A dual composite of WC-Co
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I. INTRODUCTION
THE WC-Co materials have a good combination of high modulus, high wear resistance, and adequate fracture toughness, making them the choice material for a wide range of industrial applications from metal-cutting inserts to rockdrilling tools. However, cemented tungsten carbide often suffers from catastrophic brittle failure in real-life applications due to its limited fracture toughness. Improving the fracture toughness remains a driving force for cemented tungsten carbide research. The fracture toughness of WC-Co is controlled by the ductile binder-phase cobalt. It can be improved by simply increasing the cobalt content or the grain size. The higher cobalt content or larger grain size yields a larger mean free path (MFP), which is a measure of the size of cobalt pools. As MFP increases, so does the fracture toughness. On the other hand, the wear resistance of WC-Co is proportional to its hardness and, hence, inversely proportional to the fracture toughness. The classic relation between the fracture toughness, the hardness, and the wear resistance is illustrated by Figure 1.[1,2] Increases in MFP generally result in lower hardness and wear resistance. A trade-off must be made between wear resistance and toughness when selecting materials for specific applications. The challenge, for researchers, is to not only improve the fracture toughness but also improve or preserve the wear resistance. On the other hand, this relation between wear resistance, hardness, and toughness can be changed, or even reversed in some situations, depending on a specific wear environment and the microstructure of the material. The relative abrasion resistance of two different grades can be reversed depending on whether it is high stress abrasion or low stress abrasion.[1] In particular, higher fracture toughness is beneficial for abrasive situations where microchipping is prevalent. Coarser grain sizes (or particle sizes) are preferred for their high abrasion resistance and good toughness.[3] All these
ZHIGANG FANG, Director of Materials Research and Development, GREG LOCKWOOD, Project Engineer, and ANTHONY GRIFFO, Senior Materials Project Engineer, are with Smith International, Inc., Houston, TX 77032. Manuscript submitted March 24, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
intertwined relationships between the microstructure, toughness, and wear resistance present difficulties, and also opportunities, for optimizing fracture toughness and wear resistance. There are different approaches for improving the fracture toughness of cemented tungsten carbide, including the option of increasing cobalt content or carbide grain size at the expense of wear resistance. Functionally graded material is another approach for improving effective chipping resistance of components. The cobalt-enriched surface grades by Kennametal,[4] where a cutting tool is thermally processed such that its surface has higher cobalt content than the bulk of the part, is one example. Also, the Sandvik dual property (DP) carbide,[5] consisting of thre
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