In-Situ Synthesis Of WC-Co Composite Powder And Sintering Preparation Of Bulk Cemented Carbides

Compared to micro-grained alloy, ultrafine-grained and nanocrystalline cemented carbides have much higher hardness, wear resistant, fracture toughness and transverse rupture strength. Therefore, the research on the ultrafine-grained and nanocrystalline WC-Co cemented carbides becomes an important developing direction in the hardmetal industry. The synthesis of WC-Co powder and its sintering are two key steps in preparing the ultrafine structured cemented carbides.Firstly, taking advantage of spark plasma sintering (SPS) technique, we investigated the rapid route for synthesis of WC-6wt.%Co composite powder by in situ reduction and carbonization reactions and immediate consolidating the composite powder into the bulk in SPS system, using low-cost tungsten oxide, cobalt oxide and carbon black as raw materials. The whole process with the duration less than 30min could be divided into four stages, i.e., the heating of the powder mixture, in situ reduction and carbonization reactions, sintering densification and isothermal holding. The effects of milling time on the particle size and size distribution, and the effects of the amount of carbon addition in the mixture powders, the reaction temperature, the sintering temperature, the holding time and the sintering pressure on the phases and properties of the as-sintered bulk, were analyzed systematically. The cemented carbides bulks with high density and good combined mechanical properties were obtained. The mechanism of the rapid in situ preparation of cemented carbides bulks in the SPS system was proposed for the first time. According to the phenomenon of melting or boiling at the sintering neck and the thermodynamics of reactions, the reasons of the increased rate in the non-equilibrium reactions and the differences of the reaction amount and the type and amount of the resultant from equilibrium reactions were discussed. This mechanism provided an important theoretical basis for the in situ reaction synthesis in the SPS system.Secondly, the thermodynamics, the dynamics and the stepwise reaction processes in vacuum at the equilibrium state, using tungsten oxide, cobalt oxide and carbon black as raw powders, were investigated. By the thermodynamic mechanisms of the reactions in the synthesis process of WC-6wt.%Co composite powder, the starting temperature, regularity and stability of the resultants were quantitatively described. It was found that the reactions of carbon reducing tungsten oxide and cobalt oxide occurred below 8500C, and the sequence of products of WO₃ reduced by carbon was WO_2.9, WO₂, WO_2.72 and W. The intermediate product of CoWO₄, the products of Co and tungsten oxides played as the catalytic role. The reactions generated at above 850℃were the carbonization of tungsten, and theηphases disappeared with the reaction temperature increased. W₂C could not be transformed into WC at above 1127℃, as the change of the free energy of the reaction was higher than zero. The results of a series of synthetic experiments verified the mechanism predictions. In the study of the dynamics of in situ reaction synthesis of WC-Co composite powder, the process of tungsten oxide reduced by carbon consisted of the direct reaction of solid carbon with tungsten oxide and the indirect reaction of CO gas reducing tungsten. The fact that increaseing reaction temperature and porosity of the product were favorable to accelerate the reaction was stated. It was shown by further research that the formed tungsten oxide had an obvious layer-like structure, where from outer to inner layer the valence of tungsten increased and the reduction degree decreased. The carbonization of W depended on the replacement of W atom at the center of the body-centered cubic by C atom and the minor adjustment of C atom position and the lattice parameters.The carbon content in the raw powders and the effects of preparing parameters on the phase constitution and WC grain size were studied comprehensively by a serials of experiments. Increasing the milling and reaction time, reaction temperature and vacuum degree, were favorable to improve the purity of the phases. Increasing milling time, decreaseing reaction temperature and holding time, were good for refining the particle size. Based on the combination of the thermodynamics of reaction mechnism and experiments, the optimum processing parameters for synthesis of WC-Co composite powder were obtained, and an innovative route with low reaction temperature, short reaction time, pure phases and controllable WC particle size was developed.At last, the ultrafine cemented carbide bulk with high properties was prepared by spark plasma sintering the composite powder. The effects of SPS parameters on the microstructure and properties of the as-sintered bulk were investigated systematically. The experimental results showed that the densification temperature of the composite powder started at 804℃and finished at 1175℃. The WC grain size, grain size distribution and mechanical properties were influenced by the sintering temperature, the pressure and the holding time. The WC-6Co cemented carbides bulk with good combined mechanical properties of the hardness of 92.6HRA, the fracture toughness of 12MPa·m^1/2 and the transverse rupture strength of 2180MPa were obtained, with the optimal sintering parameters of the sintering temperature of 1325℃, the pressure of 50MPa and the holding time of 6-8min. When the grain growth inhibitors were added into the composite powder before sintering, the WC grain was refined and the hardness was improved. The rupture mechanism of the cemented carbides bulk was discussed at the end.