Composites boron carbide-based

Among ceramic matrix composites with boron carbide as matrix phase, SiC and TiB2 are the most cited discontinuous reinforcements. Thus, the following sections will provide a review on these two classes of reinforced material. 

B4C-SiC The presence of particulate SiC limits grain growth in sintered B4C-SiC compacts, thus leading to fine-grained materials with improved strength compared to the single-phase boron carbide materials [313, 423]. 

Since the thermal expansion mismatch is comparably small, there is no large toughening effect observable in B4C-SiC particulate composites. Nevertheless, the combination of hardness and wear resistance of boron carbide with the oxidation and thermal shock resistance of SiC may be of interest for certain applications. 

Different processing routes have been applied to synthesize this class of composite material. Besides the classical method of powder mixing using submicron SiC, B4C and carbon or carbon-containing precursors, followed by forming and subsequent heat treatment ]313], the infiltration of boron carbide preforms with organic precursors such as PCS, followed by heat treatment [400, 401] or the pressure-assisted reaction sintering of silicon borides with elemental carbon [423], have been used. 

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. 

In the case of material, the B4C matrix is exposed to air like pure B4C, 

2 Boron Carbide-Transition Metal Diboride Ceramics 

Another method of densification makes use of reaction hot-pressing or self-propagating combustion sintering of MC-B powder mixtures under pressure  

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], 

The microstructure of hot isostatically pressed samples does not differ so much from that of hot-pressed material. The average grain size is slightly smaller than 

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B4C-TiB2 Boron carbide-based composites with TiB2 as the discontinuous reinforcing phase have been studied for cutting tools and wear parts by various authors [415, 427-433]. 

Mizrahi, A. Raviv, H. Oilman, M. Alzenstein, M. Dariel and N. Frage, The effect of Fe addition on processing and mechanical properties of reaction infiltrated boron carbide-based composites, J. Mater. Sci., 42, [16], 6923-6928, (2007),... 

A breakthrough discovery was reported by Sabatini (ARDEC) in the area of green illuminants. Formulations without any heavy metal can be based on boron carbide (B4C, fuel) with a suitable oxidizer (e.g. Iboron carbide (B4C) in pyrotechnical compositions. It can be seen that flares with 100% boron carbide as fuel show longer burn times and higher luminous intensity than the control barium nitrate based flare (M125 Al) while the spectral (color) purity is slightly lower. 

As opposed to oxides, carbides such as silicon carbide (SiC) and boron carbide (B4C) are compatible with carbon fibers, and satisfactory composites are produced with these matrices and PAN-based yarn by chemical vapor infiltration (CVI).l l A boron-carbon intermediate coating provides optimum strength and toughness as it prevents fiber degradation. 

Recently, photoluminescence properties of boron carbide nanowires were being studied and reported. Photoluminescence spectrum of a thin film made out of B4C nanowires exhibited a broad band at 638 nm, which strongly suggests the potential application of boron nanowires in visible optical devices. Nonetheless, nanowire and nanorod structures based on boron carbide find their application in different areas, including field emission devices and thermoelectric energy converters, neutron adsorbent in nuclear industries, and especially in composite materials as reinforcing agents. ... 

The problem was solved quantitatively by the decomposition of the IR optical stretching mode of the three-atom chain by model calculations taking the possible compositions and the frequency shift depending on the mass distribution in natural and isotope-enriched boron carbide into account (57). The determined concentrations of B12 and BnC icosahedra and C—B—C and C—B—B chains are shown in Fig. 26. Other chain compositions can be excluded. Toward the boron-rich limit of the homogeneity range, an increasing number of unit cells without chains arise. Two alternative models are compatible with the optical spectra completely chain-free unit cells and unit cells in which single B atoms saturate the outer bonds of the equatorial atoms of the adjacent icosahedra. Theoretical calculations of reaction kinetics based on the second version (132) satisfactorily confirm the results in Fig. 26. 

Other forms of carbon-carbon composites have been or are being developed for space shutde leading edges, nuclear fuel containers for sateUites, aircraft engine adjustable exhaust nozzles, and the main stmcture for the proposed National Aerospace plane (34). For reusable appHcations, a siHcon carbide [409-21 -2] based coating is added to retard oxidation (35,36), with a boron [7440-42-8] h Lsed sublayer to seal any cracks that may form in the coating. 

The high-temperature stability of SiC-based ceramics is well-known, and therefore its composite materials have been investigated for application to high-tem-perature structural materials [19-21]. However, well-known SiC-based fibers and matrix-materials stained with alkali salt are easily oxidized at high temperatures in air [22]. This would be a serious problem when these materials are used near the ocean or in a combustion gas containing alkali elements. In particular, a silicon carbide fiber containing boron (a well-known sintering aid for SiC) over 1 wt% was extensively oxidized under the above condition. In this... 

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