May 2001 Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites 1139 laser heating"issue. The two spectra of the longitudinal mapping 2) Interphase Materials of Fig. 4(458 nm) that were recorded on the fiber crack in composite 2 are almost stress-free internal references. With 5 mw view in Part 1. 'We found that the 6H-polytype dominate exciting power, their D band mean wavenumber was 135880 SiC layer of composite 2. The wavenumbers will be (+1.00)cm-I, the mean for all other probed points being 1365.75 below in terms of possible strains, which will not be poss\ (+0. 15)cm. There is thus strong evidence that the fiber is in BN, because of a disturbing signal scattering phenomeno ompression in the matrix. The fitting of spectra that were recorded under the very same conditions on cross sections gave an verage 1366.20(+0.30)cm wavenumber(seven measurement Discussion points). Some matrix being removed on polished cross sections, one might expect greater stress effects on spectra recorded through ( Fiber Stress in the Composites the matrix. Such recording was not possible, because of celsian's The above-mentioned apparent compression is in perfect agree low transparency(20 um). But, Wu and Colomban found ment with the CTE of the Hi-Nicalon fiber (a=3.5 x 10-6roC equivalent results for fiber-reinforced mullite probed on cross between room temperature(RT) and 500C, from Nippon carbon sections and through the matrix. 2I This peculiar behavior must data sheets) and the celsian matrix(a=5.28 X 10/C between result from CMCs having reloading lengths of a few micrometers RT and 1200C). For CMCs, the reinforcement CTE should be nly, much smaller than the typical 500-1000 um values encoun- greater, to the matrix, whose tensile strength is loy tered in organic matrix composites. 22 residual compression and, in this way, prevent microcracking Yet, low a matrixes usually have loosely packed frameworks and compared with the experimental accuracy and we decided to the best matrix choice requires a chemical and mechanical analysis record additional series of spectra with the 514.5 nm wavelength, of the multiphase materials. Note that a of the monoclinic celsian for the neon line to be systematically included as a reference ase of srAlSio. was measured to be 2.5x 10o/oc- which The power was fixed at I mW on the sample, the lowest possible confirms that thermal expansion of celsian is a function of the value still giving acceptable spectra quality for reasonable record- alkali/alkaline-earth content. 26 (A) Anticipated Stress: The axial (o) and transverse(o 900 s recording did not produce different values, neither for residual stresses in an"infinite"fiber, that is to say, a long and 2四购以mF5 Spectra were recorded lated using the following expressions: 27 wavenumbers nor for bandwidths fiber sections or sections of extracted fibers the latter constitute an internal reference. Different regions were tested in composites I and 2 for statistical analysis. Plotted values are averages with error bars indicating the extremes. Figure 6 gives an example of a E7/21 1+v two-dimensional mapping performed on composite I under the conditions used for Fig. 5. Again, a correction of the apparatus Cr-am+ va(oa-amjAT The subscripts in Eqs. (6)and (7) are relative to the matrix(m)and the axial(A)or transverse(T)properties of the fiber v are Poisson ratios and AT represents the difference between room temperature and the so-called"induction temperature, Ti, at which the matrix becomes too "soft" to constrain the fiber 28 Because of the lack of availability of experimental values of Ti, we will replace it by Is= 1300C, a typical sintering temperature for aluminosilicate ma- tries. Besides, given the relative isotropy of Hi-Nicalon fibers and, again, because of the lack of data, axial and transverse properties should be considered the same(with a subscript f). If w 10 now put all Poisson ratios to zero, which is partly justified as a first approximation by the very local probing scale of Raman spectros- Length X (um) copy, Eqs. (6)and(7)simplify to GA=(am-ax△FEr (8) On crack Taking Em =96 GPa and the Er= 270 GPa Youngs modulus given by Nippon Carbon Co for Hi-Nicalon fibers, the compres sive stress should be about 600-650 MPa, axially, and greater than 150 MPa, radially. The stress measurement derived from(unpo- Far from crack larized) Raman spectra should fall in between since the method is not sensitive to the loading direction, at least in amorphous materials 800100012001400160018002000 L(B) Experimental Stress Assessment: Looking at Fig. 5, the venumbers of the Hi-Nicalon fibers before matrix embedding Wavenumber. cm-1 are rather close: uncoated and coated fibers are suitable to serve as posite 2 polished parallel to stress-free references. Their bandwidths are slightly different, direction. Spectra were recorded with the 458 nm line (5 mw, 60 but Fig. 7 in Part I showed that structural evolution of Hi-Nicalon int,(b) Examples of spectra recorded along the fiber and fibers begins around 1300-1400%C, which happens to be the temperature range of p-BN/SiC chemical vapor deposition.“laser heating” issue. The two spectra of the longitudinal mapping of Fig. 4 (458 nm) that were recorded on the fiber crack in composite 2 are almost stress-free internal references. With 5 mW exciting power, their D band mean wavenumber was 1358.80 (61.00) cm21 , the mean for all other probed points being 1365.75 (60.15) cm21 . There is thus strong evidence that the fiber is in compression in the matrix. The fitting of spectra that were recorded under the very same conditions on cross sections gave an average 1366.20 (60.30) cm21 wavenumber (seven measurement points). Some matrix being removed on polished cross sections, one might expect greater stress effects on spectra recorded through the matrix. Such recording was not possible, because of celsian’s low transparency (#20 mm). But, Wu and Colomban found equivalent results for fiber-reinforced mullite probed on cross sections and through the matrix.21 This peculiar behavior must result from CMCs having reloading lengths of a few micrometers only, much smaller than the typical 500–1000 mm values encoun￾tered in organic matrix composites.22 All shifts that we measured on composites 1 and 2 remained low compared with the experimental accuracy and we decided to record additional series of spectra with the 514.5 nm wavelength, for the neon line to be systematically included as a reference.11 The power was fixed at 1 mW on the sample, the lowest possible value still giving acceptable spectra quality for reasonable record￾ing times. We retained 180 s per spectrum, but we checked that a 900 s recording did not produce different values, neither for wavenumbers nor for bandwidths. The in situ results are presented in Fig. 5. Spectra were recorded on cross-sectional views (on polished composites), freshly cut fiber sections or sections of extracted fibers; the latter constitute an internal reference. Different regions were tested in composites 1 and 2 for statistical analysis. Plotted values are averages with error bars indicating the extremes. Figure 6 gives an example of a two-dimensional mapping performed on composite 1 under the conditions used for Fig. 5. Again, a correction of the apparatus shift was done using the neon line as a reference. (2) Interphase Materials SiC and Si spectra were presented from a qualitative point of view in Part I.1 We found that the 6H-polytype dominates in the SiC layer of composite 2. The wavenumbers will be discussed below in terms of possible strains, which will not be possible for BN, because of a disturbing signal scattering phenomenon.1 V. Discussion (1) Fiber Stress in the Composites The above-mentioned apparent compression is in perfect agree￾ment with the CTE of the Hi-Nicalon fiber (a 5 3.5 3 1026 /°C between room temperature (RT) and 500°C, from Nippon carbon data sheets) and the celsian matrix (a 5 5.28 3 1026 /°C between RT and 1200°C23). For CMCs, the reinforcement CTE should be greater, to put the matrix, whose tensile strength is low, under residual compression and, in this way, prevent microcracking.24 Yet, low a matrixes usually have loosely packed frameworks and the best matrix choice requires a chemical and mechanical analysis of the multiphase materials. Note that a of the monoclinic celsian phase of SrAl2Si2O8 was measured to be 2.5 3 1026 /°C,25 which confirms that thermal expansion of celsian is a function of the alkali/alkaline-earth content.26 (A) Anticipated Stress: The axial (sA) and transverse (sT) residual stresses in an “infinite” fiber, that is to say, a long and nonfragmented fiber, embedded in a given matrix, can be calcu￾lated using the following expressions:27 sA 5 F 2nA EA ~aT 2 am! 1 S 1 2 nT ET 1 1 1 nm Em D~aA 2 am!G 3 EAzDTYS 2nA 2 EA 2 1 2 nT ET 2 1 1 nm Em D (6) sT 5 @aT 2 am 1 nA~aA 2 am!#zDT 2nA 2 EA 2 1 2 nT ET 2 1 1 nm Em (7) The subscripts in Eqs. (6) and (7) are relative to the matrix (m) and the axial (A) or transverse (T) properties of the fiber. n are Poisson ratios and DT represents the difference between room temperature and the so-called “induction temperature,” Ti , at which the matrix becomes too “soft” to constrain the fiber.28 Because of the lack of availability of experimental values of Ti , we will replace it by TS 5 1300°C, a typical sintering temperature for aluminosilicate ma￾trixes. Besides, given the relative isotropy of Hi-Nicalon fibers and, again, because of the lack of data, axial and transverse properties should be considered the same (with a subscript f). If we now put all Poisson ratios to zero, which is partly justified as a first approximation by the very local probing scale of Raman spectros￾copy, Eqs. (6) and (7) simplify to sA 5 ~am 2 af!zDTzEf (8) sT 5 ~am 2 af!zDTzS EmEf Em 1 Ef D (9) Taking Em 5 96 GPa23 and the Ef 5 270 GPa Young’s modulus given by Nippon Carbon Co. for Hi-Nicalon fibers, the compres￾sive stress should be about 600–650 MPa, axially, and greater than 150 MPa, radially. The stress measurement derived from (unpo￾larized) Raman spectra should fall in between since the method is not sensitive to the loading direction, at least in amorphous materials. (B) Experimental Stress Assessment: Looking at Fig. 5, the wavenumbers of the Hi-Nicalon fibers before matrix embedding are rather close: uncoated and coated fibers are suitable to serve as “stress-free” references. Their bandwidths are slightly different, but Fig. 7 in Part I showed that structural evolution of Hi-Nicalon fibers begins around 1300–1400°C, which happens to be the temperature range of p-BN/SiC chemical vapor deposition.1 Fig. 4. (a) Photomicrograph of composite 2 polished parallel to the fiber direction. Spectra were recorded with the 458 nm line (5 mW, 60 s) at each point. (b) Examples of spectra recorded along the fiber and on the fiber crack. May 2001 Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites 1139