Some Trends in Improving WC–Co Hardmetals. I. Hybrid and Coarse-Grained Hardmetals

The paper reviews and analyzes the areas for improving WC–Co hardmetals. One of the areas was initiated by Grygorii Samsonov in the mid-1950s and has proved to be relevant up to now. It deals with the development and study of hardmetals with an inhomogeneous structure resulting from nonuniform distribution of the carbide phase in samples with a cobalt binder. Samsonov showed in 1956 that the materials consisting of VK3 hardmetal grains embedded in a VK15 hardmetal matrix had very high wear resistance, surpassing the wear resistance of uniform VK3 and VK15 hardmetals by 8–10 times. Similar hardmetals but with a cobalt matrix were developed in the 2000s in the USA and were called ‘hybrid’ or ‘double’ hardmetals and later, in Ukraine, ‘mesostructural’ hardmetals. At a given level of hardness, such hardmetals show increased fracture toughness that determines their high wear resistance. Along with hybrid or mesostructural hardmetals, research efforts focusing on coarse-grained and ultracoarse-grained hardmetals, also promoting high wear resistance through increased fracture toughness and plastic strain energy, were resumed. The wear resistance of coarse-grained hardmetals was additionally increased in Germany through hardening of the cobalt matrix, specifically thick cobalt layers, by nanosized hard inclusions such as complex Co 2 W 4 C carbide (θ phase). Comparative wear resistance tests of the coarse-grained and ultrafinegrained hardmetals indicated that those with coarse WC particles were much more preferable. At the same time, the high wear resistance of ultrafine-grained hardmetals was found in abrasive wear tests at low loads.