Investigations in the boron-carbon system with the aid of electron probe microanalysis

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Abstract
For boron carbide the limit of the homogeneity range at the carbon‐rich side yet has not been firmly established. During the last twenty years carbon‐rich limiting compositions have been reported corresponding to B4.3C (i.e., ‘‘B13C3’’), B4.0C (i.e., ‘‘B12C3’’) and B3.6C (i.e., ‘‘B11C3’’). By means of quantitative EPMA the composition of the boron carbide phase in the following samples has been determined: i) Boron‐carbon diffusion couples, prepared by hot isostatic pressing of the pure elements at ≳2000 °C, ii) Fused, regular boron carbide, obtained by carbothermic reduction of boric oxide at ≳2500 °C (ESK‐electrofurnace process), iii) Dense boron carbide compacts made by pressureless sintering of carbothermic boron carbide powder or by hot‐isostatic‐pressing (HIP) of magnesiothermic boron carbide powder at ≳2000 °C. In each sample the carbon‐rich limiting composition has been determined as B4.3C (18.8 at.%C) which corresponds to B13C2.C and is probably a solid solution of carbon in B13C2. The B12C3 and B11C3 compositions could not be found. For quantitative analysis a standard sample has been used, which was obtained by arc melting the pure elements with a B/C atomic ratio of 4.39. Also indirect B/C atomic ratio determinations via chemical analyses of crushed samples have produced evidence to support the limit 4.3 found by EPMA.

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