Croissance sélective du carbure de bore B 4C par CVD réactive. Etude du système BCl 3-H 2-C (graphite)

Abstract

The interaction between boron trichloride and graphite in presence of hydrogen has been studied in order to obtain an adherent layer of boron carbide. The theoretical conditions for the formation of pure B 4C have been determined from free energy minimization calculations of the chemical system, in the range 1000-1800 K and under atmospheric pressure. These theoretical conditions are not compatible with a development of the process. The results have been compared with experimental data obtained by X-ray diffraction, by EMA microprobe and MEB microscopy. For all temperatures considered, the first step corresponds to the substrate being covered by an uniform layer of boron carbide that isolates it from the gaseous phase. The second step, according to the thermodynamical predictions, corresponds to boron formation but its appearance may be delayed by using H 2/BCl 3⩽ 1.5 mixtures and by limiting the gaseous flow rate. Boron carbide coatings of 100 μm, without boron, have been obtained on plane substrates of graphite. The stoichiometry of carbide on the layer surface depends on the temperature; it varies from B 4C (1800 K) to B 10.5C (1400 K).