Oxidation resistant carbon materials derived from boronated carbon–silicon alloys

Precursors to C–B–Si alloys have been produced by boronation of a carbon silicon alloy precursor derived from petroleum pitch and polydimethylsilane. A monofunctional boronation reagent, catechol·borane, was used in combination with the siliconised pitch precursor to produce meltable C–B–Si precursors. Use of the trifunctional borane·pyridine led to cross-linking and an insoluble infusible C–B–Si alloy precursor. The C–B–Si alloys have excellent built-in oxidation resistance compared with carbon materials and even when compared with C–Si alloys derived from the siliconised pitch precursor. The improved oxidation resistance arises from the formation of a glassy borosilicate coating on the alloys as shown by X-ray photoelectron spectroscopy, SIMS and energy dispersive analysis of X-rays. Optical microscopy and SEM shows that the coating does not have a well-defined interface with the sample core. SIMS provides strong evidence for a surface composition that gradually varies with depth after oxidation. Oxidation protection is maintained in the C–B–Si alloy system under thermal cycling conditions in dry air. In moist air a preformed borosilicate barrier still offers protection against oxidation but it appears that moisture prevents the initial formation of a coherent glassy protective layer, possibly via gasification of silicon and boron hydroxides.