Influence of heat treatment on microstructure and mechanical properties of CVI-CFC composites with medium and highly textured pyrocarbon matrices
Guellali, M; Oberacker, R; Hoffmann, MJ
COMPOSITES SCIENCE AND TECHNOLOGY, 2008, Band 68, Heft 5, S. 1115-1121
The influence of heat treatment up to 2900 degrees C on microstructure and mechanical properties of CVI-infiltrated carbon fiber felts with either medium (NIT) or highly (HT) textured pyrocarbon matrices was Studied by optical and scanning electron microscopy, X-ray diffraction, and three-point bending tests. The inicrostructural investigations of the HT-samples reveal a pronounced decrease of the interlayer spacing d(002) with a simultaneous increase of the apparent layer stack height Lc with increasing heat treatment temperature. On the contrary, the d002 and Le values of the MT-samples changed only slightly after heat treatment. But they exhibit distinct concentric cracks. The density of these cracks increases with increasing heat treatment temperature. The HT-samples show nearly no cracks after heat treatment. Three-point bending tests carried out on the HT-samples show a distinct quasi-ductility increase and a flexural strength decrease with increasing heat treatment temperature. In contrast, the mechanical properties of the MT-samples show despite cracks no significant changes after heat treatment. (c) 2007 Elsevier Ltd. All rights reserved.