Tungsten Carbide Cobalt

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Tungsten carbide-cobalt (WC-Co) is a so-called hard metal which is characterised by an extraordinary hardness and wear resistance. Hard metals are mainly used in the production of tools for milling, drilling and pressing/punching processes. In addition surgical instruments are made of tungsten carbide-cobalt and the best-known product is the rotating balls used in ball point pens.

How can I come into contact with this material?

Tungsten carbide-cobalt carbide cutter © FhG IKTS

Tungsten carbide-cobalt carbide cutter © FhG IKTS

Tungsten carbide-cobalt is produced from tungsten carbide powder to which metallic cobalt is added. Humans mainly get in contact with tungsten carbide-cobalt particles when handling the powdered form or during the grinding of workpieces. These released (nano)particles can then be taken up through the skin or via inhalation. Protection measures for workers handling these materials are mandatory at the workplace and include the use of respiratory protection masks and the installation of suction exhaust pipes. Within the finished processed workpiece tungsten carbide-cobalt particles are rigidly bound and no nanoparticles are being released during normal usage preventing uptake by the human body.

Is there any risk from this material to humans and the environment?

Tungsten carbide-cobalt is a toxic substance regardless of the size of the particles. The toxic effects are caused by the percentage of cobalt within the material. Tungsten carbide-cobalt is classified as probably carcinogenic and can cause a lung disease known as “Hard metal disease” in exposed workers. Nowadays this disease occurs only rarely since various protection measures and regulations have been put into place when handling this material at the workplace. Users are safe when handling finished workpieces containing tungsten carbide-cobalt nanoparticles since there is no particle release from these products. Modern factories producing this material are obliged to have filter facilities installed to prevent an environmental release of these materials. All waste products are collected and disposed of as hazardous waste.

Conclusion

In general, the human body as well as the environment are only exposed to very small amounts of tungsten carbide-cobalt nanoparticles. Therefore only a very low risk has been associated with this material.

By the way…

  • Hard metals such as tungsten carbide-cobalt are synthetic materials and not found in nature.
  • Hard metals are generally recycled to a large extent.
  • Tungsten carbide-cobalt is also used in armour-piercing ammunition.

Properties and Applications

The hardness of tungsten carbide cobalt (WC-Co) is influenced by WC grain size and cobalt content. © Fraunhofer IKTS.

The hardness of tungsten carbide cobalt (WC-Co) is influenced by WC grain size and cobalt content. © Fraunhofer IKTS.

Tungsten carbide cobalt (WC-CO) is an alloy of a hard ceramic phase, the tungsten carbide (WC) and a ductile metallic phase, the cobalt (Co).

Important properties of WC-Co are hardness, strength, high breaking and beating ductility as well as high electrical and thermal conductivity. By varying the metallic cobalt content and the tungsten carbide grain size, important properties of the alloy can be specifically adapted. By varying the cobalt content between 0 and 20 mass per cent, hard metals can cover a hardness range from tempered steel to super-hard alloys. The smaller the carbide grain the harder an object can penetrate the surface and the higher is its hardness.

From hard metals, mainly tools are produced for chipping (turning, milling, drilling) or non-chipping shaping (drawing, rolling, spinning), as well as for cutting (stamping) or breaking of metals, wood, paper, plastic, stone, and coal. Further, surgery equipment is produced from WC-Co.

Denotation of different grain sizes. © Fraunhofer IKTS.

Denotation of different grain sizes. © Fraunhofer IKTS.

Likewise, different technical wear-resistant parts like nozzles or reinforcements (e.g. for mills) are made of WC-Co. Well-known products include hard metal driller or hard metal coated circular saw blades for hobby craftsmen as well as the small balls in ball pens.

For hard metal grain sizes below 1 µm, exceptional combinations of hardness and strength are achievable, which makes such alloys interesting for the production of hard materials and very fine tools.

In 1999, the working group "Hard metals" of the German industrial union for powder metallurgy suggested a classification of WC-Co hard metals according to grain size, which is today accepted worldwide.

Tungsten carbide cobalt is not self-igniting. As a mixture with air (dust) under the influence of an ignition source, tungsten carbide is possibly flammable (dust explosion).

Occurence and Production

Hard metals such as WC-Co are non-natural materials not occurring in nature. They are produced by mixing carbide and metal powders, a forming step and a thermal treatment (sintering).

Ground breaking for the development of hard metal industries was the work of the Osram Studiengesellschaft in Berlin and the patent filed in 1923 by Karl Schröter, which was bought by the Krupp company later on. As soon as 1926, Krupp introduced the first alloy under the name WIDIA to the market. WIDIA is short for “wie Diamant” (German for `diamond-like`) and refers to the extraordinary hardness of the material. Nowadays, large production facilities are located in China, the USA, Sweden, Israel, Luxembourg, Austria and Germany.

After usage and at the end of life cycle hard metals are being recycled to a large extent. In case of scrap containing WC-Co it can be either fully processed and re-enter the hard metal production or used for the production of various steel alloys.


Literature

  1. Kieffer, R & Benesovsky, F (1963). Hartstoffe, Springer-Verlag, Wien.
  2. Schedler, W (1988). Hartmetall für den Praktiker, VDI-Verlag GmbH, Düsseldorf.
  3. Richter, V & Mueller, K (2007). In Schatt,Wieters, Kieback: Pulvermetallurgie,Technologien und Werkstoffe, Springer Verlag, 2. erweiterte Auflage. ISBN 978-3-540-23652-8.
  4. Design for PM - E-Learning Kurs (EN). Hartmetals of the EPMA, Moseley, S & Richter, V. (last access date: Apr 2011).
  5. Casmet Supply Limited (EN) (Apr 2006). Material Safty Data Sheet for "Sintered Tungsten Carbide with Cobalt Binder".

General Hazards

However, this disease pattern only occurs, when workers are exposed to tungsten carbide-cobalt (WC-Co), not to tungsten carbide or cobalt alone. If exposed to only one of the compounds, no lung diseases were observed. The “hard-metal-lung” is an approved occupational disease in Germany, which very rarely occurs due to extensive work protection measures.

Besides the occurrence of lung diseases, a possible link of WC-Co exposure with an increased rate of childhood leukaemia in Fallon (Nevada, USA) has been discussed, since the temporal occurrence of the diseases coincided well with an increase in airborne tungsten and cobalt levels [3]. However, this potential link has been controversially discussed and viruses as alterniative cause for the enhanced leukaemia rate have been suggested.


Literature

  1. Moulin, JJ et al. (1998), Am J Epidemiol, 148(3): 241-248.
  2. Lison, D (1996), Crit Rev Toxicol, 26(6): 585-616.
  3. Sheppard, PR et al. (2007), Environ Health Perspect, 115(5): 715-719.

Studies on Living Organisms – in vivo

To minimise exposure, suitable protective clothes like gloves and masks should be worn. For the users of tools made of WC-Co an exposure to the particles is unlikely, since the sintered tools are extraordinary hard and wear-resistant. Hence, new release of particles during usage of drillers or others tolls is not relevant.

A mixture of WC and cobalt, WC-Co particles, exhibit a higher toxicity than cobalt alone. Cobalt is a compound of known human toxicity [1] and a daily dose of 20 mg and above may induce severe health problems. Hence, the International Agency for Research on Cancer (IARC) classified cobalt alone as "possibly carcinogenic" to humans and the combination of tungsten carbide and cobalt, WC-Co, as "probably carcinogenic" to humans [2]. An mechanism called "contact-corrosion" between cobalt and WC is assumed as a reason for this enhanced toxicity, inducing the formation of free radicals in the presence of oxygen, which in turn may damage cells of the body (mostly lung cells) [3]. Alternative or additionally, particles may act as a "trojan horse" and increase the availability of cobalt in the cells.

In the small town of Fallon (Nevada, USA) human exposure by airborne WC and Co particles has been reported. A hard metal facility located in the town or a close by military base (WC-Co is also used for ammunition) have been discussed as potential sources for WC-Co release [4]. Modern hard metal facilities are equipped with filter systems which prevent particle release.


Literature

  1. ATSDR TOX FAQs (Apr 2004): Cobalt. Agency for Toxic Substances and Disease Registry, U.S. Public Health Service(ATSDR).
  2. IARC (2006), IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, No. 86:Cobalt in Hard-metals and Cobalt Sulfate, Gallium Arsenide, Indium Phosphide and Vanadium Pentoxide.
  3. Gries, B et al. (2007), Euro PM 2007, Powder Metallurgy World Congress & Exhibition, 2007 Oct 15-17; Toulouse (France) Vol. 1, 189-196.
  4. Sheppard, PR et al. (2007), Environ Health Perspect, 115(5): 715-719.

Studies Outside of Organisms – in vitro

Many of the results described here on WC-Co nanoparticles were obtained by the project INOS, a research project funded by the German Ministry of Education and Research (BMBF).

In in vitro assays, the enhanced toxicity of WC-Co is marked by a decrease in metabolic activity of cells, damage of the cell membrane and in some studies also with DNA damage [1,2]. In a study on various human cell lines originating from lung, liver, intestine and skin, nano-sized WC-Co showed similar results, but cells differed in their sensitivity.

Which genes are influenced by WC-Co particles was investigated on human skin cells. Such genes are able to unravel which functions of an organism may be influenced by particle uptake. The majority of influenced genes can be associated to cobalt exposure, and the observed effects were all known cobalt-effects; no new mechanisms of cobalt were found [3].


Literature

  1. Bastian, S et al. (2009), Environ Health Perspect, 117(4): 530-536.
  2. Anard, D et al. (1997), Carcinogenesis, 18(1): 177-184.
  3. Busch, W et al. (2010), BMC Genomics, 11(1): 65.

Until the 1950s, environmental protection was barely considered and WC and Co were released to the environment via air or water during the production process. Today, a release of hard metal particles into the environment is strictly forbidden. Modern hard metal facilities are equipped with filter systems, which significantly reduce the release of airborne particles. The waste is collected and recycled or deposited as hazardous waste. Theoretically, WC-Co particles might be released by accidents e.g. during powder transport. However, up to now such accidences have not been reported.

In the vicinity of Fallon (Nevada, USA) an enrichment of tungsten and cobalt in air and water as well as in lichen and wood has been detected [1]. A hard metal facility in the town or a close by military base are discussed as potential source.


Literature

  1. Sheppard, PR et al. (2007), Environ Health Perspect, 115(5): 715-719.

Uptake via the Lung - Inhalation

Results obtained by in vitro assays indicate an uptake of nano-sized WC-Co particles into lung cells. WC-Co nanoparticles were never detected in the cell nuclei [1] .


Literature

  1. Bastian, S et al. (2009), Environ Health Perspect, 117(4): 530-536.

Uptake via the Skin - Dermal Uptake

Results obtained by in vitro assays indicate an uptake of nano-sized WC-Co particles into cultured skin cells. WC-Co nanoparticles were never detected in the cell nuclei [1].


Literature

  1. Bastian, S et al. (2009), Environ Health Perspect, 117(4): 530-536.

Uptake via the Gastro-Intestinal Tract

Results obtained by in vitro assays indicate an uptake of nano-sized WC-Co particles into cells of the large intestine. WC-Co nanoparticles were never detected in the cell nuclei [1].


Literature

  1. Bastian, S et al. (2009), Environ Health Perspect, 117(4): 530-536.

An accumulation of tungsten and cobalt in lichen and trees was observed, but there is no information on possible biological effects available from this study [1]. Nevertheless, this illustrates a particle uptake by these organisms via air or water.

Investigations using rainbow trout gill cell cultures indicated an enhanced toxicity of WC-Co compared to WC alone [2]. Similar results have been previously shown in studies using human cells. The enhanced toxicity is likely to be caused by the cobalt fraction. Cobalt metal is a well-known toxicant and an additional increase in toxicity is due to the chemical interactions resulting from the mixing with WC. Cobalt alone is classified as hazardous for the environment and for aquatic organisms [3]. Hence, there are strict regulations for the handling and application of cobalt to keep its bioavailability limited, e.g. discharge via the waste water is forbidden. These regulations also apply for WC-Co.

An uptake of WC-Co nanoparticles into cells, but not in the nuclei of cells was demonstrated. Laboratory studies on the behaviour of particles in liquids indicated an agglomeration of nanoparticles into bigger assemblies, which may also occur in surface waters. Laboratory experiments demonstrated that WC-Co toxicity is not prevented by agglomeration. Even though particle agglomerates are bigger then single particles they were taken up by the cells.

To date, there are no data available on the effects of WC nanoparticles towards additional test organisms, such as algae or bacteria.


Literature

  1. Sheppard, PR et al. (2007), Environ Health Perspect, 115(5): 715-719.
  2. Kuehnel, D et al. (2009), Aquat Toxicol, 93(2-3): 91-99.
  3. ATSDR TOX FAQs (Apr 2004): Cobalt. Agency for Toxic Substances and Disease Registry, U.S. Public Health Service(ATSDR).

Behaviour inside the Body

For hard metal particles investigations on micro-size particles have been conducted for comparison. According to investigations conducted up to now, the assumption of differing modes of actions of nanoparticles compared to micro-sized material due to their small size has not been confirmed. The comparison of the biological effects showed merely differences in the effect strength [1]. WC-Co particles are likewise toxic, but exposure is nearly exclusively occupational and can be avoided by suitable protection clothes.


Literature

  1. Ding, M et al. (2009), Toxicol Appl Pharmacol, 241(3): 260-268.

Behaviour of Uptake in somatic cells

Micro-scale WC-Co particles were observed in macrophages and nano-scale particles in different vertebrate cell lines [1,2].


Literature>

  1. Lison, D et al. (1990), Environ Res, 52(2): 187-198.
  2. Busch, W et al. (2010), BMC Genomics, 11(1): 65.

To date, there are no data available regarding the environmental behaviour of tungsten carbide-cobalt (WC-Co) nanoparticles.

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