Boron ceramics, physical properties

Non-oxide ceramics such as silicon carbide (SiC), silicon nitride (SijN ), and boron nitride (BN) offer a wide variety of unique physical properties such as high hardness and high structural stability under environmental extremes, as well as varied electronic and optical properties. These advantageous properties provide the driving force for intense research efforts directed toward developing new practical applications for these materials. These efforts occur despite the considerable expense often associated with their initial preparation and subsequent transformation into finished products. 

Silicon carbide is noted for its extreme hardness [182-184], its high abrasive power, high modulus of elasticity (450 GPa), high temperature resistance up to above 1500°C, as well as high resistance to abrasion. The industrial importance of silicon carbide is mainly due to its extreme hardness of 9.5-9.75 on the Mohs scale. Only diamond, cubic boron nitride, and boron carbide are harder. The Knoop microhardness number HK-0.1, that is the hardness measured with a load of 0.1 kp (w0.98N), is 2600 (2000 for aAl203, 3000 for B4C, 4700 for cubic BN, and 7000-8000 for diamond). Silicon carbide is very brittle, and can therefore be crushed comparatively easily in spite of its great hardness. Table 8 summarizes some typical physical properties of the SiC ceramics. 

Table 4.9 Physical properties of boron carbide ceramics used in armor applications. After Karandikar et al. [576]. 

The common commercially available fibers used in composites are fiberglass, graphite (carbon), aramid, polyethylene, boron, silicon carbide, and other ceramics such as silicon nitride, alumina, and alumina silica. Many matrix choices are available, both thermosetting and thermoplastic. Each type has an impact on the processing technique, physical properties, and environmental resistance of the finished composite. The most common resin matrices include polyester, vinyl esters, epoxy, bismaleimides, polyimides, cyanate ester, and triazine. 

Silicon CarbidG Fibers. Silicon carbide (SiC) filaments are produced by a CVD technique. The y3-SiC is obtained by the reaction of silane and hydrogen gases with the carbon filament being the substrate for deposition. The SiC fibers have mechanical and physical properties equal to those of boron, and can be used at higher temperatures than the present boron fiber when available in production quantities. CVD SiC fibers are primarily used for reinforcing metal and ceramic matrices. Alternatively, SiC fibers can be made from a polycarbosilane precursor which is meltspun at 350°C. The final form is obtained by pyrolyzing the fiber at 1200°C in an inert environment. 

Multi-component ceramics allow the optimization of various physical properties. These include ceramics which form multi-component oxides as well as fiher-rein-forced ceramic matrix composites. However, the oxidation behavior of these materials is complex compared with the pure materials. The leading fiber-reinforced composites are silicon-based and contain continuous SiC fibers with coatings of graphitic carbon or hexagonal boron nitride. The oxidation of the fiber coating at intermediate temperatures is a major issue and models of this process are discussed for both carbon and boron nitride coatings. 

The carbides and nitrides are well known for their hardness and strength, and this section will briefly compare a number of these properties with those of the pure metals. Concentration will be placed here on the first row compounds, since these constitute a complete series, and Mo and W, since these are the most commonly studied metals. As will be shown, the physical and mechanical properties of carbides and nitrides resemble those of ceramics not those of metals. Comparisons will be made with boron carbide (B4C), silicon carbide (SiC), aluminium nitride (AIN), silicon nitride (Si3N4), aluminium oxide (A1203), and diamond, as representative ceramic materials. 

NISTCERAM National Institute of Standards and Techology Gas Research Institute, Ceramics Division mechanical, physical, electrical, thermal, corrosive, and oxidation properties for alumina nitride, beryllia, boron nitride, silicon carbide, silicon nitride, and zirconia... 

There are a number of boron-containing ceramics for which the development of a polymer based synthetic route that would allow the formation of the ceramic in the form of films, fibers, coatings or other shaped materials would be advantageous. Because of its unique physical and chemical properties, boron nitride, BN, is a material of particular interest. A BN unit is isoelectronic with C2 and accordingly, boron nitride can be obtained in forms related to diamond (cubic-BN) and graphite (hexagonal-BN). The c-BN has only been formed under extreme conditions, thus boron nitride materials that have been formed from chemical precursor routes (CVD or polymer) have been obtained in amorphous, turbostratic or hexagonal forms. 

Bellosi, A., Guicciardi, S., Medri, V., Monteverde, F., Sciti, D., Silvestroni, L. (2011). Processing and Properties of Ultra-Refractory Composites Based onZr- and Hf-Borides State of the Art and Perspectives. In Orlovskaya, N., Lugovy, M. (Eds.), Boron Rich Solids. Sensors, Ultra High Temperature Ceramics, Thermoelectrics, Armor Series NATO Science for Peace and Security Series B Physics and Biophysics (pp. 147-160). Springer. 

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