Microstrctures Characterizition And Properties Analyses Of Nanocrystalline WC-Co Cemented Carbides

Due to the preparation technique limitations, it is rarely reported in the literatureabout the “real” nanocrystalline tungsten cemented carbide bulk materials with grainsize below100nm. It is still very lack of systematic research on the relationshipbetween the microstructure and mechanical properties of nanocrystalline cementedcarbides. In this thesis, using tungsten oxide, cobalt oxide and carbon black as the rawpowders, the WC-Co composite powder at the nanoscale was firstly synthesized byin-situ reduction and carbonization reactions. The nanocrystalline cemented carbideswith high hardness and fracture toughness were prepared by spark plasma sintering(SPS) by optimizing the powder preparation and sintering processes. Based on thein-situ synthesized composite powder and SPS technique, a short-term route for highperformance nanocrystalline cemented carbide bulks has been proposed. The factorsthat influence the properties of nanocrystalline cemented carbides have beenextensively studied. Further, influences of composition, microstructure and microdefects of nanocrystalline cemented carbides on the hardness and fracture toughnesshave been investigated. The main differences in the toughening mechanisms betweennanocrystalline and traditional cemented carbides were indicated.Firstly, the in situ reaction process between WO2.9, Co3O4and C was theoreticallyanalyzed and predicted. The suitable mixing ratio of raw powders was determinedbased on theoretical calculations and experiments. By optimizing the powder millingand in-situ reaction parameters, the most appropriate processing for preparing nanoWC-Co composite powders was obtained. The effect of carbon addition in the rawpowders on the phase constitution of as-synthesized composite powder was studied.The total carbon content in the composite powder synthesized with16.74wt.%carbonaddition is quite close to the theoretical value of pure WC-10wt.%Co powder. Theoptimal carbon addition can be found depending on the phase constitutions ofas-synthesized composite powder and the subsequently sintered nanocrystalline bulk.Take the nano WC-Co composite powder as the starting material, by optimizingthe sintering temperature, pressure and addition of grain growth inhibitors, thenanocrystalline cemented carbide bulk with a mean grain size of65nm was prepared.As measured, its hardness and the fracture toughness are2050kg/mm2and14.5MPa m1/2, respectively. The effect of carbon addition in the raw powders on the phase constitution, microstructure and properties of the sintered bulk was studied.With WC-10wt.%Co cemented carbide as an example, the interface structureswere characterized in details. It was found that the coherent and semi-coherentinterfaces exist at the WC/WC and WC/Co boundaries in a high proportion underoptimal carbon content and processing. The interface features improve the interfacialstrength effectively, hence prevent the crack nucleation and propagation. Graingrowth inhibitor hinders the WC grain boundary migration and hence the graingrowth by the formation of WC/VC interface coherency. At some WC/WC andWC/Co interfaces, the WC1-xphase forms, and the mechanism in the viewpoint ofcrystal structure and interface energy was proposed.The difference in the orientation of WC grain at the surface and in the inner bulkwas found for the nanocrystalline cemented carbides with different content of cobaltand carbon. Phase constitution and relative intensity diffraction at the surface and inthe cross-section of the bulk were analyzed using XRD refinement results. Therelative number of WC planes of (0001),(10-10) and (10-11) at the surface and in thecross-section of the bulk is different. The hardness increases with the increase of thenumber of plane (0001). The probability of coherency state is larger with the increaseof the ratio between the numbers of plane (0001) and (10-10), thus it is advantageousin improving the fracture toughness. Both hardness and toughness reach the maximumat the surface of the bulk prepared by SPS process, based on optimized carbon contentin the synthesized WC-Co composite powder.Systematic studies on the quantification of the microstructures in thenanocrystalline cemented carbides have been performed. The relationship among thepreparation conditions, microstructure parameters and mechanical properties havebeen analyzed. The toughening mechanisms for the nanocrystalline and traditionalcemented carbides were detailedly compared. Based on the quantitativecharacterizations of the microstructure parameters, the changing tendencies of themechanical properties with the microstructure were proposed.