Materials Science and Engineering A 525(2009)121-127 Contents lists available at Science Direct Materials Science and engineering A ELSEVIER journalhomepagewww.elsevier.com/locate/msea Effects of the fiber surface characteristics on the interfacial microstructure and mechanical properties of the kd SiC fiber reinforced SiC matrix composites Haitao Liu*, Haifeng Cheng, Jun Wang, Gengping Tang, Renchao Che, Qingsong Ma Key Lab of Advanced Ceramic Fiber and Composites, College of Aerospace and Materials Engineering National University of Defense Technology, Changsha 410073, china ARTICLE INFO A BSTRACT SiC fiber reinforced SiC matrix(SiCr/SiC)composites, employing two types of KD SiC fibers(from National University of Defense Technology, China) with different fiber surface characteristics as reinforcements Received in revised form 18 June 2009 ccepted 10 July 2009 re fabricated by precursor infiltration and pyrolysis(PIP)process. The fiber surface characteristics were evaluated by SEM, XPS and Raman analysis. The effects of fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiCr/SiC composites were investigated. The results show that the tensile strength of the Kd-2 SiC fibers (with silicon-based oxide surface layers) is about 85% that of the KD-1 SiC fibers(with pyrocarbon(PyC)surface layers), but the flexural strength of the recursor infiltration and pyrolysis(PIP) KD-2 SiCr/SiC composite is only around 15% that of the KD-1 SiC /SiC composite SEM, TEM and elemental mapping analysis show that the large strength difference between the two composites is ascribed to Interfacial microstructure the interfacial microstructure and the degree of fiber damage, which are arising from the different fiber Mechanical property o 2009 Elsevier B V. All rights reserved 1. Introduction namely KD SiC fibers with PyC and silicon-based oxide surface layers, respectively, were employed to be reinforcements in the SiC/Sic composites generally exhibit excellent properties such SiC / Sic composites, and the effects of fiber surface characteristics as high strength and oxidation resistance at elevated tempera- on the interfacial microstructure and mechanical properties of the ure,and microstructural stability under neutron irradiation [1,2 KD SiCr/SiC composites fabricated by PIP process were investigated, Owing to these advantages, SiC /SiC composites are known to be one which have not been reported before to the best of our knowledge of the most promising materials for high-temperature structural applications, first wall and blanket components in fusion reactors and. For Sicr/siC composites, the interphase between the fiber 2. Experimental procedures mechanical properties, because the appropriate interphase allows The reinforcements used to prepare 2D-SiCr/SIC composites for crack deflection, fiber pullout and fiber-matrix (FM) debond- were plain-weave KD SiC fiber cloths. The properties of the KD-1 ing, which can provide excellent mechanical properties for Sic /Sic and KD-2 SiC fibers are listed in Table 1. Polycarbosilane(PCS), the ne interphase directly during composile surfae led by the fiber ceramic precursor of SiC matrix, was synthesized in our laboratory rocessing.Therefore, the reagent for PCS SiC powder was used as inert fillers, and the density fiber surface characteristics play an important part in affecting the and size were about 3. 2 g/cm and 1.0 um, respectively interfacial microstructure and mechanical properties of the Sic/sic 2D-SiCr/Sic composites were fabricated with PCS and DVB as [11-17. but the researches were mostly focused on the Sic /Sic 2D-SiCr/S The density and porosity of the Kd Sic /SiC composites were fibers as reinforcements. In this study two new types of Sic fibers, measured by the Archimedes method Three-point bend tests were carried out at ambient temperature. The sample geometry was about 60 mm x 4wmm x 3 mm. The support span was 50 mm. and the cross-head speed was 0.5 mm/min, corresponding to a strain rate of 3 x 10-6s-l. Scanning electron microscopy(SEM) E-mailaddressxzddiht@163.com(h.Liu work was done using a HITACHI FEG $4800 SEM. The specimens 3s-see front matter 2009 Elsevier B V. All rights reserved
Materials Science and Engineering A 525 (2009) 121–127 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Effects of the fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiC fiber reinforced SiC matrix composites Haitao Liu∗, Haifeng Cheng, Jun Wang, Gengping Tang, Renchao Che, Qingsong Ma Key Lab. of Advanced Ceramic Fiber and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, China article info Article history: Received 12 April 2009 Received in revised form 18 June 2009 Accepted 10 July 2009 Keywords: SiCf/SiC composite Precursor infiltration and pyrolysis (PIP) Surface characteristic Interfacial microstructure Mechanical property abstract SiC fiber reinforced SiC matrix (SiCf/SiC) composites, employing two types of KD SiC fibers (from National University of Defense Technology, China) with different fiber surface characteristics as reinforcements, were fabricated by precursor infiltration and pyrolysis (PIP) process. The fiber surface characteristics were evaluated by SEM, XPS and Raman analysis. The effects of fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiCf/SiC composites were investigated. The results show that the tensile strength of the KD-2 SiC fibers (with silicon-based oxide surface layers) is about 85% that of the KD-1 SiC fibers (with pyrocarbon (PyC) surface layers), but the flexural strength of the KD-2 SiCf/SiC composite is only around 15% that of the KD-1 SiCf/SiC composite. SEM, TEM and elemental mapping analysis show that the large strength difference between the two composites is ascribed to the interfacial microstructure and the degree of fiber damage, which are arising from the different fiber surface characteristics. © 2009 Elsevier B.V. All rights reserved. 1. Introduction SiCf/SiC composites generally exhibit excellent properties such as high strength and oxidation resistance at elevated temperature, and microstructural stability under neutron irradiation [1,2]. Owing to these advantages, SiCf/SiC composites are known to be one of the most promising materials for high-temperature structural applications, first wall and blanket components in fusion reactors [3–5]. For SiCf/SiC composites, the interphase between the fiber and matrix is one of the key factors that determine the composite mechanical properties, because the appropriate interphase allows for crack deflection, fiber pullout and fiber-matrix (FM) debonding, which can provide excellent mechanical properties for SiCf/SiC composites [6–10]. The interphase is usually controlled by the fiber surface characteristics, because the fiber surface layer can form the interphase directly during composite processing. Therefore, the fiber surface characteristics play an important part in affecting the interfacial microstructure and mechanical properties of the SiCf/SiC composites. Several papers have reported the effects of fiber surface characteristics on the mechanical properties of SiCf/SiC composites [11–17], but the researches were mostly focused on the SiCf/SiC composites employing the Nicalon, Hi-Nicalon and Tyranno SiC fibers as reinforcements. In this study, two new types of SiC fibers, ∗ Corresponding author. Tel.: +86 731 4576440; fax: +86 731 4576440. E-mail address: xzddlht@163.com (H. Liu). namely KD SiC fibers with PyC and silicon-based oxide surface layers, respectively, were employed to be reinforcements in the SiCf/SiC composites, and the effects of fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiCf/SiC composites fabricated by PIP process were investigated, which have not been reported before to the best of our knowledge. 2. Experimental procedures The reinforcements used to prepare 2D-SiCf/SiC composites were plain-weave KD SiC fiber cloths. The properties of the KD-1 and KD-2 SiC fibers are listed in Table 1. Polycarbosilane (PCS), the ceramic precursor of SiC matrix, was synthesized in our laboratory. Divinylbenzene (DVB) was used as the solvent and cross-linking reagent for PCS. SiC powder was used as inert fillers, and the density and size were about 3.2 g/cm3 and 1.0 m, respectively. 2D-SiCf/SiC composites were fabricated with PCS and DVB as precursors and SiC powder as inert fillers. The painting slurry, containing PCS, DVB, SiC powder and xylene (1:0.4:0.75:0.2, wt%), was dispersed homogeneously by ball milling for 4 h. The fabrication route of the 2D-SiCf/SiC composites is shown in Fig. 1. The density and porosity of the KD SiCf/SiC composites were measured by the Archimedes method. Three-point bend tests were carried out at ambient temperature. The sample geometry was about 60l mm × 4w mm × 3t mm. The support span was 50 mm, and the cross-head speed was 0.5 mm/min, corresponding to a strain rate of 3 × 10−6 s−1. Scanning electron microscopy (SEM) work was done using a HITACHI FEG S4800 SEM. The specimens 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.07.018
H Liu et al/ Materials Science and Engineering A 525(2009)121-127 Table 1 Painting Stacking Molding Properties of KD SiC fibers. Slurry KD-2 Curing Diameter (um) 14-16 Sic fiber cloths 14-1 Number of filaments(fil/yarn) 1200 2D-SiCf/SiC Pyrolysis Tensile strength(MPa) 800-22001500-1900 Infiltration 2D-SiCmSic Chemical compositions of fiber surface layer Pyc Silcon-based oxide PCS solution for transmission electron microscope(TEm)observation followed Fig. 1. Preparation route of the 2D-SiC/Sic composites JEM-20io ation procedure described by Appiah et al.[18JJEOL JEM-2010 and Philips CM 200 FEG equipped with a Gatan imag- and d) is relatively rough, and some strumae-like structure defects ing filter(GIF) system were used to characterize the interfacial re found. These defects reduce the tensile strength of Sic fibers microstructure and element distributions in the interphase region. [19 such that the tensile strength of KD-2 Sic fibers is about 85% X-ray photoelectron spectroscopy(XPS )analysis was done using a that of KD-1 Sic fibers VG ESCA-LAB MK ll apparatus with al Ko radiation and calibrated Survey XPS spectra recorded from the surface of KD-1 and KD-2 gainst Au 4f 7/2 and Cu 2p3/2 lines. Raman spectra were recorded Sic fibers are shown in Fig 3a. In the spectrum of KD-1 Sic fibers. vith a Raman spectrometer HR 800 ( obin-Yvon Company, France) no Si lines are observed. a single C ls peak at 284.5 ev is detected, in backscattering geometry at 532 nm excitation wavelength. The which is attributed to the C-Cbond in the PyC. In addition, the pI laser beam was focused in air at normal incidence on a small area ence of a weak o1s peak at 532. 5 ev indicates that some oxygen of the fiber surface(ca. 1 um2), and the laser beam power was exists on the Kd-1 SiC fiber, which originate from a contamination 1 mw. Raman spectra were recorded at wavenumbers from 800 by the glue or sample holder. So, it can be concluded that the Pyc is 2000 cm, and the acquisition time was 30s. The phases of KD the main constituent of the surface layer of the KD-1 SiC fiber Con- d KD-2 SiC fibers were characterized by X-ray diffraction(XRD) cerning KD-2 SiC fibers, the Si 2p and Si 2s peaks are all detected. analysis using monochromatic Cu Ka radiation with a D8 ADVANCE In the Si 2p spectrum of KD-2 SiC fibers( Fig. 3b). only a single Si diffractometer(Bruker, Germany ) 2p peak at 103.6ev is detected which is assigned to the si-o bond in silicon-based oxide, and the si 2p peak cannot be fitted into sub 3. Results and discussion peaks, which shows that no Si-C(100.5 ev) or O-Si-C(101.8ev) bonds 20 exist in the surface layer of the KD-2 SiC fiber. The o1s 3. 1. Fiber surface characteristic analysis peak detected at 532.9ev in the survey XPS spectrum of KD-2 Sic SEM images ofKD- and Kp- 2 sic ibers ar represented in fig 2. tioe. hs presence of anweakx c is ne ak :at 2847ev indicates that a As shown in Fig 2a and b, the surface of KD-1 SiC fibers is rath small quantity of distributed on the Kd-2 SiC fiber. There- mooth, and no obvious defects are observed Compared with the fore, it can be confirmed that the silicon-based oxide is the main surface morphology of KD-1 SiCfibers, that of KD-2 SiCfibers(Fig 2c constituent of the surface layer of the KD-2 SiC fiber. 1O um Fig. 2. SEM images of KD-1(a and b)and KD-2(c and d)siC fibers
122 H. Liu et al. / Materials Science and Engineering A 525 (2009) 121–127 Table 1 Properties of KD SiC fibers. Type KD-1 KD-2 Diameter (m) 14–16 14–16 Number of filaments (fil/yarn) ∼1200 ∼1200 Tensile strength (MPa) 1800–2200 1500–1900 Density (g cm−3) −2.54 −2.55 C/Si atom 1.25 L23 Chemical compositions of fiber surface layer PyC Silicon-based oxide for transmission electron microscope (TEM) observation followed the preparation procedure described by Appiah et al. [18]. JEOL JEM-2010 and Philips CM 200 FEG equipped with a Gatan imaging filter (GIF) system were used to characterize the interfacial microstructure and element distributions in the interphase region. X-ray photoelectron spectroscopy (XPS) analysis was done using a VG ESCA-LAB MK II apparatus with Al K radiation and calibrated against Au 4f7/2 and Cu 2p3/2 lines. Raman spectra were recorded with a Raman spectrometer HR 800 (Jobin-Yvon Company, France) in backscattering geometry at 532 nm excitation wavelength. The laser beam was focused in air at normal incidence on a small area of the fiber surface (ca. 1 m2), and the laser beam power was 1 mW. Raman spectra were recorded at wavenumbers from 800 to 2000 cm−1, and the acquisition time was 30 s. The phases of KD-1 and KD-2 SiC fibers were characterized by X-ray diffraction (XRD) analysis using monochromatic Cu K radiation with a D8 ADVANCE diffractometer (Bruker, Germany). 3. Results and discussion 3.1. Fiber surface characteristic analysis SEM images of KD-1 and KD-2 SiC fibers are represented in Fig. 2. As shown in Fig. 2a and b, the surface of KD-1 SiC fibers is rather smooth, and no obvious defects are observed. Compared with the surface morphology of KD-1 SiC fibers, that of KD-2 SiC fibers (Fig. 2c Fig. 1. Preparation route of the 2D-SiCf/SiC composites. and d) is relatively rough, and some strumae-like structure defects are found. These defects reduce the tensile strength of SiC fibers [19], such that the tensile strength of KD-2 SiC fibers is about 85% that of KD-1 SiC fibers. Survey XPS spectra recorded from the surface of KD-1 and KD-2 SiC fibers are shown in Fig. 3a. In the spectrum of KD-1 SiC fibers, no Si lines are observed. A single C 1s peak at 284.5 eV is detected, which is attributed to the C-C bond in the PyC. In addition, the presence of a weak O1s peak at 532.5 eV indicates that some oxygen exists on the KD-1 SiC fiber, which originate from a contamination by the glue or sample holder. So, it can be concluded that the PyC is the main constituent of the surface layer of the KD-1 SiC fiber. Concerning KD-2 SiC fibers, the Si 2p and Si 2s peaks are all detected. In the Si 2p spectrum of KD-2 SiC fibers (Fig. 3b), only a single Si 2p peak at 103.6 eV is detected, which is assigned to the Si–O bond in silicon-based oxide, and the Si 2p peak cannot be fitted into sub peaks, which shows that no Si–C (100.5 eV) or O–Si–C (101.8 eV) bonds [20] exist in the surface layer of the KD-2 SiC fiber. The O1s peak detected at 532.9 eV in the survey XPS spectrum of KD-2 SiC fibers is attributed to the O–Si bond in silicon-based oxide. In addition, the presence of a weak C1s peak at 284.7 eV indicates that a small quantity of carbon is distributed on the KD-2 SiC fiber. Therefore, it can be confirmed that the silicon-based oxide is the main constituent of the surface layer of the KD-2 SiC fiber. Fig. 2. SEM images of KD-1 (a and b) and KD-2 (c and d) SiC fibers.
H Liu et aL/ Materials Science and Engineering A 525(2009)121-127 KD.2 0100200300400500600700800 Binding energy/ev Fig 4. Raman spectra of KD-1 and KD-2 SiC fibers D-2 KD-1 Binding energy/ev Fig 3. Survey XPS spectra of KD-1 and KD-2 SiC fibers (a)and the Si 2p spectrum of KD-2 SiC fibers(b). Fig. 5. XRD patterns of KD-1 and KD-2 SiC fibers. 4 Raman spectra of KD-1 and KD-2 SiC fibers are shown in Fig 4 he Raman spectrum of KD-1SiC fibers, only carbon spectra con- The broad bands around a 20 value of 35.7 in KD-1 and KD-2Sic sisting of broad peaks at about 1350 and 1600 are detected which are assigned to the sp and sp 2 bonded carbon respectively fibers are attributed to p-Sic[24] For KD-2 SiC fibers, besides the typical d and G bands assigned to carbon, a peak at about 1150 is detected, which may be due 3. 2. Mechanical properties of the KD Sicy/SiC composites to the disordered and hydrogenated sp bonded carbon [21, 22).A peak detected at about 1030cm-I should be assigned to the Si-o-Si The properties of the KD-1 and KD-2 SiC /Sic composites are symmetric stretching vibration bands, which are due to Si-o-Si shown in Table 2. It can be seen that the density and porosity of linkages in a network structure having a smaller bond angle [23]. the two composites are nearly the same, but the flexural strength In Fig 4, no Sic spectra( mainly consisting of peaks around 796 and of the KD-1 Sicr/SiC composite is about 7 times that of the KD-2 972cm-l)were detected. That is because the Raman scattering effi- SiCr/SiC composite. The load /displacement curves of the two com- ciency of carbon is at least ten times that of Sic [21 whereas the posites are represented in Fig. 6. It is observed that the KD-2 SiCr/Sic enetration depth of the laser beam in SiC fibers can reach about composite shows a standard brittle fracture behavior, but the KD-1 0.1 um. The lack of Sic spectra is due to the carbon excess of Kd Sic SiC/Sic composite exhibits a toughened fracture behavior, and fails bers(see Table 1). So, it can be concluded that KD-1 and KD-2 SiC fibers have PyC and silicon-based oxide surface layers respectively, Table 2 which are in agreement with the results of the XPs analysi Properties of the KD-1 and KD-2 SiC/SiC composites. XRD patterns of KD-1 and KD-2 SiC fibers are represented in Fig. 5. No obvious XRD peaks are observed, which show that both Composites Density (gcm-) Porosity(%) Flexural strength(MPa) KD-1 and KD-2 SiC fibers are amorphous structures, and the PyCand KD-1 silicon-based oxide surface layers are also noncrystalline structures
H. Liu et al. / Materials Science and Engineering A 525 (2009) 121–127 123 Fig. 3. Survey XPS spectra of KD-1 and KD-2 SiC fibers (a) and the Si 2p spectrum of KD-2 SiC fibers (b). Raman spectra of KD-1 and KD-2 SiC fibers are shown in Fig. 4. In the Raman spectrum of KD-1 SiC fibers, only carbon spectra consisting of broad peaks at about 1350 and 1600 cm−1 are detected, which are assigned to the sp3 and sp2 bonded carbon respectively. For KD-2 SiC fibers, besides the typical D and G bands assigned to carbon, a peak at about 1150 cm−1 is detected, which may be due to the disordered and hydrogenated sp3 bonded carbon [21,22]. A peak detected at about 1030 cm−1 should be assigned to the Si–O–Si asymmetric stretching vibration bands, which are due to Si–O–Si linkages in a network structure having a smaller bond angle [23]. In Fig. 4, no SiC spectra (mainly consisting of peaks around 796 and 972 cm−1) were detected. That is because the Raman scattering effi- ciency of carbon is at least ten times that of SiC [21], whereas the penetration depth of the laser beam in SiC fibers can reach about 0.1 m. The lack of SiC spectra is due to the carbon excess of KD SiC fibers (see Table 1). So, it can be concluded that KD-1 and KD-2 SiC fibers have PyC and silicon-based oxide surface layers respectively, which are in agreement with the results of the XPS analysis. XRD patterns of KD-1 and KD-2 SiC fibers are represented in Fig. 5. No obvious XRD peaks are observed, which show that both KD-1 and KD-2 SiC fibers are amorphous structures, and the PyC and silicon-based oxide surface layers are also noncrystalline structures. Fig. 4. Raman spectra of KD-1 and KD-2 SiC fibers. Fig. 5. XRD patterns of KD-1 and KD-2 SiC fibers. The broad bands around a 2 value of 35.7◦ in KD-1 and KD-2 SiC fibers are attributed to -SiC [24]. 3.2. Mechanical properties of the KD SiCf/SiC composites The properties of the KD-1 and KD-2 SiCf/SiC composites are shown in Table 2. It can be seen that the density and porosity of the two composites are nearly the same, but the flexural strength of the KD-1 SiCf/SiC composite is about 7 times that of the KD-2 SiCf/SiC composite. The load/displacement curves of the two composites are represented in Fig. 6. It is observed that the KD-2 SiCf/SiC composite shows a standard brittle fracture behavior, but the KD-1 SiCf/SiC composite exhibits a toughened fracture behavior, and fails Table 2 Properties of the KD-1 and KD-2 SiCf/SiC composites. Composites Density (g cm−3) Porosity (%) Flexural strength (MPa) KD-1 2.04 18.4 211.7 KD-2 2.06 17.6 30.5
H Liu et al/ Materials Science and Engineering A 525 (2009 )121-127 better understanding of the relationship between the fiber surface characteristics and the interfacial microstructure, TEM and elemen- tal mapping analysis were done in the interphase regions of the Kd Sic/SiC composites. TEM cross-sectional images of the KD-1 SiC/SiC composite shown in Fig 8, indicating that a turbostratic Py interphase around 30 nm thick is found, which is formed by the Py C surface layer of the KD-1 SiC fiber. The interfaces between the fiber and the Pyc interphase and between the Pyc interphase and the matrix can b defined clearly. It can be seen that the highly aligned basal planes of the Pyc appear to be almost parallel to the interfaces, as already observed by Appiah et al. [18] in C fiber reinforced laminated KD.2 C-SiC matrix composites. Generally speaking, the approximately perfect orientation of the PyC interphase is very favorable for the mechanical property improvement of the Sic/SiC composites. In Displacement/mm his orientation, load can be more easily transferred, and the FM Fig. 6. The load/ displacement curves of the KD-1 and KD-2 SiC/SiC composites. debonding easily occurs due to the low van der Waals force between the basal planes of the Py c[18 Furthermore as shown in Fig. 8b.no obvious phenomena of elemental interdiffusion or chemical reac trophically. As shown in Table 1, the tensile strength of the tions are observed within the Pyc interphase, so the Pyc interphase is a diffusion barrier for protecting the Sic fibers from chemical KD-2 Sic fiber is around 85% that of the KD-1 SiC fiber, but the flex- damage during composite processing From the analysis above the KD-1 SiC/SiC composite. In order to make clear the strength dif- appropriate interfacial bonding strength for the Sic/Sic compos- ference between the two composites, the microstructure of the two ites, which gives the KD-1 Sic /SiC composite adequate fracture composites was investigated by SEM, TEM and elemental mapping toughness KD-2 SiCr/SiC composite, as shown in Fig. 9a and b, a silicon-based oxide interphase about 30 nm thick is observed 3.3. Microstructural analysis of the KD Sic/Sic composites which is formed by the surface layer of the KD-2 SiC fiber. The interfaces between the fiber and the interphase and between the The fracture surface of the KD-1 and KD-2 SiC /Sic composites interphase and the matrix cannot be defined clearly In order to is given in Fig. 7. Concerning the KD-1 Sicr/Sic composite the frac- understand the element distributions in the interphase, carbon ture surface shows an evident fiber pullout( Fig. 7a), and the pullout elemental mapping(Fig. 9d) of the interphase region as shown lengths can exceed 20 um. The surface of the pulled out fibers is in Fig. 9c was done. As observed in Fig. 9d, a carbon-poor region rather smooth and free of any matrix. The fm debonding is evi- about 10 nm thick is detected, the thickness of which is less than denced by a higher magnification SEM observation( Fig. 7b). In the the interphase and some carbon is also found dispersed in this of the KD-2 SiC/Sic composite, the fracture surface is very region. Therefore, elemental interdiffusion and or chemical reac- and few pulled out fibers can be found ( Fig. 7c). Fig 7d shows tions between the fiber and the interphase and between the strong interfacial bonding occurs in the KD-2 composite. For interphase and the matrix can be confirmed, which will result in 10 Fig. 7. Fracture surface of the KD-1(a and b)and KD-2(c and d)sic Sic composites
124 H. Liu et al. / Materials Science and Engineering A 525 (2009) 121–127 Fig. 6. The load/displacement curves of the KD-1 and KD-2 SiCf/SiC composites. non-catastrophically. As shown inTable 1, the tensile strength of the KD-2 SiC fiber is around 85% that of the KD-1 SiC fiber, but the flexural strength of the KD-2 SiCf/SiC composite is only about 15% that of the KD-1 SiCf/SiC composite. In order to make clear the strength difference between the two composites, the microstructure of the two composites was investigated by SEM, TEM and elemental mapping analysis. 3.3. Microstructural analysis of the KD SiCf/SiC composites The fracture surface of the KD-1 and KD-2 SiCf/SiC composites is given in Fig. 7. Concerning the KD-1 SiCf/SiC composite, the fracture surface shows an evident fiber pullout (Fig. 7a), and the pullout lengths can exceed 20 m. The surface of the pulled out fibers is rather smooth and free of any matrix. The FM debonding is evidenced by a higher magnification SEM observation (Fig. 7b). In the case of the KD-2 SiCf/SiC composite, the fracture surface is very even, and few pulled out fibers can be found (Fig. 7c). Fig. 7d shows that strong interfacial bonding occurs in the KD-2 composite. For better understanding of the relationship between the fiber surface characteristics and the interfacial microstructure, TEM and elemental mapping analysis were done in the interphase regions of the KD SiCf/SiC composites. TEM cross-sectional images of the KD-1 SiCf/SiC composite shown in Fig. 8, indicating that a turbostratic PyC interphase around 30 nm thick is found, which is formed by the PyC surface layer of the KD-1 SiC fiber. The interfaces between the fiber and the PyC interphase and between the PyC interphase and the matrix can be defined clearly. It can be seen that the highly aligned basal planes of the PyC appear to be almost parallel to the interfaces, as already observed by Appiah et al. [18] in C fiber reinforced laminated C–SiC matrix composites. Generally speaking, the approximately perfect orientation of the PyC interphase is very favorable for the mechanical property improvement of the SiCf/SiC composites. In this orientation, load can be more easily transferred, and the FM debonding easily occurs due to the low van der Waals force between the basal planes of the PyC [18]. Furthermore, as shown in Fig. 8b, no obvious phenomena of elemental interdiffusion or chemical reactions are observed within the PyC interphase, so the PyC interphase is a diffusion barrier for protecting the SiC fibers from chemical damage during composite processing. From the analysis above, the PyC interphase decreases the fiber damage and provides the appropriate interfacial bonding strength for the SiCf/SiC composites, which gives the KD-1 SiCf/SiC composite adequate fracture toughness. For the KD-2 SiCf/SiC composite, as shown in Fig. 9a and b, a silicon-based oxide interphase about 30 nm thick is observed, which is formed by the surface layer of the KD-2 SiC fiber. The interfaces between the fiber and the interphase and between the interphase and the matrix cannot be defined clearly. In order to understand the element distributions in the interphase, carbon elemental mapping (Fig. 9d) of the interphase region as shown in Fig. 9c was done. As observed in Fig. 9d, a carbon-poor region about 10 nm thick is detected, the thickness of which is less than the interphase, and some carbon is also found dispersed in this region. Therefore, elemental interdiffusion and/or chemical reactions between the fiber and the interphase and between the interphase and the matrix can be confirmed, which will result in Fig. 7. Fracture surface of the KD-1 (a and b) and KD-2 (c and d) SiCf/SiC composites.
H Liu et aL/ Materials Science and Engineering A 525(2009)121-127 Fiber Fig 8. TEM cross-sectional images of the KD-1 SiC/SiC composite the strong chemical interfacial bonding and serious chemical dam- and FM debonding also cannot take place within the silicon-based age to fibers From Fig 2c and d, it can be found that the surface of oxide interphase. Consequently the desired mechanical reinfore <D-2 SiC fibers is relatively rough, which would lead to the strong ing mechanisms such as fiber pullout and fm debonding are not physical interfacial bonding: at the same time, the defects on the available, and a brittle fracture behavior occurs in the KD-2 SiC/Sic D-2 SiC fibers can result in a large stress concentration, and the composite. larger stress may fracture the Sic fiber during composite process- Schematic representations of the interphase structures ing (as shown in Fig. 10), which causes physical damage to Sic matrix crack propagation paths in the KD-1(Fig. 11a) and fibers. Therefore, the strong interfacial bonding and fiber damage(Fig. 11b)SiCr/SiC composites are described in Fig. 11, which cause undesirable interfacial stress. fiber fracture. and low in situ trate the effects of fiber surface characteristics on the interfacial ber strength, respectively. It is well known that the silicon-based microstructure and mechanical properties of the Kd Sic/Sic com- oxide is not a layered crystal structure, so that the crack deflection posites. Interp M 10 Fig 9. TEM images of the interphase in the KD-2 SiC/SiC composite(a-c)and the carbon elemental mapping ofc(d)
H. Liu et al. / Materials Science and Engineering A 525 (2009) 121–127 125 Fig. 8. TEM cross-sectional images of the KD-1 SiCf/SiC composite. the strong chemical interfacial bonding and serious chemical damage to fibers. From Fig. 2c and d, it can be found that the surface of KD-2 SiC fibers is relatively rough, which would lead to the strong physical interfacial bonding; at the same time, the defects on the KD-2 SiC fibers can result in a large stress concentration, and the larger stress may fracture the SiC fiber during composite processing (as shown in Fig. 10), which causes physical damage to SiC fibers. Therefore, the strong interfacial bonding and fiber damage cause undesirable interfacial stress, fiber fracture, and low in situ fiber strength, respectively. It is well known that the silicon-based oxide is not a layered crystal structure, so that the crack deflection and FM debonding also cannot take place within the silicon-based oxide interphase. Consequently, the desired mechanical reinforcing mechanisms such as fiber pullout and FM debonding are not available, and a brittle fracture behavior occurs in the KD-2 SiCf/SiC composite. Schematic representations of the interphase structures and matrix crack propagation paths in the KD-1 (Fig. 11a) and KD-2 (Fig. 11b) SiCf/SiC composites are described in Fig. 11, which illustrate the effects of fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiCf/SiC composites. Fig. 9. TEM images of the interphase in the KD-2 SiCf/SiC composite (a–c) and the carbon elemental mapping of ‘c’ (d)
126 H Liu et al/ Materials Science and Engineering A 525(2009)121-127 Matri Matrix Fiber iber Crack 20 5H Fig. 10. SEM images of physical damage to KD-2 SiC fibers during composite processing. Matrix Matrix Fiber Silicon-based oxide interphase Fig. 11. Schematic representations of the interphase structures and the matrix crack propagation paths in the KD-1 (a)and KD-2(b)SiC /SiC composites. 4. Conclusions 4. For the KD Sic fibers with high oxygen and free carbon content, a PyC fiber surface layer is an ideal choice for improving the 1. XPS and Raman analysis show that the main constituents of the mechanical properties of the Sic/Sic composites, but a silicon- surface layers of the KD-1 and KD-2SiC fibers are PyC and silicon- based oxide fiber surface layer should be avoided. based oxide, respectively. 2. The tensile strength of the KD-2 Sic fiber is about 85% that of Acknowledgements ne KD-1 SiC fiber, but the flexural strength of the KD-2 Sic/Sic composite is only around 15% that of the KD-1 SiCr/Sic composite. The authors would like to thank Processor K Jian, w. Zhou and In the KD-1 SiC /SiC composite, fiber pullout and FM debonding E Wang for help with experiments and valuable discussions re obvious, and the fiber reinforcement mechanisms are oper- able. Contrarily, the KD-2 SiC/SiC composite exhibits a standard brittle fracture behavior and fails in a catastrophic manner. References 3. The fiber surface characteristics play a key role in determining the interfacial microstructure and the degree of fiber damage [1 M. Kotani, T. Inoue, A. Kohyama, K Okamura, Y Katoh, Comp. Sci. Technol. 62 for the KD Sic/SiC composites. Due in part to the layered crys- [2] M. Kotani, T. Inoue, A Kohyama, K Okamura, Y. Katoh, Mater. Sci. Eng. A 357 tal structure, crack deflection and FM debonding occur within the Pyc interphase: at the time, the Pyc interphase PIAl 141 A19993467-47 [3 R Yamada, T. Taguchi, N Igawa. J Nucl. Mater. 283-287(2000)574-578 vides protection for SiC fibers during composite processing. All C Sutoh, S Suyama, Y Itoh, S Nakagawa, J. Nucl. Mater. 271-272 those factors endow the KD-1 Sicr/SiC composite with excellent 5I K Yoshida, M Imai, T Yano, Comp. Sci. Technol. mechanical properties. For the KD-2 SiC/SiC composite, the non tte. Acta Mater. 48( yered crystal structure of the silicon-based oxide interphase N. Yu, A Kohyama, ether with rough surface morphology results in strong inter- D.B. Marshall, B N Cox, A G. Evans. Acta Metall. 33(11)(1985)2013-2021. I bonding and serious fiber damage which makes the Kd-2 Sci.29(15)(1994) Sic/Sic composite exhibit a brittle failure behavior. Ol B. Budiansky, A.G. Evans, J.W. Hutchinson. Int J Solids Struct. 32 (3-4)(1995)
126 H. Liu et al. / Materials Science and Engineering A 525 (2009) 121–127 Fig. 10. SEM images of physical damage to KD-2 SiC fibers during composite processing. Fig. 11. Schematic representations of the interphase structures and the matrix crack propagation paths in the KD-1 (a) and KD-2 (b) SiCf/SiC composites. 4. Conclusions 1. XPS and Raman analysis show that the main constituents of the surface layers of the KD-1 and KD-2 SiC fibers are PyC and siliconbased oxide, respectively. 2. The tensile strength of the KD-2 SiC fiber is about 85% that of the KD-1 SiC fiber, but the flexural strength of the KD-2 SiCf/SiC composite is only around 15% that of the KD-1 SiCf/SiC composite. In the KD-1 SiCf/SiC composite, fiber pullout and FM debonding are obvious, and the fiber reinforcement mechanisms are operable. Contrarily, the KD-2 SiCf/SiC composite exhibits a standard brittle fracture behavior and fails in a catastrophic manner. 3. The fiber surface characteristics play a key role in determining the interfacial microstructure and the degree of fiber damage for the KD SiCf/SiC composites. Due in part to the layered crystal structure, crack deflection and FM debonding occur within the PyC interphase; at the same time, the PyC interphase provides protection for SiC fibers during composite processing. All those factors endow the KD-1 SiCf/SiC composite with excellent mechanical properties. For the KD-2 SiCf/SiC composite, the nonlayered crystal structure of the silicon-based oxide interphase together with rough surface morphology results in strong interfacial bonding and serious fiber damage, which makes the KD-2 SiCf/SiC composite exhibit a brittle failure behavior. 4. For the KD SiC fibers with high oxygen and free carbon content, a PyC fiber surface layer is an ideal choice for improving the mechanical properties of the SiCf/SiC composites, but a siliconbased oxide fiber surface layer should be avoided. Acknowledgements The authors would like to thank Processor K. Jian, W. Zhou and F. Wang for help with experiments and valuable discussions. References [1] M. Kotani, T. Inoue, A. Kohyama, K. Okamura, Y. Katoh, Comp. Sci. Technol. 62 (2002) 2179–2188. [2] M. Kotani, T. Inoue, A. Kohyama, K. Okamura, Y. Katoh, Mater. Sci. Eng. A 357 (2003) 376–385. [3] R. Yamada, T. Taguchi, N. Igawa, J. Nucl. Mater. 283–287 (2000) 574–578. [4] A. Sayano, C. Sutoh, S. Suyama, Y. Itoh, S. Nakagawa, J. Nucl. Mater. 271–272 (1999) 467–471. [5] K. Yoshida, M. Imai, T. Yano, Comp. Sci. Technol. 61 (2001) 1323–1329. [6] F. Rebillat, J. Lamon, A. Guette, Acta Mater. 48 (2000) 4609–4618. [7] W. Yang, T. Noda, H. Araki, J.N. Yu, A. Kohyama, Mater. Sci. Eng. A 345 (2003) 28–35. [8] D.B. Marshall, B.N. Cox, A.G. Evans, Acta Metall. 33 (11) (1985) 2013–2021. [9] A.G. Evans, F.W. Zok, J. Mater. Sci. 29 (15) (1994) 3857–3896. [10] B. Budiansky, A.G. Evans, J.W. Hutchinson, Int. J. Solids Struct. 32 (3–4) (1995) 315–328
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