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Chromium carbide

Chromium carbide
Cr3C2structure.jpg
Chromium carbide Cr3C2.JPG
Names
IUPAC name
Chromium(II) carbide
Identifiers
12012-35-0 YesY
3D model (Jmol) Interactive image
ChemSpider 21171152 YesY
ECHA InfoCard 100.031.420
PubChem 3650773
Properties
Cr3C2
Molar mass 180.009 g/mol
Appearance gray orthorhombic crystals
Density 6.68 g/cm3
Melting point 1,895 °C (3,443 °F; 2,168 K)
Boiling point 3,800 °C (6,870 °F; 4,070 K)
reacts
Structure
Orthorhombic, oP20
Pnma, No. 62
Hazards
NFPA 704
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g., diesel fuel Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
US health exposure limits (NIOSH):
PEL (Permissible)
 TWA 1 mg/m3
REL (Recommended)
 TWA 0.5 mg/m3
IDLH (Immediate danger)
 250 mg/m3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY  (what is YesYN ?)
Infobox references

Chromium carbide is a ceramic compound that exists in several different chemical compositions: Cr3C2, Cr7C3,and Cr23C6. At standard conditions it exists as a gray solid. It is extremely hard and corrosion resistant. It is also a refractory compound, which means that it retains its strength at high temperatures as well. These properties make it useful as an additive to metal alloys. When chromium carbide crystals are integrated into the surface of a metal it improves the wear resistance and corrosion resistance of the metal, and maintains these properties at elevated temperatures. The hardest and most commonly used composition for this purpose is Cr3C2.

Related minerals include tongbaite and isovite, (Cr,Fe)23C6, both extremely rare. Yet another chromium-rich carbide mineral is yarlongite, Cr4Fe4NiC4.

There are three different crystal structures for chromium carbide corresponding to the three different chemical compositions. Cr23C6 has a cubic crystal structure and a Vickers hardness of 976 kg/mm2. Cr7C3 has a hexagonal crystal structure and a microhardness of 1336 kg/mm2. Cr3C2 is the most durable of the three compositions, and has an orthorhombic crystal structure with a microhardness of 2280 kg/mm2. For this reason Cr3C2 is the primary form of chromium carbide used in surface treatment.

Synthesis of chromium carbide can be achieved through mechanical alloying. In this type of process metallic chromium and pure carbon in the form of graphite are loaded into a ball mill and ground into a fine powder. After the components have been ground they are pressed into a pellet and subjected to hot isostatic pressing. Hot isostatic pressing utilizes an inert gas, primarily argon, in a sealed oven. This pressurized gas applies pressure to the sample from all directions while the oven is heated. The heat and pressure cause the graphite and metallic chromium to react with one another and form chromium carbide. Decreasing the percentage of carbon content in the initial mixture results in an increase in the yield of the Cr7C3, and Cr23C6 forms of chromium carbide.


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