Carbide

About: Carbide is a research topic. Over the lifetime, 36331 publications have been published within this topic receiving 503586 citations.

Stoichiometric limits of carbon-rich boron carbide phases

Abstract: The stoichiometric boron-to-carbon ratios of various carbon-rich boron carbide phases were investigated. Many powders or sintered pellets were analysed and the total boron, total carbon, free-boron and free-carbon concentrations were measured for each specimen before calculating the stoichiometric ratio. The methods described were applied to a large number of analyses and the standard deviation of the results was evaluated. The stoichiometric boron-to-carbon ratio for carbon-rich boron carbide phases was found to depend on the synthesis conditions. Boron dissolves up to 21.6 at.% C at low temperatures and up to 24.3 at.% C near the melting point. Free carbon is deposited as the result of a solid state phase transformation. The stoichiometry of boron carbide prepared by the magnesothermal reaction is B 3.63 C (approximately B 11 C 3 ). Boron carbide produced by arc melting boron oxide and carbon is a solid solution. This single carbide melts congruently at a temperature of approximately 2450°C with a carbon content of 18.4 at.% (B 4.45 C).

The crystal structure of a boron-rich boron carbide

Abstract: Structural changes accompanying extreme boron-enrichment of boron carbide were deduced with Mo K0¢ X-ray diffraction data obtained from a twinned aggregate containing a minimal amount of carbon. The crystal system remains rhombohedral, probable space group R-3m, as for B4C, but unit-cell parameters show significant expansions compared to those of a 20 at. % C material. Fourier, difference Fourier, and least-squares calculations produced a final weighted R value of 9"04 % and a al of 0"985 for a structure that retains (BI~) icosahedra and randomly replaces about one fourth of the [CBC] chains with [B4] groups. Terminal atoms of the latter are fixed along a threefold axis and have fivefold coordination, being bonded to two central (or bridge) atoms of the group and to three icosahedral atoms. Bridge atoms lie in two of twelve equivalent positions near a plane normal to the threefold axis; they also have fivefold coordination, being bonded to the two terminal atoms of the group and to three icosahedral atoms. The chemical composition of the proposed structure, 9-7 at.% C, agrees with published phase diagrams, but exceeds the analyzed carbon contents of 4.8 at. % C (bulk chemical analysis) and 8 at. % C (ion microprobe analysis). Possible reasons for the location of the boron-rich terminus of the boron carbide phase field are discussed in view of the proposed structure.

Microstructural characterization of ?REFEL? (reaction-bonded) silicon carbides

Abstract: Quantitative characterization of the microstructure of a number of samples of reactionbonded (REFEL) silicon carbide has been undertaken employing transmission and scanning electron microscopy, optical microscopy, and electron and X-ray diffraction techniques. Impurity-controlled secondary electron SEM image contrast has proved particularly useful in differentiating between the SiC present in the initial compact and that formed during the reaction-bonding process, and, in contrast to previous descriptions of the microstructure, it has been found that the newly-formed SiC is deposited from the supersaturated solution of carbon in molten silicon both epitaxially on the original SiC grains, maintaining the sameα-polytypic stacking sequences, and by nucleation of fine cubicβ-SiC elsewhere. The relative quantities of material occurring by these two mechanisms have been found to vary from sample to sample, although the epitaxial growth on the original grains always occurs to some extent and is responsible for the bulk cohesion of the material. Some conclusions have been drawn concerning the reaction model and the process parameters controlling the microstructure of this type of material.

Machinability study of pure aluminium and Al–12% Si alloys against uncoated and coated carbide inserts

Abstract: An investigation has been undertaken to study the compatibility of cutting materials in dry machining of aluminium and Al–Si alloys. Mono or multilayer coated carbide tools with a top coating of TiC, TiN, TiAlN, Al2O3, TiB2, MoS2 etc. on WC–Co inserts already made a major breakthrough in dry machining of ferrous materials. But in contrast dry machining of aluminium and Al-alloys is a great challenge. But wide application of aluminium different parts has increased the need to find out the correct cutting tool. Experimental results of turning test, SEM pictures and chip morphology investigation of the cutting tool after machining clearly reveals the inefficiency of TiC, TiN, TiB2, Al2O3, and AlON in dry machining of aluminium. This is because of the formation of very large amount of metal built-up in both rake and flank surface leading to high magnitude of cutting forces and high roughness of the work-piece during machining. The natural diamond and polycrystalline diamond (PCD) can be used as a cutting tool, when the required shape is attached on the edge/tip for machining non-ferrous materials. But both of them are limited for finishing cut because of high cost. So CVD diamond coated tool is a better option to machine these materials. CVD diamond coated tool was free from built-up edge formation leading to clean cut, low magnitude of cutting forces and improved surface finish of the work-piece. However, performances of the diamond tool depend mainly on adhesion of the diamond coating with the carbide substrate.