《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_The yttria-stabilized zirconia and interfacial coating on NicalonTM fiber

CIENCEODIRECT. E≈RS ELSEVIER Journal of the European Ceramic Society 26(2006)1725-1736 www.elsevier.com/locate/jeurcera The yttria-stabilized zirconia and interfacial coating on Nicalon fiber N I Baklanova a, * A.T. Titoy b.A. l Boronin.s v. Kosheey c a Institute of Solid State Chemistry and Mechanochemistry, SB RAS, Kutateladze St. 18, Novosibirsk 630128, Russian Federation b General Institute of Geology, Geophysics and Mineralogy, SB RAS, Nou 630090. Russian Federation Received 15 November 2004: received in revised form 24 February 2005: accepted 5 March 2005 Available online 3 May 2005 Abstract Sols of yttria-stabilized zirconia may be used as simple, readily processable and accurate controllable precursors for the ZrO2 interfacial oatings on SiC-based Nicalon fibers. The ZrO2 interfacial coatings of predictable crystal phase compositions were obtained in dependend of yttria dopant level. The morphology, composition and oxidation resistance of coated fibers were evaluated by SEM, EDs, XPS, XRD and Raman analysis. All coatings obtained are uniform, continuous and adherent to substrates. The delamination within the ZrO2 interfacial coating was found. Possible reasons of this phenomenon are discussed. The peculiarities of the behavior of Y-stabilized ZrO2-coated fibers in air at elevated temperature are considered. o 2005 Elsevier Ltd. All rights reserved Keyword: Y ttria-stabilized zirconia: Fibres; Interfaces; SiC; ZrO2; Coatings 1. Introduction within ZrO2 layer occurred as result of the martensitic trans- formation of t-ZrO nuclei to m-ZrO, on reaching a critical Ceramic matrix composites(CMCs) reinforced by Sic grain size and the development of significant compressive based fibers such as Nicalon, Hi-Nicalon, Sylramic@ oop stresses due to volume dilation and shear associated iC/SiC) achieve high toughness and damage tolerance with the martensitic transformation. The delamination to be through the disposal of weak fiber coating which can deflect bserved for the ZrO2 coating provides the retention of fiber cracks and promote debonding at the fiber/matrix region. It strength. It should be noted that the delamination process can is stated that conventional interphase materials, such as car- lead to breaking of the integrity of the fiber coating. It is un bon and bn exhibit the environmental instability at operating desirable feature considering the necessity of protection the temperatures. Therefore, there is a strong interest to study fiber surface from matrix infiltration on following stages of alternative interfaces that would be more oxidation-resistant the fabrication CMCs. Therefore. a dense undelaminated but than carbon and Bn coatings. Among alternative interphases compressively stressed coating might be preferred before in- the refractory oxide-based systems are considered as the most filtration. Thus, in order to optimize the interfacial properties of ZrO2 as the fiber coating for SiC/SiC composites it is nec- The feasibility of using a CVD ZrO2 fiber coating as an essary accurately to control phase composition, morphology oxidation-resistant and weak interphase for SiC/SiC com- and an extent of phase transformation in the ZrO2 coating posites was thoroughly examined by Lee and coworkers7-9 layer They found that the CVd ZrO2 coating exhibited desired Numerous studies demonstrated(see, e.g. review ) that tensile failure behavior and extensive crack deflection within an addition of Y203 or other oxides, such as CaO, Mg the interface region. They concluded that the delamination CeO results in an appearance of oxygen vacancies and the formation of the stabilized tetragonal phase of ZrO. The Corresponding author. Tel. +7 3832 363839: fax: +7 3832 32284 toughness of Ysz ceramics is strongly dependent on the E-mail address: baklanova@ solid nsc. ru(NI. Baklanova volume fraction of tetragonal phase with toughness de 0955-2219/S-see front matter 2005 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2005.03.241

Journal of the European Ceramic Society 26 (2006) 1725–1736 The yttria-stabilized zirconia and interfacial coating on NicalonTM fiber N.I. Baklanova a,∗, A.T. Titov b, A.I. Boronin c, S.V. Kosheev c a Institute of Solid State Chemistry and Mechanochemistry, SB RAS, Kutateladze St. 18, Novosibirsk 630128, Russian Federation b General Institute of Geology, Geophysics and Mineralogy, SB RAS, Novosibirsk 630090, Russian Federation c Boreskov Institute of Catalysis, SB RAS, Novosibirsk 630090, Russian Federation Received 15 November 2004; received in revised form 24 February 2005; accepted 5 March 2005 Available online 3 May 2005 Abstract Sols of yttria-stabilized zirconia may be used as simple, readily processable and accurate controllable precursors for the ZrO2 interfacial coatings on SiC-based NicalonTM fibers. The ZrO2 interfacial coatings of predictable crystal phase compositions were obtained in dependence of yttria dopant level. The morphology, composition and oxidation resistance of coated fibers were evaluated by SEM, EDS, XPS, XRD, and Raman analysis. All coatings obtained are uniform, continuous and adherent to substrates. The delamination within the ZrO2 interfacial coating was found. Possible reasons of this phenomenon are discussed. The peculiarities of the behavior of Y-stabilized ZrO2-coated fibers in air at elevated temperature are considered. © 2005 Elsevier Ltd. All rights reserved. Keyword: Yttria-stabilized zirconia; Fibres; Interfaces; SiC; ZrO2; Coatings 1. Introduction Ceramic matrix composites (CMCs) reinforced by SiC￾based fibers such as NicalonTM, Hi-NicalonTM, Sylramic® (SiC/SiC) achieve high toughness and damage tolerance through the disposal of weak fiber coating which can deflect cracks and promote debonding at the fiber/matrix region.1 It is stated that conventional interphase materials, such as car￾bon and BN exhibit the environmental instability at operating temperatures.2,3 Therefore, there is a strong interest to study alternative interfaces that would be more oxidation-resistant than carbon and BN coatings. Among alternative interphases the refractory oxide-based systems are considered as the most promising ones. 4–6 The feasibility of using a CVD ZrO2 fiber coating as an oxidation-resistant and weak interphase for SiC/SiC com￾posites was thoroughly examined by Lee and coworkers7–9 They found that the CVD ZrO2 coating exhibited desired tensile failure behavior and extensive crack deflection within the interface region. They concluded that the delamination ∗ Corresponding author. Tel.: +7 3832 363839; fax: +7 3832 322847. E-mail address: baklanova@solid.nsc.ru (N.I. Baklanova). within ZrO2 layer occurred as result of the martensitic trans￾formation of t-ZrO2 nuclei to m-ZrO2 on reaching a critical grain size and the development of significant compressive hoop stresses due to volume dilation and shear associated with the martensitic transformation. The delamination to be observed for the ZrO2 coating provides the retention of fiber strength. It should be noted that the delamination process can lead to breaking of the integrity of the fiber coating. It is un￾desirable feature considering the necessity of protection the fiber surface from matrix infiltration on following stages of the fabrication CMCs. Therefore, a dense undelaminated but compressively stressed coating might be preferred before in- filtration. Thus, in order to optimize the interfacial properties of ZrO2 as the fiber coating for SiC/SiC composites it is nec￾essary accurately to control phase composition, morphology and an extent of phase transformation in the ZrO2 coating layer. Numerous studies demonstrated (see, e.g. review10), that an addition of Y2O3 or other oxides, such as CaO, MgO, CeO2 results in an appearance of oxygen vacancies and the formation of the stabilized tetragonal phase of ZrO2. The toughness of YSZ ceramics is strongly dependent on the volume fraction of tetragonal phase, with toughness de- 0955-2219/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2005.03.241

1726 N.I. Baklanova et al /Journal of the European Ceramic Sociery 26(2006)1725-1736 creasing as a fraction of either the monoclinic or cubic phase 97 mol% ZrO2. The coating stage involved firstly the immer increases. Besides, it was detected the relationships between sion of the Nicalon fabrics into sols with different content the oxygen partial pressure and the nucleation and morpho- of yttria. Then the specimens were dried on air at ambient by CVD or magnetron sputtering methods. -9, These at atmospheric pressure. To increase of thickness of interfa- relationships were attributed to the grain size and oxygen cial coating the dipping-annealing procedure was repeated deficiency effects, which appear to cause the stabilization of several times t-ZrO2. Thus, one can see that the Zro2 coating composed Also the air-dried powders with different Y2O3 content of predominantly tetragonal or cubic transformable phases were prepared using sol-gel approach to compare the prop- can be fabricated with different experimental approaches erties of coatings and powders of the same composition. Early we reported about the particularities of the formation of refractory oxide interfacial coatings including ZrOz coat- 2.2. Specimen characterization ing on Nicalon fibers by sol-gel technique. A mixture of the monoclinic and tetragonal modifications was detected in The phases in sol-gel derived ZrO2 and Y-PSZ powders the ZrO2 coating derived from sol-gel precursors, with the and coatings were characterized by x-ray diffraction analysis monoclinic being predominant. The columnar morphology (XRD)using monochromatic Cu Ko radiation with DRON-3 was observed that could provide an easy, low-energy path for diffractometer(Russia). Scanning electron microscope SEM crack propagation. In order to either tough transformation or LEO 1430VP, supplied by EDX(Oxford)spectrometer was crack deflection mechanism to operate within interface zone used for studying of morphology and composition of coated the columnar morphology of ZrO2 coating must be modified fibers The aim of this work is to develop an approach to the The FT-Raman spectra of sol-gel derived ZrO2 and Y-pSZ formation of yttria-stablilized Zro, interfacial coatings on powders were recorded using a Bruker RFS 100/S spectrom- Nicalon TM fiber and to study the peculiarities of their compo- eter equipped with Nd- YAG laser operating at the 1064 nm sition, morphology and oxidation resistance. For this purpose excitation wavelength. The laser output was 100 m V. For each a sol-gel approach was chosen as main. The sol-gel pre spectrum 100 scans were accumulated. Micro Raman spectra could be considered as one of the most convenient technique of the Y-PSZ-coated Nicalon fibers were recorded using allowing us to produce the oxide interfacial coatings includ- a Triplemate SPEX spectrometer equipped with CCD spec ing zirconia coatings with controlled content of yttria and as ometric detector and microscope attachment for back scat- a consequence with predictable phase composition and mor tering geometry. The 488 nm radiation from an argon laser phology. was used for spectral excitation. XPS spectra were measured using VG ESCALAB spec trometer using al Ko irradiation and calibrated against au 2. Experimental 4f7/2(Eb=840eV) and Cu 2p3/2(Eb=932.7eV) lines Be- fore measurements specimens were heated for 30 min at 2.1. Substrate and coating preparation 600C in vacuum. All of spectra are represented in bind- ing energy scale that was obtained with respect to C ls peak Woven Nicalon TM NLM202(Nippon Carbon Co Japan) of carbon at 284.8eV For spectroscopic analysis of electron fiber cloths were used as substrate materials. Prior to coat- spectra the original software CALC was applied to extract the ing, Nicalon TM fiber cloths were immersed for 24 h in 50: 50 detailed information about electron structure of coated fiber acetone-ethanol mixture for removing a sizing agent, after that they were dried at ambient temperature. Then they were 23. Oxidation tests thermally treated in air at 450C. The coating process was based on the dipping of Thermal oxidation resistance of coated NicalonTM fab- NicalonM fabrics into sols of hydrated oxide metals.Pre- rics was examined in air under static conditions at 1000C cursors for the Y203-partially-stabilized zirconia (Y-Psz The samples of Nicalon fabrics(100-200 mg)were placed in the preliminary heated furnace(Ko-14, German)and oatings were the sols of hydrated yttrium-zirconium oxides kept during definite time intervals. Then the samples were with different Zro2: Y2O3 ratio. The preparation of initial sols taken out, cooled in dessicator and weighted with accu- was similar to that described by Yan and coworkers. 4The coating solution was prepared by dissolving yttrium nitrate racy +O 1 mg. A total time of testing was 26h hexahydrate Y(NO3)3-6H2O and zirconyl chloride octahy drate ZroCl2-8H20(CG, the Hf content not more than 1% 3. Results at given molar ratio in appropriate amount of ethanol-water solution. The coating solution contained metal ion concen- 3.1. SEM/EDS analvsis trations of 0. 1 M. Sols with the 3 and 9 mol% Y203 content were prepared Samples derived from these sols were named SEM images of the undoped and Y-doped ZrO2 coatin after the yttria content, i.e. 3Y-Zr02 means 3 mol%Y2O3 and Nicalon M fiber are represented in Fig. 1a-d.One can see

1726 N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 creasing as a fraction of either the monoclinic or cubic phase increases. Besides, it was detected the relationships between the oxygen partial pressure and the nucleation and morpho￾logic characteristics of the ZrO2 coating that was formed by CVD or magnetron sputtering methods.7–9,11 These relationships were attributed to the grain size and oxygen deficiency effects, which appear to cause the stabilization of t-ZrO2. Thus, one can see that the ZrO2 coating composed of predominantly tetragonal or cubic transformable phases can be fabricated with different experimental approaches. Early we reported about the particularities of the formation of refractory oxide interfacial coatings including ZrO2 coat￾ing on NicalonTM fibers by sol–gel technique.12 A mixture of the monoclinic and tetragonal modifications was detected in the ZrO2 coating derived from sol–gel precursors, with the monoclinic being predominant. The columnar morphology was observed that could provide an easy, low-energy path for crack propagation. In order to either tough transformation or crack deflection mechanism to operate within interface zone the columnar morphology of ZrO2 coating must be modified. The aim of this work is to develop an approach to the formation of yttria-stablilized ZrO2 interfacial coatings on NicalonTM fiber and to study the peculiarities of their compo￾sition, morphology and oxidation resistance. For this purpose a sol–gel approach was chosen as main. The sol–gel process could be considered as one of the most convenient technique allowing us to produce the oxide interfacial coatings includ￾ing zirconia coatings with controlled content of yttria and as a consequence with predictable phase composition and mor￾phology. 2. Experimental 2.1. Substrate and coating preparation Woven NicalonTM NLM202 (Nippon Carbon Co. Japan) fiber cloths were used as substrate materials. Prior to coat￾ing, NicalonTM fiber cloths were immersed for 24 h in 50:50 acetone–ethanol mixture for removing a sizing agent, after that they were dried at ambient temperature. Then they were thermally treated in air at 450 ◦C. The coating process was based on the dipping of NicalonTM fabrics into sols of hydrated oxide metals. Pre￾cursors for the Y2O3-partially-stabilized zirconia (Y-PSZ) coatings were the sols of hydrated yttrium–zirconium oxides with different ZrO2:Y2O3 ratio. The preparation of initial sols was similar to that described by Yan and coworkers13,14 The coating solution was prepared by dissolving yttrium nitrate hexahydrate Y(NO3)3·6H2O and zirconyl chloride octahy￾drate ZrOCl2·8H2O (CG, the Hf content not more than 1%) at given molar ratio in appropriate amount of ethanol–water solution. The coating solution contained metal ion concen￾trations of 0.1 M. Sols with the 3 and 9 mol% Y2O3 content were prepared. Samples derived from these sols were named after the yttria content, i.e. 3Y-ZrO2 means 3 mol% Y2O3 and 97 mol% ZrO2. The coating stage involved firstly the immer￾sion of the NicalonTM fabrics into sols with different content of yttria. Then the specimens were dried on air at ambient temperature and then slowly heated till 960 ◦C in argon flow at atmospheric pressure. To increase of thickness of interfa￾cial coating the dipping–annealing procedure was repeated several times. Also the air-dried powders with different Y2O3 content were prepared using sol–gel approach to compare the prop￾erties of coatings and powders of the same composition. 2.2. Specimen characterization The phases in sol–gel derived ZrO2 and Y-PSZ powders and coatings were characterized by X-ray diffraction analysis (XRD) using monochromatic Cu K
radiation with DRON-3 diffractometer (Russia). Scanning electron microscope SEM LEO 1430VP, supplied by EDX (Oxford) spectrometer was used for studying of morphology and composition of coated fibers. The FT-Raman spectra of sol–gel derived ZrO2 and Y-PSZ powders were recorded using a Bruker RFS 100/S spectrom￾eter equipped with Nd-YAG laser operating at the 1064 nm excitation wavelength. The laser output was 100 mV. For each spectrum 100 scans were accumulated. Micro Raman spectra of the Y-PSZ-coated NicalonTM fibers were recorded using a Triplemate, SPEX spectrometer equipped with CCD spec￾trometric detector and microscope attachment for back scat￾tering geometry. The 488 nm radiation from an argon laser was used for spectral excitation. XPS spectra were measured using VG ESCALAB spec￾trometer using Al K
irradiation and calibrated against Au 4f7/2 (Eb = 84.0 eV) and Cu 2p3/2 (Eb = 932.7 eV) lines. Be￾fore measurements specimens were heated for 30 min at 600 ◦C in vacuum. All of spectra are represented in bind￾ing energy scale that was obtained with respect to C 1s peak of carbon at 284.8 eV. For spectroscopic analysis of electron spectra the original software CALC was applied to extract the detailed information about electron structure of coated fibers. 2.3. Oxidation tests Thermal oxidation resistance of coated NicalonTM fab￾rics was examined in air under static conditions at 1000 ◦C. The samples of NicalonTM fabrics (100–200 mg) were placed in the preliminary heated furnace (KO-14, German) and kept during definite time intervals. Then the samples were taken out, cooled in dessicator and weighted with accu￾racy ± 0.1 mg. A total time of testing was 26 h. 3. Results 3.1. SEM/EDS analysis SEM images of the undoped and Y-doped ZrO2 coating on NicalonTM fiber are represented in Fig. 1a–d. One can see

N.I. Baklanova et al / Journal of the European Ceramic Society 26 (2006)1725-1736 1727 Fig 1(a-d) SEM of the undoped (one cycle)and 9Y-ZrO2 coating on Nicalon M fiber(two cycles). (Fig. la), that the surface of undoped ZrO2 coating is rather other elements were detected in EDS spectra. They appear to smooth and adherent to fiber. No fiber bridging, no spalling of originate from a contamination by glue and sample holder. coating was observed. The non-uniformities shaped as crys- An increase of number of dipping-annealing till to tallized traces of coating sol that was retained between fila- 5 leads to strong nonuniformity of coating not only on ments within a tow bundle are seen on the surface of separate length but cross-section of a filament too(Fig. 3a-d), filaments. It should be noted that this is a rare occurrence. the thickness being about 1 um or more and variable at The thickness of coating after the first cycle, determined by different filaments. a great number of large well-developed SEM. is less than 80 nm crystals and aggregates are observed on the surface of A doping by yttrium(9Y-PSZ, two cycles) leads to sig filaments. One could be proposed that nonuniformaties nificant change in the surface morphology(Fig. Ib). Sepa- arising from the first cycles(see, e.g. Fig. 1b-d) could rate well-developed crystals and aggregates of crystals can serve as centers of crystallization on the following steps of be observed at the edge of cross-section and the surface of processing. Upon a closer view one can see that aggregates each filament, especially on contacts of filaments in a bundle are consisted from nanosized (less than 100 nm) crystals (Fig. Ic). Nevertheless, no bridging was observed(Fig. 1d).(Fig. 3b). They have isometric round-shaped form. One can The thickness of coating evaluated from SEM image is about observe places where a delamination within coating occurs 300-400 nm. Upon a closer view one can see that the mor(Fig. 3c). A crack deflection phenomenon is distinctly phology of crystals is isometric unlike of columnar one that detected at interfacial zone(ig. 3d). Crack direction is was observed for undoped ZrO2 coating. The presence of Si, changed from perpendicular to parallel to a fiber surface Zr, Y,O at areas which are marked in Fig. Id is detected by This is accompanied by debonding at the coating/fiber inter EDS analysis(Fig. 2). In addition, the peaks belonging to face

N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 1727 Fig. 1. (a–d) SEM images of the undoped (one cycle) and 9Y-ZrO2 coating on NicalonTM fiber (two cycles). (Fig. 1a), that the surface of undoped ZrO2 coating is rather smooth and adherent to fiber. No fiber bridging, no spalling of coating was observed. The non-uniformities shaped as crys￾tallized traces of coating sol that was retained between fila￾ments within a tow bundle are seen on the surface of separate filaments. It should be noted that this is a rare occurrence. The thickness of coating after the first cycle, determined by SEM, is less than 80 nm. A doping by yttrium (9Y-PSZ, two cycles) leads to sig￾nificant change in the surface morphology (Fig. 1b). Sepa￾rate well-developed crystals and aggregates of crystals can be observed at the edge of cross-section and the surface of each filament, especially on contacts of filaments in a bundle (Fig. 1c). Nevertheless, no bridging was observed (Fig. 1d). The thickness of coating evaluated from SEM image is about 300–400 nm. Upon a closer view one can see that the mor￾phology of crystals is isometric unlike of columnar one that was observed for undoped ZrO2 coating. The presence of Si, Zr, Y, O at areas which are marked in Fig. 1d is detected by EDS analysis (Fig. 2). In addition, the peaks belonging to other elements were detected in EDS spectra. They appear to originate from a contamination by glue and sample holder. An increase of number of dipping–annealing till to 5 leads to strong nonuniformity of coating not only on length but cross-section of a filament too (Fig. 3a–d), the thickness being about 1m or more and variable at different filaments. A great number of large well-developed crystals and aggregates are observed on the surface of filaments. One could be proposed that nonuniformaties arising from the first cycles (see, e.g. Fig. 1b–d) could serve as centers of crystallization on the following steps of processing. Upon a closer view one can see that aggregates are consisted from nanosized (less than 100 nm) crystals (Fig. 3b). They have isometric round-shaped form. One can observe places where a delamination within coating occurs (Fig. 3c). A crack deflection phenomenon is distinctly detected at interfacial zone (Fig. 3d). Crack direction is changed from perpendicular to parallel to a fiber surface. This is accompanied by debonding at the coating/fiber inter￾face.

1728 N.I. Baklanova et al /Journal of the European Ceramic Sociery 26(2006)1725-1736 Zr zr key Full Scale 1139 cts kev Full Scale 581 cts Fig. 2. EDS spectrum of the 9Y-zrO2( two cycles)coating on NicalonM fiber 3. 2. XPS analysis elemental composition of surface layer can be evaluated. 5 The results are represented in Table 1. One can see from Fig 4 Survey XPS spectrum recorded from the 9Y-PSZ (five cy- that traces of sodium and nitrogen are present in coating. Ni- cles)coated Nicalon fabrics revealed a presence of oxy- trogen can be originated from the initial yttrium nitrate. Ear- gen, silicon, carbon, zirconium, yttrium as main components lier, the traces of sodium were revealed by XPS analysis in (Fig 4). After correction of intensities of C ls, O 1s, Si 2p, Zr initial Nicalon M fiber. 2 They could be introduced during 3d and Y 3d lines for their atomic sensitivity factors(ASF)the processing of fiber or handling Fig3(a-d)SEM images of the 9Y-zrO2 coating on Nicalon M fiber(five cycles)

1728 N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 Fig. 2. EDS spectrum of the 9Y-ZrO2 (two cycles) coating on NicalonTM fiber. 3.2. XPS analysis Survey XPS spectrum recorded from the 9Y-PSZ (five cy￾cles) coated NicalonTM fabrics revealed a presence of oxy￾gen, silicon, carbon, zirconium, yttrium as main components (Fig. 4). After correction of intensities of C 1s, O 1s, Si 2p, Zr 3d and Y 3d lines for their atomic sensitivity factors (ASF) the elemental composition of surface layer can be evaluated.15 The results are represented in Table 1. One can see from Fig. 4 that traces of sodium and nitrogen are present in coating. Ni￾trogen can be originated from the initial yttrium nitrate. Ear￾lier, the traces of sodium were revealed by XPS analysis in initial NicalonTM fiber.12 They could be introduced during processing of fiber or handling. Fig. 3. (a–d) SEM images of the 9Y-ZrO2 coating on NicalonTM fiber (five cycles)

N.I. Baklanova et al / Journal of the European Ceramic Society 26 (2006)1725-1736 1729 O1s coated Nicalon M fiber is only slightly shifted to lower bind ing energy in comparison with peak for uncoated Nicalon M fiber. A slight shift to lower binding energy is observed also for Si 2p peak (Table 1). Analogous shifts O ls and si C1s 2p lines in XPS spectrum of Y203 film on silicon was found 11s by Chambers et al. 7 They detected also a shift of the Y 3ds/ Z3z30 peak to higher binding energy (158.3 eV). On this basis they concluded that the formation of interfacial structure with the Y3d Y-O-Si bonds possible to occur. According to data listed in Table I and in Fig 5b, the Y 3dsn peak for Y-PSZ-coated Nicalon is in a good accordance with data by Moulder et al.for pure Y203 and data reported by Zou et al.8There fore, one could be reliably proposed that the formation of the Binding energy, ev Zr-0-Si, but not the Y-o-Si bonds is more expectab\e TM The XPS Zr 3dsn spectrum of Y-PSZ-coated Nicalon fiber is shown in Fig. 5c. Deconvolution of this spectrum re- 4. Survey XPS spectrum of the 9Y-PSZ (five cycles) coated Nical sults in detection of two spin-orbital doublets. The position of one of them is in a good accordance with that reported for t YSZsingle crystal 6 Moreover, a strong broadening of peaks Fig 5 shows theO 1s, Y 3d and Zr 3d photopeaks Decon- is observed. It can be connected with disordering of the oxide volution of theo Is spectrum gives two components at 530.6 structure due to yttrium doping. 9, 20 Actually, from the SEM and 532.8eV. Earlier Boronin et al. 6 studied a XPS spe analysis results it follows that the oxide coating is strongly trum of the Y-stabilized ZrO2 single crystal in details. They nonuniform one and its integrity is disturbed(Fig. 3).An detected a single 1s peak at 5306e V which was assigned to additional doublet appears to be an evidence of presence of oxygen in the metal-oxygen-metal bond, namely Y-o-Zr. zirconium bonded with Sio2 surface layer of Nicalon fiber. One can propose that the low binding energy component that was observed in spectrum of Y-PSZ-coated Nicalon M fiber 3.3. Xrd and raman studies in this study also belongs to oxygen in the Y-o-zr bond Broadening of this peak (2.7e V) can be as evidence in The XRD patterns of the 3Y-( three cycles) and 9Y-PSZ favor of strong disordering of Y-doped zirconia phase. The ( five cycles)coated Nicalon fiber are represented in Fig. 6 second component at 532. 8eV inO ls spectrum of Y-PSz Very small intensity peaks are present in XRD pattern of sam- O1s Zr 3d 5328 535 150155160165178 181 184 187 Fig. 5. XPS spectra of the 9Y-ZrO2(four cycles) coating on Nicalon fiber: O Is, Y 3d(c)and Z 3d photopeaks Table 1 Elemental composition and binding energies for the 9Y-ZrO2- coated Nicalon fibers Elemental composition(at %o) 63 124 Binding energies(ev 5306,5328(0ls)102.5(Si2p)281.0,2848,289.6(ls)182.3(r3d5/2)182.3(Y3d5/2)10727(Nals) 1537(Si2s)

N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 1729 Fig. 4. Survey XPS spectrum of the 9Y-PSZ (five cycles) coated NicalonTM fiber. Fig. 5 shows the O 1s, Y 3d and Zr 3d photopeaks. Decon￾volution of the O 1s spectrum gives two components at 530.6 and 532.8 eV. Earlier Boronin et al.16 studied a XPS spec￾trum of the Y-stabilized ZrO2 single crystal in details. They detected a single O 1s peak at 530.6 eV which was assigned to oxygen in the metal–oxygen–metal bond, namely Y–O–Zr. One can propose that the low binding energy component that was observed in spectrum of Y-PSZ-coated NicalonTM fiber in this study also belongs to oxygen in the Y–O–Zr bond. Broadening of this peak (∼2.7 eV) can be as evidence in favor of strong disordering of Y-doped zirconia phase. The second component at 532.8 eV in O 1s spectrum of Y-PSZ￾coated NicalonTM fiber is only slightly shifted to lower bind￾ing energy in comparison with peak for uncoated NicalonTM fiber.12 A slight shift to lower binding energy is observed also for Si 2p peak (Table 1). Analogous shifts O 1s and Si 2p lines in XPS spectrum of Y2O3 film on silicon was found by Chambers et al.17 They detected also a shift of the Y 3d5/2 peak to higher binding energy (158.3 eV). On this basis they concluded that the formation of interfacial structure with the Y–O–Si bonds possible to occur. According to data listed in Table 1 and in Fig. 5b, the Y 3d5/2 peak for Y-PSZ-coated NicalonTM is in a good accordance with data by Moulder et al.15 for pure Y2O3 and data reported by Zou et al.18 There￾fore, one could be reliably proposed that the formation of the Zr–O–Si, but not the Y–O–Si bonds is more expectable. The XPS Zr 3d5/2 spectrum of Y-PSZ-coated NicalonTM fiber is shown in Fig. 5c. Deconvolution of this spectrum re￾sults in detection of two spin-orbital doublets. The position of one of them is in a good accordance with that reported for the YSZ single crystal.16 Moreover, a strong broadening of peaks is observed. It can be connected with disordering of the oxide structure due to yttrium doping.19,20 Actually, from the SEM analysis results it follows that the oxide coating is strongly nonuniform one and its integrity is disturbed (Fig. 3). An additional doublet appears to be an evidence of presence of zirconium bonded with SiO2 surface layer of NicalonTM fiber. 3.3. XRD and Raman studies The XRD patterns of the 3Y- (three cycles) and 9Y-PSZ (five cycles) coated NicalonTM fiber are represented in Fig. 6. Very small intensity peaks are present in XRD pattern of sam￾Fig. 5. XPS spectra of the 9Y-ZrO2 (four cycles) coating on NicalonTM fiber: O 1s, Y 3d (c) and Z 3d photopeaks. Table 1 Elemental composition and binding energies for the 9Y-ZrO2-coated NicalonTM fibers O Si C Zr Y Na Elemental composition (at.%) 63 12.4 10 10 3.6 1 Binding energies (eV) 530.6, 532.8 (O 1s) 102.5 (Si 2p) 281.0, 284.8, 289.6 (C 1s) 182.3 (Zr 3 d5/2) 182.3 (Y 3d5/2) 1072.7 (Na 1s) 153.7 (Si 2s)

1730 N.I. Baklanova et al /Journal of the European Ceramic Sociery 26(2006)1725-1736 a0.5Y-ZrO -3Y-zro 2 1000 3000 4000 Raman shift, cm C-ZrO. Fig. 7. Raman spectra(A=1064 nm)of IY, 3Y, and 9Y-doped ZrO2 pow- tetragonal modification only. The other important peculiar- ity of Raman spectrum of 3Y-ZrO2 is in the fact that all of fundamentals are broadening. It can be assigned to struc tural disordering associated with oxygen vacancies in Y- doped samples. Additional features can be detected in the 1000-3500 cm- region, namely, small intensity bands at Fig. 6. The XRD patterns of the 3Y- and 9Y-PSZ-coated Nicalon fibers. about 1000-1300cm and 1600cm and very intensive and narrow band at 2860cm-I together with new features ple3Y. They can be assigned to tetragonal zirconia phase about 2770, 3058 and 3400cm ICSD no. 42-1164) Sample 9Y"is cubic as expected. The The increase of yttria content in precursor up to 9 mol XRD pattern coincides with that reported for fluorite-type results in the following change in Raman spectra of the 9Y- structure (ICSD no. 27-0997) No traces of other zirconia ZrO2 powder(Fig. 7c). A major peak at 614 cm- is ob- phases are detected by X-ray analysis. Besides the ZrO2 served, the width being about 150 cm. This is an evidence peaks, very broad features belonging to B-SiC phase of fiber of the presence of cubic ZrO2 modification. The width of are also observed in both XRD patterns. this peak is anomalous owing to structural disordering in the For better interpretation of Raman results the spectra of Y- oxygen sublattice. Interestingly, the Raman spectrum shows poNo WY Raman spectra of powders are represented modification not detected by XRD(Fig. 6). The same addi- in Fig. 7. For comparison a Raman spectrum of 0.5 mol% tional features as in the 3Y-zi pectrum are seen sol-gel derived zirconia powder is also shown Clear changes 1000-3500 cm"" region, namely, very intensive and narrow in the main features of Raman spectra were observed for pow peak at about 2870 and less intensive 2780(shoulder), 2580 ders of various compositions. As one can see, in spectrum of and 3067 cm- peaks 0.5Y-ZrO at least 13 well-defined narrow bands are detected Micro Raman spectra of 3Y-(three cycles) and 9Y-PSz- in the 100-800cm-Iregion(Fig 7a). In accordance with data coated(four cycles)Nicalon M fiber(488nm excitation reported by Lopez et al.,all of these bands can be assigned wavelength) are represented in Fig. 8. All features in Ra to monoclinic modification. Hardly noticeable feature to be man spectrum of 3Y-PSZ-coated Nicalon fiber can be as- comparable to noise can be detected at about 260cm-. It is signed to t-zrO2. As one can see from Fig. 8( spectrum 9Y), probably attributable to the strongest band of the tetragonal the very broad and asymmetric peak at about 610cm-is zirconia impurity present together with several additional peaks. This behavior a Spectrum of the 3Y-ZrOz is sharply distinct from that of is attributed to disorder in the system due to ionic defects 0.5Y-ZrO2(Fig. 7b). It clearly shows six peaks at 148, 261, in the oxygen sublattice induced by acceptor. High defect 320, 466, 609(shoulder)and 641 cm-I. Their number and concentration can lead to a breakdown of the selection rules positions are in a good coincidence with data reported by and allow the additional modes that are forbidden for fluorite Lopez et al. for tetragonal modification. So we can asso- structure. According to Raman data, the main component ciate them with Raman active fundamentals for t-zrO2 and of coating is cubic ZrO2 modification. The affinity of the po- propose that the 3Y-ZrO2 powder specimen is composed of sitions of the rest peaks to those to be attributable to tetragonal

1730 N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 Fig. 6. The XRD patterns of the 3Y- and 9Y-PSZ-coated NicalonTM fibers. ple “3Y”. They can be assigned to tetragonal zirconia phase (ICSD no. 42-1164). Sample “9Y” is cubic as expected. The XRD pattern coincides with that reported for fluorite-type structure (ICSD no. 27-0997). No traces of other zirconia phases are detected by X-ray analysis. Besides the ZrO2 peaks, very broad features belonging to -SiC phase of fiber are also observed in both XRD patterns. For better interpretation of Raman results the spectra of Y￾PSZ powders derived from the coatings precursors were taken preliminarily. Raman spectra of powders are represented in Fig. 7. For comparison a Raman spectrum of 0.5 mol% sol–gel derived zirconia powder is also shown. Clear changes in the main features of Raman spectra were observed for pow￾ders of various compositions. As one can see, in spectrum of 0.5Y-ZrO2 at least 13 well-defined narrow bands are detected in the 100–800 cm−1 region (Fig. 7a). In accordance with data reported by Lopez et al.,21 all of these bands can be assigned to monoclinic modification. Hardly noticeable feature to be comparable to noise can be detected at about 260 cm−1. It is probably attributable to the strongest band of the tetragonal zirconia impurity. Spectrum of the 3Y-ZrO2 is sharply distinct from that of 0.5Y-ZrO2 (Fig. 7b). It clearly shows six peaks at 148, 261, 320, 466, 609 (shoulder) and 641 cm−1. Their number and positions are in a good coincidence with data reported by Lopez et al.21 for tetragonal modification. So we can asso￾ciate them with Raman active fundamentals for t-ZrO2 and propose that the 3Y-ZrO2 powder specimen is composed of Fig. 7. Raman spectra (λ = 1064 nm) of 1Y-, 3Y-, and 9Y-doped ZrO2 pow￾ders. tetragonal modification only. The other important peculiar￾ity of Raman spectrum of 3Y-ZrO2 is in the fact that all of fundamentals are broadening. It can be assigned to struc￾tural disordering associated with oxygen vacancies in Y￾doped samples. Additional features can be detected in the 1000–3500 cm−1 region, namely, small intensity bands at about 1000–1300 cm−1 and 1600 cm−1 and very intensive and narrow band at 2860 cm−1 together with new features at about 2770, 3058 and 3400 cm−1. The increase of yttria content in precursor up to 9 mol% results in the following change in Raman spectra of the 9Y￾ZrO2 powder (Fig. 7c). A major peak at 614 cm−1 is ob￾served, the width being about 150 cm−1. This is an evidence of the presence of cubic ZrO2 modification. The width of this peak is anomalous owing to structural disordering in the oxygen sublattice. Interestingly, the Raman spectrum shows a peak at ∼260 cm−1 which can be attributed to tetragonal modification not detected by XRD (Fig. 6). The same addi￾tional features as in the 3Y-ZrO2 spectrum are seen in the 1000–3500 cm−1 region, namely, very intensive and narrow peak at about 2870 and less intensive 2780 (shoulder), 2580 and 3067 cm−1 peaks. Micro Raman spectra of 3Y- (three cycles) and 9Y-PSZ￾coated (four cycles) NicalonTM fiber (488 nm excitation wavelength) are represented in Fig. 8. All features in Ra￾man spectrum of 3Y-PSZ-coated NicalonTM fiber can be as￾signed to t-ZrO2. As one can see from Fig. 8 (spectrum 9Y), the very broad and asymmetric peak at about 610 cm−1 is present together with several additional peaks. This behavior is attributed to disorder in the system due to ionic defects in the oxygen sublattice induced by acceptor. High defect concentration can lead to a breakdown of the selection rules and allow the additional modes that are forbidden for fluorite structure.22 According to Raman data, the main component of coating is cubic ZrO2 modification. The affinity of the po￾sitions of the rest peaks to those to be attributable to tetragonal

N.I. Baklanova et al / Journal of the European Ceramic Society 26 (2006)1725-1736 1731 eliminary he Raman shift, cm Fig 8. Micro Raman spectra(i=488 nm) of the 3Y- and 9Y-PSZ-coated Fig 9. Dependences of the relative mass Am/mo on time at 1000C for the 9Y-doped ZrO2-coated Nicalon fibers modification suggests the presence of t-ZrO2 modification in trum (o=514.5 nm) of nanocrystalline zirconia, whereas these peaks in Raman spectrum taken at A0=488 nm were coating layer too. The difference of Raman spectra of both absent. They unambiguously attributed these features to flu coatings from those of powders derived from the same sols is in an absence any peaks in the 1000-3500cm-Iregion orescence peaks due to presumably defects resulting from Summing up, one can state three groups of results to be discussed. The first group is concerned in the difference Raman spectra in dependence on yttria content. With increas- ing of yttria content, the formation of monoclinic(0.5 mol% yttria), tetragonal (3%o)and cubic(9%)modifications for sol- derived powders and coatings are clearly detected by raman spectroscopy Raman spectra of this study are in good ac- cordance with those reported elsewhere for different zirconia modifications Further, the appearance of additional features in the 1000-1600cm and 2500-3500cm- are observed for yttria-doped samples. As was noted by Strekalovsky et al.,23 the peaks in the 1000-1300cm region can be assigned to the bending vibration of Me-O-H grouping (Me--ZI Y). Taking into attention that this peak is observed only for 3Y-and 9Y-doped samples one can propose that the struc 3oμm tures of gel particles of zroz with high (3Y and 9)and low cb (0.5Y) dopant level are different from each other. A greater number of water molecules bonding to terminal hydroxyl roups of Y-doped zirconia particles in comparison with low level doped appear to be retained between adjacent parti cles till up to 1000C. The appearance of small intensity stretching) peaks in Raman spectrum can be as evidence in favor of this proposition. 24, 25 The group of very intensive and narrow peaks in the 2500-3100 cm" region is a subject of special considera- tion. The fact that these peaks are observable only in spectra taken using the 1064 nm and are absent in spectra taken us ing the 488 nm excitation wavelength suggests that the nature of these features is distinct from Raman scattering. Jurado et al.26 and ref. 17 therein also mentioned the presence of in- Fig 10. SEM images of the surface of the 9Y-ZrO2-coated Nicalon TM fibers tense peaks between 1000 and 1400cm- in Raman spec after exposition to air at 1000 C for 30h(b: clear-up image)

N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 1731 Fig. 8. Micro Raman spectra (λ = 488 nm) of the 3Y- and 9Y-PSZ-coated NicalonTM fiber. modification suggests the presence of t-ZrO2 modification in coating layer too. The difference of Raman spectra of both coatings from those of powders derived from the same sols is in an absence any peaks in the 1000–3500 cm−1 region. Summing up, one can state three groups of results to be discussed. The first group is concerned in the difference in Raman spectra in dependence on yttria content. With increas￾ing of yttria content, the formation of monoclinic (0.5 mol% yttria), tetragonal (3%) and cubic (9%) modifications for sol￾derived powders and coatings are clearly detected by Raman spectroscopy. Raman spectra of this study are in good ac￾cordance with those reported elsewhere for different zirconia modifications. Further, the appearance of additional features in the 1000–1600 cm−1 and 2500–3500 cm−1 are observed for yttria-doped samples. As was noted by Strekalovsky et al.,23 the peaks in the 1000–1300 cm−1 region can be assigned to the bending vibration of Me–O–H grouping (Me—Zr, Y). Taking into attention that this peak is observed only for 3Y- and 9Y-doped samples one can propose that the struc￾tures of gel particles of ZrO2 with high (3Y and 9) and low (0.5Y) dopant level are different from each other. A greater number of water molecules bonding to terminal hydroxyl groups of Y-doped zirconia particles in comparison with low level doped appear to be retained between adjacent parti￾cles till up to 1000 ◦C. The appearance of small intensity ∼1600 cm−1 (H2O bending) and ∼3050–3400 cm−1 (H2O stretching) peaks in Raman spectrum can be as evidence in favor of this proposition.24,25 The group of very intensive and narrow peaks in the 2500–3100 cm−1 region is a subject of special considera￾tion. The fact that these peaks are observable only in spectra taken using the 1064 nm and are absent in spectra taken us￾ing the 488 nm excitation wavelength suggests that the nature of these features is distinct from Raman scattering. Djurado et al.26 and ref. 17 therein also mentioned the presence of in￾tense peaks between 1000 and 1400 cm−1 in Raman spec￾Fig. 9. Dependences of the relative mass m/m0 on time at 1000 ◦C for the 9Y-doped ZrO2-coated NicalonTM fibers. trum (λ0 = 514.5 nm) of nanocrystalline zirconia, whereas these peaks in Raman spectrum taken at λ0 = 488 nm were absent. They unambiguously attributed these features to flu￾orescence peaks due to presumably defects resulting from Fig. 10. SEM images of the surface of the 9Y-ZrO2-coated NicalonTM fibers (two cycles) after exposition to air at 1000 ◦C for 30 h (b: clear-up image).

1732 N.I. Baklanova et al /Journal of the European Ceramic Sociery 26(2006)1725-1736 initial preparation. It is well-known that doping of zirconium significant decrease in oxidation rate. As one can see from oxide by yttrium leads to the formation of defects, namely, Fig 9, an increase in number of dipping-annealing cycle(in oxygen vacancies. So one could be discreetly proposed that different terms, the thickness of coating) gives rise to more the peaks detected in Raman spectrum of Y-doped zirconia oxidation resistant coated fibers in the 2500-3000cm- region could be attributed to fluores- cence phenomenon. In any case, this phenomenon deserves 3.5. SEM/EDS, X-ray and micro Raman analysis of to be more carefully and precisely studied in future oxidized coated fibers 3. 4. Oxidation tests es of the surface of 9Y-Zr0,-coated NicalonTM ion to air at 1000C for 30 h Dependences of the relative mass Am/mo on time at 1000"C for 9Y-doped ZrO2-coated Nicalon" fibers(one, not strongly distinguished from nonoxidized one. It is nonuni- four and five cycles)are represented in Fig 9. One can see, form and new formations of well-faceted shape and macro de- that the Y-doped Zro2-coated samples exhibit a similar over- fects as traces from sol which was retained between separate all behavior during oxidation test, namely: (i)first a sharp monofilaments can be seen. The coating retains its integrity weight loss and (i) a more progressive weight increase up and no spalling is observed. It is seen from clear-up image to the test completion. The time corresponding to the transi- of the surface of oxidized coated Nicalon TM fiber(Fig. 10b) tion between regimes ()and (i)is dependent on number of that it is composed of isometric crystals in despite of the dipping-annealing cycles. For the 9Y-doped ZrO2( one cy- columnar microstructure that was early observed by Bak- cle)sample this transition is detected approximately I h after lanova and coworkers2/for oxidized undoped ZrO2-coated the beginning of oxidation whereas for the 9Y-doped ZrO2 NicalonM fiber. The elemental microanalysis by SEM/EDS (five cycles) the transition takes place later. As the number of dipping-annealing cycle during preparation of samples is increased the mass loss at the beginning of the oxidation be- comes more pronounced. Beyond transition time the general trend in the Am/mo-t dependence is a weight increase but with a much lower rate Preliminary heat treatment of the Y-PSZ-coated Nicalon fibers for 3 h at 1000oC in vacuum o1 Pa before oxidation testing leads to an insignificant mass loss(e.g 2.5% wt. for 9Y-Zro2, four dipping-annealing cycles) The following exposure of samples to air at 1000C leads to () Y Zr r""r"""""r"""r""" Full Scale 271 cts Fig. 11. EDS spectrum of the surface of the oxidized 9Y-ZrO2( two cycles) Fig 12. SEM images of the 9Y-ZrO2-coated Nicalon "M fibers(five cycles) coating on Nicalon M fiber after exposition to air at 1000"C for 30h

1732 N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 initial preparation. It is well-known that doping of zirconium oxide by yttrium leads to the formation of defects, namely, oxygen vacancies. So one could be discreetly proposed that the peaks detected in Raman spectrum of Y-doped zirconia in the 2500–3000 cm−1 region could be attributed to fluores￾cence phenomenon. In any case, this phenomenon deserves to be more carefully and precisely studied in future. 3.4. Oxidation tests Dependences of the relative mass m/m0 on time at 1000 ◦C for 9Y-doped ZrO2-coated NicalonTM fibers (one, four and five cycles) are represented in Fig. 9. One can see, that the Y-doped ZrO2-coated samples exhibit a similar over￾all behavior during oxidation test, namely: (i) first a sharp weight loss and (ii) a more progressive weight increase up to the test completion. The time corresponding to the transi￾tion between regimes (i) and (ii) is dependent on number of dipping-annealing cycles. For the 9Y-doped ZrO2 (one cy￾cle) sample this transition is detected approximately 1 h after the beginning of oxidation whereas for the 9Y-doped ZrO2 (five cycles) the transition takes place later. As the number of dipping-annealing cycle during preparation of samples is increased the mass loss at the beginning of the oxidation be￾comes more pronounced. Beyond transition time the general trend in the m/m0–t dependence is a weight increase but with a much lower rate. Preliminary heat treatment of the Y-PSZ-coated NicalonTM fibers for 3 h at 1000 ◦C in vacuum 0.1 Pa before oxidation testing leads to an insignificant mass loss (e.g. ∼2.5% wt. for 9Y-ZrO2, four dipping–annealing cycles). The following exposure of samples to air at 1000 ◦C leads to Fig. 11. EDS spectrum of the surface of the oxidized 9Y-ZrO2 (two cycles) coating on NicalonTM fiber. significant decrease in oxidation rate. As one can see from Fig. 9, an increase in number of dipping-annealing cycle (in different terms, the thickness of coating) gives rise to more oxidation resistant coated fibers. 3.5. SEM/EDS, X-ray and micro Raman analysis of oxidized coated fibers SEM images of the surface of 9Y-ZrO2-coated NicalonTM fibers (two cycles) after exposition to air at 1000 ◦C for 30 h are represented in Fig. 10. The surface of oxidized filaments is not strongly distinguished from nonoxidized one. It is nonuni￾form and new formations of well-faceted shape and macro de￾fects as traces from sol which was retained between separate monofilaments can be seen. The coating retains its integrity and no spalling is observed. It is seen from clear-up image of the surface of oxidized coated NicalonTM fiber (Fig. 10b) that it is composed of isometric crystals in despite of the columnar microstructure that was early observed by Bak￾lanova and coworkers27 for oxidized undoped ZrO2-coated NicalonTM fiber. The elemental microanalysis by SEM/EDS Fig. 12. SEM images of the 9Y-ZrO2-coated NicalonTM fibers (five cycles) after exposition to air at 1000 ◦C for 30 h.

N.I. Baklanova et al / Journal of the European Ceramic Society 26 (2006)1725-1736 of oxidized 9Y-ZrO coating taken from different areas indi cates the presence of Si, Zr, Y, Al(Fig. ll, see correspondin △m-ZrO areas in Fig. 10a). The appearance of Al probably to be con- The oxidized Y-doped ZrOz coatings (five cycles)show a 0 /24 4y nected with a contamination from crucibles which were used for oxidation tests nonuniformity in greater extent in comparison with of coat- ings which were prepared using two dipping-annealing cy cles(Fig. 12a and b). They exhibit an increased nonunifor along length and diameter of monofilament. The other uliarity is a presence of a great number of agglomer Raman shift, cm-1 ates composed from large crystals. The elemental analysis by SEM/EDS of oxidized 9Y-ZrO2 coating(five cycles) taken from l and 2 areas(specified on Fig. 12a)indicates the pres- ence of Si. Zr. Y as main constituents and Al as contamination Fig. 13a and b). According to XRD data, at least c-Zro and Alh in air at 1000C for 30h. Raman spectroscopy revealed c crystobalite phases are present after exposition of coated fib ZrO2 and possibly the tetragonal modification in coated fibers after oxidation Raman spectra taken from separate areas of the oxidized 3Y-PSZ-coated fibers clearly demonstrate the presence of peaks belonging to t-ZrO2 together with intensive peaks of Full Scale 154 cts kev monoclinic ZrOz modification( Fig. 14). One can note that no peaks other than belonging to tetragonal ZrOz modification were detected in Raman spectrum for nonoxidized specimen 4. Discussion The opportunity for fabrication of the Y-doped zirconia in- facial coatings on Nicalon fibers using sol-gel approach was clearly demonstrated in this study In dependence of the yttrium dopant level the c-or t-zrO2 coatings on Nicalon fiber were obtained using a simple way. However, in course of this study some peculiarities of interface chemistry, coat ing morphology and behavior in air at elevated temperatures of sol-gel derived Y-PSZ coatings on NicalonM fibers were revealed and they must be discussed first of all. As was mentioned above, the XRD and Raman analysis of 3Y-doped zirconia-coated Nicalon fiber showed that the t-ZrO2 is a main crystal phase of this coating. As to the 9Y-doped zirconia-coated Nicalon"M fiber, the XRD patterns show peaks which are characteristic features of cubic Zro2 crystal phase. An analysis of Raman spectra of 9Y-doped coated fibers and powders derived from the same precursors Fig. 13 confirm that the cubic phase is a main component. Ho

N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 1733 of oxidized 9Y-ZrO2 coating taken from different areas indi￾cates the presence of Si, Zr, Y, Al (Fig. 11, see corresponding areas in Fig. 10a). The appearance of Al probably to be con￾nected with a contamination from crucibles which were used for oxidation tests. The oxidized Y-doped ZrO2 coatings (five cycles) show a nonuniformity in greater extent in comparison with of coat￾ings which were prepared using two dipping–annealing cy￾cles (Fig. 12a and b). They exhibit an increased nonunifor￾mity along length and diameter of monofilament. The other peculiarity is a presence of a great number of agglomer￾Fig. 13. Fig. 14. ates composed from large crystals. The elemental analysis by SEM/EDS of oxidized 9Y-ZrO2 coating (five cycles) taken from 1 and 2 areas (specified on Fig. 12a) indicates the pres￾ence of Si, Zr, Y as main constituents and Al as contamination (Fig. 13a and b). According to XRD data, at least c-ZrO2 and crystobalite phases are present after exposition of coated fiber in air at 1000 ◦C for 30 h. Raman spectroscopy revealed c￾ZrO2 and possibly the tetragonal modification in coated fibers after oxidation. Raman spectra taken from separate areas of the oxidized 3Y-PSZ-coated fibers clearly demonstrate the presence of peaks belonging to t-ZrO2 together with intensive peaks of monoclinic ZrO2 modification (Fig. 14). One can note that no peaks other than belonging to tetragonal ZrO2 modification were detected in Raman spectrum for nonoxidized specimen (see Fig. 8, 3Y). 4. Discussion The opportunity for fabrication of the Y-doped zirconia in￾terfacial coatings on NicalonTM fibers using sol–gel approach was clearly demonstrated in this study. In dependence of the yttrium dopant level the c- or t-ZrO2 coatings on NicalonTM fiber were obtained using a simple way. However, in course of this study some peculiarities of interface chemistry, coat￾ing morphology and behavior in air at elevated temperatures of sol–gel derived Y-PSZ coatings on NicalonTM fibers were revealed and they must be discussed first of all. As was mentioned above, the XRD and Raman analysis of 3Y-doped zirconia-coated NicalonTM fiber showed that the t-ZrO2 is a main crystal phase of this coating. As to the 9Y-doped zirconia-coated NicalonTM fiber, the XRD patterns show peaks which are characteristic features of cubic ZrO2 crystal phase. An analysis of Raman spectra of 9Y-doped￾coated fibers and powders derived from the same precursors confirm that the cubic phase is a main component. How-

1734 N.I. Baklanova et al /Journal of the European Ceramic Sociery 26(2006)1725-1736 ever,as one can see from Figs. 7 and 8, the main peak at taking into consideration of the XPS results of this study. Fu about 614 cm- is very broad and asymmetric one. Besides, ther careful investigation must be carried out to clarify this in Raman spectra of both coatings and powders the additional question features in the 100-1000cm"are registered. Earlier it was According to Raman spectroscopy results, a small quan reported that widening and asymmetry of main peak and the tity of residual OH-groupings and H2O molecules can be appearance of additional peaks in Raman spectrum of cu- present in as-received Y-PSZ coating structure and contribute bic ZrO2 phase can be attributed to disorder in the system to disordering of coating structure. Besides, the traces of due to the oxygen vacancies induced by Y+ acceptor 21-23 nitrogen atoms(derived from the initial yttrium nitrate) High defect concentration in the oxygen sublattice can lead in coating was determined by the XPS analysis. Although to a breakdown of the selection rules and allow the addi- chlorine atoms were not detected by XPS, nevertheless the tional modes. As was mentioned above, the appearance of presence of residual chloride ions in as-received coating must low binding energy O ls photopeak at 530.6eV in the XPs not be excluded too. Owing to preliminary heat treatment of pectrum ofY-PSZ-coated Nicalon M fiber indicates to pres- coated Nicalon M fiber at 600C in vacuum 10-6Pa before ence of the Y-o-zr bonds in coating structure and large the XPs measurements, the gaseous chlorine-containing width of this peak to a strong disordering structure. Thus, compounds could be escaped and as result not be detected by taking into consideration of the XRD, XPS and Raman re- XPS analysis. Thus, as was stated in this study, the sol-gel sults, one can conclude that the composition of the 9Y-doped derived Y-doped Zro2-based coatings are characterized ZrO2 interfacial coating on Nicalon fiber is represented by a complex elemental and crystal phase composition by strongly disordered cubic zirconia phase induced by Y This circumstance must be taken into consideration on doping examination of the oxidation test results. Actually, as was e One can note the affinity of the positions of the rest shown in this study, preliminary heat treatment in vacuum of aman peaks observable in spectra of 9Y-doped zirconia- the as-received Y-PSZ-coated fibers with following exposure coated NicalonM fibers to those reported for tetragonal to air at 1000C results in a significant decrease of the modification.21,2 It suggests that the t-ZrO2 modification oxidation rate. During preliminary annealing a removal of possible to be present too in despite of the fact that the tetrag- residual groupings derived from sol precursors, an ordering onal zirconia impurities were not detected by XRD. As was of the coating structure and an improvement of the coating noted by many authors(see, e.g. ), Raman spectroscopy texture appear to occur. As a result, higher quality coatings seems to be more sensitive than XRD in detecting of these with improved oxidation resistance are formed. impurities. Thus, taking into consideration of the XRD, XPS The other question to be discussed is a weak interface and Raman results, one can conclude that the composition behavior of Y-PSZ coating on Nicalon M fiber that w of the 3Y-doped ZrO2 interfacial coating on Nicalon M fiber demonstrated in this study and a possible role of ZrO2 phase is represented only by tetragonal crystal phase, whereas the ransformations on this behavior as was shown in this 9Y-doped ZrO2 interfacial coating on Nicalon fiber is rep- study, a controlled addition of yttria to zirconia results in the resented mainly by strongly disordered cubic zirconia phase formation of Zro2-based coating, consisting of metastable of induced by Y+ doping and tetragonal modification as an tetragonal phase or metastable tetragonal phase in the cubic impurity. It should be emphasized that the question concern-"matrix".If a stress due to some cracks is applied to the ing an accurate control of phase composition of ZrO2-based metastable tetragonal particles, a stress induced phase tran- coating is of great interest because it is well-established that sition from metastable tetragonal to the stable monoclinic the zirconia crystal phases are strongly differ from each other phase occurs. As the phase transformation is accompanied by their ability to"low-temperature"degradation by large volume dilation and shear, further growth of crack is therein and a tough transformation. 0 suppressed. o It should be noted that the c-t transforma Another reason resulting in more complex composition of tion also occurs with the volume dilation, the volume dilation oating is a possible interaction of Y-doped zirconia coating being smaller than for the t-m transformation. If this trans- with support during processing and operating. The XPS re- ations take place within interface sults obtained in this study showed that the Zr-O-Si bonds compressive stresses might cause the ZrO2 coating to delam appear to be formed at the fiber-coating interface region. Ac- inate. Actually, a delamination phenomenon was observed tually, according to Jarvis and Carter,29 adhesion energy at within thick 9Y-PSZ (five cycles)coating on Nicalon"M fiber the ZrO2/Sio2 interface is quite high and a very strong bond-(Fig 3). As was mentioned above, in Raman spectra taken ing and a dramatic rearrangement of the atomic coordinates from some areas of oxidized 3Y-PSZ-coated fibers the peaks exist at this interface. They concluded that chemical bond- belonging to monoclinic modification and yttrium oxide ing provides a significant source of interface strengthening separate phase together with tetragonal modification were even at ambient temperature and in the absence of a new re- detected( Fig. 14), with neither m-zrO2, nor Y203 were action phase. Earlier, White and coworkers examined the detected in this coating before oxidation tests. This fact is Zro2/SiO2 interfacial zone of thin Zro2 films on silicon us- a serious argument in favor of delamination induced by the ing XPS and presumed that the formation of ZrSiO4 occurs. t-m phase transformation. Nevertheless, a contribution As to the formation of the Y-O-Si bonds, it is questionable of the CtE mismatch of Y-PSZ and Sio2 phases must not

1734 N.I. Baklanova et al. / Journal of the European Ceramic Society 26 (2006) 1725–1736 ever, as one can see from Figs. 7 and 8, the main peak at about 614 cm−1 is very broad and asymmetric one. Besides, in Raman spectra of both coatings and powders the additional features in the 100–1000 cm−1 are registered. Earlier it was reported that widening and asymmetry of main peak and the appearance of additional peaks in Raman spectrum of cu￾bic ZrO2 phase can be attributed to disorder in the system due to the oxygen vacancies induced by Y3+ acceptor.21–23 High defect concentration in the oxygen sublattice can lead to a breakdown of the selection rules and allow the addi￾tional modes. As was mentioned above, the appearance of low binding energy O 1s photopeak at 530.6 eV in the XPS spectrum of Y-PSZ-coated NicalonTM fiber indicates to pres￾ence of the Y–O–Zr bonds in coating structure and large width of this peak to a strong disordering structure. Thus, taking into consideration of the XRD, XPS and Raman re￾sults, one can conclude that the composition of the 9Y-doped ZrO2 interfacial coating on NicalonTM fiber is represented by strongly disordered cubic zirconia phase induced by Y3+ doping. One can note the affinity of the positions of the rest Raman peaks observable in spectra of 9Y-doped zirconia￾coated NicalonTM fibers to those reported for tetragonal modification.21,23 It suggests that the t-ZrO2 modification possible to be present too in despite of the fact that the tetrag￾onal zirconia impurities were not detected by XRD. As was noted by many authors (see, e.g.21), Raman spectroscopy seems to be more sensitive than XRD in detecting of these impurities. Thus, taking into consideration of the XRD, XPS and Raman results, one can conclude that the composition of the 3Y-doped ZrO2 interfacial coating on NicalonTM fiber is represented only by tetragonal crystal phase, whereas the 9Y-doped ZrO2 interfacial coating on NicalonTM fiber is rep￾resented mainly by strongly disordered cubic zirconia phase of induced by Y3+ doping and tetragonal modification as an impurity. It should be emphasized that the question concern￾ing an accurate control of phase composition of ZrO2-based coating is of great interest because it is well-established that the zirconia crystal phases are strongly differ from each other by their ability to “low-temperature” degradation28 and refs. therein and a tough transformation.10 Another reason resulting in more complex composition of coating is a possible interaction of Y-doped zirconia coating with support during processing and operating. The XPS re￾sults obtained in this study showed that the Zr–O–Si bonds appear to be formed at the fiber-coating interface region. Ac￾tually, according to Jarvis and Carter,29 adhesion energy at the ZrO2/SiO2 interface is quite high and a very strong bond￾ing and a dramatic rearrangement of the atomic coordinates exist at this interface. They concluded that chemical bond￾ing provides a significant source of interface strengthening even at ambient temperature and in the absence of a new re￾action phase. Earlier, White and coworkers30 examined the ZrO2/SiO2 interfacial zone of thin ZrO2 films on silicon us￾ing XPS and presumed that the formation of ZrSiO4 occurs. As to the formation of the Y–O–Si bonds, it is questionable taking into consideration of the XPS results of this study. Fur￾ther careful investigation must be carried out to clarify this question. According to Raman spectroscopy results, a small quan￾tity of residual OH-groupings and H2O molecules can be present in as-received Y-PSZ coating structure and contribute to disordering of coating structure. Besides, the traces of nitrogen atoms (derived from the initial yttrium nitrate) in coating was determined by the XPS analysis. Although chlorine atoms were not detected by XPS, nevertheless the presence of residual chloride ions in as-received coating must not be excluded too. Owing to preliminary heat treatment of coated NicalonTM fiber at 600 ◦C in vacuum 10−6 Pa before the XPS measurements, the gaseous chlorine-containing compounds could be escaped and as result not be detected by XPS analysis. Thus, as was stated in this study, the sol–gel derived Y-doped ZrO2-based coatings are characterized by a complex elemental and crystal phase composition. This circumstance must be taken into consideration on examination of the oxidation test results. Actually, as was shown in this study, preliminary heat treatment in vacuum of the as-received Y-PSZ-coated fibers with following exposure to air at 1000 ◦C results in a significant decrease of the oxidation rate. During preliminary annealing a removal of residual groupings derived from sol precursors, an ordering of the coating structure and an improvement of the coating texture appear to occur. As a result, higher quality coatings with improved oxidation resistance are formed. The other question to be discussed is a weak interface behavior of Y-PSZ coating on NicalonTM fiber that was demonstrated in this study and a possible role of ZrO2 phase transformations on this behavior. As was shown in this study, a controlled addition of yttria to zirconia results in the formation of ZrO2-based coating, consisting of metastable tetragonal phase or metastable tetragonal phase in the cubic “matrix”. If a stress due to some cracks is applied to the metastable tetragonal particles, a stress induced phase tran￾sition from metastable tetragonal to the stable monoclinic phase occurs. As the phase transformation is accompanied by large volume dilation and shear, further growth of crack is suppressed.10 It should be noted that the c→t  transforma￾tion also occurs with the volume dilation, the volume dilation being smaller than for the t→m transformation. If this trans￾formations take place within interfacial zone a significant compressive stresses might cause the ZrO2 coating to delam￾inate. Actually, a delamination phenomenon was observed within thick 9Y-PSZ (five cycles) coating on NicalonTM fiber (Fig. 3). As was mentioned above, in Raman spectra taken from some areas of oxidized 3Y-PSZ-coated fibers the peaks belonging to monoclinic modification and yttrium oxide as separate phase together with tetragonal modification were detected (Fig. 14), with neither m-ZrO2, nor Y2O3 were detected in this coating before oxidation tests. This fact is a serious argument in favor of delamination induced by the t→m phase transformation. Nevertheless, a contribution of the CTE mismatch of Y-PSZ and SiO2 phases must not