1996 Noranda Award Lecture Thermal properties of solids: Étude in three-part anharmony

Journal of the European Ceramic Society

2001

The paper provides a comprehensive study on Raman spectroscopy versatility as a fast and non destructive tool for the prediction of the mechanical properties of SiC fibers derived from a polymeric precursor (NLM™, Hi™, Hi-S™, SA™ and Sylramic™ grades) or produced by CVD (SCS-6™ fiber), including in situ analysis in CMCs or MMCs. We show how the results of very simple spectra fitting are correlated with Young's modulus, tensile strength and microhardness. The reason why such a correlation exists, the common dependency of Raman signal and mechanical behavior to the micro/nanostructure of ceramics, is discussed. Keywords :SiC-fibers / Raman spectroscopy / Mechanical properties / Microstructure / Carbon ∂ Author to whom correspondence should be addressed Fax : 33 (0) 1 49 78 13 18 e-mail : colomban@glvt-cnrs.fr IntroductionThe reinforcement of ceramic materials by long ceramic fibers leads to low density and refractory materials, of high damage tolerance, that should be appropriate for metal alloys substitution in advanced engines (turbines) and waste treatment energy plants [1] . Ceramic fibers can also be incorporated directly in metal matrices to increase their high temperature mechanical properties [1] .SiC fibers, which are among the most stable fibers, have always been produced by the 3D reticulation of a polymeric precursor. This synthesis route leads to a high matter homogeneity and very smooth surfaces, the lack of defects explaining tensile strengths σ r as high as 3 GPa. SiC fibers have been widely studied, especially their aging [2][3][4][5][6][7] which evidenced that densification is linked to compositional changes (for instance the loss of some residual hydrogen [8] ). A peculiarity of ceramics issued from polymeric precursors is no impurities are concentrated at the grains boundaries. Abnormal grain growth is very common in the lack of such usual diffusion regulators [9] and the know-how of SiC fibers manufacturers consists in postponing the onset of SiC crystallization and grain growth, since a high correlation is anticipated with mechanical degradation. A close relationship has been evidenced, for instance, between the micro-hardness and short-range ordering in sol-gel prepared nanocrystalline oxides [10] and some authors have already pointed out fibers overall mechanical ability is governed by their microstructure [5,7,11] .The purpose of this paper is to try correlating, to the best extent possible, the tensile strength (σ r ), the Young's modulus (E) and the micro-hardness (μH) of different SiC fibers, either to Raman spectra or to "Raman Extensometry" S coefficient (measuring the straininduced shifts of Raman bands). Unlike most experimental methods intended to measure directly σ r and E, Raman analysis does not require extracting fibers from the matrix, either by mechanical grinding/crushing [12] or by chemical attack [13] . Only the most resistant fibers are extracted in the former case while fibers surface might be altered in the latter. Besides, it will always be difficult to extract...

Section: Young's Modulus and "Raman Microextensometry"mentioning

confidence: 86%

Non-destructive mechanical characterization of SiC fibers by Raman spectroscopy

Journal of the European Ceramic Society

2001

“…In spite of its very significant influence on the physical behaviour of materials (only it can explain thermal expansion or the finite value of thermal conductivity [421]), anharmonicity is often considered as a simple perturbation: in what is referred to as the ''quasi-harmonic'' approximation, one considers that Eqs. (37) and (38) are applicable to real potentials.…”

Section: The Mechanical Characterization Of Nanophases By Micro-ramanmentioning

confidence: 99%

Raman Spectroscopy of nanomaterials: How spectra relate to disorder, particle size and mechanical properties

Progress in Crystal Growth and Characterization of Materials

2007

943

724

The purpose of this review is to provide non-specialists with a basic understanding of the information micro-Raman Spectroscopy (mRS) may yield when this characterization tool is applied to nanomaterials, a generic term for describing nano-sized crystals and bulk homogeneous materials with a structural disorder at the nanoscale e typically nanoceramics, nanocomposites, glassy materials and relaxor ferroelectrics. The selected materials include advanced and ancient ceramics, semiconductors and polymers developed in the form of dots, wires, films, fibres or composites for applications in the energy, electronic and aeronauticseaerospace industries. The text is divided into five sections:

“…i 0 is the "stress-free wavenumber" and S i ε expressed in units of cm Ϫ1 /%, is a direct measure of bond anharmonicity. 3,4 If the investigated material obeys the "elastic deformation theory," then…”

Section: Introductionmentioning

confidence: 99%

Raman Study of Hi‐Nicalon‐Fiber‐Reinforced Celsian Composites: II, Residual Stress in Fibers

Journal of the American Ceramic Society

Bansal

2001

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 unambiguously conclude that the fibers were in a state of compressive 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 phenomenon was observed at the surface of p-BN/SiC-coated Hi-Nicalon fibers whereas the uncoated or p-B(Si)N/SiCcoated Hi-Nicalon fibers did not show any stress relaxation in the celsian-matrix composites.