Borides reaction-sintering

The halide is not the only metal compound used as source of metal. Metal oxides and sulfides are employed to prepare vanadium, chromium, iron and nickel borides in this way from sulfides at lower reaction T than those required by reaction sintering of the elements . 

This material usually contains only small amounts of residual carbon or boron carbide but no metals, and is thus the favored process for the technical synthesis of less contaminated borides. The process is carried out in tunnel furnaces under hydrogen or in a vacuum at 1600-2000°C, i.e., below the melting point of the boride. It is thus a reaction sintering procedure yielding a high-porosity product which can easily be crushed and milled. Additional refinement is obtained by multiple vacuum treatments with metallic or B4C additives to compensate nonstoichiometries. The final product is then called vacuum quality . 

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. 

To produce wear-resistant or hardened surfaces, thin layers of borides can be prepared on metal surfaces by reaction and diffusion (see Metal SURFACE treatments). Boride powders can be formed iato monolithic shapes by cold pressing and sintering, or by hot pressiag. 

A pellet is pressed of an intimate mixture of finely divided reactants and reaction induced either by arc melting and high-T annealing or by solid-state sintering in an electrical or high-frequency furnace. Isolating the borides from reactive container components can be a problem. The use of boron nitride liners has proved effective. In some cases the protective liner is made of sintered boride containing the same elements as the boride in preparation. 

I. Slynthesis via fusion of the elements entails such high heats of formation that the reaction temperatures become veiy high. As a result, there is interaction with the material of the vessel and the product boride becomes contaminated. On the other hand, all borides may be prepared by sintering the appropriate metal with amorphous boron powder, which should be as pure as possible (commercial grades now available contain 97-99% B). The reaction mixtures should be heated in alumina crucibles (W or Mo crucibles or boats may also be used) in vacuum (an argon atmosphere is occasionally also used). The reaction, which is always exothermic, starts at temperatimes of 700-1200 C the highest temperature may lie above 2000°C. In some cases, sintering under pressure in carbon tubes (mentioned as a possible method of synthesis for silicides—see p. 1796) can be used. 

Typical metal contents required for a successful liquid phase hot-pressing of TiB2 are 5-25 mass-% (i.e. 2-12 at.-%) Ni or Co. In order to avoid reactions consuming TiB2, the borides of Ni or Co have also been used. By this method, the sintering temperatures have been decreased from 2100°C to 1400°C [134, 277, 294, 295, 346]. 

Table 14. Sintering reactions of ternary boride cermets.

Methods of preparation for silicides and aluminides are very similar to those used for carbides, nitrides and borides (1) synthesis by fusion or sintering, (2) reduction of the metal oxide by silicon or aluminum, (3) reaction of the metal oxide with SiOj and carbon, (4) reaction of the metal with silicon halide or (5) fused salt electrolysis. The simplest preparation method consists of... 

Liquid phase assisted sintering has also been observed with few non-oxide sintering additives such as MoSi [Basu, 2006 Fahrenholtz, 2007 Murthy, 2006 Mukhopadhyay, 2009 Raju, 2009 Biswas, 2006 Bellosi, 2006 Silvestroni, 2011) and TaSij (Fahrenholtz, 2007 Silvestroni, 2011 Sciti, 2008). Such addition has led to improved densification for most of the refractory borides (Basu, 2006 Fahrenholtz, 2007 Murthy, 2006 Mukhopadhyay, 2009 Raju, 2009 Biswas, 2006 Bellosi, 2006 Silvestroni, 2011 Sciti, 2008) as well as carbides (Raju, 2007). While TiSi (Basu, 2006 Murthy, 2006 Mukhopadhyay, 2009 Raju, 2009 Biswas, 2006) has been observed to form due to sintering reaction between the borides and MoSi (see Equation 8 [Biswas, 2006]), addition of TaSi leads to the formation of complex phases consisting of Ta-Si-B-C-0, both of which are supposed to be in the liquid state during the final sintering temperatures. The sihcides have also been reported to react with the surface oxide phases (such as B Oj) to form SiO (see Equation 9 [Sciti, 2008]), which is transient liquid above 1850 C. 

It is worthnoting that the inhibition of sintering in non-oxide ceramics is generally attributed to the presence of oxide impurities on the powder particle surface (Zou, 2011). In this respect, the above mentioned reactions help the removal of surface oxides, like B2O3, from the borides particle surfaces. 

Reaction between elemental boron and metal powder is the simplest route to produce boride powder. It can have excellent control on the stoichiometry of the resultant boride. Although this route results in pure ZrB /HfB but cannot be employed for industrial production because the starting materials used are expensive. This route can be exploited to get dense shapes of borides by hot pressing or spark plasma sintering of mixed powder (metal and boron), if the reaction with die is avoided (Tamburini et al., 2008). To avoid any reaction with the die, the graphite die is coated with boron nitride. 

Synthesis temperature affects the particle size of the boride powder which is extremely important for densification. Powders obtained at lower synthesis temperature have finer size and better sinterability. Particle size of boron carbide also affects the reaction temperature and finally the product quality. As boron carbide grain size increases, the oxide rednction processes and diffusional processes slow... 

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