Preparation Of Nanosized WC、VC And Its Composite Powders And Characterization Of Their Sintered Block Properties

The strength and hardness of WC-Co alloys would increase with decreasing the grain size of WC. The mechanical properties will increase manifold as the grain size of WC is decreased to nanoscale. At present, the fabrication of nanocemented carbides have become the focus in the field of super hard materials because of their excellent properties, such as high strength, high hardness, good wear and fracture toughness. The key technologies involved in the study of nanocemented carbide are as follows:(1) Manufacturing the nanocomposite powder of nanocemented carbide with pure phases, small size and homogeneity;(2) Inhibiting the grain growth in the sintering process by the addition of grain growth inhibitors and through the development of low-temperature, fast and pressure sintering process.In this dissertation, a type of simple and rapid technical method with low cost is put forward for the preparation of nanocemented carbide. The hydrothermal method has been used in the process of preparing nanocemented carbide. The precursor with a carbon-coated core-shell structure was synthesized via the hydrothermal method. A series of nanomaterials, such as WC, VC, WC–Co, and WC–Co–VC, were prepared via in situ reduction and carbonization of the precursor. The composite powders were then prepared to obtain the cemented carbide bulks via the powder metallurgy technology. With this technology, the uniformity of composite materials has been effectively improved and the high-quality nanocemented carbide powders were obtained. Furthermore, the mechanical properties have been improved through adding the grain growth inhibitors in the early, which were dispersed more uniform in the powders. The processes for the preparation of nanosized WC and its composite material, along with the properties of the cemented carbide bulks and preparing mechanisms, were systematically analyzed in this study. The detailed contents are shown as follows:1. The process for the preparation of carbon-coated core-shell structure via the hydrothermal method and its mechanism were systematically studied. The structure and composition of the production were studied via scanning electron microscopy(SEM), transmission electron microscopy(TEM), high resolution Transmission electron microscopy(HRTEM), Fourier-transform infrared spectroscopy(FT-IR), and X-ray powder diffraction(XRD), respectively. The yield ratio of carbonization, characteristics of the morphology of the products, and core-shell structure of several organic matters in the hydrothermal reaction were studied. Soluble starch was selected as the organic carbon source. The effects of the hydrothermal reaction time, reaction temperature, pH value of the solution, dispersant, and reaction promoters on the products were analyzed. The results showed that the hydrothermal reaction time, pH value, and dispersant were the main factors that affect the yield ratio of the nucleation of metal salt and grain size. The hydrothermal temperature and the addition amount of the reaction promoters primarily affected the carbonization process of soluble starch. The preparation mechanism was discussed, and the optimized conditions for preparing the core-shell structure were obtained. The spherical particles with a core-shell structure were synthesized, and their diameters were in the range of 30 nm to 60 nm.2. The WC and VC nanopowders, together with their nanocomposite powders such as WC-Co, and WC-Co-VC, were prepared via in situ reaction method by using the core-shell structure with different component, and the technical parameters for synthesizing nanopowders were studied. According to the thermodynamic theory, the thermodynamic process of each reaction system was calculated and studied. Based on the thermodynamic analysis results, the reaction scheme was designed, and the reaction process and technical parameters were systematically analyzed. At 950~1000 °C and the holding time 0.5 h to 1 h, WC, VC, WC-Co, and WC-Co-VC composite powders were prepared with pure phases, homogeneity, and average grain sizes of 40 nm to 80 nm. Compared with the conventional process, the reaction temperature of this process was reduced by 200 °C to 500 °C, and the reaction time was shortened by 20 h to 70 h. Analysis the reaction mechanism were showed that the contact area of the reactants increased, whereas the diffusion distance of the atoms involved in the reaction decreased and the activation energy decreased because of the uniform mixing of all raw materials in the core-shell precursors. In addition, the Co metal was as a catalyst in carbonation reaction to enable the reaction was successfully completed at a relatively low temperature and in a short time period.3. The nanosized WC–Co and WC–Co–VC composite powders were sintered into sample bulks via powder metallurgy technology. The effects of sintering temperature, sintering time, and VC addition amount on the microstructure and mechanical properties of the products were analyzed. The experimental results indicated that the grain size of the obtained bulk sample with WC was 520 nm, Vivtorinox hardness(HV30) was 1720 kg/mm2, and the transverse rupture strength was 2035 MPa when the sintering temperature of WC-10 wt%Co was 1260 °C and the heat preservation was 30 min. In the same conditions, the average grain size of WC in WC-10wt%Co-0.6wt%VC bulk was 280 nm, Vivtorinox hardness(HV30) was 1830 kg/mm2, and transverse rupture strength was 2000 MPa. This finding indicated that the addition of VC can effectively inhibit the growth of WC grains in the sintered composite samples and can improve the hardness of the alloy. Compared with the sample which has the same component prepared by traditional method, the hardness of this process has improved 6%~8.6% and the transverse rupture strength has increased 16.7~18.3%%.4. The mechanism for WC grain growth and the action mechanism of VC inhibitor in the sintering process were studied. In the liquid phase sintering process, the irregular, large WC grains had large activation energy, which was the energy source for the two-dimensional nucleation. The dissolved WC easily obtains the surface energy to form many two-dimensional nucleations, and then the liquid phase precipitated along these nucleations, which resulted in the growth of WC grains. However, the VC inhibitor would preferentially precipitate along the WC/Co and WC/WC interfaces. The energy that impedes the two-dimensional nucleation at the WC interface increased. Thus, the growth process of WC was retarded, which inhibited the WC grain growth.