Borides powder

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. 

In order to obtain dense products, fine powders must be used, i.e., with a mean particle size of 0.1-10 u.m, or preferably 0.1-5 /rm, achieved by milling the raw boride powder. 

The compressibility of group-IVA and -VIA transition-metal boride powders is measured by the dimensions and weights of the blanks, by measuring the stroke of the punches with a cathetometer, or alternatively by electrical conductivity (based upon the metallic conductivity of most borides). The process of densiheation by pressing is defined by ... 

When electrical conductivity is used to investigate the compressibility of boride powders at a pressure of 0.19-1.7 X 10 N m , the variation of the specific electrical conductivity x with the relative density 0 (apparent density of the blank/density of the bulk material) is ... 

The stability of borides, as the fact that so many exist attests, is a reason for their not appearing as components in cermet systems few, if any, metal matrices will not react with boride powders or fibers to form ternaries that lead to matrix degradation. Indeed, when boron fibers are to be used they have to be coated, usually with carbon, to prevent matrix interactions. 

When heated in vacuum to 700°C, the sodium chloride evaporates, leaving the crystalline boride powder behind. They have also made SiC and B4C by this method. 

Kislyi, P. S., Kuzenkova, M. A. (1965). Gas-impermeable protective thermocouple sheaths from zirconium boride. Powder Metallurgy and Metal Ceramics, 4,23-26. doi 10.1007/BF00773783. 

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. 

This route is the most preferred for synthesis of diborides, as it involves relatively less expensive charge material. Boride powder is prepared by reaction (1) at a temperature of higher than 1800°C and holding time of more than 1 hour (Sonber etal., 2010 2011). 

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... 

Sintering additives may be ceramics or metals. They can either form a liquid phase at the sintering temperature (liquid phase sintering) or may react with and remove oxidized surface film from boride powder particles inducing a solid state sintering densification. 

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