Borides compacting

The uncured compacts are presintered in vacuum or in a stream of neutral gas (Ar, H2) at 800-1400°C. Presintering ensures the removal of the organic binder to avoid later contamination of the sintering furnace by pyrolysis of the by-products. It also facilitates machining and finishing, which are difficult and expensive after final sintering because of the hardness of borides. 

Boride Binder nature and content (wt%) Compacting pressure X 10 (N m ) Sintering T (°C) Holding time (min) Sintering atmosphere or vacuum V (pressure in torr) Relative density (%) Remarks Ref. 

Another way of production is the coating of c-BN by electro-less plating with Ni-P, Ni-B, Ni-Fe-P, Ni-Cr-P, Ni-Cu-P, or Ni-W-P alloys, and mixing these powders with >1% of various carbides, borides, nitrides, silicides, and/or oxides. These powders are compacted and pre-sintered at 700-900 °C. Finally, hot-isostatic pressing at 1000-1400 °C and 1000-2000 bar is performed to reduce porosity [264]. 

Boron W + B (amorphous) powder mixture compacts SOO C inHj/l hour 800-1200 °C in Ar/2 hours Individual boride formation depends on W/B ratio W2B, WB, W2B3, WB4... 

PR By solid state reaction between W and amorphous B powder pressed in compacts at elevated temperature pretreatment at 500 °C in hydrogen for 1 horn followed by 800-1200 °C in argon for 2 hours. The formation of the individual boride depends on the W/B ratio. 

In this section, details of an easily controllable, safe method for producing high-purity Hz gas are described. This method of generating Hz gas is particularly suitable for providing a clean source of Hz gas for use as an anodic fuel in fuel cells or as a fuel for internal combustion engines in transportation applications. This compact, portable Hz generator is based on a non-pressurized, aqueous solution of alkaline sodium borohydride (NaBH, tetrahydroborate). As found by Schlesinger et al., when aqueous NaBH, solutions contact selected metal (or metal boride) catalysts, these solutions hydrolyze to yield Hz gas and water-soluble, sodium borate. Overall reaction stoichiometry can be represented in a simplified form as ... 

In February, 1980, Sumitomo from Japan filed the patent Sintered compact for a machining tool and a method of producing the compact [161]. This patent basically covers any compact with 10-80 vol% cBN and a balance of binder material that can comprise any carbides, nitrides, borides, or silicides of metals of groups IVa, Va, or Via. Specifically mentioned are titanium, zirconium, hafnium, vanadium, niobium. 

SHS is particularly suited to the synthesis of refractory ceramic powders and compacts such as carbides of Ti, Si, Cr, Ta, and B, borides of Ti, V and Cr, nitrides of B, Ti, Al, Si, sihcides of Mo, Ti and V, or even more complex compounds such as YBa2Cu307-,. The thermodynamic basis of the process, the individual types of SHS processing techniques, and the equipment and post-synthesis processing to obtain powder compacts have been reviewed by Yukhvid (1992). 

Combustion synthesis (44) is an efficient method for making a wide variety of materials such as carbides (45,46), borides (47), and nitrides (48,49) and is superior to conventional methods with respect to higher purity and more reactivity (Fig. 3a). When the SHS reaction is carried out with powders containing nonvolatile contaminants, significant amounts of these impurities remain in the product (50). The initial reactant mixtures (chemical composition, shape and size of reactant particles, and shape, size, and density of samples) and combustion conditions (composition and pressure of the environment, initial temperature of the compact, the method and intensity of combustion initiation, or additional external effects) determine the properties of synthesized materials (45-49). 

Cubic BN composite materials with TiC, TiN, AI2O3, AIN, Si3N4, WC, etc. are also produced. The mechanical properties of these materials depend on their microstructure such as grain size distribution and voids in the composites (258). Because these composite compacts are useful as cutting tools, they are usually described in the patent literature. Some reported elements for these composites are listed in Table 10 (7). A BN material is sintered with these elements and/or their nitrides, carbides, borides, and carbonitrides. Sintering of carbon precoated cBN grains and formation of BxCyNz grain boundaries were claimed in a patent, as quoted in Ref. 7. 

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