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Tungsten carbide is a compound of tungsten and carbon atoms. It is used alone or with up to 20 percent of other metals to harden cast iron, to cut edges of saws and drills, alongside others. In its most simple form, the compound is a fine gray powder, but it can be pressed and shaped into various shapes.
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Key Terms: Tungsten Carbide, Heavy Stone, Tungsten Trioxide, Tungsten Hexachloride, Decomposition
What is Tungsten Carbide?
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Chemically, tungsten carbide, symbolized as WC, is a compound of tungsten and carbon atoms. Due to its stiffness and density, tungsten carbide is similar in hardness to corundum and can only be polished and finished using superior hardness abrasives. Its hardness is comparable to that of lead and gold. Historically known as Wolfram/Wolf Rahm, it was later carburized and cemented with a binder to create tungsten carbide, which translates to "heavy stone".
Tungsten Carbide Structure
WC is available in hexagonal and cubic high-temperature forms, including one with a rock salt structure. In fact, the hexagonal structure is composed of metal atoms arranged in layers that are stacked directly on top of one another, with the carbon atoms filling half the interstices, giving tungsten and carbon their normal six-coordinate trigonal prism structures.
In a hexagonally packed layer, the bond lengths between tungsten atoms are 291 pm, and the shortest distance between tungsten atoms in neighboring layers is 284 pm. The tungsten carbon bond length is 220 pm, according to the unit cell dimensions. Thus, the tungsten-carbon bond length is equivalent to the single bond in W(CH3)6 (218 pm), where tungsten is highly twisted trigonal prismatically coordinated.
Also Read: Versatile Nature of Carbon
Synthesis of Tungsten Carbide
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Tungsten carbide can be synthesized by using,
- From tungsten metal and carbon
- The tungsten metal is reacted with carbon at temperatures between 1400 and 2000ºC to create tungsten carbide.
- The tungsten metal can also be reacted with CO/CO2 mixture and H2 between 900 and 1200ºC.
- From Tungsten trioxide
- WO3 is reacted with graphite at 900ºC, directly or in the presence of hydrogen at 670ºC, followed by carburization in argon at 1000ºC.
- The WO3 can also be reacted with CO/CO2 mixture and H2 between 900 and 1200ºC.
- From Tungsten hexachloride
- WCl6 is reacted with methane CH4 and hydrogen gas at 670 ºC.
- From Tungsten hexafluoride
- WF6 is reacted with methanol CH3OHand hydrogen gas at 350 ºC.
Tungsten Carbide
Physical Properties of Tungsten Carbide
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The physical properties of tungsten carbide are:
| Property | Details |
|---|---|
| Melting point | Tungsten carbide has a melting point of 2870ºC. |
| Boiling point | Tungsten carbide has a boiling point of 6000ºC. |
| Density | 15.63 g/cm3 |
| Solubility | It is insoluble in water |
| Thermal conductivity | Tungsten carbide has a thermal conductivity of 110 Wm-1K-1 at 1 atm. |
| Thermal expansion | The thermal expansion coefficient of tungsten carbide is 5.5 µm-1K-1 |
| Hardness | The hardness of tungsten carbide is represented by Mohs hardness (9 to 9.5) and Vickers’s harness (2600) |
| Electrical resistivity | Tungsten carbide has low electrical resistivity, of 0.2m |
Chemical Properties of Tungsten Carbide
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The chemical properties of Tungsten carbide are:
| Parameter | Description |
|---|---|
| Other forms | W2C and WC are two well-characterized tungsten and carbon compounds. Both compounds are present in coatings, with the proportion varying according to the coating method. It is also possible to create tungsten and carbon meta-stable compounds by heating WC to high temperatures using plasma, then quenching in an inert gas (plasma spheroidization). During this process, WC particles become spheroidized and form WC1-x which exists in a meta-stable form at room temperature. |
| Decomposition | When heated, WC decomposes into tungsten and carbon, such as when used in high-velocity oxygen fuel (HVOF) and high energy plasma (HEP) processes. |
| Oxidation | The WC oxidizes at 500-600 °C (773-873 K). Aqueous hydrogen peroxide solutions readily oxidize finely ground WC. |
| Resistance | A hydrofluoric acid/nitric acid (HF/HNO3) mixture above room temperature is the only acid that attacks it. |
| Reactivity | During high temperatures and pressures, it reacts with aqueous sodium carbonate to produce sodium tungstate, a procedure used for recovering scrap cement. It reacts with fluorine and chlorine above 400 °C (673 K) and is unreactive with dry H2 up to its melting point. |
Also Check: Chemical Bonding
Toxicity of Tungsten Carbide
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The primary health risk for tungsten carbide is exposure to dust, which can cause silicosis-like pulmonary fibrosis. The American National Toxicology Program has also determined that cobalt-cemented tungsten carbide is a potential cancer-causing agent.
In Silicosis, tiny pieces of silica, a mineral that occurs in many rocks such as sand, quartz, and granite, are inhaled into the lungs, causing pulmonary fibrosis. Silicosis is most frequently found among workers in construction and mining.
Applications of Tungsten Carbide
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- Sintered tungsten carbide is often used to make a cutting tool. These cutting tools are extremely hard, abrasion-resistant, and can withstand high temperatures, unlike high-speed steel tools.
- Armor-piercing ammunition commonly uses tungsten carbide, either from monolithic sintered material or from tungsten carbide cobalt composite materials, particularly where depleted uranium is unavailable or politically unacceptable.
- Tungsten carbide is used for a variety of applications such as top hammer rock drill bits, downhole hammers, roller cutters, shearer picks, raise boring reamers, and tunnel boring machines.
- To mount the shoes, carbide-tipped hoof nails can be used, which increase traction on slick surfaces like roads or ice. Tungsten carbide is used in farriery and horseshoeing.
- A surgical device made from tungsten carbide is used in open surgery (scissors, forceps, hemostats, and so on) or laparoscopic surgery (graspers, scissors/cutter, needle holders, cauteries, and so on).
Things to Remember
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- Tungsten carbide, symbolized as WC, is a compound of tungsten and carbon atoms. Due to its stiffness and density, tungsten carbide is similar in hardness to corundum and can only be polished and finished using superior hardness abrasives.
- Tungsten carbide can be synthesized using 4 methods, tungsten and carbon, tungsten trioxide, tungsten hexachloride, and tungsten hexafluoride.
- In a W-C-Co system, nickel and cobalt readily wet tungsten carbide. Cobalt and WC show a pseudo-binary eutectic phase.
- Sintering of tungsten carbide has been observed to produce abnormal grain growth when molten phases, such as cobalt, are present.
- The primary health risk for tungsten carbide is exposure to dust, which can cause silicosis-like pulmonary fibrosis.
Sample Questions
Ques. What is tungsten carbide? Describe its structure. (3 marks)
Ans. Tungsten carbide, symbolized as WC, is a compound of tungsten and carbon atoms. It is extremely stiff and hard. Due to its stiffness and density, tungsten carbide is similar in hardness to corundum and can only be polished and finished using superior hardness abrasives. Its hardness is comparable to that of lead and gold.
It’s available in a hexagonal structure composed of metal atoms arranged in layers that are stacked directly on top of one another, with the carbon atoms filling half the interstices, giving tungsten and carbon their normal six-coordinate trigonal prism structures. The bond lengths between tungsten atoms are 291 pm, and the shortest distance between tungsten atoms in neighboring layers is 284 pm.
Ques. How to synthesize tungsten carbide from tungsten metal and tungsten trioxide? (2 marks)
Ans. The tungsten metal is reacted with carbon at temperatures between 1400 and 2000ºC to create tungsten carbide. The tungsten metal can also be reacted with CO/CO2 mixture and H2 between 900 and 1200ºC. Tungsten oxide, WO3 is reacted with graphite at 900ºC, directly or in the presence of hydrogen at 670ºC, followed by carburization in argon at 1000ºC.
Ques. How to synthesize tungsten carbide from tungsten halides? (2 marks)
Ans. The tungsten halides commonly used are tungsten hexachloride and tungsten hexafluoride. WCl6 is reacted with methane CH4 and hydrogen gas at 670 ºC, and WF6 is reacted with methanol CH3OHand hydrogen gas at 350 ºC to produce tungsten carbide.
Ques. Write a short note on the uses of tungsten carbide. (2 marks)
Ans. Tungsten carbide is used for a variety of applications such as top hammer rock drill bits, downhole hammers, roller cutters, plow chisels, shearer picks, raise boring reamers, and tunnel boring machines. It is most commonly found as an insert in a steel matrix.
Ques. Write a short note on the physical properties of tungsten carbide. (2 marks)
Ans. Tungsten carbide has a melting point of 2870ºC, a boiling point of 6000ºC, and a density of 15.63 g/cm3. It is insoluble in water. Tungsten carbide has a thermal conductivity of 110 Wm-1K-1 at 1 atm. The thermal expansion coefficient of tungsten carbide is 5.5 µm-1K-1. The hardness of tungsten carbide is represented by Mohs hardness (9 to 9.5) and Vickers’s harness (2600).
Ques. What other forms of tungsten carbide are available? (3 marks)
Ans. W2C and WC are two well-characterized tungsten and carbon compounds. Both compounds are present in coatings, with the proportion varying according to the coating method. It is also possible to create tungsten and carbon meta-stable compounds by heating WC to high temperatures using plasma, then quenching in an inert gas (plasma spheroidization). During this process, WC particles become spheroidized and form WC1-x which exists in a meta-stable form at room temperature.
Ques. What kind of reactions does tungsten carbide undergo? (5 marks)
Ans.
- When heated, WC decomposes into tungsten and carbon, such as when used in high-velocity oxygen fuel (HVOF) and high energy plasma (HEP) processes. The WC oxidizes at 500-600 °C (773-873 K).
- Aqueous hydrogen peroxide solutions readily oxidize finely ground WC. A hydrofluoric acid/nitric acid (HF/HNO3) mixture above room temperature is the only acid that attacks it.
- During high temperatures and pressures, it reacts with aqueous sodium carbonate to produce sodium tungstate, a procedure used for recovering scrap cement.
- It reacts with fluorine and chlorine above 400 °C (673 K) and is unreactive with dry H2 up to its melting point.
Ques. Write a short note on tungsten carbide poisoning. (2 marks)
Ans. The primary health risk for tungsten carbide is exposure to dust, which can cause silicosis-like pulmonary fibrosis. In Silicosis, tiny pieces of silica, a mineral that occurs in many rocks such as sand, quartz, and granite, are inhaled into the lungs, causing fibrosis of the lungs. Silicosis is most frequently found among workers in construction and mining. The American National Toxicology Program has also determined that cobalt-cemented tungsten carbide is a potential cancer-causing agent.
Ques. What are tungsten carbide rings? (2 marks)
Ans. In recent years, tungsten carbide has become a popular material for bridal jewelry. Despite having a high impact resistance, its extreme hardness can cause it to break under certain conditions. In spite of its high impact resistance, tungsten rings can be broken under certain conditions due to their extreme hardness. Some people think this is a benefit since the rings crumble in an impact, whereas precious metal rings would bend flat and need to be cut.
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