Effects of thermal and thermomechanical induced mechanical changes of C/C composites

Carbon fiber reinforced carbon composites are produced by modified chemical vapor infiltration (CVI) of a textile fiber preform. The modified process called rapid CVI (r-CVI) allows a fast production of the matrix as well as the adjustment of graphene orientation of the matrix. At a processing temperature of 1200 degrees C graphene stacks are produced which are oriented roughly in fiber orientation. Mechanical testing at different temperatures as well as after temperature treatment and optional surface siliconization was carried out on C/C. Thermal treatment leads to growth and realignment of the graphene stacks in the matrix resulting in an intensified anisotropy of the composite. High temperature tests beyond processing temperature on C/C lead to mechanically induced straightening of graphene stacks in loading direction. The improved orientation of the graphene structure which is proven by XRD measurements leads to higher loading capacity and therefore to better mechanical properties in 0/90 degrees fiber direction and to reduced mechanical performance in +/- 70 degrees fiber orientation. This effect is also shown in transverse tensile tests when the graphene layers in the matrix are loaded perpendicular to their orientation. Furthermore a new method to microstructural assessment is presented. (C) 2010 Elsevier Ltd. All rights reserved.