Issue 30

F. Burgio et alii, Frattura ed Integrità Strutturale, 30 (2014) 68-74; DOI: 10.3221/IGF-ESIS.30.10 70 distributor, in a top down direction and the continuous flow is obtained by means of two volumetric vacuum pumps. The furnace, with the gas delivery system, is shown in Fig. 1. Figure 1 : CVI/CVD plant. Sample characterization Polarized - light microscopy was employed to characterize the anisotropy of Py-C deposited around the carbon fibres, by the measurement of the extinction angle (A e ). The used apparatus consisted of a Zeiss microscope equipped with an halogen light source, x16 and x40 objectives, and with two rotating polarizer/analyzer. Extinction angle is the measure, expressed in deg, obtained rotating the analyzer from the Maltese cross condition to the maximum extinction of the first quadrant [11]. Fig. 2 shows the typical PLM micrographs, where are evident the Maltese crosses of the Py-C matrix around the carbon fibres. In particular these micrographs are related to the C f /C composites obtained at 1300 °C. Figure 2 : PLM micrographs of Py-C deposited around the carbon fibres at 1300 °C. The extinction angle is related to the Py-C anisotropy and has been long used to classify the large number of Py-C [11, 12]. Tab. 2 summarizes the typical values of A e and the density of the different Py–C optical textures [11, 13, 14]. Optical texture Domain of extinction angle A e Density Rough Laminar (RL) ≥ 18° 2.0 – 2.2 Smooth Laminar (SL) 12° - 18° 1.8 – 1.9 Dark Laminar (DL) 4° - 12° 1.6 – 1.8 Isotropic (ISO) < 4° < 1.6 Table 2 : classification of Py-C optical texture. 40 µm 40 µm