Sheremet V. Regularities of the formation of the structure and physicomechanical properties of WC-Co cemented carbides under isostatic pressure

The PhD thesis is devoted to the study of the regularities of the influence of cold isostatic pressing (CIP) of unsintered compacts of WC-Co cemented carbides on the formation of structure, phase formation kinetics, a set of physical and mechanical properties, and performance characteristics of sintered products from these materials. Particular attention is paid to the search for technological methods to improve the performance properties of sintered products by applying high hydrostatic pressure (HHP) treatment. Thus, it has been established that an actual objective is to create scientific and practical principles for the development of the cemented carbides production and processing technology which would ensure an increase in the level of physical and mechanical properties and performance characteristics and would be devoid of the disadvantages inherent in conventional methods. Based on the established dependencies, the choice of the optimal CIP pressure is substantiated, which ensures the highest density of the compact, the formation of the cemented carbides structure with smaller WC particles, and an increase in the level of residual stresses in WC. The correlation between the change in the stress-strain state of WC-8Co and WC-15Co cemented carbides compacts produced by CIP and the kinetics of phase formation processes during their sintering was investigated. It is shown that unlike the sintered cemented carbides compacts of WC-8Co and WC-15Co produced by uniaxial pressing, in which cobalt was mainly in the FCC polymorphic modification, cobalt in composites of identical chemical composition that were subjected to CIP before sintering was mainly in the HCP polymorphic modification. The XRD analysis results indicate a change in the ratio of diffraction intensities of the WC hexagonal lattice planes and the total diffraction intensity of the FCC-Co planes in the WC-8Co-0.3VC cemented carbide produced by CIP compared to the composite produced by uniaxial pressing, indicating a decrease in the average WC grain size and a change in the distribution of the cobalt binder in the structure of the composite produced by applying CIP. It was found that the change in polymorphic modification from FCC to HCP of a significant amount of cobalt and a decrease in the average grain size of WC in the WC 8Co cemented carbide produced by using CIP led to an increase in the hardness and strength of the composite compared to the alloy obtained by uniaxial pressing. It is shown that the CIP of the cemented carbide with the addition of VC grain growth inhibitor formed conditions that ensured the formation of a bimodal structure of the sintered composite with homogeneous areas of the binder phase - cobalt pools. It was found that with a decrease in WC average grain size in the composite that were subjected to CIP as well as the tendency to increase hardness atypically as for cemented carbides with grain growth inhibitors the crack resistance of the composite increased by 1,5 times compared to the WC-8Co-0.3VC cemented carbide produced by uniaxial pressing. It was found that the change in the type of crystal lattice from FCC to HCP of a significant amount of cobalt of WC-15Co cemented carbide initiated by CIP affected the decrease in the ability to resist crack formation and propagation but led to an increase in the strength and hardness of the composite.It has been established that in comparison with the tools made of WC-6Co, WC-8Co, and WC-15Co cemented carbides produced by uniaxial pressing, the cold isostatically pressed tools made of identical chemical composition composites showed better wear resistance during its operational testing. Plastic deformation of the surface layers of drilling, cutting, and deforming tools with subsequent delamination of the deformed material volume during adhesive wear was inherent in the composites produced by uniaxial pressing, and gradual abrasive wear of the tool surface layers with subsequent crumbling of individual WC grains was typical for the composites produced by CIP. The method for cemented carbides processing by high hydrostatic pressure (HHP) is proposed. It is shown that the HHP treatment of WC-15Co cemented carbide initiates a martensitic phase transition in the binder component which is accompanied by an increase in the number of HCP polymorphic modification of cobalt in the composite. It was found that the HHP treatment forms a gradient distribution of properties over the volume of the composite, so the changes caused by the treatment are most pronounced on the surface of the composite and less pronounced in its volume as it moves away from the surface. It has been experimentally established that HHP treatment of WC-15Co cemented carbide increases its strength, hardness, and wear resistance of deforming tools made of this material.