Raman Spectroscopy of nanomaterials - institut de chimie et des ...

G. Gouadec, Ph. Colomban / Progress in Crystal Growth and Characterization of Materials 41xx (2007) 1e56As for r m , it corresponds to the effective matrix thickness responsible for the fibre stress. Avalue b w 0.03 mm 1 gave the best fit and the variation in Fig. 23c shows the axial compressionto rise from zero (at the tip) to its maximum value over a typical IL w100 mm.5.4. ‘‘MicroeMacro’’ correlationsARTICLE IN PRESS+ MODELIn this section, we will demonstrate how the local information accessed by RS through verysimple spectra fittings correlates with ‘‘macroscopic’’ parameters such as Young’s modulus,tensile strength and micro-hardness. The reason why such a correlation exists is the commondependence of the Raman signal and the mechanical behaviour on the micro/nanostructureof materials.5.4.1. Comparison of Raman Spectra with micro-indentation resultsStrong correlations between Raman and microhardness data have been reported by Gogotsiet al. [133,486] for semiconductors, quartz and carbon and by Amer et al. [487] for ‘‘diamondlikecarbon’’. Fig. 24 illustrates how ‘‘mechanical changes’’ (Fig. 24c) are similarly related toa change of the Raman spectrum of either carbon or SiC nanoprecipitates (Fig. 24a and b) in anSCS-6 fibre. 36 The variations in Berkovich’s micro-hardness (mH; three-sided diamond tip) andYoung’s modulus were measured by Mann et al. [489] and we highlighted different zones withvertical dots. From core to periphery, they correspond to:(I) The 32-mm diameter carbon core consisting of graphitic units 1e5 nm in size [475].(II) A 1.5-mm thick layer of pyrolytic carbon (grains are 25e50 nm in size). It has been identifiedas the weakest part of the fibre because its graphitic planes are parallel to the fibreaxis [489].(III) A zone in which carbon coexists with SiC and evolves over 30 mm towards a highly disorderedcarbon.(IV) A zone where only SiC is detected (CeC bonds abundance 0.1%). The TO and LOband widening and the increase of the background level reveal stacking faults are allthe more frequent as thickness increases [475]. In spite of this, the mechanical propertiesare constant, which suggests they are governed by free carbon in insertion.(V) The interfacial carbon deposit (thickness 3 mm).5.4.2. Comparison of Raman Spectra with fibre strengthFig. 25 shows the correlation of the strength measured on NLM [370] and Hi-Nicalon [383]SiC fibres annealed at different temperatures (up to about 1500 C) with their Raman spectrarecorded at room-temperature. The linearity suggests that the macroscopic (strength measurement)and microscopic (Raman spectrum) responses to stress have the same dependence ongrain size. A strength reduction resulting from grain boundary creeping is ruled out since itwould not alter the Raman spectra. Two atmospheres were tested for the NLM grade. The mechanicalfailure is postponed in a reducing atmosphere but this does not seem to modify the‘‘microemacro’’ mechanical correlation.36 This is a 140-mm diameter fibre used for metal-matrix reinforcement [365]. It has a high heterogeneity along theradius, due to a preparation by CVD of SiC (and carbon), on a carbon core [488].Please cite this article in press as: G. Gouadec, Ph. Colomban, Prog. Cryst. Growth Charact. Mater. (2007),doi:10.1016/j.pcrysgrow.2007.01.001