Pyrocarbon anisotropy as measured by electron diffraction and polarized light
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Pyrocarbon anisotropy as measured by electron diffraction and polarized light Xavier Bourrat,a) Be´atrice Trouvat, Guillaume Limousin, and Ge´rard Vignoles Laboratoire des Composites Thermosructuraux (UMR 5801 CNRS-SNECMA-CEA-UB1), Universite´ Bordeaux 1, 3 Alle´e La Boe´tie, F-33 600 Pessac, France
Franc¸ois Doux SNECMA, BP 37, F-33 165 Saint Me´dard-en-Jalles cedex, France (Received 3 June 1999; accepted 14 October 1999)
This work deals with the measurement of pyrocarbon anisotropy on very thin fiber coatings used to control the interfacial behavior in carbon/carbon composites. Differentiation of the various pyrocarbons was performed through computerized image analysis of the electron diffraction patterns by measuring the azimuth opening of the carbon 002 diffraction arcs. This orientation angle decreases when the texture switches from rough to smooth laminar. The relationship with the polarized light measurement technique at a lower resolution is discussed.
I. INTRODUCTION
In the field of carbon/carbon composites, the relationship between structure and mechanical properties is of prime interest. This is mainly due to the high anisotropy of the hexagonal form of carbon involved in those materials, especially in the different forms used in carbon/ carbon composites. A good example is the carbon fiber itself Young’s modulus of which has been shown to be directly related to the orientation angle of the basal planes along the fiber axis.1 In the case of pyrocarbon, it is already known that most of the physical properties change with their texture. In the peculiar case of carbon composites, it has been shown2 that changing the texture of pyrocarbon between fiber and matrix by means of an interphase makes it possible to change the tensile behavior from brittle to nonbrittle. It is therefore especially important to measure and control the anisotropy in order to monitor the interface,2,3 e.g., fiber/matrix chemical bonding strength, coefficient of thermal expansion (CTE), etc. Different methods have been developed to measure the anisotropy. Most are based on x-ray diffraction (XRD).1,4–7 The others involved optical methods,8–11 by measuring the reflecting power anisotropy. For example, an “optical anisotropy factor” has been introduced which is defined as the ratio of maximum-to-minimum intensity of the reflected light by rotating the stage on a 15-mlarge field. This was performed with a microphotometer and was later developed by Koizlik and Gru¨bmeier10 on a smaller target: from 10 m down to 5 m. This factor a)
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J. Mater. Res., Vol. 15, No. 1, Jan 2000 Downloaded: 04 Feb 2015
was well related to the x-ray “Bacon’s factor of anisotropy.”7 Technical aspects have been further developed. For example, Stevens has used a synchronous micropolarimeter9 to approach the bireflectance (Re–Ro). However, the reflectance anisotropy level (Ro /Re) can be easily studied on
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