S. Jacques

LPCVD and characterization of boron-containing pyrocarbon materials

Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BC13C3H8H2 precursor mixtures. By increasing the BCl3(C3H8 + BCl3) ratio up to 85%, the incorporation of boron can reach 33 at.%. A small amount of boron (e.g. 8 at.%) highly enhances the structural anisotropy of pyrocarbon, as evidenced by optical microscopy, X-ray diffraction and transmission electron microscopy (selected area diffraction and lattice fringes techniques). X-ray photoelectron spectroscopy has shown that a large fraction of the boron atoms are included by substitution in the carbon layers; the remaining boron atoms belong to a boron-rich amorphous part of the material. As the boron content increases beyond 8 at.%, the structural anisotropy of the boron-rich pyrocarbon decreases, due to the limited growth and stacking of the carbon layers. Also, amorphous boron-rich regions are more and more abundant as the total amount of boron increases. The oxidation resistance of the C(B) materials is better than that of pure pyrocarbon. This is mainly due to the improvement of the structural organization for the low boron content materials and to the coating of the whole material with a stable boron oxide for materials with a higher boron content.

SiC/SiC minicomposites with structure-graded BN interphases

Abstract BN interphases in SiC/SiC minicomposites were produced by infiltration of fibre tows from BF 3 –NH 3 –H 2 gaseous system. During interphase one-step processing, the tow travels through a reactor containing a succession of different hot areas. By TEM characterization, the BN interphases were found to be made of a structural gradient: from isotropic to highly anisotropic. The very first coating is poorly organised and allows to protect the fibre from a further chemical attack by the reactant mixture. The minicomposites were tensile tested at room temperature with unloading-reloading cycles. The BN interphases act as mechanical fuses; the fibre/matrix bonding intensity ranges from weak to rather strong depending on the tow travelling rate during interphase infiltration. The specimen lifetimes at 700°C under a constant tensile loading were measured in dry and moist air. Compared to a pyrocarbon reference interphase, the BN interphases significantly improve the oxidation resistance of the SiC/SiC minicomposites.

Preparation and characterization of 2D SiC/SiC composites with composition-graded C(B) interphase

2D SiC/C(B)/SiC composites were prepared by CVI. The C(B) interphase is made of five successive C(B) layers with increasing boron content from the fibre to the matrix. This composition-graded interphase leads to good mechanical properties similar to those obtained with pure pyrocarbon interphase. However, the lifetime in air under load at high temperature of 2D SiC/C(B)/SiC composites is not improved despite the high percentage of boron in several sublayers of the interphase. By TEM characterization, the boron-rich layers were found to exhibit a nano-porous texture probably due to a bad control of the growth process within the fibrous preforms. This nanoporous texture might be responsible for the poor oxidation resistance of these sublayers and consequently the rather short lifetime of the real 2D composites with respect to those previously reported for 1D SiC/C(B)/SiC model microcomposites.

C(B) materials as interphases in SiC/SiC model microcomposites

A specific test procedure has been developed to compare the high temperature lifetimes of SiC/SiC microcomposites with various interphases in air and under mechanical loading. The interphases, namely pure pyrocarbon (PyC) or C(B) materials with uniform or variable boron contents in the thickness, were prepared by chemical vapour deposition (CVD). Uniform addition of boron in PyC interphases improved their oxidation resistance and consequently the lifetimes of the microcomposites. However, room temperature tensile tests have shown that this improvement occurs to the detriment of the mechanical properties even when a non-brittle behaviour is maintained. In the case of variable boron contents, compositional gradient interphases (CGI) in which boron content increases from the fibre interface to the matrix interface allow the mechanical fuse properties of PyC to be combined with the oxidation resistance of a C(B) material.

SiC/SiC minicomposites with (PyC/TiC)n interphases processed by pressure-pulsed reactive CVI

Abstract(Pyrocarbon/titanium carbide)n multilayered interphases were prepared within SiC/SiC minicomposites by a new method: pressure-pulsed reactive chemical vapour infiltration (P-RCVI). This method combines P-CVI with reactive chemical vapour deposition (RCVD). Minicomposite tensile tests with unload-reload cycles have shown that the interfacial shear stress depends on the number of TiCl4 gas pulses used for the processing of TiC sub-layers. TEM observations have shown, that with a few gas pulses, the carbide nucleates as isolated grain islands which disturbs the structural anisotropy of the pyrocarbon. This structure results in a good mechanical fibre/matrix load transfer. By increasing the number of gas pulses, the TiC sub-layers become continuous and it is possible to partially consume the highly ordered pyrocarbon sub-layers, but, in that case, the load transfer is poor. The specimen behaviour in air at 700°C under a constant tensile loading was assessed. Compared with a pure pyrocarbon reference interphase, the interphases containing TiC significantly improve the lifetime of the SiC/SiC minicomposites.