t.J. Appl Ceram. Technol.,8/2/308-316(2011) DO:10.I116.1744-7402.2010.02588 International Journal o pplied Ceramic TECHNOLOGY ceramic Product D Microstructure and Mechanical Properties of SiC and Carbon Hybrid Fiber Reinforced SiC Matrix Composite Shanhua Liu, Litong Zhang, Xiaowei Yin, Laifei Cheng, and Yongsheng Liu Naidongiy, a m, shanxi 710072, Pepl e composite Materials, Northwestern Polytechnical Silicon carbide(SiC) matrix composite reinforced by both SiC and carbon fibers([SiC-C]/pyrolytic carbon [Py C]/SiC) was fabricated by chemical vapor infiltration for reducing matrix microcracks. Microstructure, mechanical properties, and oxidation resistance of the composite were compared with those of C/PyC/SiC and C/PyCH /SiC composites in which PyC interphase was untreated and heat-treated at 1800.C in argon, respectively. Compared with the C/PyC/SiC C)/PyC/SiC composite shows considerable improvements in Flexural strength, fracture toughness, and oxidation resistance The different mechanical behaviors of the three composites were analyzed based on the He and Hutchinson's model Introduction vanced thermostructural materials for use in the aero- space industry, owing to their low density, excellent Silicon carbide(SiC)matrix composites reinforced thermal-shock resistance, and high mechanical proper with carbon fiber(C/SiC)and silicon carbide fiber(Sic/ ties at high temperatures . 2 Thermal residual stresses SiC)fabricated by chemical vapor infiltration( CVI)(TRS)are always generated in C/SiC composite during process have attracted considerable attentions as ad cooling from processing to room temperature due to an extensive mismatch of the coefficients of thermal This work was financially supported by The Centre for Foreign Talents Introduction and pansionCTEs)between fiber and matrix, which results cademic Exchange for Advanced Materials and Forming Technology Discipline and The in spontaneous cracking of the matrix. Using pyrolytic Research Fund of State Key Laboratory of Solidification Processing( Grant No. 44-QP. carbon(PyC) as an interphase between fiber and matrix 2009), Northwest Polytechnical University, Xi'an, China. can reduce the TRS of the C/SiC composite to some C 2010 The American Ceramic Society extent, which can be named as C/PyC/SiC composite
Microstructure and Mechanical Properties of SiC and Carbon Hybrid Fiber Reinforced SiC Matrix Composite Shanhua Liu, Litong Zhang, Xiaowei Yin,* Laifei Cheng, and Yongsheng Liu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China Silicon carbide (SiC) matrix composite reinforced by both SiC and carbon fibers ([SiC–C]/pyrolytic carbon [PyC]/SiC) was fabricated by chemical vapor infiltration for reducing matrix microcracks. Microstructure, mechanical properties, and oxidation resistance of the composite were compared with those of C/PyC/SiC and C/PyCHT/SiC composites in which PyC interphase was untreated and heat-treated at 18001C in argon, respectively. Compared with the C/PyC/SiC composite, (SiC– C)/PyC/SiC composite shows considerable improvements in flexural strength, fracture toughness, and oxidation resistance. The different mechanical behaviors of the three composites were analyzed based on the He and Hutchinson’s model. Introduction Silicon carbide (SiC) matrix composites reinforced with carbon fiber (C/SiC) and silicon carbide fiber (SiC/ SiC) fabricated by chemical vapor infiltration (CVI) process have attracted considerable attentions as advanced thermostructural materials for use in the aerospace industry, owing to their low density, excellent thermal-shock resistance, and high mechanical properties at high temperatures.1,2 Thermal residual stresses (TRS) are always generated in C/SiC composite during cooling from processing to room temperature due to an extensive mismatch of the coefficients of thermal expansion (CTEs) between fiber and matrix, which results in spontaneous cracking of the matrix. Using pyrolytic carbon (PyC) as an interphase between fiber and matrix can reduce the TRS of the C/SiC composite to some extent, which can be named as C/PyC/SiC composite. Int. J. Appl. Ceram. Technol., 8 [2] 308–316 (2011) DOI:10.1111/j.1744-7402.2010.02588.x Ceramic Product Development and Commercialization r 2010 The American Ceramic Society This work was financially supported by The Centre for Foreign Talents Introduction and Academic Exchange for Advanced Materials and Forming Technology Discipline and The Research Fund of State Key Laboratory of Solidification Processing (Grant No. 44-QP- 2009), Northwest Polytechnical University, Xi’an, China. *yinxw@nwpu.edu.cn
wwceramics. org/ACT Properties of Sic Matrix Composite However, there still exists a great deal of matrix micro- Experimental Procedure cracks in the C/Py C/SiC composite. The microcracks in SiC matrix are regarded as one of the obstacles to the Preparation Process widespread use of C/SiC an inward diffuse through the matrix microcracks, Two kinds of fibers, Hi-Nicalon SiC fiber from fibers at 700oC, which led to the minimum strength carbon fiber from Japan Toray Tokyo, Japan)consist- retained ratio at the range of room temperature to 1250C. Therefore, it is necessary to reduce the ma- diameter of 14 and 7 um per-filament were used to pre trix microcracks of C/PyC/SiC composite for long-time pare fiber preforms using four-step three-dimensional wide temperature range. techniques by Nanjing Fiberglass Research and Design It is indicated that the trs can be controlled Institute, People's Republic of China. The major prop through the appropriate choice of the fiber-matrix erties of the raw materials are summarized in Table IIn pair. The TRS in C/PyC/SiC composite has been cal order to understand the effect of hybrid fibers on the ulated and measured in the previous work. compare mechanical properties of the composite, two types of with the C/PyC/SiC composite, the SiC/Py C/Sic preforms were braided. The first type contained both cause the Sic fiber has nearly the same CTE as the togethe arbon ibers, in which two iber tows were put composite yielded a negligible tRS (close to zero value) fiber bundles for braiding, and the sec- SiC matrix. The properties of fiber, interphase, ond one only contained carbon fiber. The volume frac- will occur or not when the compos under the preforms. PyC interphase and SiC matrix were depos- ited by CVI process. PyC interphase was deposited on Based on the above considerations, SiC matrix the surfaces of fibers by decomposition of C3H at composite reinforced by both SiC and carbon hybrid 900C for 144h at reduced pressure of 5kPa in fibers([SiC-CJ/Py C/SiC)using the PyC as the inter- CVI reactor, obtaining a thickness of 150 nm.One of phase may have a lower residual thermal stress and less the preforms only containing carbon fiber with the Pyc matrix cracks than C/PyC/SiC composite. Moreover, interphase was heat treated at 1800.C in argon for 1h heat treatment of the PyC interphase in C/Py C/SiC to increase the crystalline degree of PyC interphase. Sic composite may improve the crystalline degree of inter matrix was prepared at 1000C at a reduced pressure of phase, and the composite named as C/Pyc/SiC may using also have lower TRS and less matrix cracks than C/PyC/ with a molar ratio of 10 between H2 and MTS, which SiC composite. The major objectives of this work are to was carried by bubbling hydrogen in gas phase and have a knowledge on the microstructure and mechanical argon as the dilute gas to slow down the chemical re- action rate during deposition. For each CVI cycle, the oxidation behaviors at 700%C in air, which were com- deposition time was 80h. After CVI SiC for 6 cycles, pared with those of C/Py C/SiC and C/PyCHT/SiC the as-received composites were machined and polished ino3mm×4mm×40 mm samples. Consequently Table L. Properties of Raw Materials.11-16 Tensile Fracture Diameter Poisson's Modulus CTE strength energy onstituent ratio(v) (GPa) (X 10/K (GPa) T ( /m) Hi-Nicalon SiC fiber 2.74 14 4.6 20 T300 carbon fiber 0.3 1.12 8.6 0.23 Sic matrix 3.21 350
However, there still exists a great deal of matrix microcracks in the C/PyC/SiC composite. The microcracks in SiC matrix are regarded as one of the obstacles to the widespread use of C/SiC composite. Oxidizing species can inward diffuse through the matrix microcracks, leading to the oxidation of PyC interphase and carbon fibers at 7001C, which led to the minimum strength retained ratio at the range of room temperature to 12501C.3–5 Therefore, it is necessary to reduce the matrix microcracks of C/PyC/SiC composite for long-time use in a wide temperature range. It is indicated that the TRS can be controlled through the appropriate choice of the fiber–matrix pair.6 The TRS in C/PyC/SiC composite has been calculated and measured in the previous work.7 Compared with the C/PyC/SiC composite, the SiC/PyC/SiC composite yielded a negligible TRS (close to zero value) because the SiC fiber has nearly the same CTE as the SiC matrix.8 The properties of fiber, interphase, and matrix decide whether the debonding of interphase will occur or not when the composite is under the loading. Based on the above considerations, SiC matrix composite reinforced by both SiC and carbon hybrid fibers ([SiC–C]/PyC/SiC) using the PyC as the interphase may have a lower residual thermal stress and less matrix cracks than C/PyC/SiC composite. Moreover, heat treatment of the PyC interphase in C/PyC/SiC composite may improve the crystalline degree of interphase, and the composite named as C/PyCHT/SiC may also have lower TRS and less matrix cracks than C/PyC/ SiC composite. The major objectives of this work are to have a knowledge on the microstructure and mechanical properties of the (SiC–C)/PyC/SiC composite and its oxidation behaviors at 7001C in air, which were compared with those of C/PyC/SiC and C/PyCHT/SiC composites. Experimental Procedure Preparation Process Two kinds of fibers, Hi-Nicalon SiC fiber from Japan Nippon Carbon (Takauchi, Japan) and T300 carbon fiber from Japan Toray (Tokyo, Japan) consisting of bundles of 500 and 1000 filaments with the diameter of 14 and 7 mm per-filament were used to prepare fiber preforms using four-step three-dimensional techniques by Nanjing Fiberglass Research and Design Institute, People’s Republic of China. The major properties of the raw materials are summarized in Table I. In order to understand the effect of hybrid fibers on the mechanical properties of the composite, two types of preforms were braided. The first type contained both SiC and carbon fibers, in which two fiber tows were put together as one fiber bundles for braiding, and the second one only contained carbon fiber. The volume fraction of the fibers was approximately 40 vol% in both preforms. PyC interphase and SiC matrix were deposited by CVI process. PyC interphase was deposited on the surfaces of fibers by decomposition of C3H6 at 9001C for 144 h at reduced pressure of 5 kPa in a CVI reactor, obtaining a thickness of 150 nm. One of the preforms only containing carbon fiber with the PyC interphase was heat treated at 18001C in argon for 1 h to increase the crystalline degree of PyC interphase. SiC matrix was prepared at 10001C at a reduced pressure of 5 kPa by using methyltrichlorosilane (MTS, CH3SiCl3) with a molar ratio of 10 between H2 and MTS, which was carried by bubbling hydrogen in gas phase and argon as the dilute gas to slow down the chemical reaction rate during deposition. For each CVI cycle, the deposition time was 80 h. After CVI SiC for 6 cycles, the as-received composites were machined and polished into 3 mm 4 mm 40 mm samples. Consequently, Table I. Properties of Raw Materials7,11–16 Constituent Density (g/cm3 ) Diameter (lm) Poisson’s ratio (n) Modulus (GPa) CTE ( 106 /K) Tensile strength (GPa) Fracture energy C (J/m2 ) Hi-Nicalon SiC fiber 2.74 14 0.2 270 4.6 2.8 20 T300 carbon fiber 1.76 7 0.3 230 1.12 3.1 8.6 PyC 1.76 — 0.23 35 5.57 — 2–6 PyCHT 1.76 — 0.23 35 5.57 — 0–2 SiC matrix 3.21 — 0.21 350 4.6 — 6 www.ceramics.org/ACT Properties of SiC Matrix Composite 309
310 International Journal of Applied Ceramic Technolog-Liut, et al. Vol.8,No.2,2011 another two cycles of CVI SiC were essential for cov- followed by ion beam milling. The samples were exam- ering the porosity and the ends of fibers to form the final ined used field emission transmission electron micro- composites. Three kinds of composites were prepared. scope(Tecnai F30, FEI Company, Hillsboro, OR)with One was SiC matrix com posite reinforced with both an operated voltage of 300 kV SiC and carbon fibers([SiC-C]/Py C/SiC), which was named as hybrid composite and denoted as sample A C/PyC/SiC and C/PyC / SiC composites, denoted p The other two kinds of composites were named as Oxidation Tests Oxidation tests were conducted in static air in a samples B and C, respectively. The superscript HT tube furnace at 700C for 10h, and the weight changes means the PyC interphase was heat treated at 1800c of the samples were obtained by analytical balance(AG before the deposition of SiC matrix. It should be noted 204, Mettler Toledo, Schwerzenbach, Switzerland)and that PyC interphase in the hybrid composite was not be recorded as a function of oxidation time.Cumulative heat treated, because the strength of Hi-Nicalon SiC fi- weight change and the strength retained ratio of the ber deteriorate tes at temperatures exceeding 1400.C. samples were calculated according to the weight and flexural strength of the samples before and after oxida- tion tests, respectively Flexural Strength Tests and Microstructur Observation Results and Discussion Density and open porosity of the composites were btained by the archimedes method. The fexural Microstructure strength of the composites before and after oxidation was measured by the three-point Flexural method, and As shown in Table II, sample a had a higher den- the fracture toughness was determined by the single- sity than the other two kinds of composites. This result ge-notched-beam method using a fexural testing ma- was due to the significantly higher density of SiC fiber hine (SANS CMT 404, Sans Materials Testing, (2.74g/cm) than that of the carbon fiber(1.76g/cm) Shenzhen, China). The exural modulus was calculated Moreover, the diameter of SiC fber was larger than that using the slope of the load-displacement curves of the of carbon fiber, which made the spaces among the SiC composites according to the ASTM C1341-00 stan- fibers and SiC tows larger than those among carbon dard. The density, porosity, and Flexural strength of fibers and carbon tows, and hence it was easier for SiC the three kinds of the composites were measured after matrix to be deposited on the surfaces of SiC fibers and the sixth, seventh, and eighth cycles of CVI SiC matrix, SiC tows in the inner of the preform. As a result, the respectively. The microstructure of the composites and average value of open porosity the fracture surface morphologies of the tested samples lower than those of the other two kinds of composites, were analyzed by scanning electron microscopy (S- as shown in Table II 2700, Hitachi, Tokyo, Japan). For transmission elec The polished morphologies of SiC matrix in three on microscope (TEM) observation, the composites kinds of composites are shown in Fig. 1. There were were cut into samples with a thickness of 1000 um, no matrix microcracks found in sample A, as shown in and then mechanically thinned to a thickness of 30 um Figs. la and b. However, both samples B(Fig. Ic)and Table Il. Properties of the Three Kinds of Composites Flexural Fr acture Flexural ens porosity strengt modulus MPa) (MPam (GPa) ratio(%) A 6 530(16 175(2) 96(4.9) B 117(16) 33(0.5) 6(6.3) 18.6(1) 64(3.4 andard deviations are given in parentheses
another two cycles of CVI SiC were essential for covering the porosity and the ends of fibers to form the final composites. Three kinds of composites were prepared. One was SiC matrix composite reinforced with both SiC and carbon fibers ([SiC–C]/PyC/SiC), which was named as hybrid composite and denoted as sample A. The other two kinds of composites were named as C/PyC/SiC and C/PyCHT/SiC composites, denoted as samples B and C, respectively. The superscript HT means the PyC interphase was heat treated at 18001C before the deposition of SiC matrix. It should be noted that PyC interphase in the hybrid composite was not be heat treated, because the strength of Hi-Nicalon SiC fi- ber deteriorates at temperatures exceeding 14001C.9,10 Flexural Strength Tests and Microstructural Observation Density and open porosity of the composites were obtained by the Archimedes method. The flexural strength of the composites before and after oxidation was measured by the three-point flexural method, and the fracture toughness was determined by the singleedge-notched-beam method using a flexural testing machine (SANS CMT 4304, Sans Materials Testing, Shenzhen, China). The flexural modulus was calculated using the slope of the load-displacement curves of the composites according to the ASTM C1341-00 standard.17 The density, porosity, and flexural strength of the three kinds of the composites were measured after the sixth, seventh, and eighth cycles of CVI SiC matrix, respectively. The microstructure of the composites and the fracture surface morphologies of the tested samples were analyzed by scanning electron microscopy (S- 2700, Hitachi, Tokyo, Japan). For transmission electron microscope (TEM) observation, the composites were cut into samples with a thickness of 1000 mm, and then mechanically thinned to a thickness of 30 mm followed by ion beam milling. The samples were examined used field emission transmission electron microscope (Tecnai F30, FEI Company, Hillsboro, OR) with an operated voltage of 300 kV. Oxidation Tests Oxidation tests were conducted in static air in a tube furnace at 7001C for 10 h, and the weight changes of the samples were obtained by analytical balance (AG 204, Mettler Toledo, Schwerzenbach, Switzerland) and recorded as a function of oxidation time. Cumulative weight change and the strength retained ratio of the samples were calculated according to the weight and flexural strength of the samples before and after oxidation tests, respectively. Results and Discussion Microstructure As shown in Table II, sample A had a higher density than the other two kinds of composites. This result was due to the significantly higher density of SiC fiber (2.74 g/cm3 ) than that of the carbon fiber (1.76 g/cm3 ). Moreover, the diameter of SiC fiber was larger than that of carbon fiber, which made the spaces among the SiC fibers and SiC tows larger than those among carbon fibers and carbon tows, and hence it was easier for SiC matrix to be deposited on the surfaces of SiC fibers and SiC tows in the inner of the preform. As a result, the average value of open porosity of hybrid composites was lower than those of the other two kinds of composites, as shown in Table II. The polished morphologies of SiC matrix in three kinds of composites are shown in Fig. 1. There were no matrix microcracks found in sample A, as shown in Figs. 1a and b. However, both samples B (Fig. 1c) and Table II. Properties of the Three Kinds of Composites Samples Density (g/cm3 ) Open porosity (%) Flexural strength (MPa) Fracture toughness (MPa m1/2) Flexural modulus (GPa) Strength retained ratio (%) A 2.56 6 530 (16) 17.5 (2) 96 (4.9) 99 B 2.23 10 117 (16) 3.3 (0.5) 66 (6.3) 67 C 2.25 9 505 (37) 18.6 (1) 64 (3.4) 78 Standard deviations are given in parentheses. 310 International Journal of Applied Ceramic Technology—Liu, et al. Vol. 8, No. 2, 2011
wwceramics. org/ACT Properties of Sic Matrix Composite 311 carbon fiber Sic fibe 150k13.0mm×500sE(M carbon fiber carbon fiber microcrack Fig. I. Scanning electron microscopic(SEM) images of polished cross-section morphologies of SiC matrix in three kinds of composites: (a) SiC matrix around carbon fiber bundle in sample A; (b) SiC matrix around SiC fiber bundle in sample A;(e) SiC matrix in sample B; and(d) SiC matrix in sample C. C(Fig. ld)had matrix microcracks. The TRS in the sample a decreased significantly. Compared with sam- ple B, the opening width of matrix microcracks in sam- ▲ Sample C le c decreased which indicated that the trs in sample C was lower than that in sample B. Mechanical Properties composites as a function of open porosity. As shown in Fig. 2, for samples A and C, when the porosity of the composites decreased from 15% and 16%to 6% and 9% he fexural strength increased from 385 and 359 MPa to 530 and 505 MPa. However, the fexural strength of sam ple b was nearly independent of the decrease of the Fig. 2. The flexural strength as a fuanction of the porosity for the porosity. As shown in Table Il, samples A, B, and C three kinds of composites
C (Fig. 1d) had matrix microcracks. The TRS in the sample A decreased significantly. Compared with sample B, the opening width of matrix microcracks in sample C decreased, which indicated that the TRS in sample C was lower than that in sample B. Mechanical Properties Figure 2 compares the flexural strength of the three composites as a function of open porosity. As shown in Fig. 2, for samples A and C, when the porosity of the composites decreased from 15% and 16% to 6% and 9%, the flexural strength increased from 385 and 359MPa to 530 and 505MPa. However, the flexural strength of sample B was nearly independent of the decrease of the porosity. As shown in Table II, samples A, B, and C SiC matrix a b c d carbon fiber SiC fiber SiC matrix microcrack carbon fiber microcrack carbon fiber Fig. 1. Scanning electron microscopic (SEM) images of polished cross-section morphologies of SiC matrix in three kinds of composites: (a) SiC matrix around carbon fiber bundle in sample A; (b) SiC matrix around SiC fiber bundle in sample A; (c) SiC matrix in sample B; and (d) SiC matrix in sample C. Fig. 2. The flexural strength as a function of the porosity for the three kinds of composites. www.ceramics.org/ACT Properties of SiC Matrix Composite 311
312 International Journal of Applied Ceramic Technolog-Liut, et al. Vol.8,No.2,2011 Sample B carbon fiber 0.00,2040608101 150kv128mn Fig 3. The load-displacement curves of the three kinds of chieved a flexural strength of 530, 117, and 504 MPa, and a fracture toughness of 17.5, 3.3, and 18.6 Mpa" respectively. The elastic modulus of SiC fiber(270 GPa) was higher than that of the carbon fiber(230 GPa)and the matrix porosity of sample a was smaller than those of samples B and C. As a result, the fexural modulus of sample A was 96GPa, much higher than those of sample arbon fiber B(66 GPa)and sample C Typical failure curves of three kinds of composites are shown in Fig 3. The Flexural curve of sample A was similar to that of SiC/Py C/SiC composite& The flexural 15owV 13.6mm x200 SE/(M) curve of sample A showed an initially linear elastic be- havior and then a nonlinear beh avioN th there existed debonding and sliding at the fiber/matrix nterphase zone, which led to the pullout of fibers However, there was no nonlinear stage in the fexural curve of sample B, which exhibited a brittle fracture behavior like monolithic ceramics, indicating that no debonding and sliding occurred at the interphase zone Flexural load-displacement curve of sample C showed early linear behavior with the increasing load, and then a noncatastrophic failure behavior was observed after the maximum loa nd 4 shows the fexural fracture surface mor phologies of three kinds of composites In sample A, SiC 15.0 fibers showed apparent pullout in the fiber bundle, while carbon fibers showed no pullout, as shown in Fig. 4a. These fracture features indicated that PyC works flexural fracture surface morphologies of three kinds of composites. (a) sample A showing SiC fibers were pulled out and no carbon on SiC fibers and not on carbon fibers. As a result, the fibers were pulled out;:(b)sample B showing no carbon fibers debonding and sliding of Py C interphase mainly occurred were pulled out; and(c)sample C showing carbon fibers were between SiC fiber and SiC matrix In sample B, the frac- pulled out. ture morphology of carbon fiber bundles was fat
achieved a flexural strength of 530, 117, and 504 MPa, and a fracture toughness of 17.5, 3.3, and 18.6Mpa m1/2, respectively. The elastic modulus of SiC fiber (270 GPa) was higher than that of the carbon fiber (230 GPa) and the matrix porosity of sample A was smaller than those of samples B and C. As a result, the flexural modulus of sample A was 96 GPa, much higher than those of sample B (66 GPa) and sample C (64 GPa). Typical failure curves of three kinds of composites are shown in Fig. 3. The flexural curve of sample A was similar to that of SiC/PyC/SiC composite.8 The flexural curve of sample A showed an initially linear elastic behavior and then a nonlinear behavior, indicating that there existed debonding and sliding at the fiber/matrix interphase zone, which led to the pullout of fibers. However, there was no nonlinear stage in the flexural curve of sample B, which exhibited a brittle fracture behavior like monolithic ceramics, indicating that no debonding and sliding occurred at the interphase zone. Flexural load-displacement curve of sample C showed nearly linear behavior with the increasing load, and then a noncatastrophic failure behavior was observed after the maximum load. Figure 4 shows the flexural fracture surface morphologies of three kinds of composites. In sample A, SiC fibers showed apparent pullout in the fiber bundle, while carbon fibers showed no pullout, as shown in Fig. 4a. These fracture features indicated that PyC works on SiC fibers and not on carbon fibers. As a result, the debonding and sliding of PyC interphase mainly occurred between SiC fiber and SiC matrix. In sample B, the fracture morphology of carbon fiber bundles was flat, as Fig. 3. The load–displacement curves of the three kinds of composites. a b c SiC fiber carbon fiber carbon fiber carbon fiber Fig. 4. Scanning electron microscopic (SEM) images of the flexural fracture surface morphologies of three kinds of composites: (a) sample A showing SiC fibers were pulled out and no carbon fibers were pulled out; (b) sample B showing no carbon fibers were pulled out; and (c) sample C showing carbon fibers were pulled out. 312 International Journal of Applied Ceramic Technology—Liu, et al. Vol. 8, No. 2, 2011