How to… maintain superhard strength

Materials World magazine
,
7 Aug 2019

Learn how cobalt binders can improve material strength. 

The discovery of tungsten carbide-cobalt (WC-Co) metal-ceramic composites in the 1920s was an important technological revolution last century. Such functional hard materials have a combination of strength and fracture toughness, and hardness and wear-resistance. This comes as a result of combining a hard ceramic phase WC with a ductile and tough Co binder. Functional hard and superhard materials with an improved combination of hardness, toughness and wear-resistance are needed in numerous applications. But hardness and wear-resistance are contradictory and incompatible properties compared with toughness, so that in conventional hard and superhard materials, an increase of hardness and wear-resistance can be achieved only at the expense of fracture toughness.

Finding balance

Technology, such as cemented carbides, can be used to enhance the cobalt binder matrix. The hierarchical structure of such cemented carbides comprises ultra-coarse WC grains with a mean grain size of about 5µm and a nanostructured Co-based binder phase, reinforced and hardened by hard nanoparticles of 2-6nm in size. The nanoparticles consist of a metastable WC-Co phase with the cubic crystal lattice of the copper and gold alloy auricupride (Cu3Au) lattice type.

The combination of an ultra-coarse-grained microstructure structured on the µm-level and the binder phase structured on the nm-level has been found to provide a combination of transverse rupture strength and fracture toughness, and hardness and wear-resistance. For example, a novel cemented carbide grade for the construction industry is characterised by a transverse rupture strength of about 2,500MPa, which is nearly 20% higher than that of a conventional carbide grade, and a nearly threefold increased wear-resistance resulting in its revolutionary improved performance properties.

Nanostructured materials with 100nm grain sizes are becoming common for many applications. The trend of employing finer WC powders in the cemented carbide industry with the objective of entering the region of nanomaterials has resulted in nanostructured cemented carbides, or so-called near-nano cemented carbides with a WC mean grain size of about 150nm.

The near-nano cemented carbides are characterised by a significantly improved combination of hardness, wear-resistance and fracture toughness in comparison with conventional ultra-fine carbide grades. The wear-resistance of the novel near-nano carbides is high due to their high hardness and low thickness of Co interlayers among WC grains. For example, the near-nano carbide grade with 10% Co has a hardness of nearly 19GPa compared with 16GPa for a conventional submicron carbide grade. As a result, the difference in wear-resistance between these two materials is about seven times, which is achieved with minimal sacrifice of fracture toughness.

Alloying binders

The development of nanostructured cemented carbides is based on novel approaches to their interfacial design on the atomic level, which allows the suppressing of WC nanopowder grain growth without weakening WC-Co grain boundaries. Alloying the Co binder with large amounts of chromium leads to the segregation of chromium at WC-binder interfaces, resulting in the formation of interlayers of mixed carbides of several atomic monolayers. Such interlayers can suppress the WC grain growth without degradation of fracture toughness and transverse rupture strength of nanostructured cemented carbides.

Polycrystalline diamond

Pure diamond in applications characterised by high impact loads, for example in mining and construction, is limited. To solve this problem, polycrystalline diamond (PCD) comprising diamond grains with a Co-based binder is fabricated at elevated temperatures and ultra-high pressures on a large scale. Results of the ASTM B611 wear test on a range of PCD materials indicated the wear-resistance of those materials exceed that of conventional WC-Co cemented carbides by more than two orders of magnitude. Such a dramatic increase in abrasion resistance is achieved at a reasonably high value of fracture toughness, ensuring the effective employment of the novel PCD grades in mining and construction applications. Results of numerous field tests provide evidence that construction tools with inserts comprising PCD layers on carbide substrates can achieve up to 50 times longer tool life.