Abstract:
The discovery of WC-Co metal-ceramic composites, cemented carbides, was one of the most important technological revolutions of the last century [1, 2]. Such functional hard materials possess a unique combination of strength and fracture toughness on the one hand, and hardness and wear-resistance on the other hand as a result of combining a hard ceramic phase, tungsten carbide, with a ductile and tough cobalt binder [3]. Nevertheless, presently, for many applications there is a need for hard materials with a significantly improved combination of hardness, toughness and wear-resistance. However, traditional wisdom indicates that hardness and wear-resistance are contradictory and incompatible material properties when compared to toughness, so that in conventional hard materials an increase of hardness and wear-resistance can be achieved only at the expense of fracture toughness [3, 4].
A number of new approaches to the fabrication of novel hard materials with improved combinations of hardness, fracture toughness and wear-resistance were elaborated and implemented in industry.
One of these approaches is based on creating functionally graded WC-Co materials, known as ‘Gradient Carbides’, with a tailored gradient of Co contents from a near-surface layer towards a core region. The novel hard materials comprise a hard surface layer containing much WC phase and a tough core containing lots of Co, which results in an exceptionally high combination of hardness and fracture toughness of the surface layer.
The second approach ensuring the hardness/toughness/wear-resistance trade-off to be overcome is based on employing nanotechnology. A range of novel hard materials with nano/micro hierarchical structure were developed and are presently widely employed in the mining and construction industry. An unusual combination of the ultra-coarse-grain microstructure structured on the µm-level and the binder phase structured on the nm-level provides an extraordinarily high combination of both transverse rupture strength/fracture toughness and hardness/wear-resistance.
The third approach comprises the development and implementation of nanostructured cemented carbides also known as ‘near-nano carbides’ with a WC mean grain size of about 150 nm. These hard materials are characterized by a significantly improved combination of hardness, wear-resistance and fracture toughness, which ensures their dramatically prolonged lifetime in different applications.
|
