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 coated1694 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