Study On WC-Al₂O₃ Composites Reinforced By Additives

Cemented carbide is a high-performance alloy material made by powder metallurgy technology,which is widely used in many fields of mechanical processing,metallurgy,aerospace and electronic communication.It usually composed of two or more hard compounds of refractory metals and bonding metals,the most typical of which is tungsten-cobalt cemented carbide with the bonding phase of Co.Since it was discovered,it has been widely used due to its high hardness,high wear resistance,strong corrosion resistance and a series of excellent properties.However,Co is a non-renewable resource.In 2019,China accounted for just 1%of world production of cobalt,while high-quality Co resources are mostly need imported.Therefore,if a material can be found and made into a composite material with comprehensive properties similar to tungsten-cobalt cemented carbide,it will have high application value for the development of cemented carbide in China.In recent years,the preparation technology of WC and Al ₂O₃ composites has gradually matured and has become one of the research directions for exploring the substitution of Co-based cemented carbides.However,the mechanical properties of the new WC-Al₂O₃ composite still have a large gap,and there is a huge space for optimization of the comprehensive properties.This study uses graphene,which has been highly rese arched in recent years,as the reinforcing phase in the composite matrix,and cooperates with the widely used rare earth oxide CeO₂ in the ceramic field and the grain growth inhibitor VC as an additive to enhance the performance of WC-Al₂O₃ composites.The mixed powder is prepared by dry grinding and then wet grinding.The WC-Al₂O₃composite material containing graphene in different proportions and the graphene-containing WC-Al₂O₃ composite material doped with CeO₂ and VC are sequentially prepared by the method of vacuum hot pressing and sintering.The effects of these three additives on the densification,microstructure and mechanical properties of WC-Al₂O₃ composites were studied,and analyzed the influence of CeO₂ and VC on the grain growth process and it s toughening mechanism during the sintering process.This article is a basic research to provide theoretical basis and practical experience for further improving the mechanical properties of WC-Al₂O₃ cemented carbide materials.It also provides improved ideas for the unsatisfactory aspects of the experiment.The main work and conclusions of the thesis are as follows:1.Mixed powders prepared by high-energy ball milling which contain the dry-milled WC and Al₂O₃ powders and the ultrasonically dispersed graphene alcohol suspension.A series of WC-15vol%Al₂O₃-(0,0.1,0.3,0.6,0.9,1.2)wt.%-GPL(graphene)sintered specimens.Observed and analyzed the morphology and indentation crack pattern of each specimen;tested its hardness,fracture toughness,density and grain size,etc.The results showed that the mechanical properties of the composite material increased first and then decreased with the addition of graphene.When the amount of graphene added is0.1wt%,the mechanical properties of the WC-Al₂O₃ composite are the best,the grain size is the smallest,the Vickers hardness is 18.257GPa,the fracture toughness is 9.3263MPa/m²,but density is reduced to 95.536%.These are trans granular fractures,crack deflection and bifurcation,and bridging and micro-cracks near some cracks,which improve the fracture toughness of the material.Due to the stable mechanical properties of the graphene sheet,the material matrix has a splitting effect,so that there are pore s between the superimposed graphene,and the gas cannot be discharged during the sintering process,so the density is reduced.2.Adding the active sintering powder CeO ₂ to the WC-Al₂O₃ composite material containing graphene,the method of grain refinement is used to increase the density of the sintered sample,and studied the effect of CeO ₂ on the mechanical properties of the WC-Al₂O₃ composite material containing graphene.The results show that by refining the grain size of the material with CeO ₂,the density of the sintered specimen(0.1wt%CeO ₂)is increased to 98.652%,while the hardness is increased,the fracture toughness is also increased to11.76MPa/m²,and the flexural strength is 1009.3 MPa.According to the XRD pattern,CeO₂ reduces the generation of brittle phase W₂C,at the same time,part of Ce⁴⁺are reduced to Ce³⁺during the sintering process,which promotes the solid solution of Ce in Al₂O₃ crystals,increases the vacancies in the crystal lattice,and accelerates the migration speed of grain boundaries.Facilitate the discharge of pores and improve the density.Ce ions that tend to be distributed near the grain boundary can also reduce the free energy on the surface of the grain boundary,inhibit the growth of crystal grains,and reduce the g rain size of WC.3.In order to explore the effect of grain size on the properties of WC-Al₂O₃composites containing graphene.The graphene/WC-Al₂O₃ composite material and the VC/CeO₂/graphene/WC-Al₂O₃ composite material were prepared by added with the grain growth inhibitor VC respectively.Observ ed the cross-sectional structure of the sintered specimen and analyzing the process of grain growth during the sintering process,the results show that VC reduces the grain size of the sintered block before hea t preservation and achieves the effect of grain refinement.However,too fine grain size of the material will cause cracks to tend to fracture along the grain,which is not conducive to the improvement of the fracture toughness of the material.In addition,inadequate growth in the holding stage will cause pores to exist between the crystal grains due to insufficient grain boundaries,which affects the density of the material.