LPCVD and characterization of boron-containing pyrocarbon materials

Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BCl3–C3H8–H2 precursor mixtures. The growth rate is increased with respect to pure pyrocarbon deposition. By increasing 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 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 belongs to a boron-rich amorphous part of the material. As boron content increases beyond 8 at.%, the 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. It 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.