four he fracture surface. Fiber matrix debonding could categories: metals, nitrides, carbides and oxides. One also be found where the fibers were parallel to the or two examples of each type are described below to fracture surface. However, many matrix fragments demonstrate the type of calculations required to adhered to the fiber surface. All these phenomena are evaluate the potential of the material as a fiber attributable to the non-uniform carbon coating. The coating matrix was bonded to the fibers at the sites where the fiber surface was not covered by carbon. Such a strong 4.1.I Metals interfacial bonding may prove detrimental to the A metal could react with Si-C-O to form an oxide toughness of a CMC. Polymeric carbon could possibly carbide or silicide. One potential fiber coating be used to modify the interface in Nicalon/Si Co material is boron. The formation of B2O3 by the composites if the carbon precursor can be uniformly reaction coated on the fiber ace B+2 SiO2=B2O3+2 Si 4 THERMODYNAMIC CALCULATIONS FOR is thermodynamically unfavorable and boron does not INTERFACE COMPATIBILITY form a stable silicide. However, the reaction of boron with carbon As indicated earlier, the composites containing CVD carbon-coated fibers exhibited mechanical properties 4B+C=B,C superior to those of the composites containing is favorable and may result in strong matrix/fib unmodified fibers. Unfortunately, the coated carbon bonding, so boron is not a suitable coating material temperatures higher than 500C. In order to prevent 4.1.2 Nitrides degradation of the mechanical properties of The reaction of a nitride with Si-C-O is similar to the Nicalon/Si-C-O composites at high temperatures, reaction of a metal with Si-C-o except that an other materials may have to be utilized for replacing addition phase will be formed. For example Bn has carbon. The most important factor to consider for been demonstrated to impart significant improvemen evaluating the suitability of a candidate fiber coating in the work of fracture as well as the strength to material is the possibility that chemical reactions may several composite systems, such as Nicalon/mullite, strengthen the bonding at the fiber/ coating or SiC/mullite, and Nicalon/CVI-SiC. The reaction matrix/coating interfaces. Therefore, thermodynamic BN with SiOz compatibility calculations for various interfaces are uscful to evaluate potcntial coating matcrials. 4BN+3SiO2=2B2O3+ Si3N4 X-ray diffraction patterns of the matrix indicated (similar to reaction(1)above)is thermodynamically that the crystallinity of Si-C-O materials depe unfavorable. In addition, the standard free energy for the heat treatment temperature. When the heat the formation of B,C by the reaction treatment temperature was lower than 1000.C, the Si-C-O material was amorphous. The crystallinities 4BN +C- B4C+ 2N2 of SiC and C in Si-C-O increased if a higher heat is positive. However, the nitrogen partial pressur treatment temperature was used. .3 To simplify the not likely to be 1 atm, so the equilibrium nitrogen calculations, the Si-C-O has been treated as a partial pressure at a given temperature pr mixture of the most stable crystalline phases of C, Sic better indication of the possibility of reaction. The and SiO2. Expect where noted, free energy data from equilibrium nitrogen partial pressures for reaction (4) he JANAF Thermochemical Tables has been used are less than 10-atm for temperatures between 600 for the following thermodynamic calculations and 1100.C so the formation of B.C is unlikely
1346 L. R. Hwang et al. Table 4. Hexural strengths of Nicalon/Si-C-O composites with different densities Thickness of carbon coating (pm) 0.1 Number of re-impregnations 4 RI 5 RI Flexural strength (MPa) 151.2 158.9 Standard deviation 7.9 16.9 Density (g/cm’) 1.99 2.09 Fiber volume fraction 0.38 O-38 pull-out and fiber matrix debonding could be observed at the positions where the fibers were perpendicular to the fracture surface. Fiber matrix debonding could also be found where the fibers were parallel to the fracture surface. However, many matrix fragments adhered to the fiber surface. All these phenomena are attributable to the non-uniform carbon coating. The matrix was bonded to the fibers at the sites where the fiber surface was not covered by carbon. Such a strong interfacial bonding may prove detrimental to the toughness of a CMC. Polymeric carbon could possibly be used to modify the interface in Nicalon/Si-C-O composites if the carbon precursor can be uniformly coated on the fiber surface. 4 THERMODYNAMIC CALCULATIONS FOR INTERFACE COMPATIBILITY As indicated earlier, the composites containing CVD carbon-coated fibers exhibited mechanical properties superior to those of the composites containing unmodified fibers. Unfortunately, the coated carbon can react with oxygen to form CO or CO, at temperatures higher than 500°C. In order to prevent degradation of the mechanical properties of Nicalon/Si-C-O composites at high temperatures, other materials may have to be utilized for replacing carbon. The most important factor to consider for evaluating the suitability of a candidate fiber coating material is the possibility that chemical reactions may strengthen the bonding at the fiber/coating or matrix/coating interfaces. Therefore, thermodynamic compatibility calculations for various interfaces are useful to evaluate potential coating materials. X-ray diffraction patterns of the matrix indicated that the crystallinity of Si-C-O materials depended on the heat treatment temperature. When the heat treatment temperature was lower than lOOO”C, the Si-C-O material was amorphous. The crystallinities of Sic and C in Si-C-O increased if a higher heat treatment temperature was used.“,13 To simplify the calculations, the Si-C-O has been treated as a mixture of the most stable crystalline phases of C, SIC and SiOz. Expect where noted, free energy data from the JANAF Thermochemical Tables33 has been used for the following thermodynamic calculations. 0.18 0.5 3 RI 4 RI 3 RI 4 RI 177.1 198.6 176.5 239.4 16.1 4.3 3.6 23.9 1.88 1.98 1.83 1.9 0.32 0.32 O-30 0.33 4.1 Assessment of potential coating materials Potential coating materials can be divided into four categories: metals, nitrides, carbides and oxides. One or two examples of each type are described below to demonstrate the type of calculations required to evaluate the potential of the material as a fiber coating. 4.1.1 Metals A metal could react with Si-C-O to form an oxide, carbide or silicide. One potential fiber coating material is boron. The formation of B203 by the reaction 2B+sSiO,=B,O,+$Si (1) is thermodynamically unfavorable and boron does not form a stable silicide. However, the reaction of boron with carbon is favorable and bonding, so boron 4.1.2 Nitrides 4B+C=B,C (2) may result in strong matrix/fiber is not a suitable coating material. The reaction of a nitride with Si-C-O is similar to the reaction of a metal with Si-C-O except that an addition phase will be formed. For example, BN has been demonstrated to impart significant improvement in the work of fracture as well as the strength to several composite systems, such as Nicalon/mullite, SiC/mullite, and Nicalon/CVI-Sic. The reaction of BN with SiOz 4BN + 3Si02 = 2B,O, + S&N, (3) (similar to reaction (1) above) is thermodynamically unfavorable. In addition, the standard free energy for the formation of B4C by the reaction 4BN+C=B,C+2N, (4) is positive. However, the nitrogen partial pressure is not likely to be 1 atm, so the equilibrium nitrogen partial pressure at a given temperature provides a better indication of the possibility of reaction. The equilibrium nitrogen partial pressures for reaction (4) are less than lo-‘atm for temperatures between 600 and 1100°C so the formation of B,C is unlikely
Interface compatibility in ceramic-matrix composites 1347 These thermodynamic calculations suggest that BN is not be reduced by carbon between 600 and 1100C a promising material for fibcr coating in the Nicalon/ Ilowever, Al2O3 and SiO2 can react Lo form mullite Si-C-O composite Although this reaction is sluggish between 600 and Another potential nitride coating is Si, N,, which can 1100.C react with carbon to form SiC according to the which may degrade the mechanical properties of the reaction composite Si3N4+ 3C=3SiC 2N2(g) the equilibrium nitrogen partial pressures of which are 5 CONCLUSIONS given in Table 5. The nitrogen partial pressure The effect of interface modification on the mechanical becomes significant at 1127 C, so reaction may occur behavior of CMCs was investigated. It was further at low ambient pressures. In addition, the formation confirmed that minimal interaction between the fiber of an oxynitride according to the reaction and the matrix was essential to achieving good SiNa+sio,= 2Si N,O (6) toughness in CMCs. A thin barrier material can be coated on the fiber surface to minimize such is favorable. There is evidence that silicon oxynitride raction The flexural strength of a Si-C-O is actually a solid solution with a varying N/O ratio composite containing CVD carbon-coated Nicalon between SiO2 and Si3N4. Thus, a strong bond may fibers was found to be five times higher than that of form between a SiaN4 coating and the Sioz in the composite containing uncoated Nicalon fibers Si-C-O Chemical reactions between the barrier coating and the matrix and those between the coating and the fiber 4.1.3 Carbides must be avoided in such a three-phase material. A One potential carbide coating is TiCx, where x is methodology for assessing and selecting candid between 0. 56 and 0. 98 for a temperature of about fiber coating materials using thermodynamic chem 1000'C. As a conservative estimate, the titanium-rich compatibility calculations was described omposition(approximately TiC) will be used for the alculations. The formation of TiO according to the DGEMENT is favorable, so TiC may not be a suitable fiber NSFl roject was under the sponsorship of the TiC +Ti+ SiO,=2T10+SiC coating. authors are are grateful for this support. Another possible carbide coating is Zrc which may also react with Sio2 according to the reaction REFERENCES ZrC+SiO2= ZrO2+SiC 1. Evans, A G.& Faber, K T, Crack-growth resistance of o form zrO, microcracking brittle materials. J. Am. Ceram Soc., 67 (1984)255-260. 2. Rice, R. W, Ceramic matrix composite toughening 4.1.4 Oxides sms: An update. Ceram. Engng Sci. Proc., 6 One of the limitations in the use of metals, carbides 1985)589-607 and nitrides is that they may react with oxygen in the 3. Wiederhorn, S. M, Brittle fracture and toughening mechanisms in ceramics, Ann. Rev. Mater. Sci., 14 atmosphere to form oxides during service. Thus, oxide (1984)373-403 coatings are attractive coating materials for 4. Evans. A. G. Heuer. A. H. Porter. D. L. The tion in oxidizing atmospheres. An oxide tracture toughness of ceramics. Fracture 1977, Vol. 1 be reduced by the carbon in Si-C-O or ICF4, Waterloo, 19-24 June 1977 SiO2 in Si-C-O 5. Hillig, w.B. Strength and toughness of ceramic mat composites, Ann. Rev. Mater. Sci., 17(1987)341-383 One potential oxide coating is Al2 O3, which will 6. Yajima, S, Okamura, K, Hayashi, J.&Omori, M strength.J. Am. Ceram Soc, 59(1976)324-2> tensile Table 5. Equilibrium nitrogen partial pressure for the re- 7. Hasegawa, Y, Hayashi, J, Iimura, M.& Ya action SiN,+ 3C=3SiC+ 2N Synthesis of continuous silicon carbide Conversion of polycarbosilane fibre into silicon carbide Temperature (C) Equilibrium N2 partial pressure fibre /. Mater. Sci., 15(1980)720-728 (atm) 8. Yajima, S, Hasegawa, Y, Hayashi, J.& Emura, M Synthesis of continuous carbide fiber with high 63×10-8 lensile strength and hig gs modulus: 1. Synthesis 38×10 carbosilane J. Mater. Sci., 13 (1978) 1027 3·1×10 1127 1·7×10 9. S. Yajima et al., US Patent 4,052, 430, 4 October 1977. 10. Yajima, S, Special heat-resisting materials from
Interface compatibility in ceramic-matrix composites 1347 These thermodynamic calculations suggest that BN is a promising material for fiber coating in the Nicalon/ Si-C-O composite. Another potential nitride coating is Si3N4, which can react with carbon to form Sic according to the reaction S&N4 + 3C = 3SiC + 2N,(g) (5) the equilibrium nitrogen partial pressures of which are given in Table 5. The nitrogen partial pressure becomes significant at 1127”C, so reaction may occur at low ambient pressures. In addition, the formation of an oxynitride according to the reaction S&N4 + SiO, = 2Si2N20 (6) is favorable.34 There is evidence that silicon oxynitride is actually a solid solution with a varying N/O ratio between Si02 and Si3N4.35 Thus, a strong bond may form between a Si3N4 coating and the SiO;! in Si-C-O. 4.1.3 Carbides One potential carbide coating is Tic,, where x is between O-56 and 0.98 for a temperature of about 1000°C. As a conservative estimate, the titanium-rich composition (approximately Tic) will be used for the calculations. The formation of TiO according to the reaction TiC + Ti + Si02 = 2TiO + Sic (7) is favorable, so TiC may not be a suitable fiber coating. Another possible carbide coating is ZrC which may also react with SiOz according to the reaction ZrC + Si02 = Zr02 + Sic (8) to form ZrO*. 4.1.4 Oxides One of the limitations in the use of metals, carbides and nitrides is that they may react with oxygen in the atmosphere to form oxides during service. Thus, oxide coatings are attractive coating materials for application in oxidizing atmospheres. An oxide coating could be reduced by the carbon in Si-C-O or reaction with SiOz in Si-C-O. One potential oxide coating is A1203, which will Table 5. Equilibrium nitrogen partial pressure for the reaction Si& + 3C = 3SiC + 2Nz Temperature (“C) Equilibrium N, partial pressure (atm> 627 6.3 x 1O-8 827 3.8 x 1O-5 1027 3.1 x 1o-3 1127 1.7 x 1o-2 not be reduced by carbon between 600 and 1100°C. However, A1203 and Si02 can react to form mullite. Although this reaction is sluggish between 600 and llOo”C, it may still result in matrix/fiber bonding which may degrade the mechanical properties of the composite. 5 CONCLUSIONS The effect of interface modification on the mechanical behavior of CMCs was investigated. It was further confirmed that minimal interaction between the fiber and the matrix was essential to achieving good toughness in CMCs. A thin barrier material can be coated on the fiber surface to minimize such interaction. The flexural strength of a Si-C-O composite containing CVD carbon-coated Nicalon fibers was found to be five times higher than that of the composite containing uncoated Nicalon fibers. Chemical reactions between the barrier coating and the matrix and those between the coating and the fiber must be avoided in such a three-phase material. A methodology for assessing and selecting candidate fiber coating materials using thermodynamic chemical compatibility calculations was described. ACKNOWLEDGEMENT This project was under NSF/Alabama Advanced authors are are grateful for REFERENCES the sponsorship of the EPSCoR Program. The this support. 1. 2. 3. 4. 5. 6. 7. 8. Evans, A. G. & Faber, K. T., Crack-growth resistance of microcracking brittle materials. J. Am. Cerum. Sot., 67 (1984) 2.55-260. Rice, R. W., Ceramic matrix composite toughening mechanisms: An update. Ceram. Engng Sci. Proc., 6 (1985) 589-607. Wiederhorn, S. M., Brittle fracture and toughening mechanisms in ceramics. Ann. Rev. Mater. Sci., 14 (1984) 373-403. Evans, A. G., Heuer, A. H. & Porter, D. L., The fracture toughness of ceramics. Fracture 1977, Vol. 1, ICF4, Waterloo, 19-24 June 1977. Hillig, W. B. Strength and toughness of ceramic matrix composites., Ann. Rev. Mater. Sci., 17 (1987) 341-383. Yajima, S., Okamura, K., Hayashi, J. & Omori, M., Synthesis of continuous SIC fibers with high tensile strength. .I. Am. Ceram. Sot., 59 (1976) 324-327. Hasegawa, Y., Hayashi, J., Iimura, M. & Yajima, S., Synthesis of continuous silicon carbide fiber: 2. Conversion of polycarbosilane fibre into silicon carbide fibre. J. Muter. Sci., 15 (1980) 720-728. Yajima, S., Hasegawa, Y., Hayashi, J. & Emura, M., Synthesis of continuous silicon carbide fiber with high tensile strength and high young’s modulus: 1. Synthesis of polycarbosilane as precursor. .I. Muter. Sci., 13 (1978) 2569-2576. 9. S. Yajima et al., US Patent 4,0.52,430, 4 October 1977. 10. Yajima, S., Special heat-resisting materials from
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