Boron Carbide Hardness and Fracture Toughness

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Boron carbide is a material with a Mohs hardness of 9.49, making it the third hardest material after diamond and cubic boron nitride. It is also one of the most wear-resistant materials known, making it a good choice for industrial and military applications.

Characteristics of boron carbide

Unlike most crystalline metals, which can withstand very high temperatures without breaking, boron carbide has an extremely low melting point of 2763 deg C (5005 deg F). Its density is 2.52 g/cm3.

Complex crystal structure

Boron carbide’s crystal structure is based on B12 icosahedra in a rhombohedral unit cell, around a bridging three-atom chain, and is similar to the B11C icosahedral boron (space group: R3m, lattice constants: a = 0.56 nm and c = 1.212 nm). All carbon atoms bridge the adjacent icosahedra and the bridging carbon atoms form a network plane running parallel to the c-plane, forming a layered structure.

Susceptibility to amorphization

Shear-driven amorphization of boron carbide occurs as a result of interactions between the C atoms in the polar sites of the icosahedra and the B atoms in the chains, which unzip the icosahedra and destroy the crystallinity. Molecular dynamics simulations suggest that amorphization of boron carbide is driven by amorphous shear bands that coalesce into microcracks, which can lead to the crack opening.

The amorphous shear bands are accompanied by a low fracture toughness, which limits the armor’s ability to resist high-velocity projectiles in long-rod penetrator tests. Therefore, there is a need for boron carbide to be improved by developing methods to increase densification and improve its fracture toughness.