JAm.cerm.Sc,845l136-42(2001) journal Raman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites IL Residual stress in Fibers Gwenael Gouadec, . f Philippe Colomban, and Narottam P. Bansal**, s Laboratoire Dynamique-Interactions-Reactivite (LADIR), UMR7075-CNRS and Universite Pierre et Marie Curie, Thiais, Val de marne, 94320, France Departement Materiaux et Systemes Composites(DMSC), Office National d'Etudes et de Recherches Aerospatiales(ONERA), Chatillon, Hauts de Seine, 92322, France ohn H. Glenn Research Center at Lewis Field, National Aeronautics and Space Administrat Cleveland, Ohio 4 Band shifts in Raman spectra were used to assess, on a I. Background microscopic scale, the residual strain existing in Hi-Nicalon silicon carbide fibers reinforcing celsian-matrix composites Raman-based stress measurements rely on the anharmonic Uncoated as well as p-BN/SiC-coated and p-B(SiN/SiC-coated nature of the chemical bonds, which make normal vibrations Hi-Nicalon fibers were used as the reinforcements. We unam- sensitive to any external disturbance of the potential well, say for biguously conclude that the fibers were in a state of compres nstance a pressure or a temperature change. The shift induced in sive residual stress. Quantitative determination of the residual mode i wavenumber i(em)when a strain E" is applied to a stress was made possible by taking into account the heating induced by laser probing and by using a reference line, of fixed wavenumber. We found fiber compressive residual stress v,=Do+SE values between 110 and 960 MPa, depending on the fiber/ nomenon was observed at the surface of p-BN/SiC-coated absolute"strain. io is a matrix coating in the composite. A stress relaxation-like phe expressed in units of cm 品 coated Hi-Nicalon fibers did not show any stress relaxation in anharmonicity., 4 If the inve the celsian-matrix composites. deformation theory, then v:+S:. . Introduction and s are in units of GPa and cm /GP C ERAMIC-MATRIX composites(CMCs)are light-weight refractory aterials which are of potential interest for high-temperature respectivel nde the fiber's Young modulus in GPa. S: and Si structural components in various aerospace and industrial applica- are negative in almost all cases but depend on the mode according tions In Part I, the phases present in celsian-matrix composites to the structure anisotropy. Many of the available values were obtained through diamond anvil cell experiments on crystals. '- reinforced with (desized) uncoated or p-BN/SiC-coated Hi- Nicalon fibers were identified and characterized by Raman mi- They cannot be applied to multiphase materials like Sic fiber rospectroscopy, from a chemical and structural point of view. In because the phases are amorphous/nanocrystalline. Wavenumber hift calibrations are then mandatory and usually obtained under this second part, we further interpret the fibers spectra in an axial stress, the control parameter being the strain attempt to assess the residual strain resulting from differences in Equation(2) supposes Youngs modulus e to be the same in the coefficients of thermal expansion( CTE)(a) between the fiber compression and in tension, which we shall discuss further, on and the matrix. Modeling this stress mathematically would be account of the bond nature and 3D symmetry in silicon carbide difficult, especially in the case of coated fibers. Indeed, interpha structures. Calibrations of S: were done for carbon Raman contri- materials promote stress relaxation(due to higher compliance, butions in polymeric, carbon, and Sic fibers. --In the latter case cracking, or a mismatch). Besides, they are usually partly crystal- the results could be compared with S: of silicon carbide optical line, often metastable, materials and their expansion is difficult to modes measure. The objective of this paper is to assess stresses in Theoretical predictions of strain-induced"Raman shifts'"exist Hi-Nicalon fibers embedded in celsian-matrix composites using for crystals. >, 4 They are based on the elastic constants tensor, and their use for the determination of biaxial stresses in Cvd on films produced consistent results. The Gruneisen coefficient is sometimes preferred to S: and S:to express the wavenumber sensitivity to external stresses. It is defined as a function of the volume(n)as D. R. Clarke--contributing editor Received June 19, 2000: approved December 4, 20( If only the pressure component changing the unit cell param nteractions-Reactivite )ffice National d Etudes et de Recherches Aerospatiale ters is considered( the so-called hydrostatic component, the National Aeronautics and Space Administration other one changes bond angles), then S; and y, are linked byRaman Study of Hi-Nicalon-Fiber-Reinforced Celsian Composites: II, Residual Stress in Fibers Gwe´nae¨l Gouadec,†,‡ Philippe Colomban,† and Narottam P. Bansal**,§ Laboratoire Dynamique-Interactions-Re´activite´ (LADIR), UMR7075-CNRS and Universite´ Pierre et Marie Curie, Thiais, Val de Marne, 94320, France De´partement Mate´riaux et Syste`mes Composites (DMSC), Office National d’Etudes et de Recherches Ae´rospatiales (ONERA), Chatillon, Hauts de Seine, 92322, France John H. Glenn Research Center at Lewis Field, National Aeronautics and Space Administration, Cleveland, Ohio 44135 Band shifts in Raman spectra were used to assess, on a microscopic scale, the residual strain existing in Hi-Nicalon silicon carbide fibers reinforcing celsian-matrix composites. Uncoated as well as p-BN/SiC-coated and p-B(Si)N/SiC-coated Hi-Nicalon fibers were used as the reinforcements. We unam￾biguously conclude that the fibers were in a state of compres￾sive residual stress. Quantitative determination of the residual stress was made possible by taking into account the heating induced by laser probing and by using a reference line, of fixed wavenumber. We found fiber compressive residual stress values between 110 and 960 MPa, depending on the fiber/ matrix coating in the composite. A stress relaxation-like phe￾nomenon was observed at the surface of p-BN/SiC-coated Hi-Nicalon fibers whereas the uncoated or p-B(Si)N/SiC￾coated Hi-Nicalon fibers did not show any stress relaxation in the celsian-matrix composites. I. Introduction CERAMIC-MATRIX composites (CMCs) are light-weight refractory materials which are of potential interest for high-temperature structural components in various aerospace and industrial applica￾tions. In Part I,1 the phases present in celsian-matrix composites reinforced with (desized) uncoated or p-BN/SiC-coated Hi￾Nicalon fibers were identified and characterized by Raman mi￾crospectroscopy, from a chemical and structural point of view. In this second part, we further interpret the fibers’ spectra in an attempt to assess the residual strain resulting from differences in the coefficients of thermal expansion (CTE) (a) between the fiber and the matrix. Modeling this stress mathematically would be difficult, especially in the case of coated fibers. Indeed, interphase materials promote stress relaxation (due to higher compliance, cracking, or a mismatch). Besides, they are usually partly crystal￾line, often metastable, materials and their expansion is difficult to measure. The objective of this paper is to assess stresses in Hi-Nicalon fibers embedded in celsian-matrix composites using Raman microspectroscopy. II. Background Raman-based stress measurements rely on the anharmonic nature of the chemical bonds, which make normal vibrations sensitive to any external disturbance of the potential well, say for instance a pressure or a temperature change.2 The shift induced in mode i wavenumber n# (cm21 ) when a strain e% is applied to a material is calibrated as follows:3 n#i 5 n#i 0 1 Si ε zε% (1) The superscript “%” is a reminder that we will not use the “absolute” strain. n#i 0 is the “stress-free wavenumber” and Si ε expressed in units of cm21 /%, is a direct measure of bond anharmonicity.3,4 If the investigated material obeys the “elastic deformation theory,” then n#i 5 n#i 0 1 Si ε S s ED 5 n#i 0 1 Si s zs (2) where the stress s and Si s are in units of GPa and cm21 /GPa, respectively, and E is the fiber’s Young modulus in GPa. Si ε and Si s are negative in almost all cases but depend on the mode according to the structure anisotropy. Many of the available values were obtained through diamond anvil cell experiments on crystals.5–7 They cannot be applied to multiphase materials like SiC fibers, because the phases are amorphous/nanocrystalline. Wavenumber shift calibrations are then mandatory and usually obtained under axial stress, the control parameter being the strain. Equation (2) supposes Young’s modulus E to be the same in compression and in tension, which we shall discuss further, on account of the bond nature and 3D symmetry in silicon carbide structures. Calibrations of Si ε were done for carbon Raman contri￾butions in polymeric, carbon, and SiC fibers.8–12 In the latter case, the results could be compared with Si ε of silicon carbide optical modes.12 Theoretical predictions of strain-induced “Raman shifts” exist for crystals.13,14 They are based on the elastic constants tensor, and their use for the determination of biaxial stresses in CVD￾diamond15 or silicon16 films produced consistent results. The Gru¨neisen coefficient is sometimes preferred to Si s and Si ε to express the wavenumber sensitivity to external stresses. It is defined as a function of the volume (V) as gi 5 2 ] log n#i ] log V (3) If only the pressure component changing the unit cell parame￾ters is considered (the so-called hydrostatic component, the other one changes bond angles17), then Si s and gi are linked by D. R. Clarke—contributing editor Manuscript No. 188455. Received June 19, 2000; approved December 4, 2000. **Fellow, American Ceramic Society. † Laboratoire Dynamique-Interactions-Re´activite´. ‡ Office National d’Etudes et de Recherches Ae´rospatiales. § National Aeronautics and Space Administration. J. Am. Ceram. Soc., 84 [5] 1136–42 (2001) 1136 journal