Fig. 1. XRD patterns for nanopolycrystalline WC ceramics recovered from sintering WC nanopowder at 6 GPa and various temperatures, measured under ambient conditions by using Cu-Kα radiation with a wavelength λ of 1.5406 Å.

Fig. 2. Density and relative density of samples sintered at 6 GPa and various temperatures.

Fig. 3. Fracture surface for samples sintered at 6 GPa and (a) 800 °C, (b) 1200 °C, (c) 1400 °C, and (d) 1600 °C.

Fig. 4. Mechanical properties of nanocrystalline WC obtained at 6 GPa and various temperatures. (a) The room temperature Hv of binder-free nanocrystalline WC as a function of sintering temperature. (b) Vickers indentation of binder-free nanopolycrystalline WC sintered at 6 GPa and 1400 °C.

Fig. 5. Temperature dependence mechanical properties of nanopolycrystalline WC sintered at 6 GPa and 1400 °C. (a) Temperature dependence Hv for the binder-free nanopolycrystalline WC sample sintered at 6 GPa and 1400 °C. The literature data for SiC, Al2O3, TiC, WC-6Co, and WC-10Co alloys are also shown here for comparison [7,[24], [25], [26]]. The solid lines show the fitting curves according to Eq. (3). (b) and (d) are the Vickers indentation obtained at 1000 °C for binder-free nanopolycrystalline WC sintered at 6 GPa and 1400 °C and commercial cemented carbide YG10, respectively. (c) Comparison of the k and H0 coefficient in Eq. (3) for binder-free nanopolycrystalline WC obtained in the present work with those of traditional technical ceramics [25,26].

Fig. 6. Microstructure of nanopolycrystalline WC sintered at 6 GPa and 1400 °C. (a) STEM image of nanopolycrystalline WC sintered at 6 GPa and 1400 °C shows abundant defects in terms of dislocations and stacking faults. (b) SAED pattern of nanopolycrystalline WC sintered at 6 GPa and 1400 °C. (c) The grain size distribution of the sample sintered at 6 GPa and 1400 °C. (d) Enlarged STEM image showing the dislocation substructure in the 1400 °C sintered sample.

Fig. 7. Microstructure of the binder-free nanopolycrystalline WC was annealed at 1000 °C. (a) STEM image of the binder-free nanopolycrystalline WC subjected to 1000 °C annealing showing the microstructure evolution. The thermo-induced dislocation migration and annihilation are not obvious, and there are still many dislocations and stacking faults within grains. (b) The typical Σ2 grain boundary is seen in a WC bicrystal. The upper right inset is the SAED pattern of this grain, confirming the presence of the Σ2 boundary. (c) HRTEM image of the Σ2 boundary. The left side of (c) shows grain A viewed along [0001] direction, and the right side of (c) shows the grain B viewed along $\left[\bar{1}2\bar{1}0 \right]$ direction. Grains A and B are highly coherent, and some stacking faults exist near the Σ2 boundary.

Fig. 8. Microstructure of the binder-free nanopolycrystalline WC annealed at 1000 °C using t-EBSD. (a) The inverse pole figure and grain boundaries. The Σ2 boundaries are shown in red and the normal grain boundaries are shown in black. (b) The misorientation angle distribution for the 1000 °C annealed binder-free nanopolycrystalline WC.