Grain boron carbides

The microstructure of the composites fabricated by powder mixing consists of a fine-grained boron carbide matrix with isolated SiC grains, whereas the materials starting with organic precursors [425] or with silicon borides [423] exhibit a continuous silicon carbide phase located in the grain boundaries of the matrix phase. 

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... 

Micrograph of a B4C grain in (a) the pure boron carbide layer, and (b) the B4C-30wt%SiC layer, of the three-layered laminate. 

When nucleation takes place throughout a reacting solid particle and it is of the same speed as the growth of these grains, a different kinetic mechanism must be used. The mechanism called nucleation kinetics has been developed to combine these two steps into a single step. An example of this mechanism is the carboreduction of boron oxide to boron carbide. This overall reaction is written to imply that it follows a liquid-solid reaction pathway ... 

The addition of free graphite yields fine-grained compounds near the theoretical density (94-100 %) . Carbon is better added by the in-situ pyrolysis of a phenolfor-maldehyde resin (i 9 wt Pressure-less sintering of boron-carbide is now... 

A mixture of acetylene black, or sugar with ethylene glycol, is heated with boric acid at 1873-2073K. The resulting boron carbide is fine grained 1-5/tm (cf. ref. 3 in... 

The furnace is usually cooled externally to limit the toss of volatile materials and hence the outer mantle stays unreacted. The core contains blocky boron carbide of relatively high purity (total metallic impurities <0.5 mass-%), reproducible stoichiometry (B/C ratio = 4.3) [50], and several percent of residual graphite. The chunks are crushed and milled to the final grain size. 

Figure 37. Grain growth in boron carbide pressurelessly sintered with carbon (after [197]).

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. 

In this case, B4C also undergoes a grain size refinement which is very beneficial for the mechanical properties. In Fig. 55, a micrograph of a reacted TiC-B powder mixture is presented, which still exhibits TiB2 B2C agglomerates of the size of the initial TiC particles. Note that the average particle size of both reaction products is approximately 1 pm. Generally, this reaction can be employed for most of the transition metal boride-boron carbide composites since the borides are usually more stable than the particular carbides [92],... 

In order to avoid huge losses of volatile boron oxide compounds, the furnace is cooled externally such that the outer shell remains unreacted. The core contains low-impurity boron carbide blocks (total metal impurities <0.5 wt%) with a stoichiometry of B/C = 4.3 and residual carbon [25]. The blocks are crushed, milled to the desired final grain size, and purified by chemical leaching. 

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