Materials World Network:
Multi-Scale Study of Chemical Vapor Infiltrated Carbon/Carbon Composites

Carbon/carbon (C/C) composites combine light weight, exceptional strength and stiffness with excellent refractory properties, making them the material of choice for severe-environment applications, such as atmospheric reentry, solid rocket motor exhaust, and disk brakes in high performance military and commercial aircraft, high speed trains and racing cars. Emerging areas of applications include biomedical devices, aero-engine components, heating elements with 2000C temperature, and hardware for metal forming and glass making.

One of the major technologies for fabrication of C/C composites is chemical vapor infiltration (CVI) of porous carbon fiber performs. CVI-densified C/C composites have a complex hierarchical microstructure that consists of carbon fibers embedded in a porous matrix of pyrolytic carbon (Fig. 1a). The pyrolytic carbon (PyroC) matrix around fibers has a cylindrically layered structure with each layer having different nanostructure and anisotropic mechanical properties (Fig. 1b). These layers correspond to different preferred orientation (or texture) of basal planes in pyrolytic carbon (Fig. 1c). The distributions of layers, their width, order and structure, as well as volume fraction and topology of the porosity, are strongly influenced by the fabrication parameters including the chemical vapor infiltration parameters (temperature, pressure, gas flow rate, and duration of infiltration), the choice of precursor gas, the properties of carbon fibers and their arrangement in preform, and subsequent heat treatment of the composite.

Fig. 1. Microstructure of carbon/carbon composite on various length scales.
The outlined areas in the TEM images indicate the coherent domains
(stacks of parallel well-aligned carbon layers

Enhanced understanding of the coupling between different length scales of composite microstructure and proper modeling of the correlation between manufacturing, microstructure, and mechanical properties leads to improvements in technology for fabrication of C/C composites resulting in composites with superior material properties for various environments and applications.

The objectives of this research are:

  • to increase fundamental understanding of how nanotexture and microstructure of carbon/carbon composites is defined by the parameters of chemical vapor infiltration process;
  • develop a multi-scale numerical model relating microstructure on nano-, micro-, and meso-levels to the overall material properties (see Fig. 2);
  • propose methodology for fabrication of carbon/carbon composites with desirable microstructure and mechanical properties;
  • verify the approach by manufacturing the material under the prescribed conditions and performing microstructure characterization and mechanical testing;
  • investigate stress concentrations and fracture induced by nanotexture of pyrolytic carbon and by thermal treatment of the composite.

Fig. 2. Characterization and modeling on different length scales.