Research On The Microstructure And Properties Of Binderless Tungsten Carbide Cemented Carbides Prepared By High Pressure

With the complexity of the environment in the machining industry and the implementation of sustainable development strategies,shortcomings of tungsten carbide cobalt（WC-Co）cemented carbide such as poor corrosion resistance,low high-temperature hardness,and lack of raw materials are gradually exposed.Binderless cemented carbides with high hardness,corrosion resistance,and excellent cutting performance emerge and are widely used in the manufacture of cutting tools,precision molds,and other products.In order to understand the effects of nano tungsten carbide（WC）powder,high pressure high temperature synthesis（HPHT）method,and silicon carbide whisker（SiC_W）on the microstructure and properties of pure WC cemented carbide,research on the preparation of nano WC powder,optimization of HPHT method,Hall-Petch relationship of nanocrystalline pure WC cemented carbide,and HPHT preparation and microstructure and properties of WC-SiC_W bulk materials was studied.By exploring the effects of WC powder raw materials,HPHT process parameters,and SiC_W content on the phase composition,densification,WC grain growth,microstructure evolution process of interface and defects,element distribution,and mechanical properties of pure WC cemented carbide,the relationship between the composition,process,structure,and properties of pure WC cemented carbide was revealed.The research results are as follows:Ammonium metatungstate hydrate（AMT）and sucrose（SUC）were used as raw materials.And high-purity nano WC powders were prepared by liquid phase method and reductive carbonization method.The effects of process parameters such as molar ratio of element C and W（n（C）/n（W））and synthesis temperature on the phase composition,microscopic morphology,and average particle size and distribution of WC particles were studied.The results show that when n（C）/n（W）=4,1400℃ is the lowest temperature for synthesizing single-phase WC powder with an average particle size of 85 nm;When the temperature rises to 1450℃,the purity of WC decreases,and the number of strip-shaped particles increases,and the particles grow abnormally.The introduction of an excess C source not only facilitates the smooth progress of the carbonization reduction reaction,but also increases the reaction area and shortens the diffusion distance,thereby obtaining fine and uniform equiaxed WC particles with a minimum average particle size of 74 nm.Nano WC powder was used as raw material.And pure WC cemented carbide was prepared by HPHT method.The effects of HPHT technology on sintering densification of nano WC powders,WC grain growth,phase composition,interface and defects of the sintered body,microstructure evolution and mechanical properties were studied.The high-pressure effect of HPHT technology and the relationship between high-pressure microstructure and properties were clarified.The results show that a pure WC cemented carbide with a relative density of 98.3% and an average grain size of 103 nm was sintered at 1400℃,and there are a large number of dislocations,stacking faults,subgrain boundaries.and low energy ∑2 grain boundaries in the alloy structure.Its Vickers hardness remains 23.4 GPa at 1000℃,which is only 27%lower than that at room temperature（29.7 GPa）.Its thermal mechanical stability is significantly superior to traditional ceramic materials such as WC-Co,silicon carbide（SiC）,and titanium carbide（Ti C）.Nano or micron WC powder were used as raw material.And pure WC cemented carbide was prepared by HPHT method.High-pressure densification and grain growth processes of WC powders with different particle sizes were studied.The effects of average grain size on the hardness of pure WC cemented carbide prepared by hot pressing（HP）,spark plasma sintering（SPS）,and HPHT techniques were compared.The Hall-Petch relationship of nanocrystalline pure WC cemented carbide materials was explored.The results show that the high stress of HPHT method has a significant inhibiting effect on the grain growth of pure WC cemented carbide.When WC powder with an average particle size of 60 nm was used as raw material,the average grain size,Vickers hardness,and fracture toughness of the bulk samples sintered at 1300℃were 93 nm,29.85±0.27 GPa,and 8.15±0.18 MPa m^1/2,respectively.The average grain size of nearly fully dense pure WC cemented carbide prepared by HPHT and SPS sintering processes including bulk samples of this work is 93 nm～34μm,which all conform to Hall-Petch relationship.Nano WC powder and SiC_W were used as raw materials.And WC-SiC_W composite materials were prepared by ultrasonic mixing method and HPHT method.The effects of SiC_Wcontent on sintering densification,grain size,shape,phase composition,microstructure,and mechanical properties of WC-SiC_Wcomposites were studied.The results show that the relative density of WC-5 wt.%SiC_W bulk specimens is 98.5%,and the Vickers hardness,fracture toughness,and flexural strength reach 32.86 GPa,7.4 MPa m^1/2,and 1494 MPa,respectively.The inherent low thermal expansion coefficient,nanocrystalline structure,high density dislocations,high proportion of low energy Σ2 grain boundaries,the strengthening and toughening mechanisms of crack deflection,as well as cleavage and pull-out of SiC_Wsynergistically promote the mechanical properties of WC-SiC_W composites.