Sintering borides

Owing to the high sintering temperatures employed, losses of material (by volatilization of boron or boride) and grain growth are observed. In order to limit these losses, the part to be sintered can be embedded in a powder of the same boride. Sintering of pure refractory borides requires > 0.7T, (Tj = absolute melting temper-... 

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. 

The crystal chemistry of the borides is discussed in 6.7.2 according to this scheme. General methods of preparation, single-crystal growth and sintering of borides is considered, respectively, in 6.7.3, 6.7.4 and 6.7.5. 

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. 

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 . 

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. 

Tables 1 to 5 summarize the sintering eharaeteristies of borides of groups III A to VIA transition metals.

Table 1. Sintering Characteristics of Borides of Rare-Earth Metals... 

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. 

The sintering of boride-metal composites cannot be developed here, although it allows obtaining fully dense parts. For a review, limited to MB —M pseudo-binary systems containing more than SO vol% boride and excluding infiltrated borides, see ref. 1, 6.7.5.1.4. 

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]. 

Thermal Evaporation The easiest way of evaporating metal is by means of resistance evaporators known commonly as boats . Boats, made of sintered ceramics, are positioned side by side at a distance of approximately 10 cm across the web width (Fig. 8.1). Titanium boride TiB2 is used as an electrically conductive material with boron nitride BN (two-component evaporator) or BN and aluminum nitride AIN (three-component evaporator) as an insulating material [2]. By combination of conductive and insulating materials, the electrical properties of evaporators are adjusted. 

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