Ceramic coating of short fibers by Fluidized Bed Chemical Vapor Deposition
DA CALVA T. 1, CHOLLON G. 1, BERTRAND N. 1
1 Laboratoire des Composites ThermoStructuraux, PESSAC, France
SiCf/SiCm Ceramic Matrix Composites (CMCs) are mainly used in the aerospace industry for high-temperature structural applications where thermal stresses and corrosion are severe enough to cause the failure of most monolithic materials. A new generation of materials with discontinuous reinforcement is currently being developed for specific applications where long fiber weaving is not possible and more isotropic properties are preferred.
The fiber/matrix interphase is an essential constituent of CMCs that prevents the material from catastrophic failure and provides pseudo-ductile mechanical behavior. Due to their lamellar structures, pyrolytic carbon (PyC) or boron nitride (BN) can deflect cracks parallel to the fiber axis. The fluidized bed CVD process was naturally chosen for uniform deposition of the interphases.
The coatings were deposited on short Hi Nicalon S fibers and powder mixtures from precursors that could be of environmental interest due to their origins and the nature of the CVD by-products: ethanol (EtOH) for PyC and borane triethylamine complex (TEAB) for BN. Compared with the traditional precursors, EtOH and TEAB have the advantages of being non-chlorinated, less hazardous and liquid at room temperature.
A pressure/temperature study was first conducted in inert gas on the short fibers/powder mixtures in order to determine the optimal fluidization operating point. Then, the CVD parameters were optimized to provide sufficient interphases thicknesses. Finally, the interphases were characterized by scanning and transmission microscopy.