Properties of binderless hardmetal densified by SinterHIP, hot pressing and SPS

The properties of binderless WC made of super ultra-fine WC powder and densified using different sintering methods (sinterHIP, hot pressing, and SPS) are compared. Binderless WC was sintered to full density by all techniques investigated. The best combination of hardness and fracture toughness was obtained by SPS (HV10=2875, KIc=6.5 MPa m(exp1/2). The microhardness HVO.1=3460 can be compared with boron carbide. Properties can be explained by the extremely fine-grained microstructure. In processing ultrafine and nanocrystalline powders spark-plasma sintering offers an interesting opportunity to reduce sintering temperature and time and to suppress grain growth. With binderless cemented tungsten carbide an outstanding high hardness HV10=2875 or HVO.1=3460 was achieved in first trials without optimization of the process. This hardness compares with boron carbide and is significantly surpassed by super-hard materials only which are produced at high pressure and high temperature. Furthermore, the fracture toughness Kic=6.5 MPam172 is slightly higher than with gas pressure sintering or hot pressing (5.0- 5.6 MPa m(exp1/2). This may be traced back to the homogeneous microstructure of the super fine material. Comparing hardness and toughness of ultra-fine WC-Co hardmetals of different binder content which were sintered by different techniques also suggests that spark- plasma sintering may result in improved mechanical properties. As the correlation of hardness and toughness is significantly influenced by grain size further experiments including grain size of the sintered material and identical fracture toughness measurement will be carried out to confirm this statement. At least with binderless cemented tungsten carbide the lower thermal load during sintering, i. e. a high heating rate, relatively low sintering temperature and a short sintering tune, are considered responsible for the improvements. The reasons why sintering is accelerated in SPS are not clear yet. Further research work is necessary to understand and to take full advantage of this process.