Why carbides are hard

References P Sarker et al, Nat. Topics Carbides Materials Matter Metals. Related articles. Research Model performs reality check on adsorbents for carbon capture TZ Tool could save researchers time by assessing new materials from a variety of angles. Load more articles. No comments yet. You're not signed in. To link your comment to your profile, sign in now. Only registered users can comment on this article.

Sign in Register. Why is it called "ultra hard" alloy? That is because it is the hardest in metals. Then why is it the hardest in metals? All the metals we can see have shapes. They are a body and parts of a car, parts of a pencil case, and a screw. They all have the shapes according to applications. In giving a shape to a metal, the metal must be cut or shaved.

The saw or knife used in such a work is the cemented carbide. Therefore, cemented carbide must be the hardest in metals. In addition, the secret of the hardness is that cemented carbide contains hard substance and tough metals that bond the substance.

The hard subsistence is tungsten carbide WC , carbide of tungsten. Diamond made of carbon is as hard as diamond is, and the composite of tungsten and carbon is the second hardest.

Tough metals are cobalt Co and nickel Ni which are the families of iron. When the metal atoms are cubic-close-packed ccp , then filling all of the octahedral interstices with carbon achieves stoichiometry with the rock-salt structure. When the metal atoms are hexagonal-close-packed, hcp , since the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves stoichiometry. As a result of the packing, they are quite stable and have very high melting points and low electrical resistance.

In intermediate transition metal carbides, the transition-metal ion is smaller than the critical pm, and the structures are not interstitial but are more complex. Multiple stoichiometries are common. The best known is cementite, Fe 3 C, which is present in steels. These carbides are more reactive than the interstitial carbides; for example, the carbides of Cr, Mn, Fe, Co, and Ni are all hydrolyzed by dilute acids and sometimes by water to yield a mixture of hydrogen and hydrocarbons.

These compounds share features with both the inert interstitials and the more reactive salt-like carbides. Metal complexes containing C n fragments are well known. These molecular carbides often have carbon-centered clusters. Some metals such as tin and lead are not believed to form carbides.