Versatility and performance are the hard metal’s trump cards 

Whether car, aeroplane, wind turbine, tunnel, smartphone, diaper or glass bottle – every day we come into contact with large and small things, in the manufacture or construction of which cemented tungsten carbide (hereafter simply called tungsten carbide, after its main constituent) plays an important role.

Most people are unaware of this, but industry and professional users value the advantages of tungsten carbides. Whenever tools and components are exposed to extreme loads, the use of tungsten carbide is an obvious choice. The high hardness, wear resistance and toughness in combination with its many other high-performance properties that can be adjusted over a wide range make tungsten carbide the ideal material for a multitude of applications.

Currently, far more than one hundred different carbide grades are available to optimally address the requirements of a wide variety of applications. And the development of tungsten carbide has not yet ceased, as new challenges for tools and components as well as the opening of new fields of application always offer room for further improvements.

The microstructure of a WC-Co carbide

As with all hard metals, the hard WC phase gives the tungsten carbide its high hardness, hot hardness and wear resistance, while the binder metal ensures good toughness of the material. With an extremely high Young's modulus, carbide hardly deforms plastically during stress. This combination of properties alone makes tungsten carbides interesting for a variety of applications. However, the enormous versatility stems from the fact that the properties can be varied over a wide range of scales, which is why tungsten carbides can be used in very diverse areas of application - with high impact or bending loads as well as with high wear loads.

By far the most frequently used hard metals are those based on WC and Co, the tungsten carbide hard metals. They are not only used in metal cutting (ISO application group K), but also in products for wood and stone working as well as for many wear parts.

In addition to the simple WC-Co carbides, there are also those with mixed carbides which also contain titanium, tantalum or niobium carbides in addition to WC. They are used for steel cutting (ISO application group P) as well as for metal saws.

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A plethora of grades

The numerous carbide grades required for a wide variety of applications differ in three basic points: the mean WC grain size (α phase), the binder metal content (β phase) and the content of other alloy compounds (γ phase). These three parameters, in particular the WC grain size and the binder metal content, can be used to vary the material properties considerably.

Depending on the grade, the WC grains are on average less than 0.2 μm up to several micrometres in size and are enclosed by the binder metal. The fact that WC is excellently wetted by Co helps here.

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A unique material with many designable properties

The variation of hardness (wear resistance), transverse rupture strength and toughness of the carbide is thus affected by the choice of WC grain size, binder metal content and the addition of alloying components (various mixed carbides). These properties can be adjusted to optimally adapt the material to the respective application. To a certain extent, this is also possible with other materials. What makes carbide so unique, however, is the huge range in which the properties can be adapted (see figure 1).

Comparison of the wear resistance and toughness of various materials, including polycrystalline diamond (PCD) coatings, cubic boron nitride (cBN), oxide-based ceramics (ceramics (o)), nitride-based ceramics (ceramics (n)), ceramic-metal composite (cermet), high-speed steel (HSS) and hard metal (cemented carbide)

However, it is not only the hardness, transverse rupture strength and toughness that can be individually adjusted. The heat resistance, chemical resistance, thermal diffusivity, scour resistance, behaviour at temperature changes and other material properties can also be adapted to the respective application through the composition of a grade.

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The properties of tungsten carbide in relation to Co content and WC grain size

How then do the properties of the tungsten carbide depend on its composition? The graphic illustrations below show that the mechanical properties of the cemented tungsten carbide mainly depend on the binder content (Co) and the WC grain size. Hardness, i.e. wear resistance, increases inversely proportional to the fracture toughness. This means that the harder the material the more it reacts to notch tensions and impact stress (the ‘impact resistance’ parameter, which cannot be precisely defined, correlates with the fracture toughness of the material).

Plots comparing the hardness, transverse rupture strength and plane strain fracture toughness (KIc) of four grades of carbide with differing WC grain size as a function of cobalt content

On the other hand, the transverse rupture strength does not directly depend on the hardness but rather on the WC grain size and the cobalt content. The adhesive wear (tendency to stick), however, decreases with the grain size and the cobalt content of the carbide used. The list of the mentioned interdependencies, which could be extended at will for other wear and failure mechanisms, show that it is only possible to choose the correct carbide grade following a systematic procedure and/or based on experience with similar applications.