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

For boron carbide the limit of the homogeneity range at the carbon-rich side has not yet been firmly established. During the last 20 years carbon-rich limiting compositions have been reported corresponding to B₄₃C (i.e. “B₁₃C₃”), B_4.0C (i.e. “B₁₂C₃”) and B_3.6C (i.e. “B₁₁C₃”). By means of quantitative electron probe microanalysis (EPMA) the composition of the boron carbide phase in the following samples has been determined:

• 1.

  (i) boron-carbon diffusion couples, prepared by hot isostatic pressing (HIP) of the pure elements at above 2000 °C;

• 2.

  (ii) fused, regular boron carbide, obtained by carbothermic reduction of boric oxide at above 2500 °C (ESK electrofurnace process);

• 3.

  (iii) dense boron carbide compacts made by pressureless sintering of carbothermic boron carbide powder or by HIP of magnesiothermic boron carbide powder at above 2000 °C.

In each sample the carbon-rich limiting composition has been determined as B_4.3C (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.