Boron-Containing Nanocrystalline Ceramic and Metal–Ceramic Materials

Abstract Boron-containing nanocrystalline composites represent a wide class of materials with unique physical properties useful for many industrial applications. This chapter considers the three seemingly most important boron-containing nanocomposites: boron carbide-based nanomaterials, metallic alloys modified with layered boron nitride nanopowder, and nanocrystalline boron-containing master-alloys. Development of the boron carbide-based heterophase ceramic and metal–ceramic materials utilizing various modifiers is considered as one of the effective solutions of the actual problem of obtaining of ceramic and metal–ceramic composite materials with hardness-to-crack-resistance ratio optimal for their industrial applications. However, todays traditional methods (e.g., alloying), opportunities to improve hardened and wear-resistant tools and parts (cutters, extrusers, fillers, shaft bearings, and others), and operating parameters are virtually exhausted. One of the promising directions of achieving the same goal is the development of technologies for obtaining metal–ceramic materials with nanostructured components. In particular, there has been developed the hard nanocrystalline alloys production in a single technological process, which includes sputtering of mixtures of soluble compounds of corresponding elements and high-molecular liquid hydrocarbons inside a reactor with reduction environment and subsequent combining of reduction and selective carbonization processes within a certain temperature interval. As the result of the use of liquid (molecular solution) charge, the high-dispersive composition of corresponding metal carbides and binder-metals with homogeneously distributed nanocrystalline particles can be obtained. Compaction by spark-plasma-sintering (SPS) allows the manufacture of products from the composite material, components of which are maintained in nanocrystalline state. Taking into account its layered crystalline structure, high thermal stability, and chemical inertness, the nanopowdered hexagonal boron nitride (h-BN) can serve as a “green” alternative to nanopowdered materials with a layered structure currently widely used in industry as solid lubricants or liquid lubricants’ additives, which contain environmental contaminants such as heavy metals, sulfur, carbon (graphite), etc. There has been developed a relatively cheap chemical technology of producing the nanograde h-BN directly usable for solid lubrication, as well as friction-modifier for so-called third bodies. In particular, tests show that nanopowdered h-BN together with gelled polyethylene glycol (PEG) form the eco-friendly lubricant compositions with working resource significantly exceeding that of other materials utilized in practice with the same purpose. There are also obtained the novel self-lubricating brass- and iron-based composites with h-BN nanoinclusions. At its optimal contents (from 1 up to 4 wt.%) the wear resistance of a composite compared to that of its metallic matrix is enhanced in several times. Due to the proper combination of high hardness with plastic properties and high chemical inertness, metal (in particular, iron) borides represent an important class of inorganic compounds useful for manufacturing of industrial tools from structural steels highly alloyed with borides. Because of strong (covalent) B–B bonds presented in metal borides, the microadditives of ferroboron ligature make a positive influence on the boron steels physical–mechanical properties: small amounts of boron atoms form solid solutions, which slow the development of porosity, promoting segregation on the crystal lattice structural defects, and reducing diffusive mobility of the basic alloying elements. Because of very high cross-section of thermal neutrons absorption by boron 10 B isotope nuclei, boron steels also have found application in boiling nuclear reactors and production of containers for storage and transportation of radioactive wastes. Thus, the ligatures enriched with the 10 B isotope can be used for microalloying of austenitic class steels, which is widely applied in nuclear equipment as construction materials. There are also presented the results of research obtaining the nanocrystalline Fe–B and Fe–B–Al 2 O 3 alloys suitable for modifying the steels. Boron-containing nanocrystalline ceramic and metal–ceramic materials have many industrial applications. In the present chapter, three important classes of these composites are considered: • boron carbide-based nanomaterials, • metals and alloys modified by nanopowdered boron nitride, • boron-containing nanocrystalline modifiers of steel.