May 2001 Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites 1137 a proportionality factor: -vdP/dr is the materials bulk modulus. In the G ase, equations of state linking v(or the cell parameters) oulder plied stress are mandatory to compare S: with Y,. In cubi Y, can also be obtained from the wavenumber shifts measured for unidirectional stresses along high symmetry The Hi-Nicalon fiber consists of several phases, which are all somewhat amorphous. It would be unrealistic to theoreticall predict strain-induced Raman shifts to them or to use reported Gruneisen coefficients. We will therefore use s: coefficients owever, the rather high measurement uncertainty necessitates the Experimental Procedure The samples and the experimental equipment have been fully described in Part I. The spectra were recorded on an"XY spectrograph(Dilor, France) with a back-illuminated nitrogen cooled CCD detector. A motorized X-Y displacement table was used for a 2D mapping of any given surface and the 556.3 nm line f a neon lamp that was installed in the spectrograph chamber was a wavenumber reference when working with the 514.5 nm laser line(the corresponding shift was 1460.4 cm, peaking in the middle of the carbon signal) Our interest focused on three unidirectional celsian-matrix composites(12 plies), prepared by hot pressing. Composite I was reinforced by desized Hi-Nicalon fibers (fiber volume fraction Vr 35%), while ce te 2 incorporated double-coated Hi Nicalon fibers(a layer of 0. 4 um pyrolytic bn(p-BN) over (b) oated by a 0. 2-0.3 um thick SiC diffusion barrier, V= 28% Composite 3 will refer to a composite similar to composite 2, but with a 12 wt% silicon doping of the p-BN layer(0. 4 um thick Wavenumber and an overcoating of sic (-0 2-0.3 um thick). Sections of each omposite were polished perpendicular and parallel to the fiber ectral deconvolutions: (a) carbon spectra b)SiC spectra;(O)optic mode, (T)transverse mode, (L) longitudinal direction. Some "reference" fibers were also extracted from mode) composite 2 by matrix crushing in an agate mortar. Such extraction was impossible in composite I because of fiber strength degrada tion from mechanical damage during composite processing ected locations and ept for the 2) Choice of the"Stress Probe"and Fiting Procedure “sp3” carbon mode( D band) and the optic SiC, which are given pure Lorentzian sh Because of the high electronic absorption of C-C bonds in the parameters are the wavenumber(v), the full half-height harmonics (pure or combined) are enhanced by a so-called (), the intensity, and the band area. resonance” phenomenon.lHi- Nicalon“ "silicon carbide” fibers ave a rather large excess of carbon(C: Si stoichiometric ratio of (3) Power and Wavelength-Induced Measurement 1. 4, that is, 40% excess carbon); thus the signal C-C will be the Disturbances nost convenientstress probe. Only in the nearly stoichiometric Thermal expansion has the same lengthening effect on bonds as fibers might the Sic spectrum be used for stress assessment. Ou tensile stress. Any localized heating induced by the laser impact sults will be based on the so-called"D band whose attribution might therefore lead one to overestimate tensile stresses an bonds has been discussed in Part I In the past, a underestimate compressive ones. Under unfavorable conditions "G band"("sp--like C-C bonds)has been preferred in fibers compression might even be confused with tensile stress. There is whose D band was either weak or less defined. Yet the carbon thus a need, before stress analysis, for a preliminary study intended atoms contributing to the d band are carbon moiety surface atoms to assess the influence of working parameters on sample heating. and should be better incorporated into the Sic network than those The first parameter is the material itself (M), which includes the corresponding to G, in the bulk. Besides, the fine structure of G fiber composition and its environment; fibers are either free- reveals a doublet in the new generations of SiC fibers, one standing in air, for S; calibrations, or embedded in a given matrix component of which, D, has wavelength-dependent intensity an for in situ measurements. A large influence of fiber surroundings position. 0 This component is only a shoulder in highly amorphous on thermal dissipation is anticipated. The matrix should act as a arbon, but is truly pronounced in SiC fibers. Besides, the double huge heat sink and dissipate most of the accumulated heat. Other fitting depends to some extent on a smaller(but much wider) ban parameters that might have an effect on heating are the wavelengt around 1530 cm(probably carbon linked to oxygen atoms"). A of the laser(because of carbon resonance), the laser power Figure I illustrates typical decompositions performed using the (P), the recording time(n), and the surface area impacted by the Labspec software(Dilor, France). The first step is the systematic spot(A) subtraction of a linear base line attached to the spectral window (i Wavenumbers are highly sensitive to the power. Eight tests limits. all known contributions are then entered close to the on the carbon d band showed a linear mode softening witha proportionality factor: Si s 5 n#i 0 z gi B (4) where B 5 2V(dP/dV)T is the material’s bulk modulus. In the general case, equations of state linking V (or the cell parameters) to the applied stress are mandatory to compare Si s with gi . In cubic materials (V 5 a3 ), gi can also be obtained from the wavenumber shifts measured for unidirectional stresses along high symmetry directions.18 The Hi-Nicalon fiber consists of several phases, which are all somewhat amorphous. It would be unrealistic to theoretically predict strain-induced Raman shifts to them or to use reported Gru¨neisen coefficients. We will therefore use Si ε coefficients; however, the rather high measurement uncertainty necessitates the determination of optimum working conditions. III. Experimental Procedure (1) Samples and Equipment The samples and the experimental equipment have been fully described in Part I.1 The spectra were recorded on an “XY” spectrograph (Dilor, France) with a back-illuminated nitrogen￾cooled CCD detector. A motorized X-Y displacement table was used for a 2D mapping of any given surface and the 556.3 nm line of a neon lamp that was installed in the spectrograph chamber was a wavenumber reference when working with the 514.5 nm laser line (the corresponding shift was 1460.4 cm21 , peaking in the middle of the carbon signal). Our interest focused on three unidirectional celsian-matrix composites (12 plies), prepared by hot pressing.1 Composite 1 was reinforced by desized Hi-Nicalon fibers (fiber volume fraction Vf 5 35%), while composite 2 incorporated double-coated Hi￾Nicalon fibers (a layer of ;0.4 mm pyrolytic BN (p-BN) over￾coated by a ;0.2–0.3 mm thick SiC diffusion barrier; Vf 5 28%). Composite 3 will refer to a composite similar to composite 2, but with a 12 wt% silicon doping of the p-BN layer (;0.4 mm thick) and an overcoating of SiC (;0.2–0.3 mm thick). Sections of each composite were polished perpendicular and parallel to the fiber direction. Some “reference” fibers were also extracted from composite 2 by matrix crushing in an agate mortar. Such extraction was impossible in composite 1 because of fiber strength degrada￾tion from mechanical damage during composite processing.19 (2) Choice of the “Stress Probe” and Fitting Procedure Because of the high electronic absorption of C–C bonds in the visible–UV range, the symmetric stretching modes and their harmonics (pure or combined) are enhanced by a so-called “resonance” phenomenon.1 Hi-Nicalon “silicon carbide” fibers have a rather large excess of carbon (C:Si stoichiometric ratio of 1.4, that is, 40% excess carbon); thus the signal C–C will be the most convenient “stress probe.” Only in the nearly stoichiometric fibers might the SiC spectrum be used for stress assessment. Our results will be based on the so-called “D band” whose attribution to “Csp3–Csp2” bonds has been discussed in Part I. In the past, a “G band” (“sp2 ”-like C–C bonds) has been preferred in fibers whose D band was either weak or less defined. Yet the carbon atoms contributing to the D band are carbon moiety surface atoms and should be better incorporated into the SiC network than those corresponding to G, in the bulk. Besides, the fine structure of G reveals a doublet in the new generations of SiC fibers, one component of which, D9, has wavelength-dependent intensity and position.20 This component is only a shoulder in highly amorphous carbon, but is truly pronounced in SiC fibers. Besides, the doublet fitting depends to some extent on a smaller (but much wider) band, around 1530 cm21 (probably carbon linked to oxygen atoms3 ). Figure 1 illustrates typical decompositions performed using the Labspec software (Dilor, France). The first step is the systematic subtraction of a linear base line attached to the spectral window limits. All known contributions are then entered close to their expected locations and are given a Gaussian shape, except for the “sp3 ” carbon mode (D band) and the optical (TO/LO) modes of SiC, which are given pure Lorentzian shapes. The adjusted parameters are the wavenumber (n), the full width at half-height (w), the intensity, and the band area. (3) Power and Wavelength-Induced Measurement Disturbances Thermal expansion has the same lengthening effect on bonds as tensile stress. Any localized heating induced by the laser impact might therefore lead one to overestimate tensile stresses and underestimate compressive ones. Under unfavorable conditions, compression might even be confused with tensile stress. There is thus a need, before stress analysis, for a preliminary study intended to assess the influence of working parameters on sample heating. The first parameter is the material itself (M), which includes the fiber composition and its environment; fibers are either free￾standing in air, for Si ε calibrations, or embedded in a given matrix for in situ measurements. A large influence of fiber surroundings on thermal dissipation is anticipated. The matrix should act as a huge heat sink and dissipate most of the accumulated heat. Other parameters that might have an effect on heating are the wavelength l of the laser (because of carbon resonance), the laser power (P), the recording time (t), and the surface area impacted by the spot (A). (i) Wavenumbers are highly sensitive to the power. Eight tests on the carbon D band showed a linear mode softening with Fig. 1. Typical examples of spectral deconvolutions: (a) carbon spectra, (b) SiC spectra; (O) optic mode, (T) transverse mode, (L) longitudinal mode). May 2001 Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites 1137