Melting boron carbides

Boron carbide is resistant to most acids but is rapidly attacked by molten alkalies. It may be melted without decomposition in an atmosphere of carbon monoxide, but is slowly etched by hydrogen at 1200°C. It withstands metallic sodium fairly well at 500°C and steam at 300°C (8). 

Properties of the deposits Almost any material which can be melted is suitable for plasma spraying, giving a vast range of possible coatings of single or mixed metallic or non-metallic substances. It is often possible to produce types of coatings which are not obtainable in any other way. Typical of the materials which are plasma sprayed are copper, nickel, tantalum, molybdenum. Stellites, alumina, zirconia, tungsten and boron carbides, and stainless steels. 

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

When boron is heated to high temperatures with carbon, it forms boron carbide, B12C3, a solid with a high melting point that is almost as hard as diamond. The solid consists of B12 groups that are pinned together by C atoms. When boron is heated to white heat in ammonia, boron nitride, BN, is formed as a fluffy, slippery powder ... 

B4C boron carbide has a melting point of 2450 °C and a hardness somewhere between those of SiC and diamond. This makes the material a suitable abrasive. It is used in heads of sand blasting equipment, in mortars and in armour plating. For the latter application a B4C plate is provided on both sides with a plastic which has been reinforced with glass fibre. This is done to reduce the risk of splintering. Boron carbide is also used as the raw material for many other boron compounds ... 

Boron also forms important compounds with two other elements, carbon and nitrogen. Boron carbide (B4C) and boron nitride (BN) are important compounds because of their hardness. In fact, boron nitride may be the hardest substance known. Both compounds have very high melting points 4,262°F (2,350°C) for boron carbide and more than 5,432°F (3,000°C) for boron nitride. 

At temperatures above 2000°C, the structural stability and strength of the boronated graphite materials degrade. Boron carbide melts at 2140-2450°C and reacts with the graphite matrix. 

A small amount of Ar was trapped in the boron carbide preform. The number of moles was estimated from the ideal gas law using the melting temperature of the metal and the void volume in a typical boron carbide preform. 

The vaporization-cold-substrate condensation process of boron carbide melted by electron-gun heating and plasma torch deposition does not give a well-organized and crystalline B4C-type structure. In CH4 [1.33.10 Pa 1 torr)] an electron-gun evaporation of boron carbide contained in a graphite crucible, followed by a condensation on a water-cooled jacket, gives crystals. Sputtering by H is possible . 

Single crystals a few mm long are obtained by chemical vapor deposition (cf. 5.3.2.2.3), or by reduction of B2O3 by graphite in an electrical arc . A 6-mm diameter sintered boron carbide rod can be zone melted under Ar... 

A thermodynamic optimization of the system was performed by Domer (1982) [167]. This dataset was later refined by Lim and Lukas (1996) [36]. Due to additional crystallographic information concerning the extended homogeneity range of the boron carbide phase [152, 168] a further assessment was necessary [33, 34,169]. Data for the calculated invariant reactions are given in Table 13. Boron carbide of composition 16.4 at.% C melts congruently at 2731 K. 

Covalent carbides, which have giant-molecular structures, as in silicon carbide (SiC) and boron carbide (B4C3). These are hard high-melting solids. Other covalent compounds of carbon (CO2, CS2, CH4, etc.) have covalent molecules. 

Corrosion by Melts Hot-pressed boron carbide is attacked by fused alkalis. The use of boron-carbide-based materials at high temperatures is limited at present, which probably results from the fact that they have been investigated primarily at room or moderate temperature [101]. 

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