Porosity boron carbides

The microstructure of a pure B4C layer of three-layered B4C/B4C-30wt%SiC laminate with 4% porosity is presented in Fig. 7.20. The three-layered B4C/ B4C-30wt%SiC tiles tested as armor material had the same microstructure and porosity level as the material shown in Fig. 7.20. As one can see, the porosity at the grain boundary of the ceramics might be a reason why threelayered laminates have not outperformed the dense monolithic boron carbide tiles. A different set of ballistic experiments are required in which fully... 

Active oxidation of B4C at a low partial oxygen pressure leads to the formation of gaseous boron oxides and removal of them from the surface of the specimen. Similar to other materials, porosity increases the reactive surface of boron carbide specimens and weight gain on oxidation [106]. 

The physical and chemical properties of boron carbide have been reviewed by Lipp [159], Thevenot and Bouchacourt [256], Thevenot [164,165], and Schwetz (1999) [223]. Special problems while presenting the physical properties arise from the large homogeneity range of boron carbide. Furthermore, its poor sinterability requests additives that are usually unspecified and results in residual porosity and various grain sizes which are often also not considered in the publications. Most variation and discrepancies in the properties reported come from the undefined composition of the materials studied. 

This material usually contains only small amounts of residual carbon or boron carbide but no metals, and is thus the favored process for the technical synthesis of less contaminated borides. The process is carried out in tunnel furnaces under hydrogen or in a vacuum at 1600-2000°C, i.e., below the melting point of the boride. It is thus a reaction sintering procedure yielding a high-porosity product which can easily be crushed and milled. Additional refinement is obtained by multiple vacuum treatments with metallic or B4C additives to compensate nonstoichiometries. The final product is then called vacuum quality . 

In equation (1.2), is the measured hardness, is the hardness at zero porosity, and 0 is the fractional porosity. Typically B is less than unity and for boron carbide, B4C, it has the value 0.35 for 0 in the range 0-1.0. The bifunctional dependence of hardness on porosity contrasts with equations used to relate ceramic porosity and strength, for example, equation (1.3),... 

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