《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_ZrO2 interpahse on SiC fiber

Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Joumal of the European Ceramic Society 28(2008)1687-1696 www.elsevier.com/locate/jeurceramsoc Microstructural features of the zro interfacial coatings on sic fibers before and after exposition to air at high temperatures N I. Baklanova,, O I Kiselyova, A.T. Titov, T M. Zima a Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze Street 18. Novosibirsk 630128. Russian Federation b Lomonosov Moscow State University, Physical Department, Moscow, Russian Federation General Institute of Geology Geophysics and Mineralogy SB RAS, Novosibirsk 630090, Russian Federation Received 30 October 2007: received in revised form 27 November 2007; accepted 30 November 2007 Available online 14 January 2008 Sols of rare earth stabilized zirconia were used as simple, readily processable and accurate controllable precursors for the tetragonal zirconia interfacial coatings on commercially available Sic-based fibers. The tetragonal zirconia interfacial coatings can be applied to different types of Sic fibers without degrading fiber strength. The morphology, composition, structure, nanorelief and oxidation resistance of coated SiC fibers were evaluated by various analytical techniques, including scanning electron microscopy/energy dispersive analysis, transmission electron microscopy, atomic force microscopy in various modes, and micro-Raman spectroscopy. It was shown that the microstructural peculiarities of the RezrO2 interfacial coatings on Sic-based fibers may explain some of the differences in the behavior of different types of fibers o 2007 Elsevier Ltd. All rights reserved. Keywords: Interfacial coatings; Microstructure; ZrO2; SiC fibers: Oxidation resistance 1. Introduction ponent of composites it remains one of the weakest links in the research of the matrix-interphase-fiber triad. Insufficient nterface is a key region determining a set of properties of comprehension of interphase functions, role and nature, is a composite materials. In fiber-reinforced composites the fibers key problem and one major bottleneck retarding the devel ensure the strength of material, while the matrix helps to keep opment of efficient CMC's for high-temperature structural the shape. The interface transfers the load from matrix to applications. To solve this problem it is necessary to study the fibers. Further, the incorporation of the reinforcing fibers thoroughly the properties of interphase and to clarify which into brittle ceramic matrix provides CMCs with a degree characteristics of the interphase and in what extent control the of pseudo-ductility, preventing catastrophic failure by several behavior of the composite. Undoubtedly, among the features echanisms, such as fiber debonding, fiber sliding and crack of the interphase zone a microstructure is one of the most bridging. In order to achieve these properties, the interphase important zone must be sufficiently weak to deflect matrix microc- In addition to above-mentioned functions (load transfer racks and allow subsequent fiber pull-out. Both functions of and crack deflection), interphase materials must be compat the interphase zone in CMCs, namely, a load transfer from ible with both matrix and fiber for long-term operation in matrix to the fibers and the matrix microcrack deflection, are oxidizing atmosphere. This is especially important for non- greatly determined by nature of the interphase zone. Despite oxide CMCs, e.g. SiC/SiC composites. The interphase can of the wide recognition of the interphase as a crucial com- be exposed to oxidizing environments when the ends of coated fibers are exposed to surrounding atmosphere or when matrix cracks are present, allowing oxidants to reach the fiber Corresponding author. Tel: +7 3832 363839: fax: +7 3832 322847 coatings. Since oxide ceramics cannot be oxidized, it is com- E-mail address: baklanova@solid nsc. ru(N 1. Baklanova) monly believed that oxide-based coatings represent the best 0955-2219/S-see front matter o 2007 Elsevier Ltd. All rights reserved. doi: 10.1016/j-jeurceramsoc20071 1.008

Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 1687–1696 Microstructural features of the ZrO2 interfacial coatings on SiC fibers before and after exposition to air at high temperatures N.I. Baklanova a,∗, O.I. Kiselyova b, A.T. Titov c, T.M. Zima a a Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze Street 18, Novosibirsk 630128, Russian Federation b Lomonosov Moscow State University, Physical Department, Moscow, Russian Federation c General Institute of Geology, Geophysics and Mineralogy SB RAS, Novosibirsk 630090, Russian Federation Received 30 October 2007; received in revised form 27 November 2007; accepted 30 November 2007 Available online 14 January 2008 Abstract Sols of rare earth stabilized zirconia were used as simple, readily processable and accurate controllable precursors for the tetragonal zirconia interfacial coatings on commercially available SiC-based fibers. The tetragonal zirconia interfacial coatings can be applied to different types of SiC fibers without degrading fiber strength. The morphology, composition, structure, nanorelief and oxidation resistance of coated SiC fibers were evaluated by various analytical techniques, including scanning electron microscopy/energy dispersive analysis, transmission electron microscopy, atomic force microscopy in various modes, and micro-Raman spectroscopy. It was shown that the microstructural peculiarities of the ReZrO2 interfacial coatings on SiC-based fibers may explain some of the differences in the behavior of different types of fibers. © 2007 Elsevier Ltd. All rights reserved. Keywords: Interfacial coatings; Microstructure; ZrO2; SiC fibers; Oxidation resistance 1. Introduction Interface is a key region determining a set of properties of composite materials. In fiber-reinforced composites the fibers ensure the strength of material, while the matrix helps to keep the shape. The interface transfers the load from matrix to the fibers. Further, the incorporation of the reinforcing fibers into brittle ceramic matrix provides CMC’s with a degree of pseudo-ductility, preventing catastrophic failure by several mechanisms, such as fiber debonding, fiber sliding and crack bridging.1 In order to achieve these properties, the interphase zone must be sufficiently weak to deflect matrix microc￾racks and allow subsequent fiber pull-out. Both functions of the interphase zone in CMC’s, namely, a load transfer from matrix to the fibers and the matrix microcrack deflection, are greatly determined by nature of the interphase zone. Despite of the wide recognition of the interphase as a crucial com- ∗ Corresponding author. Tel.: +7 3832 363839; fax: +7 3832 322847. E-mail address: baklanova@solid.nsc.ru (N.I. Baklanova). ponent of composites it remains one of the weakest links in the research of the matrix–interphase–fiber triad. Insufficient comprehension of interphase functions, role and nature, is a key problem and one major bottleneck retarding the devel￾opment of efficient CMC’s for high-temperature structural applications. To solve this problem it is necessary to study thoroughly the properties of interphase and to clarify which characteristics of the interphase and in what extent control the behavior of the composite. Undoubtedly, among the features of the interphase zone a microstructure is one of the most important. In addition to above-mentioned functions (load transfer and crack deflection), interphase materials must be compat￾ible with both matrix and fiber for long-term operation in oxidizing atmosphere. This is especially important for non￾oxide CMC’s, e.g. SiC/SiC composites. The interphase can be exposed to oxidizing environments when the ends of coated fibers are exposed to surrounding atmosphere or when matrix cracks are present, allowing oxidants to reach the fiber coatings. Since oxide ceramics cannot be oxidized, it is com￾monly believed that oxide-based coatings represent the best 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.11.008

N.I. Baklanova et al /Journal of the European Ceramic Sociery 28(2008)1687-1696 choice in terms of oxidation resistance. Several oxidation 23. Specimen characterization resistant and crack-deflecting materials including monazite, alu mina/silica, stabilized zirconia, and others were proposed as Scanning electron microscope SEM LEO 1430VP, supplied appropriate candidates for interphase zone in CMCs2-8 by EDX(Oxford) spectrometer was used for studying of mor- Some information is available in publications, describing the phology and composition of the initial and coated fibers behavior of the stabilized ZrO2-coated SiC fibers exposed to Micro-Raman spectra of the RezrO2-coated ceramic fibers air at high temperatures. 9, 10 Preliminary studying of the pecu- before and after oxidation were recorded using a Triplemate, liarities of morphology and nanorelief of two zirconia-coated SPEX spectrometer equipped with CCD spectrometric detec- fibers,namely, Hi-NicalonTM and Tyranno-SAM before and tor and microscope attachment for back scattering geometry after exposition to air at 1000 C using atomic force microscopy The 488 nm radiation from an argon laser was used for spectral (AFM) and scanning electron microscopy (SEM) showed that excitation. these features are greatly dependent on the type of Sic fibers. The topography and surface roughness of fibers was exam- After application of coating the roughness of Tyranno-SAM ined by atomic force microscopy(SolverP47Bio, NT-MDT, (nearly stoichiometric) fiber increased in comparison to that Russia) and MultiMode NanoScope Illa(Veeco, USA)using of the initial fiber, whereas the roughness parameters of Hi- Tapping Mode. Silicon cantilevers were used. Filaments were Nicalon'wfiberretained their values after application of coating. attached to metal discs double-sided adhesive tape. Dif- ASs parameters for coated ferent areas of several filaments of each type fibers were Tyranno-SAM and Hi-NicalonM fibers was enhanced after selected randomly. A roughness and other statistical parameters exposition to air at 1000C of selected areas were obtained using tool "Statistics and Fem- The purpose of this work is to study the microstructural toScan 001 software for AFM images. The AFM images were features of the ReZrO2-coated SiC fibers type Hi-NicalonM, flattened before analysis using second-order surface subtrac- Hi-Nicalon $M, and Tyranno-SAM and the evolution of these tion Parameters were calculated based on following definitions features after exposition to air at 1000 and 1200C. Mean roughness(Ra) is the arithmetic average of the absolute values of the surface height deviations, Zi, measured from mean 2. Experimental 2.1. Substrate and coating preparation R 1zl NM Hi-Nicalon, Hi-Nicalon S(both Nippon Carbon Co Ltd, Tokyo, Japan) and Tyranno-saM grade 3(Ube Indus- Mean height (Rmean)is the arithmetic average of the absolute Ltd, Yamaguchi, Japan) fiber tows were used as substrate values of the measured heights materials. Prior to coating, Hi-NicalonTM and Hi-Nicalon STM fiber tows were immersed in 50: 50 acetone/ethanol mixture for rmean 24 h for removing a sizing agent, dried at ambient temperature and then thermally treated in air at 450 C. Tyranno-SAMfiber tow was immersed in hot distilled water for desizing, dried at Maximum height roughness(Rmax)is the difference in height ambient temperature and heated in air at 500C. between the highest and lowest points a detailed description of the coating procedure is given in Rmax zi Ref. [6] but some parameters of sol were optimized. Sol-gel approach was used for the preparation of multi-component rare During the scanning, phase shifts, i.e. changes in the phase earth oxide stabilized zirconia(ReZrO2)coatings on all types contact angle of vibration with respect to the phase angle of of SiC-based fibers. At least two rare earth components were freely oscillating cantilever, were recorded simultaneously with incorporated in conventional zirconia-yttria oxide system. Total height images content of earth oxides was 3 mol%. The coating stage involved firstly the immersion of the ceramic fiber tow into sol, 2. 4. Tensile strength tests drying in air at ambient temperature, slow heating till 1000C in vacuum and heat-treatment for 2 h Mechanical tensile tests of the coated fibers were conducted at room temperature using FM-4(Hungary)testing machine. 2.2. Oxidation tests Single fibers extracted from a tow were fixed on paper frame using a hard resin. The gauge of 10mm in length was used. Thermal oxidation resistance of coated ceramic fibers was the diameter of each filament was measured in the middle examined in laboratory air under static conditions at 1000 and of length by laser interferometry and used for calculation of 1200C. The samples were placed into preliminarily heated mechanical properties of filaments. Next, the lateral sides of furnace(KO-14, Germany) and kept there during fixed time support frame were cut by a heated wire and the load was intervals. Then the samples were taken out, cooled in dessicator applied at constant crosshead speed of 1.3 mm/min. About 50 and weighted with accuracy to. I mg. The total time of testing filaments for each type of fibers were tested. The average diam- was 40h eters for the coated fibers were determined to be equal to

1688 N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 choice in terms of oxidation resistance. Several oxidation resistant and crack-deflecting materials including monazite, alu￾mina/silica, stabilized zirconia, and others were proposed as appropriate candidates for interphase zone in CMC’s.2–8 Some information is available in publications, describing the behavior of the stabilized ZrO2-coated SiC fibers exposed to air at high temperatures.9,10 Preliminary studying of the pecu￾liarities of morphology and nanorelief of two zirconia-coated fibers, namely, Hi-NicalonTM and Tyranno-SATM before and after exposition to air at 1000 ◦C using atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed that these features are greatly dependent on the type of SiC fibers. After application of coating the roughness of Tyranno-SATM (nearly stoichiometric) fiber increased in comparison to that of the initial fiber, whereas the roughness parameters of Hi￾NicalonTM fiber retained their values after application of coating. Moreover, the difference in the roughness parameters for coated Tyranno-SATM and Hi-NicalonTM fibers was enhanced after exposition to air at 1000 ◦C. The purpose of this work is to study the microstructural features of the ReZrO2-coated SiC fibers type Hi-NicalonTM, Hi-Nicalon STM, and Tyranno-SATM and the evolution of these features after exposition to air at 1000 and 1200 ◦C. 2. Experimental 2.1. Substrate and coating preparation Hi-NicalonTM, Hi-Nicalon STM (both Nippon Carbon Co. Ltd., Tokyo, Japan) and Tyranno-SATM grade 3 (Ube Indus￾try Ltd., Yamaguchi, Japan) fiber tows were used as substrate materials. Prior to coating, Hi-NicalonTM and Hi-Nicalon STM fiber tows were immersed in 50:50 acetone/ethanol mixture for 24 h for removing a sizing agent, dried at ambient temperature and then thermally treated in air at 450 ◦C. Tyranno-SATM fiber tow was immersed in hot distilled water for desizing, dried at ambient temperature and heated in air at 500 ◦C. A detailed description of the coating procedure is given in Ref. [6] but some parameters of sol were optimized. Sol–gel approach was used for the preparation of multi-component rare earth oxide stabilized zirconia (ReZrO2) coatings on all types of SiC-based fibers. At least two rare earth components were incorporated in conventional zirconia–yttria oxide system. Total content of rare earth oxides was 3 mol%. The coating stage involved firstly the immersion of the ceramic fiber tow into sol, drying in air at ambient temperature, slow heating till 1000 ◦C in vacuum and heat-treatment for 2 h. 2.2. Oxidation tests Thermal oxidation resistance of coated ceramic fibers was examined in laboratory air under static conditions at 1000 and 1200 ◦C. The samples were placed into preliminarily heated furnace (KO-14, Germany) and kept there during fixed time intervals. Then the samples were taken out, cooled in dessicator and weighted with accuracy ±0.1 mg. The total time of testing was 40 h. 2.3. Specimen characterization Scanning electron microscope SEM LEO 1430VP, supplied by EDX (Oxford) spectrometer was used for studying of mor￾phology and composition of the initial and coated fibers. Micro-Raman spectra of the ReZrO2-coated ceramic fibers before and after oxidation were recorded using a Triplemate, SPEX spectrometer equipped with CCD spectrometric detec￾tor and microscope attachment for back scattering geometry. The 488 nm radiation from an argon laser was used for spectral excitation. The topography and surface roughness of fibers was exam￾ined by atomic force microscopy (SolverP47Bio, NT-MDT, Russia) and MultiMode NanoScope IIIa (Veeco, USA) using TappingModeTM. Silicon cantilevers were used. Filaments were attached to metal discs using double-sided adhesive tape. Dif￾ferent areas of several filaments of each type fibers were selected randomly. A roughness and other statistical parameters of selected areas were obtained using tool “Statistics” and Fem￾toScan 001 software for AFM images. The AFM images were flattened before analysis using second-order surface subtrac￾tion. Parameters were calculated based on following definitions. Mean roughness (Ra) is the arithmetic average of the absolute values of the surface height deviations, Zij, measured from mean plane: Ra = 1 NxNy Nx i=1 Ny j=1 |z| Mean height (Rmean) is the arithmetic average of the absolute values of the measured heights: Rmean = 1 NxNy Nx i=1 Ny j=1 Zij Maximum height roughness (Rmax) is the difference in height between the highest and lowest points: Rmax = Zmax − Zmin During the scanning, phase shifts, i.e. changes in the phase contact angle of vibration with respect to the phase angle of freely oscillating cantilever, were recorded simultaneously with height images. 2.4. Tensile strength tests Mechanical tensile tests of the coated fibers were conducted at room temperature using FM-4 (Hungary) testing machine. Single fibers extracted from a tow were fixed on paper frame using a hard resin. The gauge of 10 mm in length was used. The diameter of each filament was measured in the middle of length by laser interferometry and used for calculation of mechanical properties of filaments. Next, the lateral sides of support frame were cut by a heated wire and the load was applied at constant crosshead speed of 1.3 mm/min. About 50 filaments for each type of fibers were tested. The average diam￾eters for the coated fibers were determined to be equal to

N I. Baklanova er al. Joumal of the European Ceramic Society 28(2008)1687-1696 1689 Fig 1. The AFM images of the initial fibers(3D height representation ):(a)Hi-Nicalon TM; (b)Hi-Nicalon STM; (c) Tyranno-SATM 13.94+0.18 for Hi-NicalonTM, 13. 110.16 for Hi-Nicalon fiber has very well-developed relief which can be due to large STM, and 7.53+0.07 um for Tyranno-SATM fibers size grains in the surface region of fiber, with lateral sizes of particles being 100-200nm Grains on the filament surface are disoriented. Phase contrast AFM images help to reveal minor 3. Results features, which are sometimes poorly resolved, but only height images provide correct topographical data. When the surface 3.1. SEM/EDS, AFM and TEM analysis of as-received Sic relief contains elements, which vertical dimensions differ by the order of magnitude, it is difficult to demonstrate both types of elements in the same image. In order to increase the contrast in a AFM image of the initial Hi-Nicalon"M fiber is represented 3D image, Fig Ic was constructed by superposition of phase data in Fig. la. Filaments have very smooth and uniform surface. over 3D topography. This type of representation shows minor The estimated roughness parameter is about 5 nm and almost details on 3D topographical relief. Roughness parameters, Ra independent on the size of the scanned area. Contrary to Hi- and Rmean, were determined as 10 and 52 nm, respectively, for NicalonTM. the surface relief of Hi-Nicalon STM filament is scanned area of 1. 2 um greatly non-uniform(Fig 1b). Some areas have a rather homoge neous relief, while other areas consist of different size nodules. 3. 2. SEM/EDS, AFM analysis of the Rezroz-coated Sic In some cases, their sizes run to several hundred nanometers. fibers The disposition of nodules appears to be rather random and ape riodic. This picture is a typical one for all tested Hi-Nicalon S SEM images of the Rezro2(one dipping-annealing cycle filaments. There is a large scattering in the roughness parameters coating on Hi-Nicalon M fiber are represented in Fig. 2a-c determined for different areas of the same filament. Roughness a distinctive feature of this coating is smoothness and unifor- Ra, was determined as 6-10 nm for scanned area of 4 um. For mity along whole length and diameter of filaments. Separate areas with nodules the roughness is increased till about 25 nm. well-developed crystals and discontinuity of the coating can The difference in quality of the surfaces of both types of fibers be seen on the surface, but this is very rare occurrence. The probably to be related not only to the chemistry of the fiber thickness of coating determined by SEM is about 200 nm. From but also to the other factors. Hi-Nicalon is already successful more close view of coated fibers one can see that the coating within commercial market and thus the production parameters is formed by the radial oriented nanosized crystallites with a are strictly controlled for large quantities, whereas, Hi-Nicalon high aspect ratio. On separate filaments we observed dual ori- sTM is a new monofilament with a limited production scale, and entation of the crystals, namely, parallel and perpendicular to thus the parameters may still require further refinement in order filament axis(Fig. 2d). Earlier, it was shown that the orienta- to stabilize properties. I tion of crystals of the RezrO2 coating on Hi-Nicalon fiber is AFM image of the initial Tyranno-SA fiber is represented greatly effected by the properties of initial sol. Close view in Fig. Ic. It is clearly seen from these picture that this type of AFM(the height phase representation) and SEM images

N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 1689 Fig. 1. The AFM images of the initial fibers (3D height representation): (a) Hi-NicalonTM; (b) Hi-Nicalon STM; (c) Tyranno-SATM. 13.94 ± 0.18 for Hi-NicalonTM, 13.11 ± 0.16 for Hi-Nicalon STM, and 7.53 ± 0.07m for Tyranno-SATM fibers. 3. Results 3.1. SEM/EDS, AFM and TEM analysis of as-received SiC fibers AFM image of the initial Hi-NicalonTM fiber is represented in Fig. 1a. Filaments have very smooth and uniform surface. The estimated roughness parameter is about 5 nm and almost independent on the size of the scanned area. Contrary to Hi￾NicalonTM, the surface relief of Hi-Nicalon STM filament is greatly non-uniform (Fig. 1b). Some areas have a rather homoge￾neous relief, while other areas consist of different size nodules. In some cases, their sizes run to several hundred nanometers. The disposition of nodules appears to be rather random and ape￾riodic. This picture is a typical one for all tested Hi-Nicalon STM filaments. There is a large scattering in the roughness parameters determined for different areas of the same filament. Roughness, Ra, was determined as 6–10 nm for scanned area of 4 m2. For areas with nodules the roughness is increased till about 25 nm. The difference in quality of the surfaces of both types of fibers probably to be related not only to the chemistry of the fiber but also to the other factors. Hi-NicalonTM is already successful within commercial market and thus the production parameters are strictly controlled for large quantities, whereas, Hi-Nicalon STM is a new monofilament with a limited production scale, and thus the parameters may still require further refinement in order to stabilize properties.11 AFM image of the initial Tyranno-SATM fiber is represented in Fig. 1c. It is clearly seen from these picture that this type fiber has very well-developed relief which can be due to large￾size grains in the surface region of fiber, with lateral sizes of particles being 100–200 nm. Grains on the filament surface are disoriented. Phase contrast AFM images help to reveal minor features, which are sometimes poorly resolved, but only height images provide correct topographical data. When the surface relief contains elements, which vertical dimensions differ by the order of magnitude, it is difficult to demonstrate both types of elements in the same image. In order to increase the contrast in a 3D image, Fig. 1c was constructed by superposition of phase data over 3D topography. This type of representation shows minor details on 3D topographical relief. Roughness parameters, Ra and Rmean, were determined as 10 and 52 nm, respectively, for scanned area of 1.2 m2. 3.2. SEM/EDS, AFM analysis of the ReZrO2-coated SiC fibers SEM images of the ReZrO2 (one dipping–annealing cycle) coating on Hi-NicalonTM fiber are represented in Fig. 2a–c. A distinctive feature of this coating is smoothness and unifor￾mity along whole length and diameter of filaments. Separate well-developed crystals and discontinuity of the coating can be seen on the surface, but this is very rare occurrence. The thickness of coating determined by SEM is about 200 nm. From more close view of coated fibers one can see that the coating is formed by the radial oriented nanosized crystallites with a high aspect ratio. On separate filaments we observed dual ori￾entation of the crystals, namely, parallel and perpendicular to filament axis (Fig. 2d). Earlier, it was shown that the orienta￾tion of crystals of the ReZrO2 coating on Hi-Nicalon fiber is greatly effected by the properties of initial sol.12 Close view of AFM (the height phase representation) and SEM images

N 1. Baklanova et al /Journal of the European Ceramic Sociery 28(2008)1687-1696 discontinuity 10 um 200nm 200 Fig.2.The images of the surface and cross-section of the ReZrOz-coated Hi-Nicalon TM fiber: (a-c)SEM and(d)AFM(height representation) suggests that the coating has nanosized porous structure(Fig. 2b structure of the coating(Fig. 2)that allows crystals of coating to d) No debonding of RezrO2 coating on Hi-Nicalon"M fiber Roughness parameters of the coated Hi-NicalonMfiber were was observed. According to Patil and Subbarao, there is an estimated after subtracting of second-order surface. Average anisotropy of thermal expansion for t-zrO2 along axis, the roughness, Ra, and average amplitude, Rmean, were found to be largest thermal expansion was determined to be along a and 2.8 and 13. 5 nm, respectively, over 3. 5 um x 3.5 um area. The c axes(11.60 x 10-6 and 1608x 10-6oC-, respectively), the are practically independent on the scanned area size. This fact smallest along b axis(1.35 x 10-6oC-). Thus, there is the large can be evidence in favor of very uniform relief of the obtained difference in CTE along two axes of t-ZrO2 and Hi-Nicalon coating fiber((3.5-4)10C). Based on this fact, one could expect The morphology and topography of the Retro coating that high thermal stresses during cooling of the coated fibers on hi-Nicalon s M fiber are somewhat distinct from those could cause debonding of coating. However, for all coated observed for Hi-NicalonTM fiber. Although the application of filaments under investigation no debonding was observed by coating on Hi-Nicalon STM fiber gives rise to a smoothing high-resolution SEM analysis. Earlier, based on the X-ray spec- of the surface relief of Hi-Nicalon STM(Ra 5nm for the troscopy studies of the Y-ZrO2 coating on NicalonM fiber, we coated fiber vs Ra 7 nm for the initial fiber for 4 um2 scanned found that the Zr-o-Si bonds were formed at the fiber-coating area), te large-size nodules were observed(Fig 3a). They interface region of the Y-ZrO2-coated Nicalon fiber. White and originate from as-received fiber. The coating is composed of coworkersexamined the Zro2/SiOz interfacial zone of thin crystals aligned perpendicularly to the surface of filaments ZrO2 films on silicon using XPS and also presumed that the and has porous structure. Again, no debonding of coating was formation of the Zr-0-Si bonds takes place. Not only chem- observed ical bonding but also a dramatic rearrangement of the atomie Although the coating on Tyranno-SATM fiber is formed by coordinates exists at the ZrO2/SiO2 interface as was shown by rather coarse crystallites, it is uniform along length and diam- Jarvis and Carter. It appears to provide a significant source eter of filaments(Fig. 3b). The presence of zirconium was of interface strengthening even at ambient temperature and in confirmed by EDX analysis taken from different parts of the the absence of a new reaction phase. Another reason for the coating(Fig 3c). Non-uniformities such as large-size pores and absence of debonding under thermal stresses could be a porous crystals are practically absent for filament batch studied. AFM

1690 N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 Fig. 2. The images of the surface and cross-section of the ReZrO2-coated Hi-NicalonTM fiber: (a–c) SEM and (d) AFM (height representation). suggests that the coating has nanosized porous structure (Fig. 2b and d). No debonding of ReZrO2 coating on Hi-NicalonTM fiber was observed. According to Patil and Subbarao13, there is an anisotropy of thermal expansion for t-ZrO2 along axis, the largest thermal expansion was determined to be along a and c axes (11.60 × 10−6 and 16.08 × 10−6 ◦C−1, respectively), the smallest along b axis (1.35 × 10−6 ◦C−1). Thus, there is the large difference in CTE along two axes of t-ZrO2 and Hi-NicalonTM fiber ((3.5–4) × 10−6 ◦C−1). Based on this fact, one could expect that high thermal stresses during cooling of the coated fibers could cause debonding of coating. However, for all coated filaments under investigation no debonding was observed by high-resolution SEM analysis. Earlier, based on the X-ray spec￾troscopy studies of the Y-ZrO2 coating on NicalonTM fiber, we found that the Zr O Si bonds were formed at the fiber–coating interface region of the Y-ZrO2-coated Nicalon fiber.6 White and coworkers14 examined the ZrO2/SiO2 interfacial zone of thin ZrO2 films on silicon using XPS and also presumed that the formation of the Zr O Si bonds takes place. Not only chem￾ical bonding but also a dramatic rearrangement of the atomic coordinates exists at the ZrO2/SiO2 interface as was shown by Jarvis and Carter15. It appears to provide a significant source of interface strengthening even at ambient temperature and in the absence of a new reaction phase. Another reason for the absence of debonding under thermal stresses could be a porous structure of the coating (Fig. 2) that allows crystals of coating to expand. Roughness parameters of the coated Hi-NicalonTM fiber were estimated after subtracting of second-order surface. Average roughness, Ra, and average amplitude, Rmean, were found to be 2.8 and 13.5 nm, respectively, over 3.5 m × 3.5m area. They are practically independent on the scanned area size. This fact can be evidence in favor of very uniform relief of the obtained coating. The morphology and topography of the ReZrO2 coating on Hi-Nicalon STM fiber are somewhat distinct from those observed for Hi-NicalonTM fiber. Although the application of coating on Hi-Nicalon STM fiber gives rise to a smoothing of the surface relief of Hi-Nicalon STM (Ra ∼ 5 nm for the coated fiber vs. Ra ∼7 nm for the initial fiber for 4m2 scanned area), separate large-size nodules were observed (Fig. 3a). They originate from as-received fiber. The coating is composed of crystals aligned perpendicularly to the surface of filaments and has porous structure. Again, no debonding of coating was observed. Although the coating on Tyranno-SATM fiber is formed by rather coarse crystallites, it is uniform along length and diam￾eter of filaments (Fig. 3b). The presence of zirconium was confirmed by EDX analysis taken from different parts of the coating (Fig. 3c). Non-uniformities such as large-size pores and crystals are practically absent for filament batch studied. AFM

N I. Baklanova er al. Joumal of the European Ceramic Society 28(2008)1687-1696 169 10um c Fig 3. The SeM/EDX analysis data of ReZrO2-coated fibers:(a)Hi-Nicalon STM and(b and c) Tyranno-SATM roughness parameters estimated for the coated Tyranno-SATM be discreetly proposed that these bright spots are belonging to fiber confirm this observation. Actually, Ra value was found the harder and less viscoelastic phase than the main phase. The to be 10-12 nm for scanned area of 1.7 um x 1. 7 um in size nature and reasons for appearance of this phase are not clear. In and only slightly higher than that for the initial Tyranno-SATM any case, this phenomenon deserves to be more carefully and fiber. Rmean values are also slightly higher than those for the precisely studied in the future initial fiber (85 nm vs. 60 nm). A comparison of the AFM Micro-Raman spectra taken from the coated Hi-Nicalon images in height and phase contrast modes taken for the same Hi-Nicalon S, and Tyranno-saM fibers showed no any addi- scanned area of the coated fiber allowed us to detect interesting tional features besides those belonging to fibers themselves. The peculiarity, namely, the appearance of nanosized bright spots at ReZrOz coatings(one dipping-annealing cycle)on SiC fibers the boundaries of main crystal phase(Fig. 4a and b). It could appeared to be too thin for Raman measurements. a b Fig4. The AFM images of the Re ZrO2-coated Tyranno-SATM fiber: height(a)and phase(b)representation of the same area of surface

N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 1691 Fig. 3. The SEM/EDX analysis data of the ReZrO2-coated fibers: (a) Hi-Nicalon STM and (b and c) Tyranno-SATM. roughness parameters estimated for the coated Tyranno-SATM fiber confirm this observation. Actually, Ra value was found to be 10–12 nm for scanned area of 1.7m × 1.7m in size and only slightly higher than that for the initial Tyranno-SATM fiber. Rmean values are also slightly higher than those for the initial fiber (∼85 nm vs. ∼60 nm). A comparison of the AFM images in height and phase contrast modes taken for the same scanned area of the coated fiber allowed us to detect interesting peculiarity, namely, the appearance of nanosized bright spots at the boundaries of main crystal phase (Fig. 4a and b). It could be discreetly proposed that these bright spots are belonging to the harder and less viscoelastic phase than the main phase. The nature and reasons for appearance of this phase are not clear. In any case, this phenomenon deserves to be more carefully and precisely studied in the future. Micro-Raman spectra taken from the coated Hi-NicalonTM, Hi-Nicalon STM, and Tyranno-SATM fibers showed no any addi￾tional features besides those belonging to fibers themselves. The ReZrO2 coatings (one dipping–annealing cycle) on SiC fibers appeared to be too thin for Raman measurements. Fig. 4. The AFM images of the ReZrO2-coated Tyranno-SATM fiber: height (a) and phase (b) representation of the same area of surface.

2 N 1. Baklanova et al /Journal of the European Ceramic Sociery 28(2008)1687-1696 ed Hi-Nicalon d Hi-Nicalon S s-received 1000C · coated1000°C coated1200°c 0 12 Fig.5. Weibull probability plots for room temperature tensile strengths of the coated Tyranno-sa fiber at 1000 and 1200"C RezrO-coated Hi-Nicalon TM and Hi-Nicalon STM fibers population was small in number. Therefore we could not extract 3.3. Tensile strength measurements the Weibull parameters from the obtained data. It is well known that mechanical properties of fiber-reinforced 3.4. The oxidized Rezro2-coated SiC fibers CMC's depend not only on the interfacial stress transfer capacity but also on mechanical properties of fibers. Because of ten Dependences of the relative mass Am/mo on time for sile strength fibers can be greatly influenced by coating, it is the as-received and RezrO2-coated Tyranno-SA fibers(one very important to determine the tensile properties of the coated dipping-annealing cycle)at 1000 and 1200C are represented SiC fibers in order to evaluate their ability as reinforcement in Fig. 6. Both the as-received and coated fibers exhibit a simi- for CMCs. In this study the single filament tensile tests of the lar behavior during oxidation test at 1000C, namely, (i) first LezrO2-coated fibers were performed at room temperature. The mass loss over a short period of time(2h); (ii) a slight mass strength data for individual fibers were obtained using measured gain; and (iii) gradual mass loss for long exposition. The mass values of diameter of each filament, after that the data were ana- loss of the initial and coated fibers at the beginning of oxida- zed using two-parameter single-modal Weibull function. The tion is not more than several percents and could be attributed to In(1/Pr)vs. In o Weibull probability plots for ReZrO2-coated Hi- burnoff of carbon. The following small mass gain could be due NicalonM and Hi-Nicalon S M fibers are represented in Fig. 5. to oxidation of SiC that results in the formation of thin silica The tensile strength and the Weibull modulus were found to be layer. The exposition of the coated fibers to air at 1200C leads 2.71+0.08 GPa and 3.73, respectively, for the ReZrO2-coated to significant mass loss due to the volatilization of silicon-and Hi-Nicalon M fiber. The tensile strength is in good agreement carbon-containing with that reported in literature 4, 16 for desized fiber. The ten- the upper layer I/ com) through the cracks and pores in sile strength for the hi-Nicalon STM fiber is 3.14+0.10GPa SEM micrographs of the coated fibers after exposition to air and this value is slightly higher that those for the initial desized at 1000C(40 h) are represented in Fig. 7a-c. As a whole, the fiber. The Weibull modulus was found to be 4.20. The increase coatings conserve their integrity, uniformity and smoothness in the fiber strength appears to be related to the elimination The round shape formations were observed on the surface of of the surface flaws by the Rezroz coating. The most part of oxidized coated Hi-Nicalon $ fiber. They were present on the the RezrO2-coated Tyranno-SA filaments were broken at the surface of coated fibers(Fig 3a)and originated from the initial edge of frame during tensile strength measurements and data fiber(Fig 1b). Contrary to Hi-Nicalon STM, the new formations 10 um 2 Fig. 7. SEM images of the ReZrO2-coated Hi-NicalonTM (a), Hi-Nicalon SM()and Tyranno-SATM (c)fibers after exposition to air at 1000C for 40h

1692 N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 Fig. 5. Weibull probability plots for room temperature tensile strengths of the ReZrO2-coated Hi-NicalonTM and Hi-Nicalon STM fibers. 3.3. Tensile strength measurements It is well known that mechanical properties of fiber-reinforced CMC’s depend not only on the interfacial stress transfer capacity but also on mechanical properties of fibers.1 Because of ten￾sile strength fibers can be greatly influenced by coating, it is very important to determine the tensile properties of the coated SiC fibers in order to evaluate their ability as reinforcement for CMC’s. In this study the single filament tensile tests of the ReZrO2-coated fibers were performed at room temperature. The strength data for individual fibers were obtained using measured values of diameter of each filament, after that the data were ana￾lyzed using two-parameter single-modal Weibull function. The ln(1/Pf) vs. ln σ Weibull probability plots for ReZrO2-coated Hi￾NicalonTM and Hi-Nicalon STM fibers are represented in Fig. 5. The tensile strength and the Weibull modulus were found to be 2.71 ± 0.08 GPa and 3.73, respectively, for the ReZrO2-coated Hi-NicalonTM fiber. The tensile strength is in good agreement with that reported in literature14,16 for desized fiber. The ten￾sile strength for the Hi-Nicalon STM fiber is 3.14 ± 0.10 GPa and this value is slightly higher that those for the initial desized fiber. The Weibull modulus was found to be 4.20. The increase in the fiber strength appears to be related to the elimination of the surface flaws by the ReZrO2 coating. The most part of the ReZrO2-coated Tyranno-SATM filaments were broken at the edge of frame during tensile strength measurements and data Fig. 6. Dependences of the relative mass m/m0 on time for the initial and coated Tyranno-SATM fiber at 1000 and 1200 ◦C. population was small in number. Therefore we could not extract the Weibull parameters from the obtained data. 3.4. The oxidized ReZrO2-coated SiC fibers Dependences of the relative mass m/m0 on time for the as-received and ReZrO2-coated Tyranno-SA fibers (one dipping–annealing cycle) at 1000 and 1200 ◦C are represented in Fig. 6. Both the as-received and coated fibers exhibit a simi￾lar behavior during oxidation test at 1000 ◦C, namely, (i) first a mass loss over a short period of time (∼2 h); (ii) a slight mass gain; and (iii) gradual mass loss for long exposition. The mass loss of the initial and coated fibers at the beginning of oxida￾tion is not more than several percents and could be attributed to burnoff of carbon. The following small mass gain could be due to oxidation of SiC that results in the formation of thin silica layer. The exposition of the coated fibers to air at 1200 ◦C leads to significant mass loss due to the volatilization of silicon- and carbon-containing compounds through the cracks and pores in the upper layer.17 SEM micrographs of the coated fibers after exposition to air at 1000 ◦C (40 h) are represented in Fig. 7a–c. As a whole, the coatings conserve their integrity, uniformity and smoothness. The round shape formations were observed on the surface of oxidized coated Hi-Nicalon STM fiber. They were present on the surface of coated fibers (Fig. 3a) and originated from the initial fiber (Fig. 1b). Contrary to Hi-Nicalon STM, the new formations Fig. 7. SEM images of the ReZrO2-coated Hi-NicalonTM (a), Hi-Nicalon STM (b) and Tyranno-SATM (c) fibers after exposition to air at 1000 ◦C for 40 h.

N I. Baklanova er al. Joumal of the European Ceramic Society 28(2008)1687-1696 1693 Crack and debonding 1 um d 2 200nm Fig8. SEM images of the initial Hi-Nicalon TM (a-c)and Hi-Nicalon STM(d)fibers after exposition to air at 1200C for 40h. of large size which were observed on the surface of Tyranno- at 1200C for 40h are represented in Fig. 9a-C, respectively SA fibers afterexposition to air at 1000C ( Fig. 7c). They are The surface of oxidized coated fibers is not strongly distin- related to oxidation process. Indeed, the surface of the coated guished from oxidized surface of the initial fibers and similar Tyranno-SATM fiber before high-temperature exposition to air pattern relief was observed. No spalling of coating was detected was rather uniform according to SEM and AFM data and no any and strong bonding between the fiber core and coating for all large-size grains were detected. pes of fiber is retained after long oxidation. The elemental The surface relief of the initial fibers under investigation is microanalysis by SEM/EDS of oxidized coated fiber taken from rustically changed after exposition to laboratory air at 1200c different areas indicates the presence of Si, Zr, Al (as contami- (Fig. 8a-d). According to SEM analysis data, the surface relief nation from crucibles), and O. Although the ReZroz coating has of the initial Hi-Nicalon fiber becomes rather rough and porous nanostructure and as a consequence, cannot protect fiber patterned. Nanosized pores are present between crystals. The from oxidation in full extent, it has positive influence on oxid thickness of oxidized upper layer was found to be -250nm tion behavior. As we observed(Fig. &a and c), silica layer formed (Fig. 8b). In some cases, cracks and debonding of upper layer on the surface of uncoated fiber was cracked and debonded, was observed(Fig. &a and c). A similar patterned relief was hence, could not serve as effective barrier to oxygen, whereas observed for the oxidized initial Hi-Nicalon $M fiber( Fig. &d). Rezro2 coating was strongly bonded to SiC fiber The surface of the oxidized initial Tyranno-SATM fiber was Micro-Raman spectra taken from the smooth surfaces of the nodul oxidized RezrO2-coated Sic fibers demonstrate only the pres- SEM images of the surface of RezrOz-coated Hi-Nicalon, ence of peaks belonging to fibers itself. The coatings appeared Hi-Nicalon SM and Tyranno-SAM fibers after exposition to air to be very thin for Raman measurements. Raman spectra taken 10m 10 um Fig9. SEM images of the ReZrO2-coated Hi-NicalonTM (a), Hi-Nicalon STM()and Tyranno-SATM fibers(c)after exposition to air at 1200C for 40h

N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 1693 Fig. 8. SEM images of the initial Hi-NicalonTM (a–c) and Hi-Nicalon STM (d) fibers after exposition to air at 1200 ◦C for 40 h. of large size which were observed on the surface of Tyranno￾SATM fibers after exposition to air at 1000 ◦C (Fig. 7c). They are related to oxidation process. Indeed, the surface of the coated Tyranno-SATM fiber before high-temperature exposition to air was rather uniform according to SEM and AFM data and no any large-size grains were detected. The surface relief of the initial fibers under investigation is drastically changed after exposition to laboratory air at 1200 ◦C (Fig. 8a–d). According to SEM analysis data, the surface relief of the initial Hi-NicalonTM fiber becomes rather rough and patterned. Nanosized pores are present between crystals. The thickness of oxidized upper layer was found to be ∼250 nm (Fig. 8b). In some cases, cracks and debonding of upper layer was observed (Fig. 8a and c). A similar patterned relief was observed for the oxidized initial Hi-Nicalon STM fiber (Fig. 8d). The surface of the oxidized initial Tyranno-SATM fiber was nodular. SEM images of the surface of ReZrO2-coated Hi-NicalonTM, Hi-Nicalon STM and Tyranno-SATM fibers after exposition to air at 1200 ◦C for 40 h are represented in Fig. 9a–c, respectively. The surface of oxidized coated fibers is not strongly distin￾guished from oxidized surface of the initial fibers and similar pattern relief was observed. No spalling of coating was detected and strong bonding between the fiber core and coating for all types of fiber is retained after long oxidation. The elemental microanalysis by SEM/EDS of oxidized coated fiber taken from different areas indicates the presence of Si, Zr, Al (as contami￾nation from crucibles), and O. Although the ReZrO2 coating has porous nanostructure and as a consequence, cannot protect fiber from oxidation in full extent, it has positive influence on oxida￾tion behavior. As we observed (Fig. 8a and c), silica layer formed on the surface of uncoated fiber was cracked and debonded, hence, could not serve as effective barrier to oxygen, whereas ReZrO2 coating was strongly bonded to SiC fiber. Micro-Raman spectra taken from the smooth surfaces of the oxidized ReZrO2-coated SiC fibers demonstrate only the pres￾ence of peaks belonging to fibers itself. The coatings appeared to be very thin for Raman measurements. Raman spectra taken Fig. 9. SEM images of the ReZrO2-coated Hi-NicalonTM (a), Hi-Nicalon STM (b) and Tyranno-SATM fibers (c) after exposition to air at 1200 ◦C for 40 h

N 1. Baklanova et al /Journal of the European Ceramic Sociery 28(2008)1687-1696 compounds. This assignment seems to be reasonable taking ★t-zro into attention the fact that the composition of the as-received near stoichiometric SiC fiber is represented by mainly SiC phase together with very small quantity of graphite-like carbon. The long exposition of coated Sic fiber to air at high temperatures results in the oxidation of sic and the formation of the si-o-c The same sol of the stabilized zirconia was applied to thi types of Sic-based fibers. Hence, one could expect that some properties of the coated silicon carbide fibers will have a sim- ilarity. Actually, as one could see, the interfacial coatings are composed of tetragonal zirconia. The application of coatings 10. Micro-Raman spectrum(=488 nm)of the Re ZrO2-coated Tyranno- resulted in a smoothing of the fiber relief. Coatings are contin- fiber xposition to air at 1200C for 40h uous, well-ordered and rather uniform. The last is confirmed by AFM data on distribution of the measured heights and roughness from large-size new formations on the surface of the oxidized parameters of the surface relief. The distribution of the measured RezrO2-coated Tyranno-SA fibers clearly show the peaks heights of relief is the same or narrower compared with that for belonging to t-ZrO2 in the 100-700cm-Iregion(Fig 10). 8, 9 the initial fibers. The roughness parameters are lower compared No peaks belonging to other ZrO2 modifications were detected. with those for the initial fibers Several peaks, which are in good agreement with those reported Based on these experimental results one can conclude that for Tyranno-SA grade 3 fiber are also present in Raman spec- microstructure of the fiber is improved during the coating pro- trum. According to data, peak(shoulder)at-760cm can be cess. This conclusion has several important consequences in assigned to stretching of Si-C bonds in a-SiC, peaks centered at terms of the mechanical behavior of composite reinforced by 792 and -are belonging to stretching of Si-C bonds the coated fibers. The first of them is a retaining or a slight in B-SiC. Together with above-mentioned peaks, two main bands increase of the filament tensile strength at room temperature of amorphous carbon(so-called D and G bands) are also present due to the improved microstructure of the coated fibers. Actu- in the 1200-1600 cm region of Raman spectrum. ally, no any macrodefects which could be able to weaken a One can note that a new feature centered at 1084 cm-I is cross-section of fibers were detected on the surface of the observed in the spectrum of oxidized RezrO2-coated Tyranno- coated fibers. Hence, one can expect that the overall strength SA fiber. The assignment of this feature is a question of of composite reinforced by stronger coated fibers will be also special consideration. As this feature was observed only in increased Raman spectra of oxidized fibers it is reasonable to assume that The other consequence is that a smooth relief of the coated it originates from the oxidation process and is related to either fibers will provide easier sliding of the coated fibers relatively products of oxidation of SiC fiber itself(SiO2 phases)or prod- matrix and pull-out of fibers during the matrix crack propaga ucts of interaction of ZrO2 with Sio phases, namely, zircon. tion. Earlier, an extensive pull-out phenomenon for SiC/SiC Indeed,according to literature data, 21,22 peak centered at about composites reinforced by the ReZrO2-coated Hi-NicalonTM 1080cm is present in Raman spectra of SiO2 phases(e.g crys- and Hi-Nicalon S fibers was observed by Baklanova and tobalite and quartz), but its intensity is very low. The other peaks Lyakhov2. It was found that the fracture surface of the RezrO2 of these SiOz phases must be observed in the low frequency coated Hi-Nicalon M fiber composite was more fibrous in nature region(400-200cm-). However, as was mentioned above, no than that of Hi-Nicalon STM fiber composite andespecially, com- any peaks other than belonging to t-ZrO2 were detected in the pared with the composites reinforced by the uncoated fibers. The 400-200cm region in Raman spectrum of oxidized coated reason for this may be related to the smaller surface roughness fiber. It suggests that this feature cannot be related to stretch- of the ReZrO2-coated Hi-Nicalon TM fiber compared with that ing of the Si-o bond in SiOz phases. According to data by for the uncoated fiber and Rezro2-coated Hi-Nicalon Sw fiber. Syme et al. 2 and Lee and Condrate24, peak at 1009 cm- Fibers with high surface roughness have been found to have present in Raman spectrum of zircon(ZrSiO4). This Raman shift pronounced influence on fiber sliding behavior in CMCs26-28 does not coincide with that observed in this work. Hence. one The third consequence is in that the narrow distribution can discreetly assume that phases other than SiOz and Zrsio4 in sizes of particles of the coating provides a good stabil- are responsible for the appearance of this peak. One can note ity of the coating microstructure during exposition at elevated the affinity of the position of peak at 1080 cm-I observable in temperatures. No significant grain growth was observed. This aman spectrum of the oxidized zirconia-coated Tyranno-SATM is especially true for Hi-NicalonTM fiber and in less extent fiber to that reported for the asymmetric Si-o-Si stretching for Tyranno-SAM and Hi-Nicalon STM fiber. As one can vibration that normally observed in spectra of organic silicon see, the roughness practically does not change for the coated

1694 N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 Fig. 10. Micro-Raman spectrum (λ = 488 nm) of the ReZrO2-coated Tyranno￾SATM fiber after exposition to air at 1200 ◦C for 40 h. from large-size new formations on the surface of the oxidized ReZrO2-coated Tyranno-SATM fibers clearly show the peaks belonging to t-ZrO2 in the 100–700 cm−1 region (Fig. 10).18,19 No peaks belonging to other ZrO2 modifications were detected. Several peaks, which are in good agreement with those reported for Tyranno-SA grade 3 fiber20 are also present in Raman spec￾trum. According to data,20 peak (shoulder) at ∼760 cm−1 can be assigned to stretching of Si–C bonds in -SiC, peaks centered at ∼792 and ∼966 cm−1 are belonging to stretching of Si C bonds in-SiC. Together with above-mentioned peaks, two main bands of amorphous carbon (so-called D and G bands) are also present in the 1200–1600 cm−1 region of Raman spectrum. One can note that a new feature centered at ∼1084 cm−1 is observed in the spectrum of oxidized ReZrO2-coated Tyranno￾SATM fiber. The assignment of this feature is a question of special consideration. As this feature was observed only in Raman spectra of oxidized fibers it is reasonable to assume that it originates from the oxidation process and is related to either products of oxidation of SiC fiber itself (SiO2 phases) or prod￾ucts of interaction of ZrO2 with SiO2 phases, namely, zircon. Indeed, according to literature data,21,22 peak centered at about 1080 cm−1 is present in Raman spectra of SiO2 phases (e.g. crys￾tobalite and quartz), but its intensity is very low. The other peaks of these SiO2 phases must be observed in the low frequency region (400–200 cm−1). However, as was mentioned above, no any peaks other than belonging to t-ZrO2 were detected in the 400–200 cm−1 region in Raman spectrum of oxidized coated fiber. It suggests that this feature cannot be related to stretch￾ing of the Si–O bond in SiO2 phases. According to data by Syme et al.23 and Lee and Condrate24, peak at 1009 cm−1 is present in Raman spectrum of zircon (ZrSiO4). This Raman shift does not coincide with that observed in this work. Hence, one can discreetly assume that phases other than SiO2 and ZrSiO4 are responsible for the appearance of this peak. One can note the affinity of the position of peak at 1080 cm−1 observable in Raman spectrum of the oxidized zirconia-coated Tyranno-SATM fiber to that reported for the asymmetric Si O Si stretching vibration that normally observed in spectra of organic silicon compounds.21 This assignment seems to be reasonable taking into attention the fact that the composition of the as-received near stoichiometric SiC fiber is represented by mainly SiC phase together with very small quantity of graphite-like carbon. The long exposition of coated SiC fiber to air at high temperatures results in the oxidation of SiC and the formation of the Si O C structures. 4. Discussion The same sol of the stabilized zirconia was applied to three types of SiC-based fibers. Hence, one could expect that some properties of the coated silicon carbide fibers will have a sim￾ilarity. Actually, as one could see, the interfacial coatings are composed of tetragonal zirconia. The application of coatings resulted in a smoothing of the fiber relief. Coatings are contin￾uous, well-ordered and rather uniform. The last is confirmed by AFM data on distribution of the measured heights and roughness parameters of the surface relief. The distribution of the measured heights of relief is the same or narrower compared with that for the initial fibers. The roughness parameters are lower compared with those for the initial fibers. Based on these experimental results one can conclude that microstructure of the fiber is improved during the coating pro￾cess. This conclusion has several important consequences in terms of the mechanical behavior of composite reinforced by the coated fibers. The first of them is a retaining or a slight increase of the filament tensile strength at room temperature due to the improved microstructure of the coated fibers. Actu￾ally, no any macrodefects which could be able to weaken a cross-section of fibers were detected on the surface of the coated fibers. Hence, one can expect that the overall strength of composite reinforced by stronger coated fibers will be also increased. The other consequence is that a smooth relief of the coated fibers will provide easier sliding of the coated fibers relatively matrix and pull-out of fibers during the matrix crack propaga￾tion. Earlier, an extensive pull-out phenomenon for SiC/SiC composites reinforced by the ReZrO2-coated Hi-NicalonTM and Hi-Nicalon STM fibers was observed by Baklanova and Lyakhov25. It was found that the fracture surface of the ReZrO2- coated Hi-NicalonTM fiber composite was more fibrous in nature than that of Hi-Nicalon STM fiber composite and especially, com￾pared with the composites reinforced by the uncoated fibers. The reason for this may be related to the smaller surface roughness of the ReZrO2-coated Hi-Nicalon TM fiber compared with that for the uncoated fiber and ReZrO2-coated Hi-Nicalon STM fiber. Fibers with high surface roughness have been found to have a pronounced influence on fiber sliding behavior in CMC’s.26–28 The third consequence is in that the narrow distribution in sizes of particles of the coating provides a good stabil￾ity of the coating microstructure during exposition at elevated temperatures. No significant grain growth was observed. This is especially true for Hi-NicalonTM fiber and in less extent for Tyranno-SATM and Hi-Nicalon STM fiber. As one can see, the roughness practically does not change for the coated

N I. Baklanova er al. Joumal of the European Ceramic Society 28(2008)1687-1696 1695 Tyranno-SAT ber and slightly increases for the coated 3. Mogilevsky, P, Boakye, E.E., Hay, R.S., Welter, J and Kerans, R. fiber even after exposition to air at 1000C. Monazite coatings on SiC fibers. Il Oxidation protection. J. A. Ceram Despite of the fact that all studied fibers are silicon carbide 4. Verdenelli, M. Parola, S, Chassagneux, F, Letoffe, J.M. Vincent,H fibers and the same coating was applied to them, some of prop- Scharff, J.-P. er al, Sol-gel preparation and thermo-mechanical properties erties of interfacial coatings are greatly distinct from each other. f porous xAl203-ySiOz coatings on SiC Hi-Nicalon fibres. J. Eur. Ceram. Firstly, the coated Tyranno-SA fiber surface is formed by Soc.,2003,23(8),1207-121 more coarse crystallites than that of Hi-Nicalon TM fiber. Besides. 5. Li, H, Lee, J, Libera, M.R., Lee, W.Y,Kebbede, A,Lance, MIet al. separate large crystallites are randomly disposed on the sur- Morphological evolution and weak interface development within chemical- face of the coated Tyranno-SATMfiber. Further,the interfacial vapor-deposited zirconia coating deposited on Hi-Nicalon M fiber.J.Am. Ceran.Soc.2002,85(6,1561-1568 oughness parameters for coated and uncoated Tyranno-SATM 6.Baklanova, N I. Titov, A. T, Boronin, A. I and Kosheev, S. V, The yttria- ber are higher than those for Hi-Nicalon fiberTM The dis- stabilized zirconia interfacial coating on Nicalon fiber. Eur Ceram. Soc. tribution of the measured heights of relief is wider than that 2006,26(9),1725-1736 for Hi-Nicalon TM fiber attesting to a wider scatter in grain 7. Meier, B: Grathwohl, G. Spallek, M and Pannhorst W Sob-gel coatings sizes. The difference in morphology of the coated fibers is 10(3),237-243 enhanced after exposition to air at 1000C and especially at 8. Colomban, Ph, Bruneton, E, Lagrange, J. L and Mouchon, E, Sol-ge 1200C. The coated Tyranno-SAM fibers exhibit very inten- mullite matrix-SiC and mullite 2D woven fabric composites with or without sive grain growth, with large grains growing due to small ones zirconia containing interphase: elaboration and properties. J. Eur Ceram. It is sufficient to create critical faws and to weaken the coated Soc.,1996,16(2),301-314. 9. Baklanova, N. L, Kolesov, B. A. and Zima, T. M, Raman study of yttria fibers stabilized zirconium oxide interfacial coatings. J. Eur Cera. Soc.. 2007. Thus, one can conclude that microstructural peculiarities 27(1),165-171. of the Retro interfacial coating on Sic-based fibers includ- 10. Baklanova, N I, Zaitsev, B N and Titov, A T, Atomic force and scanning ing morphology, porosity, nanorelief, the coating/fiber interface electron microscopy study of the zirconia-coated silicon carbon fibers. J. contribute to behavior of the coated fibers Microstructure of the Eur Ceran.Soc,2007,27(6),2503-2511. interphase may be as critical for thermostructural applications 11. Morimoto, T and Ogasawara, T, Potential strength of Nicalon, Hi Nicalon, nd Hi Nicalon Type S monofilaments of variable diameters. Composit of CMCs, even if the service time of ceramic components will anA,2006,37(3),405-412 be relatively short. 12. Baklanova, N, Zima, T, Titov, A and Zaitsev. B, Interfacial coatings on inorganic fibers for high temperature ceramic matrix composites. Key Eng Mater, 2008, in press. 5. Conclusion 13. Patil, R. N. and Subbarao, E. C, Axial thermal expansion of ZrO2 and HfO2 in the range room temperature to 1400C. J Appl Cryst., 1969, 2, Sols of rare earth stabilized zirconia were used as simple 281-288 readily processable and accurate controllable precursors for the 14.Sun, Y.M., Lozano,J.Ho,H,Park,HJ,Veldman,Sand White,JM rO2 interfacial coatings on commercially available SiC-based Interfacial silicon oxide formation during synthesis of ZrO2 on Si(100) AppL.Smrf.sci.,2000,161(2,115-122. fibers. The sol can be applied to different types of Sic fibers 15. Jarvis, E. A. A. and Carter, E. A, Exploiting covalency to enhance without degrading fiber strength. The morphology, composi- metal-oxide and oxide- oxide adhesion at heterogeneous interfaces. J. Am tion structure. nanorelief and oxidation resistance of coated Ceran.Soc.2003,86(3),373-386. fibers were evaluated by various analytical techniques, includ- 16. Sha, J. J, Nozawa, T, Park, J.S. Katoh, Y and Kohyama,A.Effect of ing SEm/eds, AFm, and micro-Raman. It was shown that eat treatment on the tensile strength and creep resistance of advanced Sic fibers.J.Mcl. Mater,2004,329-33,5992-6602 microstructural peculiarities of the Rezro2 interfacial coatings 17.Zhu, Y.T,Taylor,S.T, Stout,M.G,Butt,DPand Lowe,TC,Kinetics on Sic-based fibers may explain some of the differences in the f thermal. passive oxidation of Nicalon fibers. J. Am. Ceram. Soc., 1998. behavior of different types of fibers 81(3),655-660 18. Lopez, E F, Escribano, VS Panizza, M, Carnasciali, M. M. and Busca, G, Vibrational and electronic spectroscopic properties of zirconia powders. Acknowledgements J Mater: Chem,2001,11(7),1891-1897 19. Strekalovsky, V. N, Makurin, Yu. N. and Vovkotrub, E. G. Study of phase The authors are grateful to Mrs. T M. Naimushina for mea ansformation and defects in the ZrO2-Y2O3 system by Raman spec. troscopy Inorg. Mater. 1983. 19(6),925-929(in Russian) surements of the tensile strength of filaments and Dr. B.A. 20. Havel, M. and Colomban, Ph, Rayleigh and Raman images of the Kolesov(Institute of Inorganic Chemistry SB RAS) for mea- bulk/surface nanostructure of SiC based fibres. Composites: Part B, 2004. surements of micro-Raman spectra 35(1),139-147. 21. Socrates, G, Infrared Characteristic Group Frequencies. John Wiley and Sons. New York, 198 References 22 Nakamoto, K. Infrared Spectra and Raman Spectra of inorganic and Coordination Compounds. John Wiley and Sons, New York, 1991 G D. B. Mechanical behavior of ceramic matrix p.120 composites In Fiber Reinforced Ceramic Composites, ed K.S. Mazdiyani. 23. Syme,R. w.G., Lockwood, D J and Kerr, H J, Raman spectrum of General Atomics, San Diego, CA, 1990. Pp. 1-39. synthetic zircon(ZrsiO4)and thorite (ThSiO4).J. Phys. C: Solid State Pirys 2. Boakye, E. E, Mogilevsky, P, Parthasarathy, T. A, Hay, R.S., Wel- 977,10(6,1335-1348 ter, J. and Kerans, R. J. Monazite coatings on SiC fibers. I. Fiber 24. Lee, S. w. and Condrate Sr, R. A. The infrared and Raman spectra of ength and thermal stability. J. A. Ceram. Soc., 2006, 89(11), ZrO2-SiO2 glasses prepared by a sol-gel process. J. Mater. Sci., 1988, 3309-3324. 3(11).2951-2959

N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 1695 Hi-NicalonTM fiber and slightly increases for the coated Tyranno-SATM fiber even after exposition to air at 1000 ◦C. Despite of the fact that all studied fibers are silicon carbide fibers and the same coating was applied to them, some of prop￾erties of interfacial coatings are greatly distinct from each other. Firstly, the coated Tyranno-SATM fiber surface is formed by more coarse crystallites than that of Hi-NicalonTM fiber. Besides, separate large crystallites are randomly disposed on the sur￾face of the coated Tyranno-SATM fiber. Further, the interfacial roughness parameters for coated and uncoated Tyranno-SATM fiber are higher than those for Hi-Nicalon fiberTM. The dis￾tribution of the measured heights of relief is wider than that for Hi-NicalonTM fiber attesting to a wider scatter in grain sizes. The difference in morphology of the coated fibers is enhanced after exposition to air at 1000 ◦C and especially at 1200 ◦C. The coated Tyranno-SATM fibers exhibit very inten￾sive grain growth, with large grains growing due to small ones. It is sufficient to create critical flaws and to weaken the coated fibers. Thus, one can conclude that microstructural peculiarities of the ReZrO2 interfacial coating on SiC-based fibers, includ￾ing morphology, porosity, nanorelief, the coating/fiber interface contribute to behavior of the coated fibers. Microstructure of the interphase may be as critical for thermostructural applications of CMC’s, even if the service time of ceramic components will be relatively short. 5. Conclusion Sols of rare earth stabilized zirconia were used as simple, readily processable and accurate controllable precursors for the ZrO2 interfacial coatings on commercially available SiC-based fibers. The sol can be applied to different types of SiC fibers without degrading fiber strength. The morphology, composi￾tion, structure, nanorelief and oxidation resistance of coated fibers were evaluated by various analytical techniques, includ￾ing SEM/EDS, AFM, and micro-Raman. It was shown that microstructural peculiarities of the ReZrO2 interfacial coatings on SiC-based fibers may explain some of the differences in the behavior of different types of fibers. Acknowledgements The authors are grateful to Mrs. T.M. Naimushina for mea￾surements of the tensile strength of filaments and Dr. B.A. Kolesov (Institute of Inorganic Chemistry SB RAS) for mea￾surements of micro-Raman spectra. References 1. Evans, A. G. and Marshall, D. B., Mechanical behavior of ceramic matrix composites. In Fiber Reinforced Ceramic Composites, ed. K. S. Mazdiyani. General Atomics, San Diego, CA, 1990, pp. 1–39. 2. Boakye, E. E., Mogilevsky, P., Parthasarathy, T. A., Hay, R. S., Wel￾ter, J. and Kerans, R. J., Monazite coatings on SiC fibers. I. Fiber strength and thermal stability. J. Am. Ceram. Soc., 2006, 89(11), 3309–3324. 3. Mogilevsky, P., Boakye, E. E., Hay, R. S., Welter, J. and Kerans, R. J., Monazite coatings on SiC fibers. II. Oxidation protection. J. Am. Ceram. Soc., 2006, 89(11), 3475–3480. 4. Verdenelli, M., Parola, S., Chassagneux, F., Letoffe, J.-M., Vincent, H., Scharff, J.-P. et al., Sol–gel preparation and thermo-mechanical properties of porous xAl2O3–ySiO2 coatings on SiC Hi-Nicalon fibres. J. Eur. Ceram. Soc., 2003, 23(8), 1207–1213. 5. Li, H., Lee, J., Libera, M. R., Lee, W. Y., Kebbede, A., Lance, M. J. et al., Morphological evolution and weak interface development within chemical￾vapor-deposited zirconia coating deposited on Hi-NicalonTM fiber. J. Am. Ceram. Soc., 2002, 85(6), 1561–1568. 6. Baklanova, N. I., Titov, A. T., Boronin, A. I. and Kosheev, S. V., The yttria￾stabilized zirconia interfacial coating on Nicalon fiber. J. Eur. Ceram. Soc., 2006, 26(9), 1725–1736. 7. Meier, B., Grathwohl, G., Spallek, M. and Pannhorst, W., Sol–gel coatings on ceramic fibers for ceramic matrix composites. J. Eur. Ceram. Soc., 1992, 10(3), 237–243. 8. Colomban, Ph., Bruneton, E., Lagrange, J. L. and Mouchon, E., Sol–gel mullite matrix-SiC and mullite 2D woven fabric composites with or without zirconia containing interphase: elaboration and properties. J. Eur. Ceram. Soc., 1996, 16(2), 301–314. 9. Baklanova, N. I., Kolesov, B. A. and Zima, T. M., Raman study of yttria stabilized zirconium oxide interfacial coatings. J. Eur. Ceram. Soc., 2007, 27(1), 165–171. 10. Baklanova, N. I., Zaitsev, B. N. and Titov, A. T., Atomic force and scanning electron microscopy study of the zirconia-coated silicon carbon fibers. J. Eur. Ceram. Soc., 2007, 27(6), 2503–2511. 11. Morimoto, T. and Ogasawara, T., Potential strength of Nicalon, Hi Nicalon, and Hi Nicalon Type S monofilaments of variable diameters. Composites: Part A, 2006, 37(3), 405–412. 12. Baklanova, N., Zima, T., Titov, A. and Zaitsev, B., Interfacial coatings on inorganic fibers for high temperature ceramic matrix composites. Key Eng. Mater., 2008, in press. 13. Patil, R. N. and Subbarao, E. C., Axial thermal expansion of ZrO2 and HfO2 in the range room temperature to 1400 ◦C. J. Appl. Cryst., 1969, 2, 281–288. 14. Sun, Y. M., Lozano, J., Ho, H., Park, H. J., Veldman, S. and White, J. M., Interfacial silicon oxide formation during synthesis of ZrO2 on Si(1 0 0). Appl. Surf. Sci., 2000, 161(2), 115–122. 15. Jarvis, E. A. A. and Carter, E. A., Exploiting covalency to enhance metal–oxide and oxide–oxide adhesion at heterogeneous interfaces. J. Am. Ceram. Soc., 2003, 86(3), 373–386. 16. Sha, J. J., Nozawa, T., Park, J. S., Katoh, Y. and Kohyama, A., Effect of heat treatment on the tensile strength and creep resistance of advanced SiC fibers. J. Nucl. Mater., 2004, 329–333, 5992–6602. 17. Zhu, Y. T., Taylor, S. T., Stout, M. G., Butt, D. P. and Lowe, T. C., Kinetics of thermal, passive oxidation of Nicalon fibers. J. Am. Ceram. Soc., 1998, 81(3), 655–660. 18. Lopez, E. F., Escribano, V. S., Panizza, M., Carnasciali, M. M. and Busca, G., Vibrational and electronic spectroscopic properties of zirconia powders. J. Mater. Chem., 2001, 11(7), 1891–1897. 19. Strekalovsky, V. N., Makurin, Yu. N. and Vovkotrub, E. G., Study of phase transformation and defects in the ZrO2–Y2O3 system by Raman spec￾troscopy. Inorg. Mater., 1983, 19(6), 925–929 (in Russian). 20. Havel, M. and Colomban, Ph., Rayleigh and Raman images of the bulk/surface nanostructure of SiC based fibres. Composites: Part B, 2004, 35(1), 139–147. 21. Socrates, G., Infrared Characteristic Group Frequencies. John Wiley and Sons, New York, 1980. 22. Nakamoto, K., Infrared Spectra and Raman Spectra of Inorganic and Coordination Compounds. John Wiley and Sons, New York, 1991, p. 120. 23. Syme, R. W. G., Lockwood, D. J. and Kerr, H. J., Raman spectrum of synthetic zircon (ZrSiO4) and thorite (ThSiO4). J. Phys. C: Solid State Phys., 1977, 10(6), 1335–1348. 24. Lee, S. W. and Condrate Sr, R. A., The infrared and Raman spectra of ZrO2–SiO2 glasses prepared by a sol–gel process. J. Mater. Sci., 1988, 23(11), 2951–2959

N.I. Baklanova et al /Journal of the European Ceramic Sociery 28(2008)1687-1696 aklanova, N. I and Lyakhov, N. Z, The development of components of 27. Jero, P D, Kerans, R.J. and Parthasarathy, T.A Effect of interfacial rough- advanced te materials for structural applications. In Proceedings ess on the frictional stress measured using pushout tests. J. Am. Ceram of the /V international Technological Congress, Part 2, ed. V. V. Shalay Soc.1991,74(11),2793-2801. OmSTU, Omsk, 2007, Pp 306-310 28. Brennan, J. J, Interfacial characterization of slurry-cast melt-infiltrated 26. Parthasarathy, T.A., Barlage, D. R, Jero, P. D. and Kerans, R.J., Effect of SiC/SiC ceramic matrix composites. Acta Mate, 2000, 48(18-19) interfacial roughness parameters on the fiber pushout beh omposite. J. A. Ceram Soc., 1994, 77(12) 3236

1696 N.I. Baklanova et al. / Journal of the European Ceramic Society 28 (2008) 1687–1696 25. Baklanova, N. I. and Lyakhov, N. Z., The development of components of advanced composite materials for structural applications. In Proceedings of the IV International Technological Congress, Part 2, ed. V. V. Shalay. OmSTU, Omsk, 2007, pp. 306–310. 26. Parthasarathy, T. A., Barlage, D. R., Jero, P. D. and Kerans, R. J., Effect of interfacial roughness parameters on the fiber pushout behavior of a model composite. J. Am. Ceram. Soc., 1994, 77(12), 3232–3236. 27. Jero, P. D., Kerans, R. J. and Parthasarathy, T. A., Effect of interfacial rough￾ness on the frictional stress measured using pushout tests. J. Am. Ceram. Soc., 1991, 74(11), 2793–2801. 28. Brennan, J. J., Interfacial characterization of slurry-cast melt-infiltrated SiC/SiC ceramic matrix composites. Acta Mater., 2000, 48(18–19), 4610–4628