Engineered surfaces on cemented carbides obtained by tailored sintering techniques

The paper consists of three parts. In the first part, functionally graded cemented carbides comprising hard surface layers with low Co contents are described. Such surface layers form as a result of a tailored sintering technique based on the selective carburization of the near-surface layer of carbide green bodies with original low carbon contents and consequent liquid-phase sintering, which leads to Co drifts from the surface towards the core during sintering. This novel sintering technique allows obtaining Co gradients between the surface layer and the core of carbide articles of up 7 wt.%. As a result of significantly different contraction rates between the near-surface layer and the core of such functionally gradient cemented carbides, high residual compression stresses are created in the carbide near-surface layer. This leads to a dramatically increased combination of both hardness and fracture toughness of the near-surface layer. The presence of the hard and tough surface layers on the novel functionally gradient carbides results in their significantly improved wear-resistance and prolonged tool lifetime in mining applications. In the second part of the paper, functionally graded cemented carbides comprising surface layers with high Co contents are described. They are fabricated by a tailored sintering technique based on the selective de-carburization of the near-surface layer of carbide green bodies with original high carbon contents resulting in Co drifts from the core towards the surface during the consequent stage of liquid-phase sintering. Such functionally graded cemented carbides are successfully employed as substrates for poly-crystalline diamond (PCD) layers. In the third part of the paper, a mechanism of formation of thin Co films on the surface of carbide articles during sintering, which is designated in literature as “Co capping”, is briefly described. On the basis of understanding the mechanism of the Co capping phenomenon, an industrial technology for obtaining such thin Co films on the surface of carbide articles during sintering was developed. The wettability of non-ground carbide articles by braze alloys is dramatically improved by use of the thin Co films, resulting in better quality of various brazed wear parts and tools.