May 2001 Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites 1141 compressive nature of the stress in composite 2, but with a constant ADExbandental(about 1 cm, between"stress-free"references- either desized fibers or extracted fibers- and the wavenumber Laser measured in situ). If Sp really depends linearly on the laser energy (in electronvolts), according to Fig. 3 comments, then we must conclude after Eq.(13)that the stress is wavelength-dependent (Ssample has almost no dependency on wavelength in Table I). As a matter of fact, light penetration in resonant materials is correlated with the wavelength. Assuming that the absorption of Hi-Nicalon fibers should be close to that of amorphous silicon carbide(a-SIC) films, which is ascertained by Raman spectra similitude, the maximal penetrations can be assumed to be about 25 and 75 nm for the 458 and 647.1 nm lines, respectively. There might exist actual stress differences as a function of the distance to the sample surface or, in other words, the analyzed volume. Our observation would indicate a stress gradient with the highest stress measured at the very surface of the sample(blue laser observation). More 1355.51 detailed results would require precise measurements of the laser power and using reference lamps for red and blue excitations 13550 (2) Overcoating Analysis For SiC and Si materials, the only values at our disposal were 4 found in the literature for compressive experiments in diamond 1354,5 anvils on single-crystal pieces. Linear dependencies of 3.53 + 0.21 and 4.28 0.22 cm/GPa were calculated respectively for th TO and o modes of 6H-siC, which our SiC layer mainly consists of(see Part D). TO, and Lo modes are at their expected values(796 and 966 cm, respectively) but TO, mode points at 785-788 cm, while its reported stress-free value is about 789.2 cm-. This would correspond to a huge stress, but this"shift attribution"would be dubious and the great variety of recorded spectra are rather a consequence of great structural variations(SiC Figure 5 in Part I suggested the more intense the silicon signal the weaker the TOz contribution. Silicon unsplit (stress-free wavenumber peaks at 5206cm" It is a triply degenerate mode whose splitting has been fully reported in the literature for stresses along the [111] and [001] directions, biaxial strains in a polycrystalline silicon thin layer. 36,37 The mean value(weighted by the degeneracy)must be considered in disordered silicon and the splitting explains band widening observed in strained silicon. In our samples we detected a mean 2 cm upward shift, would indicate compressive stresses of nearly 1 GPa(th se mean 0 sensitivity is"450 MPa/cm for [100] stress). Such a would be very high for a macroscopic stress, but what is measured here is the very local stress of cross-section two-dimensional mapping(0.5 um directions)performed on a fiber/matrix interface ed fibers)using a 514.5 nm excitation(P= I mW,I VI. S 00s):(a) raph, (b) mapping based on the fitted wavenumbers for the spectra in the black rectangle, (c) bandwidth mapping. The fibers are under compressive residual stress of 950 MPa celsian-matrix composites reinforced with uncoated or p-B(Si)N/ SiC coated Hi-Nicalon fibers. In another celsian-matrix composite <1360.7cm-. Thus, the compression would be attenuated by the where Hi-Nicalon fibers have been coated with undoped p-BN/ presence of surrounding fibers, which would mean, as expected, SiC, residual stress in the fibers seems to have significantly relaxed that the matrix plays a more important role than the interphase to 110 MPa. In addition, the compression seems to reach a higher ssing the fibers level at the fiber core than in the vicinity of the fiber-matrix apparent stress relaxation about 3 m from the fiber-matrix account the effectiveAt results would require (O)taking into D Possible Edge efect: Figure 6(b) shows evidence of an s modulus in compression, (ii) per interphase in composite 1. a band widening is observed at the very forming Sn calibration compression, and (iii) using fibers interface in Fig. 6(c), which suggests a chemical alteration(and, annealed under the ons of composite fabrication as a possibly, fitting errors due to intensity lowering), but the wave- rererence number shift must have a mechanical effect for the region of onstant bandwidth. The best way to ascertain why the c-C bonds Refere edge region are different would be to record spectra on ct33A smallcompression relaxation"was detected in com- Reinforced Cel Ph Colomban, andN.P.Bansal, "Raman Study of Hi-Nicalon-Fiber- -embedded fibers, which would rule out any mechanical posite 2 when moving away from the fiber core, but no proper s" Am ceram.Soc,845]112935(2001) -Ph. Colomban and J. Corset, "Special Issue on Raman(Micro Spectrometry and edge analysis"could be performed where BN contribution was affecting the bandwidths destructive Mechanical Characterization of (E) Wavelength Influence: Rapid mappings performed with SiC Fibers by Raman Spectroscopy, J. Eur. Ceram. Soc., in pres "M. A. White,"Thermal Properties of Solids: Etude in Three-Part Anharmony, four different wavelengths(458/488/514/647 nm) confirmed the Camn.J.Chem,74,l916-21(1996;1360.7 cm21 . Thus, the compression would be attenuated by the presence of surrounding fibers, which would mean, as expected, that the matrix plays a more important role than the interphase in compressing the fibers. (D) Possible Edge Effect: Figure 6(b) shows evidence of an apparent stress relaxation about 3 mm from the fiber–matrix interphase in composite 1. A band widening is observed at the very interface in Fig. 6(c), which suggests a chemical alteration (and, possibly, fitting errors due to intensity lowering), but the wave￾number shift must have a mechanical effect for the region of constant bandwidth. The best way to ascertain why the C–C bonds of the edge region are different would be to record spectra on nickel-embedded fibers, which would rule out any mechanical effect.33 A small “compression relaxation” was detected in com￾posite 2 when moving away from the fiber core, but no proper “edge analysis” could be performed where BN contribution was affecting the bandwidths. (E) Wavelength Influence: Rapid mappings performed with four different wavelengths (458/488/514/647 nm) confirmed the compressive nature of the stress in composite 2, but with a constant Dn#Experimental D-band (about 1 cm21 , between “stress-free” references— either desized fibers or extracted fibers—and the wavenumber measured in situ). If SD ε really depends linearly on the laser energy (in electronvolts), according to Fig. 3 comments, then we must conclude after Eq. (13) that the stress is wavelength-dependent (Ssample P has almost no dependency on wavelength in Table I). As a matter of fact, light penetration in resonant materials is correlated with the wavelength. Assuming that the absorption of Hi-Nicalon fibers should be close to that of amorphous silicon carbide (a-SiC) films,34 which is ascertained by Raman spectra similitude, the maximal penetrations can be assumed to be about 25 and 75 nm for the 458 and 647.1 nm lines, respectively. There might exist actual stress differences as a function of the distance to the sample surface or, in other words, the analyzed volume. Our observation would indicate a stress gradient with the highest stress measured at the very surface of the sample (blue laser observation). More detailed results would require precise measurements of the laser power and using reference lamps for red and blue excitations. (2) Overcoating Analysis For SiC and Si materials, the only values at our disposal were found in the literature for compressive experiments in diamond anvils on single-crystal pieces. Linear dependencies of 3.53 6 0.21 and 4.28 6 0.22 cm21 /GPa were calculated respectively for the TO and LO modes of 6H-SiC,35 which our SiC layer mainly consists of (see Part I). TO2 and LO modes are at their expected values (796 and 966 cm21 , respectively) but TO1 mode points at 785–788 cm21 , while its reported stress-free value is about 789.2 cm21 . This would correspond to a huge stress, but this “shift attribution” would be dubious and the great variety of recorded spectra are rather a consequence of great structural variations (SiC polytypism).1 Figure 5 in Part I suggested the more intense the silicon signal, the weaker the TO2 contribution. Silicon unsplit (stress-free) wavenumber peaks at 520.6 cm21 . 36 It is a triply degenerate mode whose splitting has been fully reported in the literature for stresses along the [111] and [001] directions,18 or biaxial strains in a polycrystalline silicon thin layer.36,37 The mean value (weighted by the degeneracy) must be considered in disordered silicon and the splitting explains band widening observed in strained silicon. In our samples we detected a mean 2 cm21 upward shift, which would indicate compressive stresses of nearly 1 GPa (the mean sensitivity is ;450 MPa/cm21 for [100] stress16). Such a value would be very high for a macroscopic stress, but what is measured here is the very local stress. VI. Summary The fibers are under compressive residual stress of 950 MPa in celsian-matrix composites reinforced with uncoated or p-B(Si)N/ SiC coated Hi-Nicalon fibers. In another celsian-matrix composite, where Hi-Nicalon fibers have been coated with undoped p-BN/ SiC, residual stress in the fibers seems to have significantly relaxed to 110 MPa. In addition, the compression seems to reach a higher level at the fiber core than in the vicinity of the fiber–matrix interface. More precise results would require (i) taking into account the effective Young’s modulus in compression, (ii) per￾forming SD ε calibrations under compression, and (iii) using fibers annealed under the conditions of composite fabrication as a reference. References 1 G. Gouadec, Ph. Colomban, and N. P. Bansal, “Raman Study of Hi-Nicalon-Fiber￾Reinforced Celsian Composites: I, Distribution and Nanostructure of Different Phases,” J. Am. Ceram. Soc., 84 [5] 1129–35 (2001). 2 Ph. Colomban and J. Corset, “Special Issue on Raman (Micro)Spectrometry and Materials Science,” J. Raman Spectrosc., 30 [10] 861–947 (1999). 3 G. Gouadec and Ph. Colomban, “Nondestructive Mechanical Characterization of SiC Fibers by Raman Spectroscopy,” J. Eur. Ceram. Soc., in press. 4 M. A. White, “Thermal Properties of Solids: Etude in Three-Part Anharmony,” Can. J. Chem., 74, 1916–21 (1996). Fig. 6. Example of cross-section two-dimensional mapping (0.5 mm steps in X and Y directions) performed on a fiber/matrix interface in composite 1 (uncoated fibers) using a 514.5 nm excitation (P 5 1 mW, t 5 300 s): (a) photomicrograph; (b) mapping based on the fitted wavenumbers for the spectra in the black rectangle, (c) bandwidth mapping. May 2001 Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites 1141