Cemented carbides, consisting of WC grains bound by the Co phase, have been used as cutting tools, rock drill tips and other wear resistant components during the decades. The effect of the variations of Co composition and the addition of other types of cubic carbides have been investigated to improve the properties of WC-Co cemented carbides. In recent days, nanocrystalline WC-Co cemented carbides have been developed by thermochemical and thermomechanical process named as spray conversion process. Recent studies of mechanical properties of nanocrystalline WC-Co cemented carbides proposed different mechanisms for fracture and deformation of nanocrystalline WC-Co cemented carbides according to the microstructure and chemical composition. Representatively, WC-TiC-Co cemented carbides which have better hardness and wear resistance than WC-Co cemeted carbides have been introduced to improve mechanical performance for wider applications of cemented carbides. However, researches on ultra-fine and nanocrystalline WC-TiC-Co cemented carbides have not been carried out.
In this study, in order to investigate the effect of WC grain size refinement in WC-TiC-Co cemented carbides, the microstructure and mechanical properties were analyzed by varying the WC grain size in various TiC content. The WC grain size was controlled by varying initial WC powder size in range from 0.5 to 4 μm and the TiC content was varied from 0 to 20 wt.% with constant Co binder composition of 10 wt.%. The powders were mixed and wet-ball-milled with paraffin for 24 hours and dried in vacuum furnace at 100℃ for 10 hours. In order to investigate the effect of consolidation process on mechanical performance, several compaction and sintering process were introduced including cold compaction and cold isostatic pressing(CIP), liquid phase sintering(LPS), hot isostatic pressing(HIP), and sinter-HIP. Mechanical properties were evaluated by Vickers indentation method for hardness and fracture toughness.
The microstructure of WC-TiC-Co cemented carbides consisted of several phases including WC, TiC, (Ti,W)C, Co. Between TiC and (Ti,W)C phase, there exist core-rim structure in which TiC forms core phase and (Ti,W)C rim phase. The fraction of (Ti,W)C rim phase increases as increasing the TiC content, while the WC grain size and volume fraction decreases as increasing the TiC content. In given TiC content and grain size, the microstructure of WC-TiC-Co cemented carbides depended on the WC grain size; in ultra-fine WC-TiC-Co cemented carbides, the WC grains were surrounded by Co binder phase and there exist binder-free region arround TiC, (Ti,W)C core-rim phase, while in conventional ones, the WC and TiC, (Ti,W)C core-rim phase surrounded by Co binder phase.
The hardness of WC-TiC-Co cemented carbides were sensitively dependent on the porosity. The hardness of cemented carbides sintered by liquid phase sintering, which have higher porosity in higher TiC contents, decreases over 15 wt.% of TiC content due to high porosity over 3-4 %. In contrast, the hardness of cemented carbides sintered with full densification by HIP and sinter-HIP process increases with increasing the TiC content. In case of given composition, the hardness of WC-TiC-10Co cemented carbides increased with decreasing the WC grain size ranged from 0.5 to 4 μm. The hardness of WC-10Co cemented carbides satisfied the Hall-Petch type relation with the WC grain size. Dislocation pile-up model was applied to confirm hardness of WC-TiC-Co cemented carbides. Relatively, calculated hardness value of WC-TiC-10Co cemented carbides was matched well with measured hardness value. However, the hardness of ultra-fine WC-TiC-10Co cemented carbides had variation from tendency of Hall-Petch relation of conventional ones.
The fracture toughness of WC-TiC-Co cemented carbides decreased with increasing TiC content and refining WC grain size. In WC-Co cemented carbides, the grain size refinement improves hardness while fracture toughness decreases very sensitively. However, in WC-TiC-Co cemented carbides, the grain size refinements improves hardness without decreasing the fracture toughness. WC-TiC-Co cemented carbides have a better hardness to fracture toughness relationship.
