Near-nano WC–Co hardmetals: Will they substitute conventional coarse-grained mining grades?

Development of nanostructured hardmetals is a task of great importance. Nevertheless, in spite of some “euphoria” with respect to nanograined hardmetals, their potential application ranges are yet not clear. In some works, near-nano and nano hardmetals are believed can potentially substitute conventional medium- and coarse-grained WC–Co grades. In the present work near-nano hardmetals with WC mean grain size of nearly 200 nm and Co contents of 10–33 wt.% were produced and examined with respect to their hardness, fracture toughness, transverse rupture strength and wear-resistance. The near-nano hardmetal with 10% Co having a hardness of 20 GPa and fracture toughness of 9.5 MPa m1/2 is characterised by exceptionally high wear-resistance obtained by use of the ASTM B611 test in comparison with an ultra-fine grade with 10%. The wear-resistance of the near-nano hardmetals in the ASTM B611 test significantly decreases with increasing the Co content and the wear rates of the difference between the wear rates of the grades with 10% and 33% Co is equal to nearly 44 times. The near-nano hardmetals with 25%, 28% and 33% Co having a moderate hardness and high fracture toughness corresponding to conventional coarse and ultra-coarse-grained mining grades have a very low wear-resistance in laboratory tests on concrete-cutting, granite-cutting and percussion drilling of quartzite. A number of grades with the very similar hardness of 13 ± 0.2 GPa, WC mean grain sizes varying from 0.2 to 4.8 μm and Co contents varying from 3% to 25% were produced and examined by use of the ASTM B611 test. The wear-resistance of the near-nano grade with 25% Co is found to be lower by more than three times compared to the coarsest grade with 3% Co at almost the same hardness. In this case, in spite of the very similar hardness of all the samples, the proportion of the soft binder phase on the surface subjected to abrasive particles when performing the test is significantly higher for the near-nano grade compared to the coarse- and ultra-coarse grained hardmetals. Thus, near-nano and presumably nano hardmetals are expected to never substitute conventional medium- and coarse-grained mining grades. The only application range, where near-nano and nano hardmetals can potentially substitute conventional grades, is an application range of hardnesses of above 18 GPa.