Influence of heat treatment on microstructure and mechanical properties of CVI-CFC composites with medium and highly textured pyrocarbon matrices

The influence of heat treatment up to 2900 °C on microstructure and mechanical properties of CVI-infiltrated carbon fiber felts with either medium (MT) or highly (HT) textured pyrocarbon matrices was studied by optical and scanning electron microscopy, X-ray diffraction, and three-point bending tests. The microstructural investigations of the HT-samples reveal a pronounced decrease of the interlayer spacing d002 with a simultaneous increase of the apparent layer stack height Lc with increasing heat treatment temperature. On the contrary, the d002 and Lc 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.