Microstructures 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... 

T. D. Claar, W. B. Johnson, C. A. Andersson, and G. H. Schiroky, Microstructure and properties of platelet reinforced ceramics formed by the directed reaction of zirconium with boron carbide. Ceram. Eng. Sci. Proc. 10(7-8) 599-609 (1989). 

E. Breval and W. B. Johnson, Microstructure of platelet reinforced ceramic prepared by the directed metal reaction of zirconium with boron carbide. J. Amer. Ceram. Soc. 75(8), 2139-2145 (1992). 

In this paper, to meet the different requirements in self-healing CMCs, typical boron carbide and boron doped carbon was prepared by CVD ftom BCla-CH -Ha-Ar and BClj-CaH -Ha-Ar mixture, respectively. Microstructures, phases and chemical bonding characters of the deposits were systematically analyzed and the relationship between microstmctures and compositions was discussed. 

In order to investigate the microstructure of boron carbide under field A and field B, TEM studies were performed. Fig.7 showed the microstructure of deposits under different temperature fields. Under field A, the space of (021) plane were 0.24nm, which showed the deposits were crystal B13C2. Under field B, no crystal phase can be found, which showed the deposits were amorphous. 

The above results showed that the morphology, composition and microstructure were different under two different temperature fields. Under field A, the deposits were crystal B13C2 with high boron concentration and crystalline-like morphologies. Under field B, the deposits were amorphous boron carbide with low boron concentration and cauliflower-like morphologies. The characteristics of deposits depended on deposition mechanism. It was apparent that there were different deposition mechanisms for boron carbide since the characteristics of deposits were very great different under... 

Figure 36. Microstructure of boron carbide pressurelessly sintered with carbon black (after [197]).

Figure 57. Microstructure of hot-pressed material. Dark matrix boron carbide, gray TiB2-solid solution, white W2B5.

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. 

While REPEL or conventionally sintered (CS) materials are the common forms, hot pressed material (HP) of almost theoretical density is also available. The HP artifacts are usually obtained at 2100°C and 50 MPa pressure with either aluminum or boron carbide, B4C, added as a sintering aid. Hot pressing with up to 1.5% aluminum produces equiaxed microstructures with grains about 2 /im in diameter, but a different microstnicture, with elongated grains 20-40 fim long, is produced in the presence of B4C. 

Boron carbide has very high structural stability under irradiation [37]. However, when used as a neutron absorber, very high absorption rates are obtained (the volume density of neutron captures by °B is most often called bumup) in SFR, up to 10 He/cm is produced, leading to drastic modifications of the microstructure and of the composition and high energy release (about 100 W/cm ). 

A powder with good characteristics (high purity, good homogeneity, fine particle size, narrow particle size distribution, absence of hard agglomerates) is a must to get the desired properties and microstructure in the final component and thus synthesis of high quality powder is extremely important. Powders of ZrB and HfB are synthesized by (a) reaction between elements (Brochu et al., 2008 Tamburini et al., 2008) (b) borothermic reduction of metal oxide (Peshev et al., 1968), (c) boron carbide reduction of metal oxide in presence of carbon (Sonber et al, 2010 2011) (d) carbothermic reduction of metal oxide and B Oj (Fahrenholtz et al., 2007) (e) Metallothermic reduction of metal oxide and B Oj (Setoudeh et al., 2006 Kobayashi et al., 1993),(f) molten salt electrolysis (Frazer et al.,1975) (g) solution based techniques (Yan et al., 2006) and (h) s3mthesis from polymer precursors (Suetal., 1991). 

Bakshi, S. R., Musaramthota, V., Lahiri, D., Singh, V., Seal, S., Agarwal, A. (2011). Spark plasma sintered tantalum carbide Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties. Materials Science and Engineering A, 52S(3), 1287-1295. doi 10.1016/j.msea.2010.10.009. 

The high rate of the recasting process gives opportunities for formation of metastable phases and considerable decreasing of grain size. The electrolyte type is of great importance for the chemical composition, microstructure and properties of the modified layer. By these experiments the electrolyte is on water basis and contains boron or silicon compounds. At short times of treatment it is not observed diffusion of elements from the electrolyte in the modified surface, but it is available diffusion process inside the workpiece between the white layer and the matrix - Table 3. The strong carbide-formed elements such as Mo, W, and V diffuse from the white layer to the matrix and Cr, Co in the op>posite side. 

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